Fatigue cracking usually links to inadequate road foundations

By Tom Kuennen, Contributing Editor

Fatigue cracking in bituminous pavements is experienced by the public so often that it is considered just part of driving. Motorists endure it and assume it’s just part of an aging pavement. But the perceptive road manager knows that fatigue cracking in less-deep asphalt pavements is a symptom of distressed base layers, and is indicative of a pavement on its way to failure.

This fatigue cracking usually, but not always, is manifested by so-called alligator cracking. Named for its similarity to the pattern on an alligator’s hide, alligator cracking appears as many sided, sharp-angled pieces, usually less than 12 inches on the longest side. This characteristic alligator or chicken wire pattern appears in later stages of deterioration.

There are three types of fatigue cracking, according to the Federal Highway Administration’s Distress Identification Manual for the Long-Term Pavement Performance Program (to view, search for FHWA-RD-03-031). A concise pocket guide for field use – Distress Identification Guide – also is available from FHWA (download by searching FHWA-RD-05-001).

Neither of these guides explain the why of pavement distresses, instead offering precise identification of distresses in order to provide a common, standard definition for use by pavement managers. In these documents FHWA categorizes types of cracking for asphalt pavements, jointed concrete pavement, and continuously reinforced concrete pavements.

• Low-severity fatigue cracking is an area of cracks with no or only a few connecting cracks; the cracks are not spalled or sealed; pumping of base materials out the cracks is not evident.

• Moderate fatigue cracking is manifested by interconnected cracks forming a complete pattern. The cracks may be slightly spalled and may be sealed, and pumping is not evident.

• High-severity fatigue cracking is an area of moderately or severely spalled interconnected cracks forming a complete pattern; pieces may move when subjected to traffic, cracks may be sealed, and pumping may be evident.

It’s easy to look at fatigue cracking in thinner pavements and assume it’s a surface problem, but subsurface investigation will find fatigue cracking is bottom-up cracking, in which stresses propagated to asphalt pavement foundations cause cracks in inadequate base layers. As the asphalt pavement structure flexes under loads, these foundation cracks work their way upward through the pavement. Typically they are found in pavements subjected to repeated traffic loadings, like wheel paths, and can be a series of interconnected cracks.

“[Fatigue cracking] generally occurs when the pavement has been stressed to the limit of its fatigue life by repetitive axle load applications,” according to Hot Mix Asphalt Materials, Mixture Design and Construction, published by the National Asphalt Pavement Association’s Research and Education Foundation. “Fatigue cracking is often associated with loads that are too heavy for the pavement structure or more repetitions of a given load than provided for in design.”

Poorly drained bases exacerbate the problem. As the cracks in the base layers work their way upward, capillary action will draw water from undrained bases up into the pavement, where it damages the pavement structure through reflection cracking, cracks along longitudinal joints, cracks in wheel paths, alligatoring, raveling and potholes.

“The problem is often made worse by inadequate pavement drainage, which contributes to this distress by allowing the pavement layers to become saturated and lose strength,” NAPA says. “The HMA layers experience high strains when the underlying layers are weakened by excess moisture and fail prematurely in fatigue. Fatigue cracking also is often caused by repetitive passes with overweight trucks and/or inadequate pavement thickness due to poor quality control during construction.”

New Look at Top-Down Cracking

Not all fatigue cracking is bottom-up; in thick pavements, cracks may start from the top of the pavement in areas of high localized tensile stresses.

“[L]oad-related top-down fatigue cracking – i.e., cracking that initiates at the surface of the pavement and propagates downward – commonly occurs in hot mix asphalt (HMA) pavement,” say Hong Joon Park and Y. Richard Kim, Ph.D., P.E., North Carolina State University-Raleigh, in their 2013 Transportation Research Board paper, Investigation into the Top-Down Cracking of Asphalt Pavements in North Carolina.

“Top-down Top-cracking cannot be explained by the traditional fatigue mechanisms that are used to explain load-associated fatigue cracking that initiates at the bottom of the pavement,” they say.

Their literature review quotes a field study that found top-down cracking occurs in pavement layers that typically are more than 6.3 inches thick. And stiffness of asphalt does not seem to be an issue, the study says; in sections that exhibit top-down cracking, falling weight deflectometer (FWD) data do not show as much reduction in structural stiffness as do sections that exhibit full-depth cracking.

The researchers used two different methods to study the structural differences between bottom-up and top-down fatigue cracking, the viscoelastic continuum damage finite element program (VECD-FEP) to calculate stresses and strains in a pavement as microcracks initiate and propagate, and the AREA parameter that is determined from falling weight deflectometer deflections, and pavement thicknesses.

No patching can cure severe fatigue cracking, which indicates profound base failure; road ultimately was in-place foam-recycled with thin overlay.

In the lab they used a mechanistic approach that uses cores to investigate crack initiation locations and the propensity of asphalt pavements to exhibit top-down cracking. They concluded the FWD-based in situ method will allow pavement engineers to identify the existence and likelihood of top-down cracking. “This simplified method will not only reduce the time and cost involved for the engineer to verify the structural soundness of the pavement, but will also lead to selecting the optimal maintenance treatment,” Park and Kim conclude.

Virginia Fixes Fatigue Cracking

Roads can be constructed to resist fatigue cracking. But as fatigue cracking will appear in existing pavements, a road manager’s first challenge will be to rebuild the existing pavement to preclude future bottom-up fatigue cracking.

In 2011 the Virginia Department of Transportation rebuilt a section of I-81 in the shadow of the Blue Ridge Mountains. Fatigue cracking, caused by years of heavy traffic loads, had deteriorated the pavement structure from bottom to top. While the symptoms of this deterioration could have been addressed by mill-and-overlay, VDOT says, the underlying condition would have remained, and the cause of this extensive wear could only be remedied by reworking all the material down to the subgrade.

This section of I-81 was constructed in the late 1960s. VDOT routinely maintained the surface asphalt with periodic patching and overlays. However, the original foundation of compacted stone aggregate and soil had weakened to the point it no longer provided a stable base for the overlying asphalt layers.

“Unless the foundation is repaired, simply repaving the road surface is a temporary improvement,” VDOT says in a public outreach statement. “If VDOT were to do nothing to this section of road, the pavement would crack more. Pieces up to the size of a golf ball could come out of the road. The fatigue cracks also allow a direct path for water to seep down to the pavement foundation. The water saturates the subgrade, further reducing its load-carrying capacity. This condition can lead to deep rutting within the wheel paths that can affect skid resistance and even steering ability.”

Revealed, failed base of Virginia I-81 is stabilized in situ using hydrated lime kiln dust; existing asphalt courses above were foam-recycled at portable cold mix plant adjacent to project and placed over this stabilized base.

“This section of I-81 is 43 years old,” says Chaz B. Weaver, P.E., district materials engineer, VDOT Staunton District Materials Section. “It was really beaten up. It had gotten more truck traffic, and lasted longer, than it was originally designed for. We had gotten to a point where our maintenance cycles were three, maybe four years long, with patching in-between. It’s very, very expensive to come out every four years, while we are looking for a 10- to 12-year cycle for a surface fix.

“Cores evaluation and pavement analysis with the falling weight deflectometer indicated that the base layer – about 24 to 26 inches down – had failed,” Weaver says. “We needed a process that could go in very deep, fix the entire pavement structure, and get out very quickly. With specialized machinery and the recycling process we can do that efficiently, and accelerate construction as much as possible.”

The existing subgrade had been built on top of impervious, plastic clayey soil, and did not have drains built alongside. “There were no underdrains or subgrade drainage in the design,” Weaver says. “The pavement structure was like a bathtub. Water got in and stayed in. We actually had some pumping of water and fines coming up through the surface. Trucks have only gotten heavier in the 43 years since the pavement was constructed and we’re up to 30 percent trucks.”

Roanoke-based Lanford Brothers Co. was prime contractor for the I-81 In-Place Pavement Recycling Project in Augusta County south of Staunton. As part of the $7.6 million project, Lanford Brothers rehabilitated and paved a 3.7-mile section of southbound I-81 in Virginia’s Shenandoah Valley that had experienced deterioration in the highway’s subbase.

After rebuilding shoulders to accommodate work zone traffic, Lanford first milled the top 10 inches of asphalt from the right-hand “truck” lane and brought it to a Wirtgen KMA 220 portable cold mix plant just off the Interstate, adjacent to the work zone. There it was foam-recycled for immediate placement on I-81 as a flexible base course.

In the meantime, the revealed, existing subgrade – which had deteriorated to the point of causing damage to the friction course – was stabilized by subcontractor Slurry Pavers Inc. using 5 percent lime kiln dust [Calciment, a reclaimed industrial byproduct] to a depth of 12 inches and compacted in-place with padfoot and smooth drum rollers.

The recycled foamed asphalt mix from the portable plant then was used to pave a new flexible base course over the restored subgrade to 6 inches compacted depth, later to be topped with a 4-inch intermediate course of conventional hot mix asphalt (HMA) and a 2-inch friction course of stone-matrix asphalt (SMA).

In the second phase of the project, subcontractor Reclamation Inc. of West Hurley, N.Y., performed in-place foam recycling in the left-hand passing lane. For this work the top 2 inches of pavement was milled off, followed by cold in-place recycling (CIR) of the next 5 inches. The foamed asphalt-stabilized base layer then was compacted using two Hamm smooth-drum vibratory rollers and one pneumatic (rubber-tire) roller. It was then topped with a 2-inch HMA intermediate course and a 2-inch friction course of SMA.

MEPDG: Building Not to Fail

As Virginia I-81 showed, there is no real way to fix bottom-up fatigue cracking without digging out the base, rebuilding, stabilizing and replacing the pavement structure.

The new Mechanistic-Empirical Pavement Design Guide (MEPDG) and associated software – now in various stages of adoption throughout the state DOTs –provides a state-of-practice mechanistic-empirical highway pavement design methodology based on actual experience from the real world, translated to a design program, and in the near future will be the first place to which agencies will go for designing fatigue-resistant pavements.

The MEPDG methodology is based on pavement responses computed using detailed traffic loading, material properties, and environmental and climatic data. The responses are used to predict incremental damage to the pavement structure over time.

“Design is an iterative process using analysis results based on trial designs postulated by the designer,” according to the 2010 report, Mechanistic-Empirical Pavement Design Guide Implementation, by R.L. Baus and N.R. Stires. “A trial design is analyzed for adequacy against user input performance criteria. These criteria are established by policy decisions and represent the amount of distress or roughness that would trigger some major rehabilitation or reconstruction activity.”

The output of the computer software is a prediction of distresses and smoothness against set reliability values. If the predictions do not meet the desired performance criteria at the given reliability, the trial design is revised and the evaluation is repeated.

“The MEPDG method provides for three hierarchical levels of design inputs to allow the designer to match the quality and level of detail of the design inputs to the level of importance of the project (or to best utilize available input data),” Baus and Stires say. “In addition to inputs required to quantify a trial pavement structure, the MEPDG requires over 100 inputs to characterize traffic loading, material properties and environmental factors.”

The MEPDG will provide pavement structure details, but it will be up to the contractor to build the road right. The subgrade soil material under most prepared bases will need to be compacted, as well as the base itself, because it acts as a platform on which the base is placed. Each needs to be compacted at or near its optimum moisture content, neither too dry nor too wet, although too dry is better than too wet for any compaction.

But if the base is not within about 2 percent of its optimum moisture content, it will never be densified or compacted to the point where it has the strength and durability to carry the prepared layers and pavement above.

That’s critical because nearly every road agency will have a spec and conduct tests to determine what the moisture content should be for the construction. But agency staff reductions and loss of qualified field personnel can result in supervisory oversights, and base problems permanently built into the completed structure, not to reveal themselves until years later.

Bases for flexible HMA and portland cement concrete roads differ greatly. The base beneath a rigid PCC slab is there to provide profile, as the loading from traffic is carried by the slab. Not much force is transmitted beneath the slab.

For the HMA pavement, traffic loads are transmitted by tires through the pavement to the underlying base. The greatest concentration of pressure will be at the surface, and then distributed in a bell-shape to depths below.

Crushed aggregate base gets its strength from the frictional interlock of the stone. Point loading tends not to dissipate so far, and deflection of the base is not as great, so the base serves excellently for profile. But when cohesive, sticky materials like clays comprise the base, stabilization with asphalt emulsion, lime, cement or fly ash, or foamed asphalt is indicated, to counter the effect of the material’s adverse reaction to the presence of water, when it can expand and destroy a pavement.

The more cohesive the soils are, the more likely they will be to react adversely in the presence of moisture. The stabilizing agent effectively seals or waterproofs the base to keep the moisture out and make it more stable, thus providing a superior foundation to resist bottom-up fatigue cracking.

By Tom Kuennen, Contributing Editor

The emerging universe of biologically based modified asphalts got significant exposure at the 92nd annual meeting of the Transportation Research Board (TRB) in Washington, D.C. Better Roads was there.

More than 11,700 transportation professionals from around the world enjoyed over 4,000 presentations in nearly 750 sessions and workshops. Here are some of the most significant papers involving bio-based modifiers and other themes of interest to the roadbuilding community. For more information, visit www.trb.org.

Performance in Asphalt Mixes

Because they fall under the umbrella of organic chemistry, strictly speaking, any petroleum-based binder or modifer might be considered a “bio-binder.” However, today the “bio” prefix is reserved for binders and modifiers derived from processed biomass or biowastes, thereby earning them “green” status.

Mixes containing “bio-binders” – when used in conjunction with conventional asphalt binders – exhibit performance very similar to regular asphalt mixes, but with the benefit of being environmentally sustainable, write Louay N. Mohammad, Mostafa A. Elseifi, Samuel B. Cooper III, and Harshavardhan Challa, Louisiana Transportation Research Center, Louisiana State University, and Prem Naidoo, Green Asphalt Technologies, Pass Christian, Miss., in their paper, Laboratory Evaluation of Asphalt Mixtures Containing Bio-Binder Technologies.

Bio-binder is an alternative asphalt binder made from non-petroleum-based renewable resources. The use of bio-oil as a replacement to petroleum-based asphalt binder has been proposed in recent years, they say. “Bio-oils have distinct advantages when compared to fossil fuel oils,” the authors write. “Bio-oils are renewable, environmentally friendly, provide energy security, and can be an economic opportunity for the United States.”

In most cases, say Mohammed, Elseifi, Cooper, Challa and Naidoo, the feed stocks for bio-based materials do not compete with food or feed supplies. Bio-based materials include industrial products, co-products, and byproducts made from agricultural or forestry feed stocks, they say. These feed stocks could be wood, wood waste and residues, grasses, crops, and co-products of crops.

Technologies for converting biomass to energy can be generally classified into either biochemical conversion or thermochemical conversion, they say. Biochemical conversions such as anaerobic digestion and fermentation typically involve large facility footprints and long processing time, on orders of days or months, and pose potential threats to surface or ground waters.

“However, thermochemical conversions, including pyrolysis, gasification and hydrothermal liquefaction, feature more compact facility size and faster reaction (usually in order of minutes) when compared to biochemical conversions,” they write.

The bio-binder considered in this study was produced by fast pyrolysis of biomass, say Mohammed, Elseifi, Cooper, Challa and Naidoo. During fast pyrolysis, biomass is heated rapidly in a high temperature environment (Fig. 1).

The resulting product is a mixture of liquid fuel (bio-oil), combustible gases, and solid char. Bio-oils produced by fast pyrolysis have many industrial uses, which include: combustible fuel, liquid smoke, preservative, base for chemicals and resins, binder for combustible fuel starter and briquettes for boilers, or an adhesive.

Fig.1: Biomass pyrolysis for production of bio-binder.

“The binding properties of the material are desirable for use in asphalt pavements,” they say. “Typically, [in practice] bio-binders are a combination of petroleum asphalt cement and bio-oils. Bio-binders are used to reduce the demand for petroleum-based bituminous binder in three ways: direct replacement (100 percent replacement), an extender (25 to 75 percent replacement), or a modifier (<10 percent replacement). Applications of bio-binder range from asphalt paving to roofing shingles to sealants.

“Several research studies have evaluated the viability of bio-binders in asphalt pavements,” they say. “However, in many of these cases the bio-binder was evaluated in minimal proportions (<10 percent). In addition, much of the research evaluated manure, oak tree and switch grass feed stock. There is a need for a comprehensive laboratory evaluation of mixtures containing bio-binder from alternative feed stocks at higher blending proportions (up to 50 percent replacement).”

This study determined if asphalt mixtures prepared with high bio-binder content from pine tree feed stock have the potential for increased use in transportation infrastructure. The researchers conducted a comprehensive laboratory evaluation of asphalt mixtures containing bio-binder technology at a content of 20, 25.5, 30 and 50 percent.

Attributes of Bitutech RAP as provided by marketer

The mechanistic properties of asphalt mixtures containing green asphalt technologies were evaluated in comparison to conventional asphalt mixtures. A suite of laboratory tests was conducted to capture the mechanical behavior of the mixtures against major distresses.

“Laboratory testing evaluated the rutting performance, moisture resistance, fracture performance, and low temperature resistance of the produced mixtures using the Hamburg loaded-wheel tester, the modified Lottman test, the semi-circular bending test, and the TSRST [thermal stress restrained specimen test].”

Based on the results of the experimental program, the researchers conclude:

• With respect to rutting performance, mixtures modified with [this] green asphalt have shown similar or improved performance when compared to the conventional mixes.

• The mean rut depths of the mixtures containing PG 64-22 and PG 76-22 [binders] were significantly improved with the addition of bio-binder. In addition, all the mixtures did not experience tertiary flow or passed the stripping inflection point, indicating moisture susceptibility.

• All mixtures, except for two, including one without an anti-stripping agent, exceeded the 80 percent tensile strength ratio. An anti-stripping agent was used for only one mixture comparison, 64-22. After the addition of the anti-stripping agent, the tensile strength ratio of those two mixes exceeded 80 percent.

• There were no significant statistical differences associated with the addition of bio-binder.

• The mixes containing bio-binder exhibited reduced intermediate temperature fracture resistance as compared to conventional mixes. This may be due to the stiffening effects of bio-binder on the mix. But the results for the PG 70-22 comparison were significantly different, and

• Bio-binder asphalt modification improved the low temperature fracture performance of the mixtures when compared to conventional mixtures of similar performance grade.

Improving RAP Mixes

Bio-binders improve the elasticity, hence performance, of bituminous mixes containing reclaimed asphalt pavement (RAP), say Elie Y. Hajj, Ph.D., Western Regional Superpave Center, University of Nevada-Reno, Mena I. Souliman, Ph.D., and Mohammad Zia Alavi, Department of Civil and Environmental Engineering, UNR, and Luis Guillermo Loría Salazar, Ph.D., Laboratorio Nacional de Materiales y Modelos Estructurales, Universidad de Costa Rica, in their paper, Influence of Hydrogreen Bio-Asphalt on Viscoelastic Properties of Reclaimed Asphalt Mixtures.

“’The inclusion of reclaimed asphalt pavement (RAP) into new asphalt mix design is becoming more popular due to the rising cost of oil, limited amount of quality virgin materials and limited space in landfills,” the authors write. “However, incorporating RAP exposes some challenges from the design perspective since the asphalt binder in RAP becomes stiffer because of aging.”

This stiffness can be addressed via use of rejuvenating additives, and the paper describes the lab evaluation of an existing bio-based rejuvenating agent, called BituTech RAP.

High content RAP mixtures used in Manitoba were evaluated to study BituTech RAP’s impact on the viscoelastic properties of asphalt mixtures to overcome any possible moisture damage – or thermal cracking problems – that may arise in such a wet-freeze environment.

“The laboratory testing consisted of producing mixtures containing 15 and 50 percent RAP, with and without BituTech RAP,” they write. “Mixtures were tested using the dynamic modulus test as well as thermal stress restrained specimen test.”

Even though RAP use has been specified by multiple highway agencies, the potential issues with binder and mixture stiffening prevented various states from using higher percentages (greater than 30 percent) of RAP in hot mix asphalt (HMA), they say. “This stiffening behavior is function of the age of the RAP material and the compatibility between the virgin and reclaimed asphalt binders,” they write. “Consequently, the mixture durability and resistance to fatigue and thermal cracking may be reduced, resulting in a poor pavement performance.”

The increased stiffness of the RAP binder is believed to be the cause of increased modulus, or stiffness, of the asphalt mixture, they say. “Likewise, the use of high RAP contents was found to possibly affect fatigue behavior and low-temperature cracking properties of asphalt mixtures,” they add.

Rejuvenating agents have been traditionally used to offset the high stiffness of the aged RAP binder, and use of rejuvenating agents could possibly increase the durability and the low-temperature performance of mixtures containing RAP without jeopardizing the overall performance of the mixture, they state.

“One such bio-asphalt additive is BituTech RAP, also sold under the brand name Hydrogreen, developed by Green Asphalt Technologies as an alternative to carcinogenic aromatic oil rejuvenants,” say Hajj, Souliman, Alavi and Salazar. “BituTech RAP is a unique combination of selected natural plant extracts reacted in a distinct process to create a powerful asphaltene dispersant. It is intended to offer maltenes without an aromatic content in order to eliminate environmental concerns associated with using oil based products.”

To investigate, an extensive laboratory evaluation was conducted to study the effect of BituTech RAP on asphalt mixtures containing RAP up to 50 percent. The impact of RAP content and BituTech RAP on dynamic complex modulus, moisture damage and thermal cracking resistance of HMA mixtures were evaluated using advanced testing techniques.

Based on the analysis of the data generated in this study, the researchers find:

• Cole-Cole analysis showed that the loss modulus (E”) values for mixtures with BituTech RAP were significantly higher than the ones without. In addition, the performance of mixture with 50 percent RAP with BituTech RAP was similar to the mixture with 15-percent RAP with the additive. Hence, HMA mixtures with high RAP percentages (up to 50 percent) can be used with BituTech RAP without sacrificing the performance of the mixture.

• Adding BituTech RAP to the RAP mixtures improved their resistance to moisture damage after three freeze-thaw cycles. Mixtures with BituTech RAP exhibited storage modulus and loss modulus values that are significantly higher than the mixtures without BituTech RAP.

• For the thermal stress restrained specimen test (TSRST), the addition of BituTech RAP restored the low temperature properties of the RAP mixtures. A reduction in the relaxation modulus was observed with a shift in the fracture temperature, micro-cracking initiation temperature, and viscous-glassy transition temperature to the colder side.

• A cost analysis showed potential savings for using BituTech RAP with 50 percent RAP besides its effect as a rejuvenating agent. Using BituTech RAP [can] promote using higher percentages of RAP without affecting the overall performance of the mixture.

Cement-Limestone Blends Benefit Concrete Performance

There are profound benefits in performance and sustainability of concrete structures made with portland cement blended with limestone, but careful attention must be given to the fineness of the limestone portion, say V. Tim Cost, P.E., Holcim (US), and Isaac L. Howard, Ph.D. and Jay Shannon, Mississippi State University, in their paper Integrated Improving Concrete Sustainability and Performance Using Portland-Limestone Cement (PLC) Synergies.

The use of fine-ground limestone as an adjunct to portland cement – with little to no change in performance with the blend – has been supported by the cement industry, as it permits increases in product shipped without a corresponding expansion of plant capacity, with all the environmental permitting required. While the cement industry has drifted toward portland-limestone cement (PLC) blends, the recent inclusion of PLC – a cementitious blend of portland cement – in the ASTM and AASHTO specification canons mean PLC is going mainstream.

Published research has documented performance synergies of cementitious mixtures with finely ground limestone (particle sizes generally smaller than for cement), especially in combination with certain supplementary cementitious materials (SCMs)

PLC is similar to ordinary portland cement, but it also contains a specific minor fraction of finely ground limestone particles, the authors say, and can be produced at any portland cement manufacturing plant. For this, crushed limestone is metered along with clinker and gypsum to the finish grinding mill.

“The sources of limestone typically used in cement making as a raw material are ideal for the limestone used in PLCs, thus no additional special materials must be acquired,” say Cost, Howard and Shannon. “Limestone is softer than clinker and grinds more easily, so the limestone particles in finished PLC are generally finer than the ground clinker, which favorably effects the overall particle size distribution of the cement.”

Compared to conventional portland cement, PLC measurably improves concrete sustainability through the reductions of greenhouse gas emissions associated with cement production (by roughly the percentage of limestone used) and by the lower embodied energy of the cement, they add.

“Increased use of portland-limestone cements in the U.S. is anticipated, as a new provision for PLCs containing up to 15-percent limestone has been added to blended cement specifications,” the authors write.

Specifications for blended cements under ASTM C595 and AASHTO M 240 now include provisions for Type IL cements (PLCs) containing up to 15 percent limestone as well as Type IT cements containing combinations of limestone and pozzolan or slag.

Synergie have been found, however, to occur under specific circumstanced.

Published research has documented performance synergies of cementitious mixtures with finely ground limestone (particle sizes generally smaller than for cement), especially in combination with certain supplementary cementitious materials (SCMs), they say. “Time of setting and strength development benefits are reported, generally in proportion to limestone fineness,” the authors write.

“It appears possible to fully develop potential for these performance synergies in mill-ground PLCs, in which limestone comprises the majority of the finest particles.”

For this research, performance trends observed in concrete with PLC were investigated using separately proportioned, commercially ground limestone and ordinary portland cement , as well as cement mill-ground PLC samples.

Influences of variables such as SCM type and limestone fineness were also evaluated using laboratory paste mixtures.

“Set acceleration increased with limestone fineness for all combinations, including mixtures without SCMs,” say Cost, Howard and Shannon. “Strength improvements were clearly evident with all SCMs, more significantly with Class C ash and slag cement than with Class F ash. All strength trends improved as limestone fineness was increased.”

Consistently enhanced setting and strength performance appear achievable with PLCs, they say. “Optimizing particle fineness will be a key factor in achieving these benefits,” they add. “Performance contributions of SCMs in combinations with PLCs may exceed those of similar mixtures with traditional portland cements, thus SCM use can be maximized and related sustainability impacts further extended.”

The authors conclude:

• Portland-limestone cements produced at up to 15 percent limestone have the potential to significantly improve concrete sustainability, with performance at least equal to – and often superior to that – of conventional cements.

• Properly optimized, portland-limestone cements clearly hydrate with synergies contributed by limestone that enable enhanced setting and strength performance, especially in combination with SCMs.

• The extent of PLC performance benefits relates to limestone fineness; clinker fineness approaching that of conventional cements must generally be maintained in the composite PLC.

• The particle size distribution of PLC, produced to optimum overall fineness in finish grinding ball mills, appears well suited for synergy-driven performance enhancement.

• SCM use in concrete with PLC, to the maximum levels that are allowed with conventional cement, must be allowed and encouraged in order for the greatest possible sustainability and performance benefits of PLC to be achieved.

• Higher-than-traditional replacement rates with some SCMs appear possible without loss of performance, further extending the sustainability benefits of PLCs.

Environmentally Sustainable Preservation

Today’s pavement maintenance and preservation practices minimize their environmental impact and thus are environmentally sustainable, according to Sustainable Pavement Maintenance and Preservation Practices: A Review of Current Practice, by Dr. Susan L Tighe, P.Eng., University of Waterloo, Ont., and Dr. Douglas Gransberg, P.E., Iowa State University-Ames.

“Pavement preservation promotes environmental sustainability by conserving energy, virgin materials, and reducing greenhouse gases by keeping good roads good,” the authors say. “Therefore, the foundation of a sustainable pavement maintenance program is to commit personnel and resources to pavement preservation.”

Pavement preservation promotes environmental sustainability by conserving energy, virgin materials, and reducing greenhouse gases by keeping good roads good.

- Sustainable Pavement Maintenance and Preservation Practices: A Review of Current Practice

Tighe and Gransberg summarize seven sustainability impact factor areas, including virgin material usage, alternative material usage, program for pavement in-service monitoring and management, noise, air quality/emissions, water quality and energy usage; and evaluate their relationship to typical pavement preservation and maintenance practices (the United States administration differentiates between pavement maintenance and preservation, while the Canada administration does not).

The authors conclude:

• Environmental sustainability research specific to post-construction operations is an emerging (but not established) field.

• Thin asphalt overlays are the most prevalent treatment for asphalt and composite pavements.

• Diamond grinding and joint sealing are the most commonly used for concrete pavements.

• Regrading and regraveling are the most prevalent treatments for graveled roads, while chip seals are the preferred treatments for surface-treated roads.

• Quantification of environmental sustainability with pavement preservation and maintenance programs is not commonly practiced in the United States and Canada.

• Various construction and design environmental sustainability initiatives are available in the literature. However, an assessment tool to properly quantify environmental sustainability in the pavement preservation and maintenance context is required.

• Environmental sustainability metrics should be developed and integrated into pavement preservation and maintenance treatment programming.

They identified a number of future research needs in quantifying the environmental sustainability of maintenance and preservation. Those needs include:

• pavement preservation- and maintenance-specific research to furnish agency engineers with fundamental quantitative data on the relative environmental sustainability of common treatments.

• development of a pavement preservation and maintenance specific lifecycle assessment and cost analysis method that includes values for factors such as carbon footprinting, resource renewability, and other salient elements of environmental sustainability, to furnish a rational approach to treatment selection.

• research quantify the importance of preservation and maintenance treatments in environmental sustainability impact areas.

• complete research in adapting recycled and alternate materials for use in pavement preservation and maintenance treatments.

• research to identify and develop appropriate noise standards for pavement preservation and maintenance operations and life cycle assessment.

Slurry surfacings provide durable pavement preservation treatments that can be environmentally sustainable, research indicates

• research to quantify the contribution that crack sealing makes to environmental sustainability of all types of pavement surfaces, and

• research policies, technology, materials, specs, standards, equipment, capacity-building, recycling/waste reduction, emissions reduction (air pollution) and life cycle cost analysis.

“In this regard, it is necessary to incorporate sustainable pavement maintenance elements into on-going and future pavement maintenance research,” they conclude.

Permeable Asphalt and ‘Heat Islands’

Permeable asphalt pavements can help urban areas reduce the local “heat island” effect – enhancing livability – but the impact depends on ambient conditions, say Hui Li, John Harvey and David Jones, University of California Pavement Research Center, University of California-Davis, in their paper Cooling Effect of Permeable Asphalt Pavement under Both Dry and Wet Conditions.

Permeable pavement contains more voids than conventional impermeable pavement and is designed to allow water to drain through the surface into the sublayers and down into the ground below. Permeable pavements include permeable asphalt pavements, porous concrete pavements, pervious cast concrete pavement, and interlocking concrete pavements, they write.

Permeable pavements include

permeable asphalt pavements, porous concrete pavements, pervious cast

concrete pavement, and interlocking

concrete pavements

“Through field measurements on pavement test sections with both conventional and alternative designs, the thermal behavior and cooling effect of permeable asphalt pavements under both dry and wet conditions were investigated,” say Li, Harvey and Jones .

The researchers found the overall seven-day average cooling effect of wetting once on permeable pavements for near-surface air is approximately 0.2 to 0.45 degrees C [0.36 degrees F to 0.81 degrees F]; for the surface it is approximately 1.2 to 1.6 degree C [2.16 to 2.88 degrees F]; and approximately 1.5 to 3.4 degrees C [2.7 to 6.12 degrees F] for the in-depth layers.

“Based on the findings, permeable asphalt pavements have the potential of being a type of cool pavement which produces lower temperatures and thus helps improve the thermal environment and mitigate the heat island effect,” they write. “However, attention should be given to this type of pavement under dry conditions. As a pavement thermal management strategy, water from rain or irrigation systems might need to be applied to the pavements to produce a better cooling effect for improving the thermal environment and mitigating the heat island.”

These permeable pavements can be used in city streets, parking lots and highway shoulders, and the type of surface could potentially enhance the evaporation from pavement, while the cooling effect depends on the moisture content and evaporation rate, they say.

“Beyond reducing temperature, permeable pavements also could potentially reduce the air/pavement noise due to reduced air pumping under high speed, and improve high-speed driving safety through reducing splashing and hydroplaning during rain,” they say. “Also, full depth permeable pavement could reduce stormwater runoff and improve water quality. However, permeable pavements also could possibly increase fuel consumption of vehicles due to higher rolling resistance caused by the rougher surface.” Permeable pavements are possibly not suitable for high speed and heavy-duty facilities such as highways and airfields due to their relatively lower structural capacity, they add.

The issue of whether rigid (white) or flexible (black) pavement is the “coolest” pavement medium has been raging for years between the portland cement concrete and the bituminous concrete industries. The authors address this issue, saying it’s not so simple.

“With respect to the pavement type, the heat island might not be a ‘black and white’ issue (asphalt vs. concrete) as being argued in the pavement industry, but might be an ‘impervious and pervious’ issue,” they say. “To mitigate heat islands and reduce the associated impacts, some impervious surface coverage should be substituted by pervious coverage.”

The minimized impervious surfaces could potentially improve the quality of life in a community and reduce other environmental impacts, such as reduced stormwater runoff and associated water pollution, reduced stormwater management facilities, and enhanced on-site infiltration for vegetation growth, recharging the underground water, they say.

Safety and the mowing-herbicide-beautification-erosion control-culvert maintenance mix

By Tom Kuennen, Contributing Editor

Integrated roadside management programs are being adopted at state and county road agencies because they combine the multiple missions of vegetation management, roadside beautification, motorist safety, erosion control and roadway appurtenance maintenance into one package.

By combining these missions, economies of scale are achieved and long-term results can be achieved. Like pavement and bridge preservation programs, or other asset management programs, roadside management programs can help administrators decide what outcomes will be best for a particular stretch of highway, and following a timeline, help the agency work toward that outcome over a period of years.

Gone are the days when roadside management meant just periodic mowing and spraying of weeds, biannual regrading of shoulders, and sporadic visits to sedimented drainage ditches with a wheel excavator. Today’s integrated plans incorporate these elements into a “tool box” of additional treatments or actions that are programmed over time for the best impact on the system and expenses.

“Vegetation is [just] one important element of roadside maintenance,” says the Minnesota DOT in its Best Practices Handbook for Roadside Vegetation Management, 2008-2020. “A healthy roadside environment reduces maintenance needs and costs, reduces erosion and improves water quality, improves water infiltration and reduces runoff, preserves the roadside surface, maximizes safety for vehicles and travelers, limits liability for the governing agency, maintains good public relations, improves the overall driving experience, and provides habitat for wildlife populations.”

Vegetation Control = Safety

Nonetheless, vegetation control as a means of ensuring motorist safety is the prime driver of the integrated roadside management movement.

“The primary objective in maintenance of roadside vegetation is to promote the safety of the highway user, preserve the highway infrastructure and control of legally designated noxious weeds where they occur on the right of way,” says the Washington State DOT. “Other considerations include protection and preservation of natural environment, preservation and enhancement of the natural scenic quality of the roadside, and the need to be a good steward of the forest along this corridor.”

Washington State DOT – a leader in roadside vegetation management – defines different zones in the roadside, each with a different management strategy.

In Washington State, roadside vegetation maintenance activities are planned and conducted to discourage or eliminate unwanted vegetation, and promote desirable vegetation. Like the pavement preservation timeline, integrated vegetation management principles include prevention of overgrowth or growth of noxious weeds, monitoring of conditions, determination of action thresholds and the proper timing of maintenance efforts, selection of the least-disruptive control and effective revegetation tactics, and continuing evaluation.

“The integrated vegetative management process provides information for the total roadside management system, which is used to analyze vegetation problems and implement long-term solutions,” WS DOT says. This broad overview approach helps vegetation managers answer key questions, such as whether treatment actions are needed, where they should take place in the system, when these actions should take place, and which mix of strategies, tactics and treatments are the best to use.

Very simply, roadside vegetation management involves caring for or controlling plants along the highway. “If managed properly, roadside vegetation can become self-sustaining over time and require less maintenance,” WS DOT says. “This helps reduce costs and minimizes herbicide use.”

Vegetation, if left alone, can grow out of control and block visibility (signs, traffic, wildlife) which could endanger motorists, WS DOT says. “Weeds must be controlled to avoid impacts on the farming community and native ecosystems,” it says. “Pride of ownership and the beauty of Washington State are also important factors, both aesthetically and economically, such as with the tourism industry.”

The plan constitutes a “how to” guide for the best way to manage roadsides in any given area. “Washington State has diverse climates and the highways have many neighbors, so the plans vary depending on location,” the state DOT says.

Echoing the mantra of the pavement preservation movement, WS DOT says “[t]he plans determine the right tool or combination of tools, for the right plant at the right place and time,” adding that is the essence of an integrated vegetation management plan.

WS DOT’s vegetation management tools include mowing and trimming, selective use of herbicides, release of weed-eating insects, improvement of soils, re-establishment of native plants, and hand-maintenance by volunteers or contract services. Their use is articulated in regional roadside vegetation management plans for each region inside each DOT district, necessary due to the extreme variation of environments within the Evergreen State, which range from high desert to alpine to urban to coastal rain forest.

Safety, Preservation in Bay State

Massachusetts’s current Vegetation Management Plan, 2009-2013 has a primary objective of providing a safe, unobstructed roadway corridor, and preserving the integrity of the highway infrastructure.

“Management of vegetation is an important element of roadside maintenance for safety and aesthetic purposes,” according to the plan. “Left uncontrolled, roadside vegetation can impede normal maintenance operations, obstruct motorists’ line of vision, threaten pedestrian safety and cause damage to structures such as median barrier, pavements, guard posts, drainage lines and waterways.”

“Left uncontrolled, roadside vegetation can impede normal maintenance operations, obstruct motorists’ line of vision, threaten pedestrian safety and cause damage to structures such as median barrier, pavements, guard posts, drainage lines and waterways.”- Massachusetts’s Vegetation Management Plan, 2009-2013

Other objectives include provision of an aesthetically pleasing roadside, pest control, creation of wildlife habitat, and stabilization of embankments and other areas prone to erosion.

The key components of Massachusetts Highway’s integrated program are identification of priorities for vegetation control, implementation of controls in an environmentally sensitive manner, and monitoring of performance to check methodology.

Like other states, Massachusetts is retaining use of herbicides. “Controls shall include mechanical, chemical, cultural, biological and roadside development methods,” the commonwealth states, adding “[i]t shall be a goal to minimize the use of chemical controls, through minimizing areas of application, quantity of chemicals, and frequency of applications. Chemical control techniques shall be limited to use on high traffic volume, high speed interstate and primary roadways in the commonwealth where safety of motorists, department employees and contractors precludes the use of mechanical methods.”

Mass Highways defines three classes of roadside vegetation:

The State of Iowa is encouraging districts, counties and cities to improve roadsides with its Living Roadway Trust Fund.

• Hazard vegetation represents the primary target material, including vegetation obscuring sightlines, growing over guardrails, creating obstacles to signs or vehicular movement, posing windfall hazards over vehicular or pedestrian ways, or creating winter shade leading to icing conditions.

• Detrimental vegetation, including grasses and woody plants that are destructive to, or compromise the function of, highway structures, including grasses in pavement and bridge joints, medians, barriers and traffic islands, and vegetation growing in and along drainage structures, compromising drainage. Such vegetation creates storm water accumulation and hazardous icing conditions.

• Nuisance/noxious vegetation, including any vegetation growing along state roadways that could potentially cause problems to the general public or Mass Highway employees or contractors, usually poison ivy. Other nuisance vegetation may be growing within 30 feet of the edge of pavement, bridge abutments, a drainage structure or swale, other structures and appurtenances requiring maintenance, within state highway rights-of-way, are considered target vegetation.

• Invasive Vegetation includes ROW infestation with invasive plant species, including introduced plants that have spread from gardens and agricultural areas into the wild, where they pose problems for the natural environment.

Nebraska: Encourage Native Plants

In Nebraska, the Department of Roads’ Plan for the Roadside Environment promotes the increased use of native plantings and vegetation management to provide a sustainable roadside. The Cornhusker State emphasizes the use of native plantings adapted specifically to the varying climate zones across the state.

Seventh grader Vicky Evans of Cold Spring, Minn., drew this poster, which was a finalist in MnDOT’s “Roadsides are for the Birds” poster contest.

Its roadside plan contains sections for each of the six landscape regions across Nebraska. Each individual landscape section contains regional maps and summarizes a variety of ecosystem information for the region, including hydrology, climate, and soil and plant communities, as well as regional history, land use, and economic features.

The plan is applicable to the entire state and includes landscaping objectives for integration into transportation system planning, safety, design and operation. Ultimate goals include good stewardship of roadsides, and maintenance of a unique and sustainable “Nebraska-style” landscape.

“The Plan for the Roadside Environment is designed to create a roadside that can better overcome the disturbances of construction, withstand the rigors of the Nebraska climate and perform the landscaping objectives that contribute to safe and maintainable roadsides that complement the surrounding landscape,” the plan states.

The plan is founded on the Nebraska Natural Legacy Project, a conservation plan published by the Nebraska Game and Parks Commission, and integrates environmental concerns, landscape objectives and mitigation and maintenance requirements.

Themes of Nebraska’s plan include:

• increased use of native plants appropriate to each landscape region of the state;

• seeding of native grasses, legumes and forbs in new ways as design elements to accomplish landscape objectives, as well as provide soil stabilization for the roadway corridor;

• use of required environmental mitigations in a manner that will accomplish landscape objectives within the highway corridor;

• use of permanent erosion control and stormwater control constructions as design features to accomplish landscape objectives within the highway corridor;

• development of additional ways to use plantings to reduce maintenance efforts and improve stewardship; and

• enhance existing partnerships and develop new partnerships with natural resource agencies and others to broaden benefits and to share knowledge and combine resources for mutual benefit.

If roadbuilding and maintenance changes the landscape, the roadside landscape must be maintained to recognize the movement of plants and animals, Nebraska DOT says. “These corridors provide a way for plants and animals to move between habitats that have been fragmented by agriculture, expanding communities and various other activities of man and nature,” Nebraska’s plan states. “Understanding this need and using thoughtful design and appropriate long-term management of these corridors will allow for safer movement of all species whether for seasonal migration or changes over longer periods of time.”

Iowa Funds County Roadsides

The state of Iowa is actively promoting integrated roadside vegetation management via a program that was established by the state in 1988.

Iowa’s tool chest includes judicious use of herbicides, spot mowing, prescribed burning, mechanical tree and brush removal and the prevention and treatment of disturbances to existing vegetation. Like Massachusetts, the plan’s long-term objective is to reduce roadside maintenance by creating stands of durable, long-lived, native plants.

Until the mid-1980s, Iowa’s roadside weed control relied exclusively on herbicides, with most counties using blanket spraying. It was expensive and potentially harmful, and it was an ineffective means of weed control, creating openings for weeds by stressing and weakening roadside grasses, and eliminating beneficial broadleaf species.

“Iowa counties were spending a lot of money putting large amounts of herbicide into the environment, and, at the same time, making little or no headway in the control of roadside weeds,” the plan says. “Clearly, this type of roadside management proved unsustainable.”

Simultaneously, the Iowa DOT began using native prairie grasses and wildflowers for erosion control. A few county conservation boards were also experimenting with this naturally adapted, alternative vegetation for roadsides. When the Iowa legislature officially adopted its integrated roadside vegetation management plan in 1988, the cornerstone of the program became the establishment and protection of native vegetation in Iowa roadsides.

To support roadside management, Iowa’s Living Roadway Trust Fund was created the following year, supporting state, city and county roadside projects.

“Since that time over 100,000 acres of state and county road right-of-way have been planted to native vegetation,” the plan says. “Diverse stands of 15 to 45 prairie grass and wildflower species – all naturally adapted to local growing conditions – provide stable, low-maintenance roadsides for Iowa.”

Simply by adopting an IRVM plan, a county or city becomes eligible to receive grant money from the trust fund for conducting an inventory of roadside conditions within the jurisdiction, and obtaining prairie grass and wildflower seed for roadside plantings.

By hiring a roadside manager, Iowa counties become eligible to receive Living Roadway Trust Fund money for equipment such as a native grass drill, hydro mulcher, straw mulch blower, GPS and software, prescribed burns, and seed harvesting. Funds also are made available for erosion control material, a seed storage facility, education and outreach materials, and for research. At this time some 39 Iowa counties now have full-time roadside managers.

According to the 2011 IRVM Technical Manual, herbicide use in Iowa roadsides has been reduced to spot-spray application, and the Iowa DOT and half of Iowa’s counties routinely plant native vegetation.

“When considering the establishment of a new IRVM program, consider: money spent in weed control, money spent in contract seeding, money spent contracting erosion control, money spent on erosion stone vs. best management practices,” says Wes Gibbs, Jones County roadside manager. “There’s lots of money to be saved with an IRVM program. Make it about money!”

Minnesota: Roadsides for Wildlife

Minnesota strives to maintain roadsides for safety and aesthetics at all levels of government, and articulates eight best management practices for roadsides at all levels of government:

• Develop an integrated roadside vegetation management plan

• Develop a public relations plan

• Develop a mowing policy and improved procedures

• Establish sustainable vegetation

• Control prohibited and restricted noxious weeds

• Manage living snow fences

• Use integrated construction and maintenance practices, and

• Manage roadside vegetation for wildlife and vehicle safety.

This last point has received special emphasis in the Gopher State. In 2011 it launched Roadsides are for the Birds, a promotion that stressed management of roadsides for all wildlife.

“Grassy roadsides can be for the birds!” MnDOT says. “Although these ribbons of green make up only a small fraction of our land area, researchers have found them to be highly productive nesting sites for more than 40 kinds of birds and animals that nest on the ground or in low vegetation. Examples include pheasants, gray partridge, rabbits, waterfowl, and songbirds.

“Roadsides represent over 500,000 acres of permanent grassland in Minnesota’s pheasant range,” the DOT says. “Unfortunately, many thousands of nest and nest sites are destroyed annually in southern and western Minnesota because of disturbance to our roadsides during spring and summer (late April through early August).”

To this end the state recommends delaying of roadside mowing of the ditch bottom and back slope until after August 1. “Each species of wildlife has its own nesting habits including when and how many times they rear young each year,” MnDOT says. “As a result, undisturbed roadside cover receives almost continuous nesting use from spring until late summer. By delaying roadside disturbance until after August 1, nests for most species can hatch successfully. A mowed strip along the shoulder is not damaging to nesting wildlife because most nests occur in the ditch bottom or back slope. Other disturbance factors that should be avoided include ‘blanket’ spraying, vehicle and tractor encroachment, and grazing. If possible, leave roadsides undisturbed year around.”

Roadsides mowed after September 1 should be clipped “high” as a minimum of 10 to 12 inches of erect, residual cover is vitally needed for next year’s early nesters, MnDOT says. Residual can also provide some roosting and escape cover. MnDOT provides free Roadsides for Wildlife signs that remind highway patrons of the mission.

Living snow fences are another way to enhance roadsides for wildlife while keeping roads clear of blowing snow, MnDOT says. “These linear shrub plantings and prairie grass snow catch areas provide significant nesting and brood cover for pheasants and other birds.” Financial incentives are available to encourage planting of living snow fences.

A road’s drainage system – however innocuous – is a major part of the integrated roadside management program. But drainage systems tend to be out-of-sight, and therefore often out-of-mind, leading to under-maintained drains that perform inefficiently or clog.

If drainage systems are not maintained, moisture will be retained in the structure and pavement deterioration will increase. If a road agency is unable to maintain its drainage systems, experts say, then it likely will be better served by designing a road structure without a drainage system.

A road drainage system can be degraded with aggregate, sand, clay or dirt from the pavement structure itself, from material and refuse dumped on the pavement, and by mud and gravel washed down in the roadside ditch.

Lastly, plastic and corrugated metal pipes can separate or be crushed, often during construction. That’s why systems should be inspected via video “snake” right after construction is completed, and then periodically thereafter. Maintenance forces should mow around the system outlet pipes at least two times each year, and should mow and clean roadside ditches as well.

A telescoping-boom, wheeled excavator is ideal for digging out clogged, silted drainage ditches, or for cutting weeds with an attachment. Headwalls protect outlet pipes, help limit erosion where there drainage system empties, and help workers identify the location of outlets, which otherwise might become overgrown or silted-in.

“The accumulation of sediment in culverts happens ‘out-of-sight and out-of-mind’ and that the very process of sedimentation, slow or sudden, is pervasive and nearly inevitable,” says Gerry Sackett, Gerryrigs, Charlottesville, Va.

“The hydraulic capacity of a culvert pipe is specified and is considered as a constant. The gradual accumulation of sediment within the culvert, and resulting reduction of design capacity are not factored in. It seems to be generally expected that the velocity of drainage flow will scour the barrel of a culvert and keep it clean, but in reality, this is not the case.”

Water may freely flow through an unencumbered culvert, and carry with it, particulate material in suspension, Sackett says. However, when the flow meets natural grade at the outlet of the pipe, the friction and irregularity of the natural grade (including stones, grass or debris) slows the current and makes it roll. Heavier particles (detritus and debris) settle out and the process of sediment accumulation is begun.

To reduce sedimentation of culverts, Sackett has developed the Culvert Inlet Protection Device, or CIPD, which was approved for testing by Virginia DOT in 2010 and by Georgia DOT in 2012.

Under new federal surface transportation law MAP-21, pavement preservation treatments like slurry surfacing (shown) and state and local policy elements like asset management are supported with federal funds

For the first time, pavement preservation is part of national surface transportation reauthorization legislation, which now explicitly supports preservation and asset management.

Pavement preservation techniques are promoted as cost-effective and environmentally sustainable strategies designed to extend the life of existing pavements before they deteriorate substantially.

These techniques include nonstructural preventive maintenance surface treatments such as slurry surfacings, crack sealing, chip sealing, micro surfacing, rejuvenation, hot and cold in-place recycling and thin-lift hot-mix asphalt paving; and structural preservation techniques used in concrete pavement restoration (CPR).

Pavement preservation methods, proponents say, prolong pavement life, avoiding high future costs of reconstruction or rehabilitation through the expenditure of lesser amounts of money at critical points in a pavement’s life. Pavement preservation pays off in both the short and long term. Experience shows that spending a dollar on pavement preservation can eliminate or delay spending $6 to $10 on future rehabilitation or reconstruction costs, reports the National Center for Pavement Preservation at Michigan State University.

At the end of June, 2012, the Moving Ahead for Progress in the 21st Century bill (MAP-21) became law, and the concepts of asset management and preservation are widely used in MAP-21. In the long run this will bring major benefits via needed federal funding to state and local government agencies trying to implement pavement preservation principles.

The new NCAT Pavement Test Track life cycle pavement preservation performance study also will include data from an instrumented local road near the track, the traffic of which is almost entirely truck traffic to and from a quarry; here in late summer, micro surfacing is placed as part of NCAT study

Pavement preservation also took a big leap forward in fall 2012 when, for the first time, preservation techniques began study at the Pavement Test Track at the National Center for Asphalt Technology (NCAT) near Auburn, Ala. As the track began a new three-year cycle of tests, a variety of preservation techniques were placed on the track and a nearby local road. This work will provide, for the first time, quantifiable data to better understand how preservation treatments relate to a pavement’s life cycle.

In August, 2012, came the National Pavement Preservation Conference – held Aug. 27-30 in Nashville – giving even more momentum to the pavement preservation movement.

At the confernce attendance exceeded expectations, exhibit space sold out, and a field demonstration of multiple pavement preservation treatments raised awareness of the potential for expansion of preservation activities in North America and in foreign countries.

Preservation in MAP-21

As a matter of policy, for years the Federal Highway Administration and the American Association of State Highway and Transportation Officials have supported pavement preservation as a means of optimizing scarce pavement dollars, but now preservation and asset management are underscored in federal legislation. MAP-21 specifically contains language that indicates the significance of preservation practice.

At the August National Pavement Preservation Conference in Nashville, all presentations were video-recorded, and can be streamed and materials downloaded at http://nationalpavement2012.org/presentation-multimedia/

For example, the term asset management is included in the language and is defined to include “a structured sequence of maintenance, preservation, repair, rehabilitation, and replacement actions that will achieve and sustain a desired state of good repair over the lifecycle of the assets at minimum practicable cost.”

The Maintenance Section of the law has been expanded to specifically define pavement preservation programs and activities. The final bill explicitly states that preservation activities are eligible for projects under the national highway and surface transportation programs.

“The new MAP-21 surface transportation legislation enacted July 6, 2012 contains language both specifically, and more generally, helpful to pavement preservation,” says Mike Buckingham, president, FP2 Inc., and director of pavement preservation for Colas.

FP2 began as the Foundation for Pavement Rehabilitation and Maintenance Research in 1992, as a nonprofit public charity to pursue and encourage research in pavement maintenance. In 2000 it changed its name to the Foundation for Pavement Preservation, which was dissolved in 2009 to form FP2 Inc., a nonprofit trade association that conducts political lobbying, among many other duties.

“Thanks to the strong support of members of the U.S. House Transportation and Infrastructure Committee, pavement preservation in the final bill was stronger than it was in either the House or Senate versions,” Buckingham says.

But even as the benefits of having preservation and asset management included in MAP-21 have become manifest, supporters say they have learned working at the federal level is never as simple as it seems. The preservation community says it now must make sure the “metrics” by which success of pavement preservation will be measured under MAP-21 are applicable to pavement preservation practice.

At the National Pavement Preservation Conference in Nashville, different surface treatments are placed and compared at an outdoor setting

“We know now that an important theme of the new law is accountability for work undertaken under the legislation,” FP2’s Buckingham says. “The operative word here is ‘metrics’ or measurement of progress toward established goals. Our work now is shifting from getting pavement preservation included in federal law, to making sure the metrics by which progress in preservation will be measured are appropriate for the techniques.”

Right now, the most widely used metric for performance at the federal level is pavement smoothness as measured by the International Roughness Index (IRI). While this is appropriate for new construction or standard overlays – nearly every survey shows ride quality is the No. 1 criterion of the public in judging consumer satisfaction – the preservation community would like an additional metric such as a health index or remaining service life for preservation treatments. FP2 and its allies are working to make that happen.

Preservation at the NCAT Track

New quantitative research on pavement life cycle effects of pavement preservation started in late 2012, as the new National Center for Asphalt Technjology (NCAT) Pavement Preservation Effectiveness Study will bring the prestige of NCAT’s research facility to pavement preservation practice.

Bexar County applies a fundamental tenet of pavement preservation, that is, for the lowest-cost, long-term performance, treat roads before they show distress.

NCAT’s Pavement Test Track is funded and directed by a multi-state research cooperative program in which the construction, trafficking, and performance evaluations are carried out on 46 different 200-foot test sections around a 1.7-mile oval test track.

Each of the test sections is constructed using the asphalt materials and design methods used by individual sponsors. A fleet of heavy trucks is operated on the track in a highly controlled manner in order to apply a design lifetime of truck traffic (10 million equivalent single axle loads, or ESALs) in two years.

Test sections are rebuilt every three years to provide experimental pavements for the next research cycle. The 2012 NCAT Pavement Test Track, which represents the fifth research cycle, is the first experiment that will include a formal pavement preservation study.

Referred to as the Preservation Group (PG) experiment, the study is designed to encompass multiple timely issues that are important to the entire pavement community.

“State departments of transportation – beset by dwindling tax revenues and rising material costs – are being forced to do more with less like never before,” says NCAT assistant director Buzz Powell, P.E., Ph.D. “Many DOTs either have a mandate to invest infrastructure dollars in pavement preservation, or have a strong interest to do so.” In the long term the NCAT research should help supporters of pavement preservation make their case for increased funding.

In Bexar County, Tex., 3/8-inch crushed basalt chips immediately follow placement of cationic, high-float, rapid-set emulsion; county was 2012 winner of FP2’s James B. Sorenson Award for Excellence in Pavement Preservation

Preservation treatments often are applied to roadways as a reaction to badly deteriorated conditions, which amounts to throwing good money after bad. Instead, as pavement preservation is defined by the mantra “the right treatment to the right pavement at the right time,” by definition preservation has to be placed in a proactive manner at the right point in a pavement’s life cycle.

“A great need exists to quantify the relationship between pretreatment condition and lifecycle for all preservation alternatives, so that DOTs can select those that provide the lowest possible lifecycle cost,” Powell says.

Therefore the goal of the Preservation Group study on the 2012 NCAT Pavement Test Track is to provide sponsoring state DOTs with that relationship (i.e., the unique curve that defines the relationship between pretreatment condition, and life cycle performance for each preservation treatment), Powell says, which can then be programmed into decision trees that objectively select alternatives as a function of pretreatment condition.

“Over time, feedback from pavement management systems will precisely calibrate these relationships to local materials, contractors and environmental conditions,” he says.

The Preservation Group study will include select test sections on the NCAT Pavement Test Track that have survived from previous research cycles. All track PG sections are supported by the same subgrade and base, and the total thickness of all bituminous lifts is 7 inches. “This thickness was chosen when these sections were originally constructed at the beginning of the 2009 research cycle,” Powell says, “because in past studies 7-inch sections exhibited significant performance differences within the standard 10 million ESAL traffic cycle as a function of the differences in mix designs and materials.”

Off-Track Applications

The new Preservation Group experiment is unique in that – again, for the first time – pavement preservation treatments are not only being studied at the test track, but also off-track on a 1/2-mile stretch of a local county road, Lee Road, that supports traffic to an aggregate quarry with a high percentage of truck traffic.

On Lee Road, relatively good condition pavement supports lighter traffic in the inbound lane, while generally distressed pavement supports heavier traffic in the outbound lane. In both lanes, performance will be differentiated between the wheelpaths because widening in the past history of the road has produced distinctly lower levels of pavement condition in the right (widened) wheelpaths.

In late summer 2012, pavement preservation treatments were applied full lane width in 100-foot test sections, producing 200 wheelpath-feet of experimental pavement surface. Each section was further differentiated into 10-ft.-long test cells, producing 20 data points per treatment.

Pavement preservation techniques placed were

• Fog seals (with and without rejuvenators),

• Crack seals (routing/filling, hot air lance, go-type),

• Chip seals (single, double, triple, scrub, FiberMat),

• Cape seals (on chip/scrub seals, FiberMat),

• Micro surfacing (single, double, Capes),

• Plant mix overlays (4.75 screening mix variations), and

• Ultra-thin bonded wearing courses, and lightweight aggregates for surface treatments.

Experiment contractors were Vance Brothers, Colas and E.D. Etnyre and Co. Instrumentation will document multi-depth pavement temperatures in each section, and records from the quarry will provide a comprehensive load history over the life of the experiment.

Pavement condition will be monitored on a weekly basis in order to determine the time and traffic needed to reduce pavement condition back to the pretreatment level. Because a distinct value will be produced for each test cell, 20 data points will define the shape of the life cycle curve for each preservation treatment.

Pavement Preservation Conference

At the August National Pavement Preservation Conference, 48 exhibitors and over 500 delegates from across the continent and around the world came together for a seminal event in the growing field of pavement preservation.

“For two years, your leadership at FP2 has been engaged with the staff of the National Center for Pavement Preservation to plan and develop this most important event in 2012 in pavement preservation,” said FP2 President Buckingham.

Better Roads’ editor-in-chief John Latta and contributing editor Tom Kuennen both participated in a panel on pavement preservation and public relations at the event.

Major pavement preservation partnerships uniting state and provincial road agencies held concurrent meetings. These included the Midwestern, Northeast, Rocky Mountain West, and Southeast Pavement Preservation Partnerships.

Plenary sessions set the stage for the conference to come. Then, seven topical “tracks” relevant to pavement preservation, asset management and pavement management featured 24 sessions spread over four days.

A hectic field demonstration held on the grounds of the Old Tennessee State Prison outside Nashville featured asphalt and concrete pavement preservation techniques such as chip seals, microsurfacing, scrub seals, surface re-texturizing, pavement rejuvenation, dowel bar load transfer retrofits, diamond grinding, and other innovative treatments.

Video of all presentations may be streamed, and materials downloaded, at http://nationalpavement2012.org/presentation-multimedia/.

Also, Bexar County, Tex., and the Tennessee DOT were honored for their pavement preservation programs with FP2’s James B. Sorenson Award for Excellence in Pavement Preservation. The Bexar County Public Works Department was honored with the Sorenson Award for 2012, and Tennessee DOT for 2011.

Receiving the award for Bexar County was Tony Vasquez, public works operations manager, for his work instituting asset management of county roads beginning in 2004, and subsequent pro-active pavement preservation practices to economically prolong the life of county roads.

Bexar County applies a fundamental tenet of pavement preservation, that is, for the lowest-cost, long-term performance, treat roads before they show distress. Bexar (pronounced “bear”) County includes urban as well as rural pavements, as the City of San Antonio is located there.

Accepting the award for the state of Tennessee was Tennessee DOT Commissioner John Schroer. The DOT was honored for its outstanding advocacy for, and implementation of, its statewide pavement preservation program. In only four years – between 2007 and 2011 – Tennessee DOT transitioned from an almost exclusively hot mix asphalt resurfacing program to one that incorporates pavement preservation principles. The result has been a significant improvement in pavement condition.

The department provided detailed pavement management systems data to prove the case for future network condition, and worked with the local hot mix industry to develop new specifications for thin hot mix overlays to gain its buy-in to the program. These thin hot-mix overlays now have become another routine pavement preservation treatment used in Tennessee.

Also recognized at the awards ceremony for his leadership and long service to the pavement preservation community was FP2 executive director James Moulthrop, inducted into the FP2 Pavement Preservation Hall of Fame.

‘Gravel,’ stone-surfaced and dirt road preservation helped by new techniques, guidelines, RAP

By Tom Kuennen, Contributing Editor

A clogged corrugated drainpipe under this Texas road results in water flowing across it, eroding the surface and creating a ford in wet weather.

If you don’t drive them often, it’s easy to overlook unpaved roads. After all, they carry significantly less traffic than rigid or flexible pavements in areas where most people don’t live. Typically, unpaved roads carry less than 150 vehicles per day, and those vehicles tend not to put excess stress on the road structure. That is, unless it’s during harvest time, when a limited number of overloaded trucks put very heavy loads on these dirt and gravel surfacings.

Unpaved roads aren’t limited to rural areas. In December 2012 the residents of Ft. Lauderdale’s South Middle River neighborhood – located off the busy Andrews Avenue corridor, just blocks from downtown Fort Lauderdale, Fla. – demonstrated for reconstruction of their dirt streets by removing road construction signage from an adjacent work zone and using it to direct motorists down their dirt street. The demonstration continued for 40 minutes before police broke up the gathering, reported the Ft. Lauderdale SunSentinel.

In recent years, declining road funds, rising traffic and loads have combined with natural calamities like floods to stress the nation’s unpaved roads. Significantly, the boom in fossil fuel extraction of oil, oil sands and natural gas from states from Pennsylvania to Texas to Wyoming, and in Canada’s western provinces, has put new stresses on unpaved roads as they carry construction equipment inbound, and extracted resources outbound. Fortunately this new boom is providing a fresh stream of tax revenue for road repair.

Even the boomlet in windmill-powered electricity generation has stressed unpaved roads. Often, access to agricultural areas where wind farms are located is entirely via unpaved roads, which suffer under heavy traffic from 50,000-pound concrete mixers, and trucks hauling 800-ton cranes.

Unpaved Road Preservation

While preservation of asphalt and concrete pavements has become quite technically advanced, preservation of unpaved gravel, stone or dirt roads is still at the maintenance stage, in which preservation is defined as preventive maintenance.

South Dakota has articulated a checklist for preservation of its many miles of gravel roads. Blading and graveling of mainline roadways and shoulders are effective at reshaping or replacing granular material lost on the surface of either a roadway or shoulder, the SD DOT says.

“Graveling of mainline or shoulders involves shaping of the surface and a periodic addition of granular material to provide a smooth driving surface or shoulder that has a proper crown slope and is free of ruts and distortions,” it maintains. Deficiencies include:

Well-built and maintained culvert keeps water off both unpaved roadways.

• Rutting and shoving of material. Wheel motion of the traffic will shove material to the outside (as well as in-between traveled lanes), leading to rutting, reduced water-runoff, and eventual road destruction if unchecked, the state says. As long as the process is interrupted early enough, simple blading is sufficient, with material being shaped to correct the deficiencies.

• Washboarding. Washboarding is the formation of corrugations across the surface at right angles to the direction of travel. They can become severe enough to cause vibration in vehicles so that bolts loosen or cracks form in components, the DOT says. Blading performed under the correct moisture conditions will aid in removing the corrugations, and the addition of a good quality gravel can help prevent them reforming.

• Cross slope. The amount of crown in the roadway or shoulder should be maintained at a rate of 0.3 foot per foot, to 0.05 foot per foot. This amount of crown will allow for adequate drainage of surface water without washing off surface materials, South Dakota says.

• Loss of fines. If a gravel-surfaced roadway or shoulder has dust blowing off the surface, it indicates a loss of fines, and material is being lost, causing the road to deteriorate. “Typically a roadway with average traffic loses about 1 inch of material per year,” the state says, “so periodic replacement with good quality material is a necessity.”

• Blading and reshaping of a roadway or shoulder should take place in moist weather conditions if possible, the South Dakota DOT says. “When blading, material should be pulled from the in-slope area back up onto the roadway or shoulder surface and smoothened, watered, and compacted to the proper grade and crown slope. The grader should be operated at a top speed of between 3 to 5 miles per hour, and the grader moldboard should be operated at the correct angle and pitch to adequately move and mix the material.

Inventory, Plan in Washington County

Maintenance of unpaved or gravel roads in a jurisdiction can be addressed in a systematic method, as Washington County, Iowa – located just west of the Quad Cities – did recently.

Beginning in 2008, when a “perfect storm” of diminished cash flow, increasing traffic and loads, and excessive flooding severely damaged many unpaved roads, the county’s engineering staff prepared a $12 million, multi-year unpaved road improvement program funded largely by an $8 million general obligation bond issue that county engineers were able to justify with the plan.

Poor drainage results in ponding and aggregate loss in unpaved roads.

“High costs and flat revenues, increased vehicle weights, and increases in rural traffic volumes have created conditions on the gravel road system in Washington County that are quickly becoming unacceptable to residents,” said David Patterson, P.E., Washington County engineer, in September 2008.

“While any one of these changes – financial, increased weight, or traffic issues – could probably be handled by the system under its current allocation of resources, the combination of all three has tipped the system into a downward spiral that will become irreparable if changes are not made,” Patterson said in terms that are familiar to the pavement preservation community. “Under its current allotment of resources, the gravel road system has reached a point where damage is being done faster than can be repaired.”

This plea was buttressed by an unpaved roads inventory. “During the summer of 2008 the Secondary Roads Department performed a complete inventory of the gravel road system in the county,” Patterson said. “The inventory marked the condition, needs, and noted any special circumstances of every gravel road in the county.”

A comprehensive review of the available solutions was then performed to see what solution would best apply to which road. These solutions included adding rock surfacing, grading the roadway to repair deficiencies, upgrading a roadway for paving, or vacating/downgrading the road classification.

As articulated in 2010, Patterson outlined four outcomes for bringing county unpaved roads up to par:

• Fix the six to eight “hot spots” where the county spends three to four times its average maintenance dollar (average of $2,500 per mile, compared to $7,500 to $10,000 per mile in these locations)

• Make system-wide improvements to gravel

• Reduce long-term maintenance cost, and

• Rebuild and recover from disasters like the extensive flooding of May-June 2010, when it rained continuously.

Ultimately the plan would provide five miles of new paving, rebuilding of 80 miles of gravel roads, and additional rock surfacing for 40 miles of roads. “By improving these 125 miles of roadway, maintenance resources will be available to make improvements on the other 800 miles of our county roads,” Patterson said. “The new gravel roads would be 30- to 50-percent cheaper to maintain, trap less drifting snow, have proper drainage, and have an appropriate surface course for the traffic they are carrying.”

GPR and Gravel Roads

The use of motorized vans carrying ground penetrating radar (GPR) is well-established in characterizing the subsurface conditions of road networks as road agencies update their pavement condition inventories, but in 2012 researchers at the University of Illinois at Urbana-Champaign described use of GPR for unpaved roads.

In Denver, this unpaved alley was topped with a mix of straight RAP and neat rejuvenator, and then compacted; the result is an inexpensive unpaved driving surface.

In their TRB paper Assessment of Subsurface Deformation in Unsurfaced Pavements Using Ground Penetrating Radar, U of I graduate research assistants Debakanta Mishra and Zhen Leng, and professors Erol Tutumluer and Imad L. Al-Qadi describe the findings of full-scale accelerated pavement testing to evaluate aggregate quality in two unsurfaced pavement sections.

“An innovative application of the nondestructive GPR technology was established for assessing subsurface deformations and distinguishing between the different rut mechanisms that contributed to the failure of unsurfaced pavements,” they write.

They observed that aggregate type and angularity clearly governed the mechanisms responsible for rut accumulation in unsurfaced pavements. “An uncrushed gravel layer with high fines underwent internal rutting through shear flow and exhibited significant heaving adjacent to the wheel path,” they say. “The pavement test section with the crushed aggregate base, on the other hand, showed significantly higher load resistance and primarily exhibited subgrade shear failure at load repetitions much higher than the gravel section.”

GPR scanning of unsurfaced pavement sections shows promise for identifying different rutting modes and assessing subsurface layer deformations and damage potentials, they conclude, adding this concept can be further expanded to quantify the accumulation of rutting in individual layers by estimating the dielectric constants of layer materials.

RAP Stabilizes Unpaved Surfaces

With a boom in natural gas extraction – as well as high production from its low-sulfur coal surface mines – Wyoming unpaved roads are suffering from very heavy truck traffic, as well as an increase in automobiles.

But increased use of reclaimed asphalt pavement (RAP) to preserve riding surfaces is an option that offers a low-cost, but substantial, way to suppress dust loss and enhance aggregate retention, say Burt Andreen and Harry Rocheville, graduate research students at the University of Wyoming, and the university’s Khaled Ksaibati, Ph.D., P.E., in their 2012 TRB paper, A Methodology for Cost/Benefit Analysis of Recycled Asphalt Pavement (RAP) in Various Highway Applications.

In recent years, RAP has been used as a surface additive on Wyoming’s unpaved roads, streets and alleys. This has been boosted by recent state legislation, which compensates Wyoming DOT for RAP donated to Wyoming counties. That being said, the DOT and local agencies needed to evaluate the cost effectiveness of blended RAP and virgin aggregate as a surfacing material for unpaved roads, and that was the goal of the research.

For the test, two half-mile sections were constructed using RAP, which was distributed using bottom discharge dump trucks and then spread with a motor grader. The RAP and existing base were then mixed by a mobile stabilizer using a toothed drum, which mixed the RAP with 6 inches of base and released the mixture back onto the road surface. This mix of RAP and existing base then was sprayed with water and compacted to achieve maximum density.

“RAP in gravel roads resulted in a savings of $17.07 per ton of RAP in this case … due to the savings from virgin aggregate and the savings through dust reduction, which will keep the road in better condition by retaining the fine particles embedded in the road,” the authors write. “The air quality will also be improved as a result of the reduction in dust.”

The City of Denver, Colo., has been using RAP to surface its unpaved alleys, where the RAP particles “knit” to a solid surface in the hot sun. For its unimproved alleys, in the spring and fall, the city removes 6 to 8 inches of surface with a mid-size cold mill, and replaces it with RAP millings. This turns them into a gravel alley and reduces dust and runoff components.

Once, the city would bring in trucks, grader and a backhoe loader to dig the alleys out. Now the cold mill does the excavation work in one pass, cutting the work time from three to one day. Following 8 inches of excavation, the cold millings are brought in, graded and compacted.

In the heat of the sun, the millings meld to form a firm surface that holds up to residential traffic loads, with no truck traffic other than the refuse trucks once a week. Denver also has experimented with mixing cold millings with rejuvenator in the equipment yard, and then placing it in the alley with a paver. The result is a simple cold mix with consolidated surface.

Dust Research in Alaska

Fugitive dust escaping from the surface of unpaved roads has severe negative effects. The dust can affect air quality because it is a PM 10 material, defined as composed of particles 10 micrometers or less. And as dust leaves the surface of a gravel, stone or dirt road, aggregates and other fines loosen, leading to surface problems and costly replacement with new gravel.

Dust palliatives suppress fugitive dust. Spread by distributor truck, they suppress dust on a driving surface, keeping moisture in the road. Some, like liquid calcium chloride, a very common dust palliative, absorb humidity from the ambient air, suppressing dust by keeping it relatively damp. Calcium chloride absorbs water vapor from the air and moisture extant in the road structure. At 77 degrees F and 75 percent humidity – common conditions during summer in the Midwest and South – it absorbs more than twice its weight in water. In addition, calcium chloride solutions attract more moisture to the road than they give up in evaporation. Thus a treated road surface can retain moisture even during the heat of summer.

In early June, a research team from the Alaska DOT&PF Statewide Research, Development, and Technology Transfer program, and the Alaska University Transportation Center, continued remote field work testing dust-reducing palliatives at rural runways and roads across Alaska.

Using a unique portable instrument and a variety of dust-control palliatives, they are helping the state compare which dust treatments are the best for specific unpaved road and runway locations throughout the state.

The instrument, the DUSTM, monitors palliatives on gravel airports and roads in over 30 remote Alaska communities. When deployed, it mounts on the rear of an ATV to measure loftable dust levels (defined by the EPA as PM 10). An air intake extends from the unit off the back of an ATV and pulls a continuous air stream as the ATV drives at specific speeds over a surface. As dust emerges, the air stream is pulled through a tube and passes by a laser, which measures the opacity of the airstream. This data is then recorded in an on-board datalogger box, which can be analyzed later. Early results show various palliatives reduce dust from 65 to 99 percent.

‘Gravel Roads Academy’

An outreach program – the Gravel Roads Academy – teaches how to both provide optimal gravel road conditions for local drivers, and optimize shrinking budgets.

Launched in 2012 by the makers of DustGard – a road stabilization and dust control product – the Gravel Roads Academy trained road engineers and maintenance crews in 16 states to build and maintain gravel roads for greater cost savings, superior stabilization and improved air quality.

Held from March through October, the free Gravel Roads Academy engaged “gravel gurus” to teach city, county and state road professionals the most efficient unpaved road construction and maintenance techniques, including the regular (usually annual) application of a road stabilizer and dust control product.

Each 1 ½-day session included a field demonstration of motor grading techniques and stabilization, and taught best practices in design, maintenance, stabilization, resource management, air quality and funding options. Approximately 35 sessions will be conducted in 2013. For more information, visit www.gravelroadsacademy.com.

When Paved Roads Go Bad

The potential of road agencies letting a paved road deteriorate to unpaved status has achieved notoriety in our age of diminished agency budgets. Whether and how to let a road “unpave” was explored in February 2012 at the 16th Annual TERRA Pavement Conference at the University of Minnesota-St. Paul.

There, Freeborn County, Minn., engineer Sue Miller described the unpaving of County Road 20, which was so far gone that county repair trucks were destroying the surface as they attempted to repair it. Unpave a road with trepidation, she warned, and, as she said “Make sure you don’t take a bad bituminous surfaced road and make it into a terrible gravel road.”

County Road 20 was disintegrating, but was not scheduled for any type of major maintenance until 2015. “We had to unpave it, reclaim the useless broken asphalt, and revert to a gravel-surfaced road until we had time to consider alternatives to traditional paving and find the required funding,” she said, as quoted by Richard Kronick in the Spring 2012 Minnesota LTAP newsletter.

When Miller used the word “unpave” in a county board meeting, she discovered that it was loaded with negative connotations, Kronick writes. “For some of my county commissioners, the word unpave means loss of service. It means we’ve failed. No one wants to tell constituents that the level of service they expect can’t be delivered. In fact, I was told not ever to use that word again!”

The dean of gravel road maintenance, South Dakota Local Technical Assistance Program’s Ken Skorseth, said removing asphalt and reverting to a gravel surface may only lead to different types of problems, Kronick reported. “If the truck traffic is 25 to 50 ADT and there is low subgrade support, you will need 14.5 inches of gravel,” Skorseth said. “That’s hard to do when trucks are knocking it off constantly and the blade needs to be out there every other day. Furthermore, the quality and availability of gravel varies greatly from place to place.”

Unpaving, or reversion of a paved road to gravel, is not widespread. The most current overview will be found in the June 2010 research synthesis, Decision Tree for Unpaving Roads. Download the report – which includes state and local agency experience – at www.lrrb.org/media/reports/trs1007.pdf.

Calculators help predict the environmental impact of roadbuilding projects

By Tom Kuennen, Contributing Editor

New tools keep appearing to help the highway, road and bridge community determine the environmental impact of the infrastructure they design and build.

Cold-in-place recycling of failed pavements optimizes use of in-place materials, relieving pressure on virgin aggregate extraction sites, on landfills, and greatly reduces truck trips with benefits to air quality, congestion, noise and reduced resource consumption.

Last month we looked at the proliferation of environmental certification programs available for highways (see Roadway Environmental Ratings: What’s Best for Your Agency?, November 2012, pp. 21-27). But in addition to these guidelines for a holistic summation of the environmental sustainability of a project, a plethora of environmental calculators now exist for a road builder to use in determining project sustainability in advance of construction.

These software-based eco-calculators allow an array of variables or “inputs” about a project – such as dimensions, materials, quantities, haul distances and equipment – and kick out data such as quantities of carbon dioxide emitted in construction or over the life of a project, other emissions and more. These data can provide a user with multiple paths to a sustainable project. In doing so a planner gets hard data projections he or she can use to justify or defend a project against those who would oppose it.

These calculators can be used in advance by an agency to establish project fundamentals, by a contractor to pose environmental alternatives that will favor his capabilities, or by consulting engineers to assist their agency clients.

For example, at the October semiannual meeting of the Asphalt Recycling & Reclaiming Association, Donald M. Matthews, P.E., of Pavement Recycling Systems in Riverside, Calif., said contractors can use the value engineering process – using environmental sustainability calculations – to suggest to clients that an alternate (in this case, cold-in-place recycling) could save time and money for a client.

“Use the calculations to suggest and prove that an alternative method will save an agency money, while providing them with an equal or better product,” Matthews says. “Sustainability calculators are valuable ways of showing even more benefit to an agency as agency personnel will get the pat on the back they deserve.”

Not surprisingly, eco-calculators first appeared in Europe, but now are making their way into North America. Some are proprietary and were developed by corporations for internal use; some are public domain, developed by government agencies; and some may promote one outcome or the other, depending on which business or national association is promoting it.

And contractors and agencies have been doing this type of evaluation in recent years without formal calculators. Using spreadsheets and their own knowledge, they’ve been able to informally determine the environmental impact of their projects.

National Asphalt Pavement Association’s Greenhouse Gas Calculator is an online tool that determines GHG emissions at the plant.

For example, in 2006, just by simple calculation that incorporated materials, quantities and truck trip inputs, Caltrans’ Joe Peterson, P.E. – now statewide chief of the Office of Roadway Materials Testing – was able to determine the benefits of an in-place recycling of busy I-80 through cities and countryside on the western slope of the Sierra Nevada.

“They saved approximately 112,200 short tons of aggregate, 2,860 tons of bitumen and 9,200 truck trips,” Robertson says in a presentation. “That’s an incredible number of truck trips, and that doesn’t include all the auxiliary equipment that would have been moving in and out of the work zone.”

He also calculated emissions benefits. “Because the project was in an urban environment, the 15,800 pounds of NOx that was not put in the environment made a lot of the people in the area a lot happier,” he says.

“That is because our mountain-and-valley areas are subject to inversion layers in the atmosphere, which trap all the smog down low. Anything they can do on a construction project to lessen that impact is a benefit to everyone.”

But today’s tools permit much greater detail in sustainability benefit reporting. Here are a few of them.

PaLATE Within Greenroads

One useful eco-calculator is found within the Greenroads certification system. As described last month, the Greenroads Rating System is a certification process for sustainable roadway and bridge construction projects, not unlike the Leadership in Environmental Excellence in Design (LEED) system for certifying buildings and developments.

Greenroads rates a project’s sustainable elements in seven categories: Project Requirements, Environment and Water, Access and Equity, Construction Activities, Materials and Resources, Pavement Technologies and Custom Credits. Accumulated points will position a project for one of four levels of certification: Bronze, Silver, Gold and Evergreen.

Under Greenroads, points may be accumulated by use of the environmental sustainability calculator PaLATE v2.2 as modified for Greenroads, or an approved equivalent. The idea is to incorporate – in advance – energy and emissions information into decision-making for pavement design alternatives.

Emissions of greenhouse gases are typically expressed in a common metric so that their impacts can be directly compared, as some gases are more potent (i.e., have a [presumed] higher global warming potential) than others, according to the Environmental Protection Agency. Thus the international standard practice is to express greenhouse gases in carbon dioxide equivalents (CO2e).

With PaLATE 2.2, for points to be obtained, a lifecycle inventory for the final pavement design alternative for the project should report total energy use and global warming potential in CO2e.

Inputs include:

• Total weight and types of virgin materials. This includes aggregates, binders, base materials and structures, according to Greenroads. These amounts can be design estimates or constructed totals.

• Total weight and types of recycled materials. PaLATE v2.2 models emissions and energy for several types of materials

• Expected transportation distances for all materials. This means distances from source to production as well as from production to site. Transportation of waste to disposal is also included.

• Expected construction vehicle types. These include, but are not limited to, pavers, mixers, hauling vehicles, excavators, rollers and finishing equipment. Obviously use of the new mobile equipment meeting Tier 4-interim emissions guidelines would be a plus.

• Estimated design life. This should reflect the same input data as used in Greenroads Lifecycle Cost Analysis section.

• Scheduled years and expected type of maintenance. Likewise, Greenroads suggests use of the same input data as used in its Lifecycle Cost Analysis, and meet project specs provided for its pavement and site maintenance plans.

FHWA’s INVEST Now a Reality

On October 10, the Federal Highway Administration’s INVEST sustainable pavements rating system was introduced. Not a calculator per se, it’s a totally free, voluntary, web-based tool for assessing the environmental sustainability over the lifecycle of a transportation project or program, including system and project planning, through design and construction, to operations and maintenance.

Opening screens of detailed National Ready Mixed Concrete Carbon Calculator, a spreadsheet that enables calculations of carbon footprint of concrete products and projects.

INVEST is intended to permit agencies to assess their own transportation plans, projects and programs, and help them make more informed decisions with limited resources to balance what FHWA calls economic, social and environmental factors.

Goals of INVEST are to support the U.S. DOT’s aims for livability and sustainable transportation, increase the body of knowledge regarding sustainability aspects of both asphalt and concrete materials in pavement design, construction, preservation and maintenance, and to boost use of sustainable technologies and practices in pavement design, construction, preservation and maintenance, FHWA says.

More information – and the program – may be downloaded at www.sustainablehighways.org

NAPA’s Greenhouse Gas Calculator

The National Asphalt Pavement Association has stepped into the fray with its Greenhouse Gas Calculator for pavements, revised in 2012 and currently under beta testing.

This online tool calculates greenhouse gas emissions related to asphalt pavement manufacturing in a gate-to-gate analysis. “The user-friendly interface provides drop-down lists of typical fuels that are linked to greenhouse gas emission factors, expressed as carbon dioxide equivalents (CO2e), the universal measure of greenhouse gas emissions,” NAPA says.

The calculator provides inputs for three separate fuels used by a rotary dryer, plus three additional fuels used inside the facility by equipment, including vehicles. “A final category addresses CO2e attributed to electrical use, with unique factors for each state,” NAPA says. “Fuel used for onsite power generation should also be included. Generally, the more fuel combusted or burned, the more CO2-equivalent gases [that] are emitted.”

FHWA’s INVEST – released in October 2012 – was tested for two years prior to launch, but the final product is radically different from the one used in beta and pilot program testing.

The beta update adds emission offset credits for those fuels and activities that either reduce the amount of CO2e released, or that sequester CO2. Credits are calculated for plant-based bio-fuels, transportation credits for waste derived fuels like recycled fuel oil, and cradle to gate credits for recycled raw materials like RAP and shingles. Credits are also calculated for reducing mix temperatures using warm mix asphalt technologies, based on user defined mix temperature.

To learn more, or to use the calculator or view an online webinar on its use, visit http://www.asphaltpavement.org/ghgc

In 2009 NAPA published an environmental sustainability report on asphalt, the first ever of its kind for the asphalt industry. Titled Black and Green: Sustainable Asphalt, Now and Tomorrow, it highlights the ways in which the asphalt industry’s everyday practices address climate change, improve air quality and water quality, provide green jobs, and reduce the carbon footprint of pavements. This 12-page report can be downloaded at http://www.asphaltpavement.org/images/stories/sustainability_report_2009.pdf

Britain’s asPECT

In Great Britain, a calculator to measure the lifecycle greenhouse gas emissions or “carbon footprint” of asphalt used in highways has been developed and maintained by TRL, the Transport Research Laboratory. This program – dubbed asPECT, for Asphalt Pavement Embodied Carbon Tool – was produced as a result of a 2008-11 collaborative research effort of the Highways Agency, Mineral Products Association, Refined Bitumen Association and TRL Limited.

asPECT provides a methodology to calculate the whole life contribution of highway products to climate change. At its core is the calculator, a downloadable spreadsheet-based product that guides the user through the program’s methodology and makes the necessary calculations using up-to-date emissions factors on the user’s behalf. Learn more at www.sustainabilityofhighways.org.uk

Concrete’s Carbon Calculator

The National Ready Mixed Concrete Association has made available its NRMCA Sustainable Concrete Carbon Calculator. It will quickly determine two environmental impact measures to use to benchmark annual RMC production, Global Warming Potential, and Primary Energy Consumption. Global Warming Potential is reported in metric tonnes CO2e and short tons CO2e, and is a reference measure to report the amount of greenhouse gases produced in extraction, processing, transportation, construction and even disposal of material.

Primary Energy Consumption is reported in mega-joules and millions of Btus, and includes all primary energy consumed, directly and indirectly, to transform and transport raw materials into concrete products and buildings, NRMCA says.

It includes inherent energy in raw or feedstock materials that are also used as common energy sources, and also captures pre-combustion (indirect) energy use associated with processing, transporting, converting and delivering fuel and energy.

A plant emissions calculator is available as well. To learn more or to use the calculator visit www.rmc-foundation.org/Env_Health_Safety_Track.htm

Last December the National Concrete Pavement Technology Center at Iowa State University released Sustainable Concrete Pavements: A Manual of Practice.

This guide provides a clear, concise and cohesive discussion of pavement sustainability concepts and of recommended practices for maximizing the sustainability of concrete pavements. Download it at www.cptechcenter.org/publications/Sustainable_Concrete_Pavement_508.pdf

Bridge Project Evaluated

The PaLATE program was used in evaluating a bridge project last year, as described by Jeralee L. Anderson, P.E., and Craig Weiland, Department of Civil and Environmental Engineering, University of Washington, in their 2012 Transportation Research Board paper, Energy and Carbon Footprint of a Single Span Concrete Bridge.

For this purpose, lifecycle assessment (LCA) – a systematic way to quantify environmental impacts for all life stages of a project – was used to inform project decision making and complement results of other decision-making tools, such as cost projections determined through bridge lifecycle cost analysis.

“PaLATE v2.2 [as modified for Greenpave] is a streamlined tool for lifecycle assessment that is non-proprietary,” Anderson and Weiland say. “It does not include vehicle energy and emissions from users of the final structure, but does address demolition, construction, production of materials and transport of materials.”

The results of this study show that LCA can be completed on bridge projects using an easily accessible and free tool and standard bridge design and construction documents, the authors conclude. “As expected, materials production and transportation are the most significant contribution to thelifetime energy consumption and CO2e emissions,” they say.

Granite’s Corporate Program

Corporations develop their own eco-calculators. Foremost among them is California-based Granite Construction whose Granite Environmental Management System (GEMS) to provide a consistent and reliable framework for managing environmental responsibilities.

GEMS is central to the firm’s environmental management efforts. “GEMS is a system of business processes that supports Granite’s environmental objectives and policies, and ensures that our actions align with our environmental vision and strategy, core values, and environmental affairs policy,” the big contractor says.

Other tools include a comprehensive environmental and land management software system, and an ongoing self-evaluation of operations. “Our strategy focuses our efforts on our environmental management procedures and systems, recycling of construction materials, energy conservation and land stewardship.”

Granite recently joined the Sustainability Infrastructure Advisory Board (SIAB) at Harvard University.

This board – with other members such as the Louis Berger Group, HNTB and Stantec, provides input to the Institute for Sustainable Infrastructure (ISI) there, developers of the new infrastructure rating system, Envision, developed to help designers, builders and infrastructure owners build and direct infrastructure projects toward increasing levels of sustainability.

More information on Envision can be found in last month’s RoadScience article.

Landmark Colas Study

Paving the way for these calculators was a landmark 2003 report. Environmental Road of the Future: Life Cycle Analysis, Energy Consumption and Greenhouse Gas Emissions, by M. Chappat and J. Bilal – was released by the Colas Group and is an in-depth analysis of energy consumption and GHG emissions of more than 20 different paving product types by ton of material placed.

Chappat and Bilal show that portland cement concrete (PCC) paving materials and processes demand the most energy, followed by hot mix asphalt (HMA) paving. The report also showed that cold-in-place (CIP) recycling is the least energy-intensive process.

“A comprehensive and realistic measure of energy use and GHG emissions of a specific type of roadwork begins at the extraction of raw materials from the earth, including all intermediate steps, such as transport, refining, manufacturing, mixing and placement,” says Jim Chehovits, P.E., vice president, operations, Crafco, Inc., and Larry Galehouse, P.E., director, National Center for Pavement Preservation. “These energy input and emissions data then can be extended via an annualized life extension basis.”

Energy consumption for aggregate production includes quarrying, hauling, crushing and screening. Chappat and Bilal demonstrate that energy consumption for aggregate production ranges from 25,850 to 34,470 Btu/t, and GHG emissions range from 5 to 20 pounds CO2/t.

Energy consumption for asphalt binder production includes crude oil extraction, transport and refining. Energy consumption for asphalt binders has been determined to be 4.2 mm Btu/t, and GHG emissions are 570 pounds CO2/t.

Download the report at www.colas.com/FRONT/COLAS/upload/com/pdf/route-future-english.pdf

By Tom Kuennen, Contributing Editor

The never-ending quest to define which pavements and highways can be considered “environmentally sustainable” has gotten a lot harder, just as it’s gotten easier.

Cold milling of aged asphalt produces reclaimed asphalt pavement (RAP), and is encouraged by various environmental sustainability rating systems

It’s gotten easier because state, county and municipal agencies can choose from a variety of programs that enable them to evaluate and rate the “green-ness” of a particular pavement.

But it’s gotten harder because, well, there are so many to choose from.

Unlike the nationally recognized LEED system – which is the only accepted environmental certification program for buildings and structures – there are a variety of different evaluation/certification programs for roads or civil engineering structures at international, national and state levels.

For example, even as the Transportation Association of Canada’s Green Guide for Roads poses sustainability guidance for road construction for Canada, the Ontario Ministry of Transportation is promulgating its own GreenPave points-based rating system.

Similarly, the New York State DOT has developed the GreenLITES (Green Leadership In Transportation Environmental Sustainability) pavement rating system, and in 2011Illinois introduced I-LAST, the Illinois-Livable and Sustainable Transportation rating guide.

Even so, in the United States Greenroads is the national leader by its entrenched position. Established in 2010, the Greenroads Foundation is developer of the Greenroads Rating System, and the foundation manages the certification process for sustainable roadway and bridge construction projects in the United States and internationally.

Greenroads rates a project’s sustainable elements in seven categories: Project Requirements, Environment and Water, Access and Equity, Construction Activities, Materials and Resources, Pavement Technologies, and Custom Credits, which are intended to accommodate good ideas that don’t fall under the previous headings. Accumulated points will position a project for one of four levels of certification: Bronze, Silver, Gold and Evergreen.

Joining these state and national programs in October 2012 was the full-scale release of INVEST by the Federal Highway Administration. An acronym for Infrastructure Voluntary Evaluation Sustainability Tool, INVEST 1.0 follows a beta version from fall 2010, and a pilot version in 2011, and is a practical, web-based collection of best practices.

“Sustainability is an opportunity for an organization to adjust its course,” said Stephen T. Muench, Ph.D., P.E., Greenroads director, and associate professor at the University of Washington-Seattle, at the National Pavement Preservation Conference in Nashville in August this year. “It permits a look at organizational priorities to see if there’s a need to adjust them. Rating systems play a role in this adjustment because they are a reasonable means to manage and communicate sustainability efforts.”

Greenroads follows LEED

Whether acknowledged or not, these programs follow the footsteps of the Leadership in Energy and Environmental Design program. LEED recognizes environmentally sustainable building and neighborhood design, and LEED certification is administered by the U.S. Green Building Council.

Drainage is important in sustainable design. In Tacoma, Wash., this roadside drainage swale keeps
exhaust byproducts, such as residual hydrocarbon compounds and heavy metals from the pavement surface, out
of water supplies. Natural bacteria in soil neutralize products in runoff prior to runoff reaching existing aquifer.

The building industry uses the LEED system to evaluate the degree of “green” design a structure or development incorporates. The LEED Green Building Rating System is a voluntary third-party rating system in which credits are earned for satisfying specified green building criteria. Projects are evaluated within five environmental categories: Sustainable Sites, Water Efficiency, Energy and Atmosphere, Materials and Resources, and Indoor Environmental Quality. Certified, Silver, Gold and Platinum levels of green building certification are awarded, based on the total credits earned.

In 2009 LEED was expanded to include complete residential developments, encompassing drives, pavements and parking areas where inclusion of reclaimed asphalt pavement (RAP) and recycled concrete aggregate (RCA) can boost sustainability points. This LEED-NR (for Neighborhood Development) rating system integrates the principles of smart growth, urbanism and green building into the first national rating system for sustainable neighborhood design.

In February 2012, the Meador Kansas Ellis Trail Project in Bellingham, Wash., became the first-ever project to achieve Greenroads certification. This project – actually a six-block area of downtown Bellingham – was reviewed by the Greenroads Foundation as an independent third party, and was certified to meet Greenroads Silver certification.

The City of Bellingham incorporated many sustainable elements into the project’s design, including recycled porcelain aggregates made from more than 400 crushed toilets that were diverted from a landfill; asphalt with recycled content of 30 percent and recycled concrete aggregates; porous pavements that naturally treat runoff and provide effective stormwater management; low-energy LED street lighting; and new amenities and improvements for pedestrians and bicycles using the Whatcom Creek Trail.

An international standard, the Greenroads Rating System is a collection of sustainable roadway design and construction best practices that address concerns about water, environment, access, community impact, construction practices and materials. There are 11 project requirements that must be completed in order for a roadway to be considered a “greenroad,” as well as 37 voluntary credits that a project team can choose to pursue.

After a rigorous review process, the Greenroads Foundation then assigns a project score based on the number of points earned by meeting the requirements and achieving credits. If certification is attained this score translates to one of the four certification levels.

“The Greenroads Rating System can be used to help manage, improve and communicate sustainability,” Muench said. “It represents an independent verification of sustainable features that truly matter and make a difference.”

Globally, 12 projects are currently pursuing Greenroads certification, ranging from new construction to reconstruction to overlay and bridge projects. Registration for project certification became available in 2011. More information is available at www.greenroads.org.

FHWA Rolls Out INVEST

The INVEST sustainable pavements rating system was rolled out in October. INVEST is a voluntary, web-based self-evaluation tool for assessing sustainability during the lifecycle of a transportation project or program. It addresses project environmental sustainability from system and project planning, through design and construction, to operations and maintenance.

INVEST 1.0 – which was radically changed from an initial 1.0 pilot version – was released via webinar on October 10. For sustainability to be fully integrated into highway and transit programs, FHWA says, it must be considered throughout the project lifecycle. Therefore INVEST focuses on three fundamental themes in agency operations: System Planning and Processes, Project Development, and Transportation Systems Management, Operations and Maintenance.

“INVEST is Federal Highway’s tool to encourage sustainability,” said Heather Holsinger, environmental protection specialist in FHWA’s Office of Planning, Environment and Realty, also at the National Pavement Preservation Conference in Nashville. “It’s a web-based, voluntary, and is not mandatory. It looks at project development through public planning, through design and construction, and finally through operations and maintenance.”

Goals of INVEST are to support the U.S. DOT’s aims for livability and sustainable transportation, increase the body of knowledge regarding sustainability aspects of both asphalt and concrete materials in pavement design, construction, preservation and maintenance, and to boost use of sustainable technologies and practices in pavement design, construction, preservation and maintenance, Holsinger said.

“The program allows agencies to assess individual or multiple projects,” she told the conference. “Projects can be looked at prospectively in a design phase, or retrospectively look back to get a sense of what was done. And it helps agencies communicate sustainability goals, which is a really important aspect of the tool itself.”

More information – and the program – may be downloaded at www.sustainablehighways.org.

Two More Rating Systems

Two other transportation infrastructure rating systems are worth mentioning, Envision and CEEQUAL

• Envision. A new infrastructure rating system, Envision was developed by the Institute for Sustainable Infrastructure (ISI) to help designers, builders and infrastructure owners build and direct infrastructure projects toward increasing levels of sustainability.

ISI develops and maintains Envision, a collaboration between ISI in Washington. D.C., and the Zofnass Program for Sustainable Infrastructure at the Graduate School of Design at Harvard University. ISI was founded by the American Council of Engineering Companies, the American Public Works Association and the American Society of Civil Engineers.

The Envision rating system evaluates, grades and gives recognition to infrastructure projects that use “transformational, collaborative approaches to assess the sustainability indicators over the course of the project’s life cycle,” ISI says, and its tools help the design team assess costs and benefits over the project lifecycle, evaluate environmental benefits, use outcome-based objectives, and reach higher levels of sustainability achievement.

Envision describes itself as a holistic rating system, in that it takes an extended, broader view of a project’s environmental impact than just the elements of the project itself. “For highways, the question ought to be, ‘What are the transportation choices for improving access and mobility in the community?’,” the ISI states. “For water treatment plants, ‘What can be done to reduce, reuse and restore the community’s water supply?’” Because either, or both, of these criteria may call the project’s very existence into question, Envision may be better suited for regional planning agencies or state DOTs, rather than local agencies. More information will be found at www.sustainableinfrastructure.org.

• CEEQUAL. The grandfather of the rating systems, developed in the United Kingdom, CEEQUAL is an evidence-based sustainability assessment and awards scheme for civil engineering, infrastructure and landscaping, and celebrates the achievement of high environmental and social performance.

CEEQUAL rewards project and contract teams in which clients, designers and contractors go beyond the legal and environmental and social minima to achieve distinctive environmental and social performance in their work. CEEQUAL was launched in 2003, and more than 130 final and 60 interim Awards have been achieved with a further 240 projects and contracts being assessed in March 2012. More information is available at www.ceequal.com.

State and Provincial Systems

National initiatives are complemented by state and provincial sustainability rating systems. These include:

• GreenLITES. Developed by the New York State DOT, GreenLITES is a sustainability rating and self-certification program that recognizes transportation projects and operations on the extent to which they incorporate sustainable choices.

GreenLITES is modeled after the LEED and Greenroads programs. NYS DOT’s certification program builds on other environmental initiatives already begun by the department. It’s the next step in a long-term commitment to evaluating and refining practices to encourage sustainable choices in project design. Most NYS DOT projects are evaluated under GreenLITES. The certification program is designed to be flexible, and as new best practices emerge and new innovative approaches are developed, they are added to the program.

GreenLITES certification categories are Sustainable Sites, Water Quality, Materials and Resources, Energy and Atmosphere, and Innovation.

Like Greenroads, GreenLITES certifies projects at increasing levels of sustainability: Certified, Silver, Gold and Evergreen. More information is available at www.dot.ny.gov/programs/greenlites.

• GreenPave. In 2010 the Ontario Ministry of Transportation launched GreenPave, a points-based rating system which focuses on pavements, not the entire right-of-way. MTO’s goal is to establish a rating system for pavement sustainability that applies to all designs of asphalt and concrete pavement structures.

Download more information at www.mto.gov.on.ca/english/transtek/roadtalk/rt16-1/#a6.

• I-LAST. The Illinois Livable and Sustainable Transportation (I-LAST) Rating System and Guide was rolled out in January 2010 and is a sustainability performance metric system developed by the Joint Sustainability Group of the Illinois DOT, the American Council of Engineering Companies-Illinois, and the Illinois Road and Transportation Builders Association, among other statewide groups.

The use of I-LAST is voluntary. I-LAST includes a point system for evaluating the sustainable measures included in a project. Download the I-LAST manual at www.dot.state.il.us/green/documents/I-LASTGuidebook.pdf

NEXT MONTH: We will again address “Green Roads” with an examination and update of existing trends in our December 2012 issue.

What Went Wrong, and Why

Forensic Studies Give Clues to Pavement Failure

By Tom Kuennen, Contributing Editor

Whether the pavement is black or white, flexible or rigid, asphalt or concrete, pavement forensic testing is the key to preventing future pavement failures in either paving medium.

In the lab or in the field, engineers examine pavement condition, cores or entire cut-out sections to ascertain what went wrong, and why.

“Forensic pavement analysis is a core function of every department of transportation,” say Paul E. Krugler, Carlos M. Chang-Albitres and Robert L. Robideau, Texas Transportation Institute, in their paper Development of a Rigid Pavement Forensics Knowledge Management System to Retain TxDOT Corporate Knowledge.

“Excellence in this technical area allows selection of proper and most cost-effective rehabilitation options, with potential monetary benefits to the department of millions of dollars annually,” they write. “Capturing and disseminating corporate forensic pavement knowledge will help assure exceptional performance in this area in the future.”

Acknowledging that staff turnover and retirements were depleting the acquired engineering expertise of Texas DOT, the writers in 2005 outlined creation of a knowledge database of rigid (portland cement concrete), and later, in 2007, flexible (bituminous concrete) pavements, all accessible to Texas DOT employees via the Texas i-Way learning content management system.

Tools for Detective Work

Poor quality construction can occur due to a number of complex and sometimes competing variables, reports the Texas DOT, including reduced inspection staffing, employee turnover, variability of inspectors’ and project managers’ experience levels, incompatibilities between new admixtures and construction materials, implementation of new technologies and construction methods, environmental constraints, recycled materials and other issues unforeseen during design and construction phases.

“To prevent, and to reduce the probability of premature pavement failures and poor long-term pavement performance, the root causes of these problems have to be identified,” Texas DOT says in its Pavement Design Guide. “In conducting forensic studies, a thorough review and analysis of existing quality construction records and tests, nondestructive testing like ground penetrating radar (GPR) and the falling weight deflectometer (FWD) are essential to identify problematic areas and probable causes.”

“When a pavement fails earlier than expected – with early cracking or rutting – we conduct forensic investigations to determine why that happened so soon,” says Timothy R. Clyne, P.E., MnROAD forensic engineer for Minnesota DOT’s MnROAD pavement test facility.

MnROAD – a full-scale accelerated pavement test facility – tests pavement materials, structural designs and construction techniques. It’s unique in that in addition to a low-volume roadway test track that simulates conditions on rural roads, it includes an actual test section of I-94 that carries live Interstate traffic.

“We will do a forensic investigation on good roads to find out what we did right, or what were the conditions that made things go so well with that section,” Clyne says. “But most of the time our forensic investigations are on early failures.” MnROAD also will conduct forensic investigations for pavements throughout the state, either for Minnesota DOT or local agencies.

Cores Provide Clues

Pavement coring is at the heart of both virgin pavement testing and pavement forensic testing. Forensic investigation studies pavement structure and materials in the event of premature deterioration, substandard materials, new materials for evaluations, investigation of pavement for overweight loading and proposed new techniques and methods, says the Indiana DOT.

Sometimes investigators will use the cores to examine thickness of the pavement structure, or deterioration at the bottom of the structure where water may be present at the interface between pavement and sub-base, eroding pavement support.

Forensic evaluation of pavement failure begins with simple field observations.

Cores will reveal if any layer of asphalt has failed to bond to the layer beneath it, or if there has been any stripping in any asphalt layer throughout the core. While pavement forensic testing likely will involve cores taken from troubled pavement, cores from newer or even fresh pavements can reveal trouble down the road.

“Early in a pavement’s life we often will take a core and undertake lab testing or performance testing for rutting, cracking or stiffness,” Clyne tells Better Roads. “We’ll core a fresh pavement to check for thickness, just to verify that it’s what it’s supposed to be. We also will look for asphalt early aging characteristics.”

When cores aren’t enough, extensive sections of pavement may be removed for forensic analysis.

Coring begins with visual examination of a pavement’s condition. “If you can see a problem at the surface, you can pinpoint exactly where you should take those cores,” Clyne says. “We never just take one core; we take several at various locations, either along one particular crack, several cracks throughout the pavement, or from a strategic grid where we take cores throughout the grid.”

That being said, quite often, the failure area will be small relative to the whole pavement length, or the engineer can choose one representative area of the failure and investigate that small area, which will give him or her an indication of what’s going on throughout the whole project.

But testing of cores often is not needed if careful visual examination will work. “We don’t always test the cores in a forensic investigation; sometimes we just take a look at the cores to give us clues to what’s happening,” Clyne adds. “We won’t test the material properties; we will be just looking to see how the pavement’s deteriorated.”

Visual Examination

Visual examination plays a big role when actual sections – not cores – are removed from pavements. Trenches and test pits cut with pavement saws can remove a larger area of pavement – up to the entire 12-foot lane width – for large samples. Depending on the size of the sample, it may be cut into smaller sections for removal, numbered and then pieced together for review.

GPS-enabled survey vehicle incorporating ground penetrating radar.

In these applications, visual examination may suffice. “Your eyes will tell you the story of what’s going in the pavement,” Clyne says. “The simpler the better, we like to say; if a simple tape measure will do, that’s what we will use.”

With visual analysis, investigators will be looking for signs that water has been in the pavement system, and scoured or leached away materials at the bottom of the core or in the core. Are the layers bonded the way they are supposed to be? Are they the proper thickness? If a pavement is a couple of inches thinner than it should be, it’s liable to crack early.

Water damage is indicated by a lack of material. “Portions of the core or pavement won’t be there anymore,” Clyne says. “When the core is taken over a pavement crack, you can see from the edges of the core that the pavement goes down 6 inches, but you will see that the core is missing material where that crack is. It’s just gone.”

“When we have a PCC pavement over a base that doesn’t drain water – an impermeable base – we will see a lot of deterioration and missing material about a third of the way up in that core,” Clyne says. “But for other pavements that drain more readily, cores taken after construction will exhibit joints that are nice and tight and don’t have the deterioration.”

In-service pavement cores also will exhibit unexpected irregularities. “A few years ago we had an asphalt paving job with whole pine cones in the asphalt mix,” he says. “We never were sure how they got there, but we were called to determine how extensive the pine cones were; were they just in one location or distributed throughout the pavement. We went out and took cores, and also made a visual observation of the pavement surface.”

Because Minnesota DOT has a spec that permits up to a certain level of organic materials in a pavement, MnROAD’s forensic evaluation had to determine whether the pavement met the specification, and would the pine cones pose a long-term performance problem. The verdict: The contractor was at fault and took a large deduct.

“A concrete pavement from a few years ago had clumps in the concrete, small balls of unmixed material, aggregates and cement that had not been fully mixed at the plant,” Clyne says. “You could see them behind the paver, small clumps that did not look the way fresh concrete should look. We hired a consultant to evaluate the project by covering every square inch of pavement with ground penetrating radar (GPR). The GPR was able to locate the unmixed clumps in the concrete, and it also found areas where tie bars were missing. The contractor was held responsible to the tune of $1 million.”

Forensic studies don’t just involve tests on fresh pavements or failed pavements; they can involve material samples taken at the time a pavement was produced, and stored. “Pavement forensics for rutting can include in-depth testing of quality control and ‘bag’ samples taken at the time of production,” says Chris Huner, P.E., assistant division engineer – materials, Alabama DOT 7th Division. “There we run volumetric tests and do Abson recovery tests on the liquid binder. We recover the liquid binder from the sample and determine the percent polymer in it if applicable. Then we cut cores from the roadway where the rutting is most severe and compare with the stored samples.”

Closer Look at Materials

Once visual examination of cores is concluded, lab analysis of the asphalt binder, cement paste or aggregates may be necessary to see if the materials confirm the results. A suite of sophisticated laboratory testing devices is available for this analysis.

Lab examination of core displays thermal crack, a problem of HMA mixes in cold-weather regions

Some asphalt paving projects may begin flushing or bleeding. In this case asphalt will rise to the surface and make slick spots on the driving course. Cores will be taken and asphalt extracted to establish the stiffness of the binder and see if the material placed matches specifications.

For this analysis, a chemical lab will extract a pavement sample from the core, heat it, crumble it and put it through a solvent extraction method using toluene, which strips the asphalt from the aggregate.

That asphalt is recovered by “washing” the toluene out of the liquidasphalt via a vacuum distillation process. This liquid asphalt then is tested in various machines to see if it met the spec.

If that binder is too “soft” it will be revealed by the dynamic shear rheometer(DSR) in the course of a lab investigation. The DSR has two parallel plates, in which one is stationary and the other rotates at a certain amount of strain and frequency (speed). This application is useful in revealing the PG spec of the binder.

The DSR is not the same as the machine used in the dynamic modulus test. This test is used to evaluate mix stiffness at different temperatures and loading speeds, and is sensitive to changes in binder grades, presence of RAP, production temperatures, or anything else that would influence stiffness.

The DSR is a test on binder; the dynamic modulus is a test on the whole mixture, including binder and aggregate. “The dynamic modulus is tested in compression, in which you push or squeeze the material together,” Clyne says. “The dynamic shear rheometer tests shear, in which the sample is twisted. If the DSR gives us the PG rating of the binder, the dynamic modulus gives us the overall stiffness of the asphalt mixture, including aggregate.

GPR vs. core data (from University of California-Davis)

“In general we’ve been trying to move away from just testing the binder,” he adds. “We want to test the whole mixture, including the aggregate, because that’s what’s happening on the road, that’s what the traffic is rolling on. Binder typically makes up just 5 percent of the mixture – an important part to be sure – but it’s not the only part.”

The dynamic modulus test is not unlike the compression test for portland cement concrete. With the compression test, though, the lab is looking for a failure strength, in which the specimen is broken to bits; the dynamic modulus is tested at much lower loads, in a low strain range, and not tested to failure.

The Asphalt Pavement Analyzer (APA) Rut Tester may predict rutting by exposing mix samples to repetitive loads. It consists of a rubber hose resting on a beam of asphalt, or cores, with a steel wheel that passes over the hose repetitively, replicating the impact of a tire. It is not unlike the Hamburg Loaded Wheel Tester, which uses the steel wheel only. “There is a lot of debate as to which rut tester is more accurate, and they each have their own advantages,” Clyne says. “Most of the time they will rate mixtures similarly.”

The solvent extraction method is one way of measuring asphalt content. The ignition oven method, as developed by the National Center for Asphalt Technology, is a quicker, less labor-intensive method, but it has a limitation.

“If you are looking just for asphalt content in the mix, either one will give you an accurate measure,” Clyne says. “If you want to run an aggregate gradation test afterwards you can run a gradation on either sample. The advantage to the solvent extraction method is that you can test the asphalt as well as the aggregate; the ignition oven quickly burns off all the asphalt so you can’t test it afterwards.”

There is a “green” element to the NCAT ignition oven: The ignition oven does not involve use of polyaromatic hydrocarbons to dissolve the asphalt from the sample, which are perceived to be atmospheric pollutants.

“As recently as 10 years ago we used more harmful chemicals, such as trichloethylene, that could be cancer-causing,” Clyne says. “But we’ve gone away from those to use much safer, much more environmentally friendly chemicals that still extract the asphalt, but don’t come with all the health and safety risks.”

Either way, workers are protected by safety garb, gloves, protective eye wear and fume hoods that pull fumes away from the work area.

Nondestructive Testing

Non-destructive testing or evaluation of pavements in the field avoids coring and section-cutting, which can compromise the long-term performance of a pavement if not done right, and certainly affect ride quality. New technologies make this possible.

“These tools have their limitations but they are very good tools all the same,” Clyne says. “If we can run equipment over the road surface without cutting a core or a trench, and it can tell you what you need to know, that equipment is a very helpful thing.”

The falling weight deflectometer (FWD) is a nondestructive testing device that evaluates physical properties of pavement, including structural capacity for overlay design, or determines if a pavement is being overloaded; a load pulse is imparted that simulates the load produced by a rolling vehicle wheel.

The trailer-mounted FWD will use a plate about a foot in diameter, which drops a load of a known weight on the pavement. Sensors at various spacings around the load plate measure the deflection of the pavement surface from the impact.

On concrete, the FWD can indicate load-transfer efficiency across joints. For all pavements it can test the entire structural capacity of the road, or by using backcalculation, use the raw load and deflection data to determine the stiffness of each of the layers in the pavement system.

“The FWD will not tell you depth,” Clyne says. “You either have to know that from the plans, take cores, or use ground penetrating radar.”

Ground penetrating radar (GPR) is a relatively new, non-invasive, nondestructive pavement testing procedure that will reveal pavement structure data. GPR is an alternate to FWD testing but also may supplement it.

Oklahoma DOT uses GPR to reduce the number of cores required

Antennae mounted on a moving vehicle transmit short pulses of radio wave energy into the pavement structure, and echoes are created at boundaries of dissimilar materials (such as the asphalt–base interface), reports the Federal Highway Administration. The arrival time and strength of these echoes can be used to calculate pavement layer thickness and other properties, such as moisture content.

“Coring may have some degree of effectiveness for specific projects, but at a network level it is costly, intrusive to traffic, and provides very limited samples of the actual pavement structure,” says Dr. Ken Maser, P.E., president of Infrasense, Arlington, Mass.

UST [is] … engineered to detect and evaluate internal reinforced concrete defects

“GPR involves transmitting short radio frequency pulses and receiving echoes from the boundaries between the pavement layers,” Maser says. “The technology has been in use for a variety of highway applications over the past 20 years, and has been adapted for routine use by a number of state agencies.”

The accuracy of the GPR pavement thickness measurements, typically ranging within 3 to 10 percent of core values, has been documented in several university, state agency, and SHRP studies, Maser says. A key advantage of GPR is the ability to collect data at highway speed, using non-contact equipment; typical survey coverage of 200 to 300 lane-miles per day on intercity roads makes this technology well suited for network-level pavement structure evaluation.

Network-level GPR pavement structure assessments have been carried out at the statewide level, as well as by various local agencies and municipalities. At the network level, GPR is now being used for network segmentation into relatively uniform pavement structures, for data inventory input into a pavement management system (PMS) database, and for layer thickness detail for use with network level FWD evaluations.

GPR won’t replace the FWD. For example, Oklahoma DOT has been implementing GPR measurements as part of its ongoing efforts in pavement management to improve its decision making process through enhanced knowledge of its pavements’ structural capacities.

Previously, Oklahoma’s PMS utilized only surface distress data to identify deficiencies at the network level and recommend appropriate treatments. More recently, the state has been acquiring pavement structural condition data using a combination of FWD and GPR measurements on its 2,765 centerline-mile, non-toll NHS system. Oklahoma DOT uses GPR to reduce the number of cores required, to identify changes in the pavement structure, and to provide information for overlay design, says Maser.

“The combination of GPR and FWD data is being exploited by other agencies to identify underlying conditions and to support pavement rehabilitation design,” he adds. “In 2006, Montana DOT acquired a combined GPR/FWD system, and since then has been using the system at a network level to obtain more accurate characterization of pavement structural properties by combining GPR layer thickness data with FWD data.

New on the horizon is ultrasonic testing, the MIRA process. Using ultrasonic shear-wave tomography (UST) technology, it’s a low-frequency (20 to 100 kHz) phased array ultrasonic system engineered to detect and evaluate internal reinforced concrete defects such as honeycombs or voids, and is useful for large concrete structures like bridge components.

Recycled materials need analysis, characterization

By Tom Kuennen, Contributing Editor

There is no stopping the growth of recycled and reclaimed materials in pavements.

The use of reclaimed asphalt pavement (RAP), reclaimed asphalt shingles (RAS), recycled concrete aggregate (RCA), and recycled granulated tire rubber (GTR) in pavement mixes and structures is growing dramatically as states accept them more and more in their specs.

But because RAP, RCA, RAS and GTR come from a variety of sources, they must be physio-chemically characterized prior to use in mixes.

• For RAP, which virgin aggregates does it contain? Do deleterious materials exist? How much residual asphalt remains after years of exposure to the elements and oxidation? How much liquid binder will the residual asphalt replace when reused in fresh asphalt mixes?

• For RCA, what is the extent and composition of the mortar or residual cement/sand blend? Were its virgin aggregates prone to alkali-silica reactivity (ASR) or is ASR present and in what degree? Are other deleterious materials present? Is the resulting RCA “good” enough to be used as aggregate in fresh asphalt or portland cement mixes, or is it going to be destined for road base, a much more common use?

• For RAS, the processed post-consumer (“tear-off”) shingle feed will come from a supplier that certifies the material meets state specs. The supplier will have sorted, ground and tested the RAS to make sure it does not contain asbestos, wood scraps or metal and is kept separate from pre-consumer (manufacturer waste) shingles (more on this below). Likewise, GTR will come from a supplier that maintains consistency.

Thus, physio-chemical analysis of beneficiated RAP, RCA, RAS and GTR by in-plant or supplier labs is essential for their continued usage. Because their source composition varies tremendously, these reclaimed materials must be chemically characterized and cataloged; then, blended stockpiles may be managed over time with more or less material added to maintain consistency.

Use of RAP and RCA as road base or fill is a less-critical application so a detailed analysis is not essential; here the research emphasis is on the possibility of leached pollutants finding their way into ground water, and long-term performance.

Processing Adds Value to RAP

Ideally the raw, stockpiled RAP or RCA will have been crushed and screened, or “beneficiated,” or screened or “fractionated” into homogenous stockpiles. While this costs the mix producer or contractor additional money, it adds value to the raw materials as they now are consistently sized.

Fractionation is the act of processing and separating raw RAP into at least two sizes, typically a coarse fraction (plus-1/2 or plus-3/8 inch) and a fine fraction (minus-1/2 or minus-3/8 inch), reports the Federal Highway Administration (FHWA) in its April 2011 publication, Reclaimed Asphalt Pavement in Asphalt Mixtures: State-of-the-Practice, by Audrey Copeland, formerly materials research engineer at FHWA, now vice president, engineering, research and technology at the National Asphalt Pavement Association.

“States allow higher amounts of RAP if it has been fractionated,” Copeland writes. “For example, in the Texas specification, unfractionated RAP is limited to 10, 20, and 30 percent by surface, intermediate and base layers, respectively. However, by special provision, fractionated RAP is allowed at up to 20, 30 and 40 percent in those same layers.”

Separately, RAP has to be chemically analyzed or characterized to determine its properties (below). That beneficiation or fractionation of RAP that’s been chemically characterized can permit significantly higher levels of RAP in Superpave mixes is borne out in a paper from the 2012 Transportation Research Board meeting, Fractionation of High Recycled Asphalt Pavement Content in Asphalt Mixtures for Superpave Mix Design Compliance, by Cory Shannon, E.I.T.; Yongjoo Kim, Ph. D.; Thomas Glueckert and Hosin “David” Lee, Ph.D., P.E., Department of Civil and Environmental Engineering University of Iowa-Iowa City.

“Due to the increased amount of fines created during the milling process and the corresponding increased surface area, high RAP content mixes have great difficulty in meeting the volumetric requirements of the Iowa DOT,” they write. “The fractionation method for this study focused on physical removal of RAP material below a certain sieve size to limit fine aggregate contribution.”

Current Iowa DOT specifications limit contractors to a maximum of 30-percent virgin asphalt binder replacement by RAP materials in the surface course for any state-regulated project, the authors write. “The main objective of this study is to develop quality standards for the inclusion of RAP contents higher than 30 percent in asphalt mixtures,” they write. “First a sieve analysis was performed on the recovered aggregate materials from ignition oven burn-off testing to determine the aggregate and asphalt binder composition of the RAP materials. To remove excessive fine materials a fractionated RAP stockpile was produced by removing RAP materials passing the No. 30 (0.60 mm) sieve.”

The Superpave mix design was then performed with RAP inclusion levels of 30, 40 and 50 percent, based on virgin asphalt binder replacement for RAP materials randomly selected from the bulk stockpile (traditional RAP) and RAP materials from the fractionated stockpile consisting of materials retained at No. 30 sieve and larger (fractionated RAP).

The fractionated RAP materials produced a lower surface area requiring lower virgin asphalt content, they say, resulting in the increased asphalt film thickness on aggregates. The fractionated RAP mixtures also exhibited higher indirect tensile strength than traditional RAP mixtures for all levels of inclusion.

“It was determined that the use of the fractionation method to remove fine recovered aggregates contributed by the RAP materials is an effective method for improving mix design criteria compliance while also reducing the requirement of virgin asphalt for asphalt surface mixtures with a high RAP inclusion level up to 50 percent,” they conclude.

Determining AC Content

Lab testing requirements and testing frequency for binder (AC) content vary according to the category of RAP and the amount of RAP used in a mixture, Copeland writes in Reclaimed Asphalt Pavement in Asphalt Mixtures (download a copy by Googling “FHWA-HRT-11-021”).

In Wisconsin, pavement demolition concrete is crushed next to a construction site. The resulting RCA will go back as base material below concrete pavement.

RAP from multiple sources may be subject to more rigorous testing than RAP from a single source, she writes. For all RAP stockpiles, the asphalt binder content and aggregate gradation must be determined. The asphalt binder content may be determined according to AASHTO T308 or AASHTO T164.

The most common method for determining the AC in a sample of RAP is to use the ignition oven method specified in AASHTO T308. A Colorado DOT survey compiled in January 2008 includes responses from 29 state DOTs, and shows that almost half of them used the ignition oven to determine the AC of the RAP fraction for mix design purposes. About 30 percent of the respondents used solvent or chemical extraction, while three out of the 29 states used both solvent extraction and the ignition oven, reported FHWA in Reclaimed Asphalt Pavement in Asphalt Mixtures.

The oven can predict future performance of RAP mixes as well. Use of a lab oven for long-term aging or oxidation of various-content RAP mixes found that as RAP content increased, HMA mixes would stiffen at a slower rate than virgin mixes say Sean Tarbox, and Jo Sias Daniel, Department of Civil Engineering, University of New Hampshire, in their 2012 TRB paper, Effects of Long-Term Oven Aging on RAP Mixtures.

“Asphalt concrete mixtures undergo aging while in place during their service lives,” the authors state. “The aging process stiffens the asphalt, changing its mechanical properties and resulting performance under traffic loading. The major factor contributing to the increase in stiffness of asphalt concrete mixtures over time is the oxidation of the asphalt binder at the molecular level.”

In this study, four plant-produced mixtures containing zero, 20, 30 and 40 percent RAP were long-term oven-aged in the laboratory to three levels. The dynamic modulus was measured for each aging level and was compared to unaged values to determine if there was a statistical difference. It was found that as RAP content increased, aging had less of an effect on stiffness.

Long-term oven aging to simulate aging in the field can be used to evaluate stiffness changes over time, they say. The impact of the measured increases in stiffness on the fatigue performance of the pavement has been shown to be influenced by the binder type, as well as the pavement structure. Aging has also been shown to reduce the stress relaxation capacity of the binder. The authors found the stiffening effect of long term oven aging on RAP mixtures is less than that of virgin mixtures. This could be due to the inclusion of already aged binder in the RAP mixtures that does not age further under laboratory conditioning.

Spectroscopic Evaluation of RAP

The ignition oven is not the only way to determine the content and chemical condition of residual binder in RAP. Spectroscopic analysis offers the chance of moving the analysis from the lab into the field.

“Spectroscopic investigation of the oxidization age hardening in asphalt products has been the focus of pavement research for more than three decades.”

- Yut and Zofka

The ongoing Second Strategic Highway Research Program (SHRP-2) project titled Evaluating Applications of Field Spectroscopy Devices to Fingerprint Commonly Used Construction Materials targets, among the other objectives, evaluation of oxidation in RAP, say Iliya Yut and Adam Zofka, Ph.D., University of Connecticut, in their 2012 TRB paper, Spectroscopic Evaluation of Recycled Asphalt Pavement Materials.

The study investigates the effect of the RAP content on the concentration of oxidized components of asphalt by using advanced, yet portable, easily interpretable spectroscopic methods. Two types of samples are prepared in the laboratory: binder blends containing 15 to 40 percent weight RAP-binder, and loose HMA samples modified by up to 80 percent weight RAP.

Spectroscopic measurements were performed using a portable attenuated total reflection Fourier transform infrared (ATR FT-IR) spectrometer. Quantitative analysis of the ATR spectra indicates that an increase in RAP content is highly associated with concentration of ketones and sulfoxides in RAP binder. It’s also possible to determine RAP content based on the analysis of extracted RAP binder, the authors write.

“Spectroscopic investigation of the oxidation age hardening in asphalt products has been focus of pavement research for more than three decades,” Yut and Zofka write. Earlier research employed spectroscopy to study long-term aging in asphalt binders. The researchers recognized three major products of oxidation, i.e., benzylic ketones, sulfoxides and free hydroxyl radicals that may interact with ketones and form carboxylic acids. Later studies confirmed that an increase in viscosity of aged binders is related to an increase in their carbonyl content, they write.

They found that a portable ATR spectrometer is capable of detecting main chemical components usually present in both binders and HMAs, namely aliphatic and aromatic hydrocarbons and mineral aggregates, thus allowing timely field tests of pavements.

RCA Needs Analysis

Recycled concrete aggregate (RCA), like RAP, must be crushed, screened and tested, and stored in blended stockpiles to ensure consistency. It should consist of mineral aggregates bonded by a hardened cementitious paste; residual mortar causes processed RCA to have a rougher surface texture, lower specific gravity and higher water absorption than similar virgin aggregates, says FHWA

The properties of recycled concrete aggregate can vary greatly, depending on the original aggregate source, and the production techniques. Therefore it’s necessary to characterize the material so it’s used properly, and if using in new concrete, appropriate adjustments are made in the structural or mix design.

Residual mortar causes processed RCA to have a rougher surface texture, lower specific gravity and higher water absorption than similar virgin aggregates – FHWA

That’s why as an engineered material, RCA must be tested and analyzed in a lab before being included in a structure or mix. In particular, the physical and mechanical properties of RCA vary with the quality and quantity of reclaimed mortar, which may affect the design of the structure or concrete mixture. These effects can be significant when making reclamation simpler by including lots of mortar, or minimal when efforts are made to eliminate as much reclaimed mortar as possible.

Late last year, the Michigan DOT produced a definitive guide for use of RCA. Prepared by Applied Pavement Technology of Urbana, Ill., for the Michigan Tech Transportation Institute, Using Recycled Concrete in MDOT’s Transportation Infrastructure: Manual of Practice (download by Googling the title) is an essential guide.

Michigan refers to RCA, the most widely used term, as crushed concrete aggregate (CCA), and observes how it’s different from virgin aggregates. “CCA has different properties than natural aggregate, largely because the resultant crushed material is composed of both the original natural aggregate and reclaimed mortar, which significantly affects the properties and behavior of materials produced with CCA unless specific steps are taken to account for it in the design and construction process,” the report says. “Moreover, the composition of CCA can be highly variable, and in addition to aggregates and reclaimed mortar may contain contaminates such as soil and clay balls, joint sealant, and asphalt or other construction waste.”

Freshly processed RCA/CCA also is highly alkaline and may contain chlorides that may limit is use or applicability, Michigan DOT says “Nevertheless, when its characteristics are properly considered and accounted for, CCA can be used effectively in a number of transportation infrastructure applications.”

Data collected from 2009 indicate that concrete pavements are recycled for transportation infrastructure applications in at least 41 states; moreover, about 140 million tons of CCA are produced in the United States per year, according to the American Concrete Pavement Association. “The material has been used in applications ranging from placement in various paving layers (surface, base, subbase) and as fill and embankment material,” MDOT says.

A major concern regarding the use of CCA in base layer applications is related to leachates, MDOT says. “CCA contains calcium hydroxide from the original cement hydration reaction,” according to the manual. “It is water-soluble, and when water flows through a CCA base, some calcium hydroxide will dissolve into the water. Subsequently, it interacts with atmospheric carbon dioxide to form calcium carbonate, precipitating out of solution and leaving deposits where the water flows. This is problematic if the precipitate clogs up elements of a pavement drainage system, such as filter fabrics, drainage pipes, and outlets.”

Some environmental concerns exist regarding the use of RCA/CCA as base material, primarily because of its alkalinity. However, the alkalinity rapidly decreases with time, and is not considered a major concern although some vegetation may be destroyed where runoff is discharged directly from a CCA base.

CCA contaminants may be encountered during the recycling process, including HMA overlays and patches, joint sealant, reinforcing steel, dowel and tie bars, and soils and foundation materials, the manual states. “Other contaminants may be present within the concrete itself, such as alkalis and chlorides from deicing salts. Efforts should be made to minimize the potential for introducing contaminants, especially if the CCA is to be considered for use in new concrete.”

RCA/CCA will exhibit lower specific gravity, which decreases with increasing amount of reclaimed mortar; higher absorption, which increases with increasing amount of reclaimed mortar; greater angularity; and increased abrasion loss, which increases with increasing amount of reclaimed mortar.

“In addition, CCA may contain unhydrated cement, which may alter its behavior and complicate stockpiling, especially the fine material,” according to the 2011 manual. “Finally, the fines produced during the crushing operation (those passing the No. 4 sieve) are coarse and angular, which tend to make CCA concrete mixtures very harsh and difficult to work.”

RCA/CCA in Pavements

RCA/CCA can be used with confidence in asphalt pavements. “Although not common, CCA can be used as an aggregate in asphalt paving layers,” the DOT says. “As with the application in base courses, CCA can produce a stable mixture because of its high angularity. And, because the asphalt cement forms a film around the aggregate, leaching and other complications from water interacting with the CCA are minimized.”

In central Texas, crushed recycled asphalt shingles (RAS) are conveyed to an asphalt drum.

For this purpose, deleterious materials such as soil, ash, or other fine organic materials should be limited, as they will increase demand for liquid asphalt and decrease overall quality.

“Soundness, abrasion resistance and volume stability should also be tested to ensure the CCA is a suitable aggregate for asphalt mixtures,” the report says. “Specific gravity and absorption are generally the properties in which CCA varies the most from natural aggregate, and should be thoroughly evaluated and properly accounted for in the mix design … due to the crushing process, CCA is generally very angular and therefore would contribute to good asphalt mixture stability. And CCA should meet the requirements for flat and elongated particles, as excessive amounts of these can lead to a weak aggregate matrix and weak asphalt mixtures. Even if ASR or D-cracking was observed in the source concrete, it is not a concern if the CCA is used as aggregate in an asphalt mixture.”

In the modern permutation of high-service concrete pavements – continuously reinforced concrete pavement (CRCP) – Texas has done major work in the field in evaluating the use of RCA with CRCP, and is confident that it works, thanks to the largest application to-date of RCA in CRCP in 1995, a very heavily traveled section of I-10 in Houston between Loop 610 and I-45 involving 10 lanes, including HOV lanes.

Today, crushed concrete is used extensively in state projects in the Houston area and is fairly common in Dallas as well. There are a number of factors affecting CRCP performance with RCA, including adequacy of pavement structure, material properties, environmental conditions during concrete placement, and construction practices.

Crumb rubber increases asphalt binder viscosity as it’s blended in ranges of 18 to 25 percent rubber, reacting to produce an asphalt-rubber binder

TxDOT found that the CRCP sections using 100-percent recycled coarse and fine aggregates have performed well. No distresses, including spalling, wide cracks, punchouts, or meandering cracks, have taken place. Transverse crack spacing distributions are comparable to those in concrete with natural siliceous river gravel.

And Now, Shingles

Because of their asphalt, fiber and mineral content, abundant, processed recycled asphalt shingles (RAS) are finding their way into hot, warm and cold asphalt mixes. Typical addition rates for RAS into hot mix asphalt can range from 3 to 6 percent by mass, reports FHWA.

RAS provides similar or enhanced properties to conventional asphalt pavements, with reductions in requirements for virgin asphalt cement by 0.5 to 1.5 percent. In most instances, raw shingles are collected by a recycling firm, and cleaned and crushed into fine aggregate.

Currently a national pooled-fund study involving multiple state DOTs – Performance of Recycled Asphalt Shingles (RAS) in Hot Mix Asphalt [TPF-5(213)] – is under way. Research in different states includes mix performance comparisons, beam fatigue testing, dynamic modulus, flow number and binder property test results.

“The use of reclaimed asphalt shingles (RAS) in asphalt paving mixtures is not a new concept,” say NAPA’s Kent Hansen and Dave Newcomb, in NAPA Information Series No. 138: Asphalt Pavement Mix Production Survey. “The combination of a high asphalt binder content, high-quality fine aggregate, mineral filler, and fibers makes roofing shingles very compatible with asphalt pavement mixtures,” they say.

The fact that the asphalt cement in shingles is generally harder than that employed in paving mixtures, and that the other ingredients impact the volumetric properties of the final mix, generally limits its incorporation in asphalt mixtures to 5 percent or less, they add.

“However, even at a relatively lower RAS content, there is somewhere on the order of 15- to 20-percent binder replacement in the final paving mixture,” Hansen and Newcomb say. “Currently, 12 states allow the use of manufacturers’ waste in asphalt mix and 10 states allow either manufacturers’ waste or roofing tear-offs in their mixtures. It is estimated that there are 10 million tons of tear-off waste and 1 million tons of manufacturer waste available on an annual basis. If all these could be incorporated into asphalt paving mixtures, it would amount to approximately 1.8 million tons of asphalt binder replacement. Thus, there is great interest in utilizing waste asphalt roofing shingles in asphalt paving mixtures.”

Rubber as Performance Modifier

Rubber from recycled tires is a common additive to asphalt on a regional basis, either as an asphalt modifier (wet process) – where it reacts with the liquid asphalt – or as a fine aggregate substitute (dry process).

As a modifier, crumb rubber increases asphalt binder viscosity as it’s blended in ranges of 18 to 25 percent rubber, reacting to produce an asphalt-rubber binder.

Asphalt mixes in which ground rubber particles are added as fine aggregate are referred to as rubberized asphalt for open-graded mixes.

The road to universal acceptance of granulated tire rubber (GTR) as an alternative to conventional polymer modifiers is long, and has been paved with doubts created by premature failures from technologies of the 1990s that were not well understood, or required too many challenges to implement, says Doug Carlson, vice president of asphalt products for Liberty Tire Recycling.

“Two more decades of research and development have dramatically changed the landscape by generating materials and process advancements that definitively position rubberized asphalt as a viable alternative to polymer-modified asphalt in terms of performance and cost,” Carlson says.

Asphalt-rubber (A-R) is defined by the American Society for Testing and Materials (ASTM) Standard D6114 as “a blend of paving grade asphalt cement, ground recycled tire (that is, vulcanized) rubber and other additives, as needed, for use as binder in pavement construction. The rubber should be blended and interacted in the hot asphalt cement sufficiently to cause swelling of the rubber particles prior to use.”

Asphalt-rubber binder is field-blended (at a hot mix plant) – requiring mobile mixing equipment to produce – or as a terminal blend. The typical rubber content for asphalt rubber ranges from 18 to 22 percent. Granulated tire rubber used in asphalt rubber is in the 10-to-16 mesh range for maximum particle size. This binder is best suited for very thin overlays and heavy duty surface treatments to prevent cracking.

“New technologies have emerged that allow GTR to be used as the primary modifier in performance-graded (PG) asphalt,” Carlson says. “These binders are manufactured with 8 to 12 percent rubber content and may include a small amount of virgin polymer or other additives. The rubber particles have a 30-minus maximum size, but are small enough to fit into PG tests. They can be made onsite or delivered by an asphalt supplier. Mechanical or chemical suspension is needed for the binders that retain GTR particles. These binders can directly replace polymer modified materials in dense graded mixes and chip seals.”

Rubber enables the use of more recycled asphalt pavement (RAP). Emerging technologies researched by Louay N. Mohammad and Samuel B. Cooper Jr. at the Department of Civil Engineering and Louisiana Transportation Research Center at Louisiana State University have shown that rubber mixes with up to 40 percent of RAP can perform as well as regular mixes with only 25 percent RAP, Carlson says.

Low-energy asphalt mixes reduce viscosity – without the heat

In the last two decades, improvements in the physiochemistry of asphalt mixes have sparked a revolution in their production and placement.

Today’s warm and cold asphalt mixes – which to minimize confusion might be better termed “low energy” mixes – are stealing the spotlight from classic hot mix asphalt.

Warm mix asphalt (WMA) is created by mixing one of a variety of solid or liquid chemical compound additives with asphalt mix in the plant, or by foaming the mix with water in the plant. WMA processes generally reduce the viscosity of the liquid asphalt through a variety of means, and enable the complete coating of aggregates at temperatures 35 to 100 degrees F lower than conventional hot mix asphalt.

As with many trends, low energy mixes began in Europe, where warm mixes were popularized in the 1990s. In 2002, the National Asphalt Pavement Association led a study tour to Europe to examine WMA technologies, and the association began exploring their attributes, most recently, at the 2nd International Warm-Mix Conference in October 2011 in St. Louis.

In 2007 a Federal Highway Administration (FHWA) international scanning tour investigated warm mix practices in Europe to gather information on technologies used to produce WMA, with particular emphasis on long-term field performance. Its report – Warm-Mix Asphalt: European Practice – was published in February 2008 (download the report by Googling its title).

In the meantime, another type of low energy asphalt mix – a warm mix created by foaming hot mix asphalt with water in a drum plant – began growing in popularity in the 1990s. And yet another foamed asphalt – a cold mix created in a portable plant, or in situ in the field, using as much as 100-percent reclaimed asphalt pavement – also is called foamed asphalt or foamed bitumen mix.

The one thing all of these components have in common is that they are “green” technologies that reduce plant emissions, lower fuel consumption at the plant, and create a better environment for workers in the field. Their future is so established that in the past year NAPA changed its web site URL from hotmix.org to asphaltpavement.org, and changed the name of its magazine from HMAT (for Hot Mix Asphalt Technology) to Asphalt Pavement.

Data indicate plant emissions are significantly reduced with low energy mixes. “Typical expected reductions are 30 to 40 percent for CO2 and sulfur dioxide (SO2), 50 percent for volatile organic compounds (VOCs), 10 to 30 percent for carbon monoxide (CO2), 60 to 70 percent for nitrous oxides (NOX), and 20 to 25 percent for dust,” according to Warm-Mix Asphalt: European Practice in 2008. “Actual reductions vary based on a number of factors. Technologies that result in greater temperature reductions are expected to have greater emission reductions.”

Burner fuel savings with WMA typically range from 11 to 35 percent, FHWA’s report states, with fuel savings possibly higher (possibly 50 percent or more) with other processes.

And workers will benefit as well, according to the scanning tour report. “Tests for asphalt aerosols/fumes and polycyclic aromatic hydrocarbons (PAHs) indicated significant reductions compared to HMA, with results showing a 30- to 50- percent reduction,” the report says, but with a caveat: “It should be noted that all of the exposure data for conventional HMA were below the current acceptable exposure limits.”

Click to enlarge

From Near-Solid to Liquid

At ambient temperatures, “liquid” asphalt or bitumen from the refinery is a near-solid material. To lower its viscosity and make it workable, it is kept hot, and classic hot mix asphalt production – in which liquid asphalt coats aggregate – takes place at 275 to more than 330 degrees F, and compaction between 260 and 300 degrees F.

More heat comes from the aggregates. Before mixing with hot-liquid asphalt, fine and coarse aggregates are heated to high temperatures to drive off moisture, to ease coating of the mineral aggregates with the liquid asphalt, and to keep the complete mix fluid enough to be workable during placement.

Low energy mixes introduce chemicals such as organic waxes or surfactants to lower asphalt mix viscosity in lieu of higher temperatures, or water to lubricate the mix.

“WMA technologies can also be classified by type,” the scanning tour report says. “Two major types of WMA technologies are those that use water and those that use some form of organic additive or wax to affect the temperature reduction.

“Processes that introduce small amounts of water to hot asphalt, either via a foaming nozzle or a hydrophilic [water-loving] material such as zeolite, or damp aggregate, rely on the fact that when a given volume of water turns to steam at atmospheric pressure, it expands by a factor of 1,673,” the report says. “When the water is dispersed in hot asphalt and turns to steam (from contact with the hot asphalt), it results in an expansion of the binder phase and corresponding reduction in the mix viscosity.”

Three different types of products comprise the chemical WMA technologies.

Generically they are solid, synthetic zeolites, which release water molecules when mixed with liquid asphalt at the plant, achieving a foamed asphalt binder, an example being Aspha-min; solid organic additives, which are synthetic paraffin waxes that reduce the viscosity of the binder at mixing and compaction temperatures, an example being Sasobit; and surfactant liquid additives, which add lubricity to individual microscopic asphalt particles, enhancing workability at various temperatures, an example being Evotherm.

Asphalt-treated permeable base (ATPB) warm mix for a new taxiway being paved at O¹Hare International Airport.

Zeolites are crystalline hydrated aluminum silicates that have large empty spaces in their structures that allow the presence of large cation groups, such as water molecules. Aspha-min, a synthetic zeolite, contains about 20 percent water of crystallization, which is released when mixed with hot aggregates or asphalt, reports FHWA. The water creates a controlled foaming effect that leads to a slight increase in binder volume, therefore reducing viscosity of the binder.

While the zeolites release water into a mix to decrease viscosity, the waxes melt and lubricate the mix, then stiffen as temperature declines. “The processes that use organic additives (e.g., Fischer-Tropsch wax, Montan wax, or fatty amides) show a decrease in viscosity above the melting point of the wax,” according to Warm-Mix Asphalt: European Practice. “The type of wax must be selected carefully so that the melting point of the wax is higher than expected in-service temperatures (otherwise permanent deformation may occur) and to minimize embrittlement of the asphalt at low temperatures.”

Fischer-Tropsch waxes are long-chain aliphatic hydrocarbon waxes with a melting point of more than 208 degrees F, high viscosity at lower temperatures, and low viscosity at higher temperatures, the report states, adding “They solidify in asphalt between 149 and 239 degrees F into regularly distributed, microscopic, stick-shaped particles. They may be used to modify binder or added directly to the mixture.”

Another wax – Montan wax – is a complex combination of nonglyceride long-chain carboxylic acid esters, free long-chain organic acids, long-chain alcohols, ketones, hydrocarbons, and resins, the scanning tour report states. It’s a fossilized plant wax, also known as lignite wax or OP wax, obtained by solvent extraction from certain types of lignite or brown coal. Its melting point is 180 to 200 deg F, and Asphaltan-B is a trade name for this type of additive.

Different from the waxes are the surfactants, such as Evotherm and Rediset. We have shown how important surfactants are for manufacturing asphalt emulsions (see Spreading the Wealth: Asphalt Emulsions Mix Oil with Water, June 2012, pp 18-29).

Evotherm is a chemical compound with surfactant activity, which adds lubricity to individual microscopic asphalt particles. The particles or micelles develop “slip planes” that let the asphalt particles move more easily, requiring lower levels of energy. Because the energy is lowered, Evotherm warm mix has the same viscosity properties at lower temperatures as conventional hot mix asphalt. Rediset-LQ is another product.

Water-based, in-plant foamed systems for low energy mixes – such as the Astec Industries, Inc.’s Double-Barrel Green System – use nozzles that precisely meter water into the drum of a drum mix plant. Injection of water, along with the liquid asphalt cement, causes the liquid asphalt to foam and expand in volume. The foaming action helps the liquid asphalt coat the aggregate at a temperature that normally is in the range of 230 to 270 degrees F.

A much different kind of foamed asphalt incorporate liquid “foamed” asphalt as a stabilizing agent, in which hot-liquid asphalt is foamed with water and air, and is then injected into RAP or aggregate in a mixing chamber, either in a self-propelled recycling machine, or portable plant.

In this cold-mix foamed asphalt process, the recycled aggregate is not completely coated, as is the process with in-plant foamed injection using mostly virgin aggregate. Instead, as 100 percent reclaimed materials are introduced to the pug mill, foamed asphalt is injected into the material stream, and acts as a binding agent to “glue” the reclaimed aggregates together.

This will permit use of less liquid asphalt and much lower mixing temperatures. With 100 percent of existing aggregates used in cold recycling, 2.2 to 2.5 percent liquid asphalt is used to partially coat the RAP and any added virgin aggregate, opposed to in-plant foamed technologies that completely coat the aggregate – as is needed for critical applications like friction courses – that use 5 percent liquid asphalt.

Moisture Sensitivity Enhanced?

At the 2nd International Warm-Mix Conference in October, presentations heralded the spread of WMA from coast to coast.

For example, NAPA director of engineering Kent R. Hansen, P.E., reported that WMA use was growing very rapidly, with a 148-percent increase in use from 2009 to 2010, the most recent year for which firm data were available, rising from 19.2 million tons to 46.7 million tons in one year. That’s drawn from estimated total volume of both HMA and WMA of 358 million tons in 2009 to 360 million tons in 2010. Download the report by Googling NAPA Information Series 138.

As usage grows, WMA has moved from a boutique product to the mainstream, and is coming under increased scrutiny, with the question being raised of whether the physiochemistry of warm mixes makes them more sensitive to moisture damage.

Injection of water, along with the liquid asphalt cement, causes the liquid asphalt to foam and expand in volume.

Moisture damage is a well-recognized phenomenon, reports The Asphalt Institute in its 2007 pamphlet, Moisture Sensitivity: Best Practices to Minimize Moisture Sensitivity in Asphalt Mixtures, with 10 of 50 states reporting fresh hot mixes being treated for moisture damage.

Because in the absence of an emulsifier oil and water don’t mix, moisture damage impairs the bond between aggregate and liquid asphalt. “While adhesion failure between the asphalt and aggregate (referred to as stripping) is the most commonly recognized [mechanism], there are others,” AI says. “These include moisture-induced cohesion failures within the asphalt mixture, within the aggregate, emulsification of the asphalt and freezing of entrapped water.”

“Although moisture damage potential is also possible in some HMA mixtures, due to its method of production, it may be more likely in WMA,” say Thomas Bennert, Ph.D., Center for Advanced Infrastructure and Transportation at Rutgers University, and Greg Brouse, Eastern Industries Inc., in their 2011 WMA conference paper, Influence of Initial Aggregate Moisture Content and Production Temperature on Mixture Performance of Plant Produced Warm Mix Asphalt.

“Inadequately dried aggregates at lower production temperatures, and even the possible introduction of additional moisture to the WMA from the various WMA foaming technologies, may affect the binder-to-aggregate adhesion, moisture susceptibility and general mixture performance,” they write. “Preliminary laboratory testing has shown that when producing warm mix asphalt, both the initial aggregate moisture content and mixing temperature can have a dramatic impact on the performance of the mixtures, especially with respect to the resistance to moisture damage. Unfortunately, it is often difficult to control these parameters outside the laboratory setting.”

That’s because, as they say, while some lab tests show WMA specimens often fail the Texas DOT Hamburg Wheel Tracking test criteria, moisture damage has not been witnessed in the field, and similar results with other tests have been reported outside Texas. “This may indicate modifications to material preparation and/or test procedures are required when evaluating the moisture damage susceptibility of WMA mixtures in the laboratory, so field conditions can be properly simulated,” they observe.

For this work, two different initial aggregate moisture contents (less than 1.5 percent and greater than 3.0 percent) were achieved by the use of covered stockpiles. Five different mixtures were produced at both moisture contents, an HMA as control; WMA via foaming system; WMA foaming plus an anti-strip; WMA foaming plus a different anti-strip; and WMA via surfactant additive.

Laboratory performance samples were sampled and compacted at the asphalt plant’s QC laboratory after similar conditioning periods. Moisture damage tests (Tensile Strength Ratio, Hamburg Wheel Tracking), permanent deformation (AMTP Flow Number, Asphalt Pavement Analyzer), dynamic modulus, and fatigue cracking via overlay tester tests were undertaken on the samples.

“Although moisture damage potential is also possible in some HMA mixtures, due to its method of production, it may be more likely in WMA,”

- Thomas Bennert, Ph.D., and Greg Brouse

The study found that at the moisture contents measured, production temperature is more significant than initial aggregate moisture content on performance. Moisture damage tests did not rank performance the same: the tensile strength ratio results were okay, but the Hamburg were bad based on general criteria.

“Not all anti-strips work the same,” Bennert and Brouse say. One anti-strip outperformed the other in the TSR test, but with mixed results in the Hamburg. It’s important to evaluate each of the materials to ensure they provide the performance required, the authors say. A 10 to 15 degrees F increase in temp clearly increased rutting resistance, but decreased fatigue resistance, they report. Field cores of an actual installation will be undertaken after one year.

Another October WMA conference paper, Moisture Damage Potential for Warm Mix Asphalt Containing Reclaimed Asphalt Pavement, by Mariely Mejías-Santiago and Ray Brown, Ph.D., P.E., U.S. Army Engineer Research and Development Center, Vicksburg; and Jesse Doyle, Ph.D. and Isaac L. Howard, Ph.D., Mississippi State University, stated that increased percentages of RAP in a mix may improve WMA’s sensitivity to moisture.

Their moisture damage study of a variety of samples indicated:

• Gravel WMA mixes showed moisture susceptibility for lower mixing temperatures. Limestone mixes did not show this problem.

• WMA additive type did not significantly affect moisture susceptibility at HMA mixing temperature

• The low mixing temperature results in increased moisture susceptibility of WMA for some WMA additives/processes, and

• Increasing RAP tended to increase resistance to moisture susceptibility.

“The use of high percentages of RAP with WMA looks promising and should be considered for selected field projects if additional research supports this effort,” they conclude.

Surfactants Exhibit Anti-Strip

In the meantime, surfactant-based warm mix additives can feature an anti-strip function. Rediset LQ from AkzoNobel Surface Chemistry allows a reduction in mixing and paving temperatures while providing built in anti-stripping effect, the manufacturer says, while also being used as a compaction aid for hot and warm mixes. Rediset LQ allows the asphalt mixing temperature to be reduced (by 25 to 55 degrees F) while ensuring good coating and workability. “Mixes prepared with Rediset LQ meet moisture resistance requirements without the need for additional liquid adhesion promoter,” Akzo says.

And last year, use of Evotherm 3G from MeadWestvaco enabled the U.S. Army Corps of Engineers to preclude use of lime as anti-strip modifier in an open-graded porous asphalt pavement. At the Marine Boot Camp at Parris Island, the U.S. Army Corps of Engineers did not have a specification for an open-graded porous asphalt mix, so it studied specs of the South Carolina DOT, says Dean Frailey, business development manager, MeadWestVaco Asphalt Innovations.

“They opted to go with South Carolina’s OGFC standard,” he said. “It’s a large-stone mix, with 6 percent polymer modified PG 76-22 asphalt binder. Typically, with a mix like this they would put in lime as a stabilizer, adhesion promoter, and for protection against moisture susceptibility, and then add fibers.”

Use of the WMA additive greatly simplified things. “They opted to remove the fibers and remove the lime while using the Evotherm warm mix technology. Since both fibers and lime were removed, they added back 8 percent washed screenings to act as fines in the mix,” Frailey says. “The additive fights moisture susceptibility, promotes adhesion and blocks drain down. It’s very unique for this application.”

Premature Aging Diminished

In the hot mix plant, the heat contributes to premature or artificial aging of the binder, which breaks down the liquid asphalt even as HMA is produced. This gives pavement oxidation a head-start which gets worse as the mix ages in-place over time.

Low energy WMA may enhance pavement durability, because with lower production temperatures, the valuable, lightest hydrocarbon fractions from liquid asphalt in the mix aren’t driven off by the heat of the burner. Thus the lower heat precludes “premature aging” of binder, with a goal of reducing pavement thermal cracking.

“By lowering the temperature of production, we are avoiding some of the potential degradation to binder that can occur at higher production temperatures,” says Everett Crews, Ph.D., technical manager for MeadWestvaco. Evotherm is different from other warm mix technologies in its ability to produce mixes around 200 degrees F, Crews says. “Other warm mix technologies produce material at temperatures up to 260 degrees Fahrenheit,” he says. “Evotherm is unique in that we can push the limit lower. We have made mixes at below 200 degrees F that have performed very well.”