Development of Aggregate Avoidance Index for Evaluating Recycled Aggregate Concrete

Early strength gain in concrete is typically associated with transition from the plastic to the semisolid state, and later to the solid state. When tensile failure occurs during the plastic and semisolid states, the paste, being the weak link in the matrix, provides the path for cracking as the aggregates are avoided. As the paste begins to strengthen in concrete with virgin aggregates, more fracture may occur through the aggregates. However, in recycled aggregate concrete, a possible enhancement of the interfacial transition zone (ITZ) at later ages may redirect the fracture path around some recycled aggregate particles. This research investigated mechanical and rheological properties of concrete with various proportions of recycled coarse aggregate ranging from 0% to 100% by weight. Gradations were as received from the crushing process but limited to a maximum size of 1.5 in. (38.1 mm) and minimum size of 0.19 in. (4.75 mm). Samples with higher percentages of recycled aggregate exhibited improved mechanical properties and freeze thaw durability. A counting of the fractured aggregates in the exposed faces of cracked flexural beams suggested an enhancement of the ITZ in recycled aggregates. For elimination of counting error, an aggregate avoidance index method was used to evaluate the fractured faces. Results indicated lower aggregate avoidance indexes for higher recycled aggregate content, a validation of the enhancement of the ITZ, possibly by the absorptive properties of the recycled aggregates. This validated, beneficial property of recycled concrete aggregates can be used to promote sustainable use of recycled concrete.