Better Roads Staff
complexity of data collection, management and analysis, including simplification, standardization of data, development of analysis software, and to training for field personnel; and acceptance by agencies and contractors to replace or augment existing QC practices.
Among other things, RAP fights dust
Reclaimed-asphalt pavement (RAP) can be used to suppress dust on local roads, but even more cost-effective applications can be justified with a cost/benefit analysis of its use, say Burt Andreen and Harry Rocheville, University of Wyoming-Laramie; and Khaled Ksaibati, Ph.D., P.E., director, Wyoming T2 / LTAP, of the Department of Civil and Architectural Engineering, University of Wyoming, in their paper, A Methodology for Cost/Benefit Analysis of Recycled Asphalt Pavement (RAP) in Various Highway Applications.
The Wyoming T2/LTAP (Local/Tribal Technology Assistance Program) recently completed a study on the use of RAP in gravel roads, which explored RAP as a means of dust suppression while considering road serviceability. The study found that RAP in gravel roads reduces dust without affecting the serviceability of roadways. This has become more important as expanded oil and gas exploration in the state has put new stresses on unpaved roads.
Now, Wyoming DOT and local agencies need to determine – out of the many possible uses – which use of RAP is the most cost-efficient. In this study, the T2/LTAP evaluated three possible RAP uses: RAP in hot plant mix, RAP in base and RAP on gravel roads. Using a method developed by the National Asphalt Pavement Association (NAPA) entitled Calculating the Value of Using Reclaimed Asphalt Pavement, a system was devised to assess the costs and benefits of using RAP in hot plant mix, and a similar process was developed to evaluate the value of using RAP in gravel roads and bases.
“The system normalizes all of the costs and benefits into savings per ton of RAP as a means-equal comparison,” the authors write. “The method includes factors such as savings from dust loss, layer coefficients, haul and decreased need of virgin aggregates. A case study was then conducted using the three different applications. It was determined that using RAP in hot plant mix was the most cost-effective in this case study. In addition, it was concluded that using RAP in gravel roads may be more cost-effective than using it in bases due to the additional benefit of dust loss reduction.”
Following the economic analysis, the authors found:
RAP can be a very effective material in highway construction applications. It is very economically feasible to use RAP because the recycled material greatly reduces the need for virgin aggregates, and does not decrease pavement performance.
Added RAP significantly reduces the dust loss on gravel roads from traveling vehicles.
For every ton of RAP included in hot plant mix, $40.87 was saved. Also there were savings of $17.07 for every ton of RAP used in gravel roads.
The implementation of RAP in road base also saved money, but it was the least effective of the three applications. For every ton of RAP used in road base, $15.71 was saved.
This analysis shows that regardless of the construction use of RAP, a savings always is realized.
“Clearly, the application of RAP in highway construction is cost-effective,” the authors conclude. “The amount of savings can increase exponentially when large quantities are used and when a greater percentage of RAP is included,” they say. “The use of RAP in any situation has no shortfalls; RAP saves money, does not impact performance, and has the ability to help the environment due to dust loss in gravel roads.”
Pervious and RCA
Limits to recycled concrete aggregate in pervious concrete
Recycled concrete aggregate (RCA) can increase compressive strength of environmentally-sustainable pervious concrete, but there’s a limit to how much can be used in place of virgin coarse aggregates, say Bradford M. Berry, Mark J. Suozzo, Ian A. Anderson and Mandar M. Dewoolkar, The University of Vermont, in their paper, Properties of Pervious Concrete Incorporating Recycled Concrete Aggregate.
Their work investigates using RCA in pervious concrete, specifically the effects on the density, strength and permeability. Cylindrical specimens of pervious concrete with different percentages of RCA and conventional aggregate were cast. The coarse aggregate was substituted with 0-, 10-, 20-, 30-, 50- and 100-percent RCA. As percent RCA increased, both compressive strength and permeability generally decreased.
The strength and hydraulic characteristics of mixes examined in this study compared generally well with other studies, on pervious concrete without RCA, found in the literature. The results indicate that up to 50-percent substitution of coarse aggregate can be used in pervious concrete without compromising strength and hydraulic conductivity significantly. Further testing evaluating freeze-thaw durability is necessary if pervious concrete with RCA is to be used in cold weather climates.
Conventional paving surfaces prevent water from entering the subsoil beneath them. “These impervious surfaces increase runoff, cause flooding and contribute to siltation and other water pollution,” the authors say. “Pervious surfaces allow stormwater to infiltrate into the ground, recharging the water table, and thus reduce the amount of runoff. This reduction in stormwater runoff also lessens resulting environmental pollution. For these reasons, the use of pervious concrete is among the Best Management Practices recommended by the Environmental Protection Agency.”
Ready-mix concrete is normally specified in accordance with the requirements of ASTM C94, Standard Specifications for Ready-Mixed Concrete, they say. For concrete in parking areas, a minimum compressive strength of 24 MPa (3,500 psi) is recommended; however, in areas where freeze-thaw durability is a concern, a minimum compressive strength of 28 MPa (4,000 psi) is advised. But typical pervious concrete mixtures can develop compressive strengths in the range of 3 to 28 MPa (500 to 4,000 psi).
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