Alternatives to mitigate the inferior performance of wet-mix shotcrete containing high recycled fine aggregate replacement rates

The thixotropy, shootability, and mechanical properties of wet-mix shotcrete (WMS) mixtures with high replacement rates of recycled fine aggregate (RFA) of 75% and 100% are examined in this study. Four different alternatives including reduced free mixing water and addition of metakaolin (MK), thixotropy enhancing agent (TEA), and polypropylene fibres (PPF) are evaluated in order to improve the WMS properties. Test results demonstrated that the build-up thickness improved by 1.3- to 2-folds with the incorporation of MK, PPF, and TEA, reflecting their suitability to overcome the inferior shootability at high RFA additions. The TEA significantly increased the development of thixotropy, which was ascribed to the chemical reactions that create a gel and dense network structure. The use of 1% TEA in the mix containing 75% RFA increased by 64% the thixotropic initial shear stress. Nevertheless, this was accompanied with the highest drop in strength, requiring proper tailoring of the dosage rate and mortar composition. Mixtures prepared with reduced water-to-binder ratio from 0.45 to 0.4 compensated the drop in strength due to high RFA rates. Yet, this alternative is not an efficient to counterbalance the decline in shootability, given the increased superplasticizer demand that promotes bleeding and sagging despite the high thixotropy.