Effects of isothermal microwave heating on the strength and microstructure of ultra-high performance concrete embedded with steel fibers

This paper presents an investigation on the adoption of isothermal microwaving in curing ultra-high performance concrete (UHPC) embedded with steel fiber reinforcement. The plain UHPC and the one with varying levels of steel fiber reinforcement are developed by microwave heating at 80 °C for 30 min. The compressive and flexural strength, hydration products, microstructure and interfacial structure of the mixes are evaluated. The results show that, besides the common reactions of hydration, pozzolanic reaction, carbonation, a reaction consuming calcium carbonate is found to occur in the system; furthermore, a Hayne-like phase and a cancrinite-like phase are formed. The formation of them is reduced by increased reinforcement. The accelerated effects of microwave on reaction are found to vary with reinforcement. Accelerating hydration is beneficial from increased fiber contents. Steel fiber reinforcement is more effective in increasing the compressive and flexural strength of UHPC cured with microwaves than cured under standard conditions.