Better Roads Digital Magazine
Issue link: https://read.dmtmag.com/i/85930
RoadScience RCA/CCA will exhibit lower specifi c gravity, which de- creases 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 stockpil- ing, especially the fi ne material," according to the 2011 manual. "Finally, the fi nes 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 diffi cult to work." crease 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. "Specifi c gravity and absorp- tion 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 fl at 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 con- crete pavements – continuously reinforced concrete pavement (CRCP) – Texas has done major work in the fi eld in evaluating the use of RCA with CRCP, and is confi dent 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 In central Texas, crushed recycled asphalt shingles (RAS) are conveyed to an asphalt drum RCA/CCA in Pavements RCA/CCA can be used with confi dence in asphalt pave- ments. "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 fi lm around the aggregate, leaching and other complications from water interacting with the CCA are minimized." For this purpose, deleterious materials such as soil, ash, or other fi ne organic materials should be limited, as they will in- 22 September 2012 Better Roads state projects in the Houston area and is fairly com- mon in Dallas as well. There are a number of factors affecting CRCP performance with RCA, including adequacy of pavement structure, material proper- ties, environmental conditions during concrete placement, and construction practices. TxDOT found that the CRCP sections using 100-percent recycled coarse and fi ne 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, fi ber and mineral content, abun- dant, processed recycled asphalt shingles (RAS) are fi nding 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 conven- tional 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 fi rm, and Photo courtesy of Tom Kuennen