Road Science Tutorial
Recycled portland cement concrete pavements have a relatively high level of water absorption, they warned, adding that relatively high level of water absorption could potentially make the proper compaction of gravel cushion and aggregate base course layers variable.
Nonetheless, South Dakota found that “recycled portland cement concrete pavements are a viable option for use in gravel cushion and aggregate base course construction.”
Acceptance of RCA
The last formal survey of RCA use among the states was released in 2004 by the Federal Highway Administration. Its purpose was to capture for technology transfer the most advanced uses of recycled concrete aggregate for use by state highway agencies.
Transportation Applications Of Recycled Concrete Aggregate: FHWA State of the Practice National Review found that concrete routinely is being recycled into the highways of the United States, and its principal application has been as base material.
State transportation agencies were surveyed to determine the current uses of RCA. Forty-one of 50 state DOTs allowed some use of RCA in their specifications, the survey reported. From the results of this survey, five states were identified as being among the highest consumers of RCA in the United States, as well as having large supplies. Their applications were spotlighted in the report, which may be easily located online by “Googling” the title.
“Of those 41 states, 38 were permitting its use in foundation layers, among other applications,” Snyder said. “Foundation layers were the most common application. But its use in pavements is increasing as aggregates costs go up, and haul distances to virgin aggregates become greater. The amount of RCA used is increasing and it’s already fairly high.”
Addressing the need for better guidance, ACPA has articulated guide specifications for use of RCA in road projects, and the American Association of State Highway & Transportation Officials (AASHTO) has a standard specification for use of RCA in foundation layers, and a draft temporary spec for its use in concrete mixes, Snyder said.
The ACPA guide spec is contained in a new publication released in October 2009, Recycling Concrete Pavements. The 102-page technical resource describes concrete pavement recycling as a proven technology that offers an alternative aggregate resource that is both economical and sustainable.
The publication begins with an executive summary and an introduction, and then continues with chapters covering production, properties and characteristics, uses of RCA, and properties, performance, and recommendations for concrete pavement structures containing RCA. Appendices follow, including guidelines for removing and crushing existing concrete; using RCA in unstabilized (granular) subbases: and using RCA in concrete pavement mixtures. Additional appendices include AASHTO and ASTM standards, as well as a glossary of terms.
The mortar content key
The mortar content of the final RCA product is key to a successful application. “Crushed concrete will come down to either large-size coarse aggregate particles, or the ‘glue’ that holds them together, the mortar, which is the cement paste plus sand particles and fly ash or any other admixture,” Snyder told Better Roads.
How much mortar should come off the RCA depends on the ultimate use of the RCA, Snyder said. “It really depends on the application,” he said. “RCA needs to be treated as an engineered material. If you want to maximize reclamation efficiency, so you are reclaiming as much mortar as possible to be as ‘green’ as possible, there are certain types of crushing processes that will remove less mortar. Then you will need to take the presence of that added mortar into account in designing the application.”
For example, concrete mix designs may need to be modified to compensate for the higher absorption capacity of the RCA, and base course gradations need to be selected with consideration of the higher abrasion or degradation properties of the material.
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