Autogenous and drying shrinkage in Ultra-High-Performance Concrete (UHPC) and the effectiveness of internal curing

Ultra-High-Performance Concrete has a high paste content, silica fume, and a low water-to-cementitious materials ratio (w/cm). All these factors can lead to increased autogenous shrinkage and risk of cracking. This work evaluates the effectiveness of internal curing (IC) using pre-saturated lightweight sand (LWS) on the autogenous shrinkage, drying shrinkage, internal relative humidity (IRH), and restrained shrinkage of UHPC mixtures with and without fibers. A reference UHPC mixture having a 28-day compressive strength of 156 MPa and four test mixtures with 10 %, 20 %, 30 %, and 40 % replacement by volume of aggregates with LWS were tested. It was observed that the mixture with 40 % LWS had 75 % less autogenous shrinkage at 56 days (-295 µε vs. −1155 µε) and a 10 % higher IRH (77 % vs. 67 %) in comparison with the reference mixture. The nonfibrous 40 % LWS mixture also showed a 26 % reduction in drying shrinkage compared to the reference mixture at 56 days. In the restrained ring test, both the reference and R-40 %-LWS mixtures showed a low cracking potential. This was due to the presence of fibers, which bridge microcracks and prevent crack localization. In contrast, the non-fibrous control mixture showed high cracking potential and cracked after one day, highlighting the crucial role of fibers in preventing cracking in UHPC.