Effect of calcined bauxite powder or aggregate on the shrinkage properties of UHPC

Abstract Commonly used internal curing (IC) agents (super absorbent polymers or lightweight aggregate) to mitigate the higher levels of autogenous shrinkage in UHPC could possibly lead to a negative effect on the mechanical properties of concrete due to the increased void created by the agents, the not-well estimated added water for IC, or low strength of the materials. In this study, a new type of calcined bauxite (CB) aggregate was found to be stronger than normal aggregate and exhibit a porous structure. Different quantities of this aggregate and its powder product were applied to estimate their effect on the mechanical and shrinkage properties of non-fibrous UHPC matrixes and UHPCs with steel fibers. The maintained higher internal relative humidity and significantly reduced autogenous shrinkage (more than 65%) demonstrate that the use of either dry or prewetted CB aggregate can both provide IC effect in UHPC. It reveals that the dry CB aggregate absorbs water during mixing and releases them after setting. Both the compressive and tensile strength of UHPC mixtures were significantly improved by using CB aggregate as compared to the reference mixture, which is due to the actual lower effective w/b, enhanced properties of the ITZ, and increased hydration by internal curing. However, the powder product of CB did not provide IC effect, but a higher content of CB powder (600 kg/m3) helped enhance the strength of UHPC matrix while the one with 300 kg/m3 of CB powder obtained similar strength both compared to the reference mixture.