Damage behavior of the multiple ITZs in recycled aggregate concrete subjected to aggressive ion environment

Recycled aggregate concrete (RAC) is inevitably subjected to various aggressive environments; however, the damage behavior of multiple interfacial transition zones (ITZs) in RAC under such environments has received little consideration in previous studies. Therefore, this paper was developed to quantify the influence of sulfate and chloride attack on the properties of multiple ITZs in RAC. A modeled RAC sample that contained multiple ITZs was first prepared, and the micro-hardness and micro-structure of multiple ITZs undergoing sulfate and chloride attack were determined. The results showed that the ITZOA-NM between old aggregate and new mortar was the weak point of RAC when the new concrete strength was lower than the old concrete strength; however, the ITZOA-OM between old aggregate and old mortar was the weakness of RAC when the new concrete strength was higher than the old concrete strength. A binomial relationship between ITZs properties and sulfate attack duration was observed, and the micro-hardness of multiple ITZs decreased linearly with increasing chloride attack duration. The degradation model for the micro-hardness of multiple ITZs was similar to that for the compressive strength of RAC subjected to an aggressive ion environment. The properties of ITZOA-NM were more sensitive to aggressive ion attack compared with that of other ITZs. Increasing the strength of new concrete is an effective method of improving the properties of multiple ITZs in RAC.