Effect of additional water content and adding methods on the performance of recycled aggregate concrete

Due to the high porosity of the old cement mortar attached to the surface of the recycled coarse aggregate (RCA), the RCA exhibits a high water absorption property, seriously affecting the workability of recycled aggregate concrete (RAC). The additional water in RCA plays a crucial role in the effective water to binder ratio and various properties of RAC. In order to determine the optimal content and addition method of additional water, this study innovatively investigates the effect of the above two variables on the workability, mechanical properties and durability of RAC. And combined with microscopic tests, the mechanism of the above two variables on the performance of RAC has been revealed. The additional water content includes 100% water absorption (WA) and 75% WA, and the adding methods includes mixing water compensation and pre-wetting aggregates. The results show that the water content of oven dry RCA in water and cement paste reaches 94.7%-95.4% and 71.0%-76.0% WA after 2 h, respectively. The water content of RCA is reduced by 4%-4.9% after immersing RCA into cement paste for the first 5 min under the initial state of 70% WA. No matter the adding methods, the 100% WA additional water has a positive effect on workability but a negative effect on the mechanical properties and durability of RAC. When the additional water content is the same, the adding method has different effects on the mechanical properties and durability of RAC. The microstructure analysis reveals that the interfacial transition zone (ITZ) in RAC prepared with pre-wetted aggregates by 75% WA is the densest of all samples. Compared to the 75% WA water compensation method, the elastic modulus of ITZ in RAC with pre-wetted aggregates by 100% WA decreases by 10.5%, while the thickness of the new ITZ increases by 14.3%. The RAC prepared by pre-wetted aggregates exhibits the lower porosity. •In oven dry state, the WA of the RCA placed in the cement pastes reaches about 70% after 2 h.•The 2-h slump loss of all the RAC meets the requirement for pumping project.•Mechanical properties and durability are different from the adding water method.•The RAC with pre-wetted RCA by 75% WA has the densest microstructure.