Replacement of recycled concrete aggregate (RCA) for natural coarse aggregate by up to 45 percent by volume had no significant effects on any of the concrete properties studied, indicating high-quality RCA can be used as a replacement for a portion of the coarse natural aggregates in new portland cement concrete pavements. That’s what Haifang Wen, Ph.D., P.E., Washington State University; David I. McLean, Ph.D., P.E., Colorado State University; and, Kim Willoughby, Washington State DOT, report in their 2015 TRB paper, Evaluation of Recycled Concrete as Aggregates in New Concrete Pavements.
“Recycled concrete aggregate produced from demolished pavements in three geographically-dispersed locations in Washington State was used to perform tests on aggregate, characteristics, fresh concrete properties, and hardened concrete properties,” Wen, McLean and Willoughby report. Variables included the source of the RCA, percent replacement of coarse natural aggregate with RCA (0 to 45 percent), and percent replacement of portland cement with Class F fly ash (0 or 20 percent). RCA from all three sources met the Washington State DOT requirements for aggregates, and all fresh and hardened concrete properties met WS DOT requirements as well.
“The effects of RCA on fresh concrete properties were evaluated by measuring slump, air content, and density,” the authors write. “Slump and air content were controlled parameters in the batching process, with targets specified by WS DOT of 1 to 3 in. for slump and 3 to 7 percent for air content.”
Slump was controlled by withholding mix water or by adding water-reducing admixture (WRA), they note, and air content was controlled by the amount of air entraining admixture used in the batch. Slump and air content for all batches were within these target ranges. RCA was found to decrease the slump and density of fresh concrete, but did not have a significant effect on air content.
The effects of RCA on hardened concrete properties were evaluated by measuring compressive strength, modulus of rupture, coefficient of thermal expansion, drying shrinkage and freeze-thaw durability. “Test results showed that up to a 45 percent replacement of coarse natural aggregate with RCA had no significant effect on any of the hardened concrete properties tested,” Wen, McLean and Willoughby conclude. “In addition, all samples tested met WS DOT minimum strength requirements for use in concrete pavements.” The authors emphasize that these results were obtained using RCA obtained from demolished pavements incorporating high-quality original materials.
Properties of the RCA can be characterized using standards tests, including resistance to abrasion and the amount of adhered mortar. While no maximum effective RCA substitution rate was established in this study, the results show that a 45 percent substitution of coarse natural aggregate with high-quality RCA meets all WS DOT requirements for use in new PCC pavements.
RCA should be washed and sieved to remove fine particles prior to being used as a replacement for natural aggregates in new concrete, Wen, McLean and Willoughby recommend. In order to address performance concerns related to alkali-silica reactivity, they add, each RCA source should be tested for reactivity potential following the crushing process and mitigated as necessary.