Influence of Silica Fume on Mechanical Properties and Microhardness of Interfacial Transition Zone of Different Recycled Aggregate Concretes

Several countries have started using recycled aggregate as a partial replacement to natural aggregate in concrete. Recycled aggregate contains adhered mortar, which distinguishes it from the natural aggregate. In the present study, natural coarse aggregates were entirely replaced by two kinds of recycled coarse aggregates. The recycled aggregates obtained from the jaw crushing method were named recycled coarse aggregate-1 and the aggregates that were further processed by the ball milling method were named recycled coarse aggregate-2. The performance of control concrete and two kinds of recycled coarse aggregate concretes were studied experimentally with respect to mechanical properties. Results indicate that the processing method to obtain recycled coarse aggregates plays an important role in developing the required mechanical properties. The ball mill processed aggregates performed better than the jaw crushed aggregates in concrete. The performance was also assessed with respect to the microhardness of the interfacial transition zone around the surface of the aggregates. The presence of adhered mortar in recycled aggregate weakens it because of the presence of an old interfacial transition zone that affects the strength of concrete. The interfacial transition zone hardness at the aggregate-mortar interface is 53.94, 34.21, and 44.08 % of bulk concrete for control concrete, recycled coarse aggregate-1 concrete, and recycled coarse aggregate-2 concrete, respectively. The addition of silica fume improved the average microhardness, and the same was reflected in the mechanical properties of both the recycled coarse aggregate concretes. It is recommended to use ball mill processed recycled coarse aggregates as a complete replacement to natural coarse aggregates along with a 5 % addition of silica fume for better performance.