Evaluation of Recycled Concrete Aggregate for Usage in Highway and Municipal Concrete Applications

摘要

Coarse recycled concrete aggregate (RCA) is produced by crushing concrete which has reached the end of its service life. Depending on its intended application, concrete as a construction material exhibits a wide range in physical and mechanical properties. Partially as a consequence of this, RCA exhibits variability in terms of physical properties which can make it less desirable for usage as coarse aggregate in new concrete infrastructure. However, one characteristic of RCA which is consistent is a higher absorptive capacity when compared with natural aggregate (NA). This higher absorptivity may indicate the potential for RCA to provide some internal curing-like benefits when cast in concrete. Internal curing is the method of entraining water in reservoirs within the concrete such that it is drawn from the reservoirs so as to be beneficial to the cement hydration process. Internal curing in concrete has been found to have many benefits including reducing the negative effects of poor external curing. In this research, two types of saturated coarse RCA have been used to study the effects of different curing practices on the performance of the concrete. Particular emphasis has been placed on those properties that are critical for concrete pavement design. Two curing regimes are used in order better understand the impact of curing practices on the properties of saturated RCA concretes. The angularity of the RCA had a pronounced negative effect on the slump values of the different concrete mixtures. The two sources oppositely affected the compressive strength: RCA 1 increased the strength while RCA 2 caused a strength decrease. While the RCA types resulted in opposite compressive strength changes, the stiffness of both RCA concrete materials were found to decrease indicating that the density of the aggregate and consequently concrete had a marked effect on elastic modulus. Studies of the RCAs' desorption characteristics indicated that the RCA sources tested do not have the properties consistent with internal curing agents according to the definition set by previous researchers focused in the field of internal curing of concrete. However, saturated high absorption RCA appeared to provide some benefit in terms of buffering compressive strength loss under MTO specified curing conditions, suggesting a possible contribution of internal curing from the saturated RCA. When tested at 28 days, the lower quality RCA 2 exhibited the lowest thermal expansion coefficients; however both RCA mixtures exhibited higher variability of results than the NA control mixture. These results indicate that some RCA concretes may be well suited for use in some pavement applications. While RCA 1 concrete exhibits a strength increase in comparison with natural aggregate concrete, it exhibits a reduction in elastic modulus with similar thermal expansion coefficients. This could reduce the magnitude of stress concentrations in a pavement exposed to thermal stresses. These results appear to confirm the findings of a University of Waterloo developed RCA classification framework which identifies RCA 1 as being suitable for use in concrete structures.