Influence of the moisture states of aggregate recycled from waste concrete on the performance of the prepared recycled aggregate concrete (RAC)-A review

Recycled aggregate (RA) presents times higher water absorption compared to the natural aggregate (NA). The moisture states of the RA can be quite variable when exposed to certain environments, which can largely influence the performance of the prepared recycled aggregate concrete (RAC). This paper reviews the published papers dealing with the effects of the RA moisture states on the workability, the mechanical performance, the long-term properties and durability of RAC, and discusses the controversy between different studies. This paper suggests that the effects of the RA moisture states on the RAC performance at the fresh state or after hardening should be considered in combination with the water supply scheme or the water-to-cement ratio as taken in certain scenarios. Basically, pre-wetting RA is reported to be beneficial for the performance of RAC at the hardened state in most cases: higher compressive/tensile/flexural strength and lower drying shrinkage/autogenous shrinkage could be achieved by using the partially saturated RA as reported in many references. However, the use of the surface-saturated dry (SSD) RA should be avoided as it can lead to great loss in the compressive/ tensile strength as well as the resistance of RAC to carbonation and chloride ingress. As for the workability, the pre-saturated RA is regarded to be favorable for the RAC slump when the mixing water supply is fixed, while in the case with a consistent total water supply, using the dry RA can lead to a larger slump. This paper proposes a model describing the water absorption/desorption behavior of RA at varying moisture states, and relates it to the microstructural changes in the ITZ and mortar phases so as to provide possible explanations for the RAC performance influenced by the RA moisture states. Recommendation is also given in terms of the shortcoming and challenges on this research topic encountered thus far.