Crushed Returned Concrete Aggregate in New Concrete: Characterization, Performance, Modeling, Specification, and Application

Abstract

Every year roughly 2% to 10% of the estimated 455 million cubic yards of ready mixed concrete produced in the USA (est. 2006) is returned to the concrete plant. The crushed returned concrete aggregate (CCA) is obtained from crushing the returned concrete that was discharged at the concrete pant and left for a period of time before crushing. It is estimated that about 60% of all returned concrete is managed with this manner by the concrete plant according to the national ready mixed concrete association report. But the reuse of the returned concrete aggregate is very much limited so that most of the returned concrete aggregate has been diverted to the landfill. The main obstacle to limit the use of the returned concrete aggregate is the current type of prescriptive specifications by controlling the concrete composition, which limits the ability to optimize concrete mixtures for performance.

The CCA aggregate has useful aggregate properties among which it is free of any contamination. Thus, CCA aggregate is distinguished from other recycled concrete aggregate (RCA) that comes out of existing old structures with high contamination from many years of exposure during the service life. The objective of this research was to develop technical data that will support the use of the CCA aggregates from the returned concrete by the ready mixed concrete industry. Three CCA aggregates at three strength levels were characterized. Thereafter, the virgin coarse/fine aggregates and the three CCA aggregates were used with various amounts to prepare concrete mixtures so as to investigate the effect on the fresh and harden concrete properties.

The second objective of this research was to develop the performance models of harden concrete properties. The harden concrete properties of a selected number of mixtures containing CCA aggregates were used for the modeling of compressive strength, drying shrinkage, elastic modulus, and rapid chloride ion penetrability. This analysis was instrumental for a better understanding of how the CCA aggregates affect the harden concrete properties.

The fine CCA aggregates were further investigated for their potential use as internal curing agent due to their unique aggregate properties (i.e. low specific gravity and high water absorption capacity). Those two properties are crucial factors for the internal curing. The fine CCA aggregates were used with mortar mixtures to evaluate the strength and autogenous shrinkage behavior along with the lightweight fine aggregate. This new approach can promote the use of CCA aggregate in a specialized application.

Another objective of this study was to demonstrate the advantages of using a performance based specification. An example of an experimental case study was used for both conventional and CCA based concrete for comparing performance and prescriptive specifications.