Tests and prediction model for time-dependent internal relative humidity of recycled aggregate concrete due to self-desiccation

The concrete internal relative humidity (RH) is the direct evidence of internal curing effect, controlling concrete shrinkage deformation and also affecting the strength development, creep and cracking risk of concrete structures. However, no studies have been conducted on the internal RH development due to self-desiccation and its model for recycled aggregate concrete (RAC). Therefore, this study aims to quantify the coarse recycled aggregate and fine recycled aggregate (CRA and FRA) influence on the internal RH and establish a prediction model. RAC specimens were designed considering the replacement ratios of CRA and FRA of 0, 50 % and 100 %, and their internal RH was monitored for 185 days. Based on the Bažant hydration model for natural aggregate concrete (NAC), the model for RAC was theoretically proposed considering the influence of CRA and FRA on the initial water-to-cement ratio and the dynamic water release process. Results showed that (1) Both CRA and FRA have a significant internal curing effect, decreasing the RH loss by 61.4 % when both CRA and FRA are fully used; (2) The influence of CRA on the internal curing effect is more pronounced than that of FRA, and a coupling effect exists between the influences of CRA and FRA; (3) The proposed model can accurately predict the time-dependent internal RH of RAC, with the maximum difference of 0.5 %.