Influence of moisture conditioning of recycled aggregates on the properties of fresh and hardened concrete

Usually, on a working site and before being placed the fresh concrete may undergo of waiting times, which may as a result a significant reduction in the workability of concrete in the interval between mixing and placing. In addition, this reduction is intensified when the recycled-concrete aggregates (RCA) are used due to their high water absorption capacity. For this purpose, RCA were controlled at three states prior to use, i.e., dry, with a pre-wetting and at a saturated-surface-dry (SSD). RCA were used in this work to replace the natural aggregates in different proportions. The influence of moisture conditioning of RCA on the slump and the properties of hardened concrete are investigated. Slump values were regularly measured immediately after mixing and wait times of 15, 30, 45 and 60 min. The test results obtained showed that the moisture conditioning of the coarse recycled aggregates is needed in order to maintain proper workability. The kinetics of water absorption by recycled aggregates is fast in the early minutes when the aggregates are in contact with water. After 30 min the workability loss, in terms of the slump loss appears to subside. The compressive and splitting tensile strengths of concrete at 7 and 28 days with recycled coarse aggregate depend on the mix proportions and moisture conditions of RCA. The SSD recycled aggregates seemed to impose the largest negative effect on the concrete strength. The use of recycled aggregates in the dry state gave the best strength.