The Influence of Expansive Agent on the Performance of Fiber Reinforced Cement-Based Composites

In this work several fibre reinforced cement-based composites (FRCCs) as well as ultra high performance concretes (UHPCs) were studied, in which CaO-based expansive agent was added in order to help in reducing the risk of cracking induced by drying shrinkage. Three different kinds of metallic reinforcing fibres were tested: flat (and flexible) steel-alloy fibres, and brass-coated either hooked or straight steel fibres. All the mixtures were characterised at both fresh and hardened state, by measuring fresh consistency, compressive and flexural strength, as well as length changes under drying shrinkage test. Moreover, FRCC microstructure was investigated by means of Mercury Intrusion Porosimetry (MIP) and Scanning Electron Microscope (SEM) observations. The effect on mechanical performance of a thermal pre-treatment at 80 °C for the first 24 h of curing after casting was also evaluated. Results obtained confirmed the effectiveness of CaO addition on material stability under drying shrinkage; moreover, it seems to produce a beneficial effect on flexural strength if used with brass-coated fibres. The reason of this synergy probably lies in the formation of calcium-hydroxy-zincate crystals at the interface between fibres and surrounding cement paste (phenomenon promoted by dezincification of brass in alkaline environment), able to significantly improve the quality of the interface fibre–matrix by increasing adhesion.