Road Science Tutorial
Two for the Price of One
Recycled concrete aggregate proves its value; but it demands an engineered approach.
By Tom Kuennen, Contributing Editor
Recycled concrete aggregate (RCA) is a valuable resource for road owners and builders, both in terms of lowering costs and in constructing a “green” highway. But RCA needs to be approached as an engineered product, with its production and reuse tailored to fit its composition and ultimate end use in a project.
Fortunately, new guidance is available that will help road owners and builders make decisions on how to use RCA, even as its utilization is growing on vast scales, for example the recently completed reconstruction and widening of I-294, the TriState Tollway in suburban Chicago.
Despite being blessed with extensive virgin aggregate sources, Texas is a strong supporter of RCA. In a September 2008 report, Recycled Concrete Aggregates Make ‘Cents’, the Texas DOT said, “In today’s environment of skyrocketing material and transportation costs encountered in road construction, recycled concrete aggregates (RCA) provide substantial savings to TxDOT and taxpayers.”
With RCA, natural resources are conserved, waste disposal is reduced, and air quality is improved due to reduced haul distances and reduced energy consumption, TxDOT said. “In many cases, allowing the use of RCA can be the most cost-effective choice for an aggregate source,” the DOT said. “This is especially true for those districts that do not have good, native aggregate sources. Using RCA can reduce time and expense of importing aggregates from other parts of the state.”
TxDOT has researched and used RCA with good success for about 17 years. In the years 2006-2008, TxDOT saved approximately 1.8 million tons of virgin aggregates by incorporating RCA in cement treated base, flexible base, continuously reinforced concrete pavement (CRCP), filter dams, gabion walls, concrete traffic barriers, flowable fill and select backfill for mechanically stabilized earth walls. “This equates to an estimated savings of $12.6 million from reduced or eliminated landfill and virgin aggregate associated costs. Savings from using RCA has the potential to increase tenfold based on current availability of RCA.”
RCA an Engineered Material
But care must be taken in specifying and using RCA in road structures and concrete mixes.
“Concrete pavement recycling is a viable, successful and proven technology,” said Mark B. Snyder, Ph.D., P.E., vice-president, Pennsylvania chapter of the American Concrete Pavement Association (ACPA), adjunct professor at the University of Pittsburgh, and 2010 president of the International Society for Concrete Pavements.
“However, RCA must be treated as an engineered material, and not as a straight-up replacement for natural aggregate,” Snyder told Better Roads. “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.”
That’s why as an engineered material – like reclaimed asphalt pavement (RAP) – 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 maximizing reclamation efficiency by including lots of mortar, or minimal when efforts are made to eliminate as much reclaimed mortar as possible.
It’s a matter of deciding what your goals are in terms of reclamation, and what the final use of the material will be, and test, test, test and characterize the material accordingly,” Snyder said.
Natural “virgin” aggregates and mortar (cement paste, sand, air and admixtures) comprise RCA. The crushing process results in aggregate particles that are often more angular and rough than typical virgin aggregates, and their properties are somewhat different. RCA typically has a higher absorption capacity, lower specific gravity, greater mass loss in tests such as L.A. abrasion and sodium and magnesium sulfate, and higher chloride content than virgin aggregates, reports ACPA.
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