Study on interfacial transition zone properties of recycled aggregate by micro-hardness test

This study was undertaken using two experimental approaches for analyzing the influence of recycled aggregate on recycled aggregate concrete: The failure shape of recycled aggregate after loading and the internal properties of recycled aggregate that influence on strength of recycled aggregate concrete; the change of mechanical properties in the interfacial transition zone between the aggregate and mortar by a micro-hardness test. The recycled aggregate concrete, contrary to fail mode of natural aggregate concrete, failed by both interfacial transition zone failure between the mortars and aggregate and cracks through the inner parts of the recycled aggregate. For the interfacial transition zone properties between the aggregate and mortar, a micro-hardness value in the old interfacial transition zone was lowest, while the micro-hardness value in the new interfacial transition zone was high next to the old interfacial transition zone. Regarding the interfacial transition zone properties of mortar, a micro-hardness value in the old mortar was similar to or somewhat lower than that of the new mortar. This study confirmed interfacial transition zone properties between old mortar and new mortar, between the aggregate and mortar, and the amount of the old mortar that influenced properties of the recycled aggregate.

► We analyzed the physical characteristics on the ITZs between RAs and mortar.
► The RAC is seriously affected by the existing cracks within the RAs.
► The ITZs properties determine the bond strength between RAs and mortar.
► The old ITZs were weak compared to the new ITZs by cracks and voids with RAs.