Effect of hybrid fibers on fresh properties, mechanical properties, and autogenous shrinkage of cost-effective UHPC

This paper investigates the effects of hybrid micro-macro steel and micro steel blended with synthetic fibers and of the fiber content on key properties of a cost-effective ultrahigh-performance concrete (UHPC). Eleven mixtures are prepared using three types of fibers: micro steel straight fibers (SF), macro steel hooked-end fibers (HF), and polyvinyl alcohol (PVA) fibers. The fiber content of SF is increased from 0 to 5%. At a fiber content of 2%, different combinations of micro-macro steel and micro steel-PVA fibers are considered. The minislump flow of all mixtures is fixed to 280 ± 10 mm by adjusting the high-range water reducer (HRWR) dosage to ensure self-consolidating characteristics. The investigated properties include the HRWR demand, plastic viscosity, compressive strengths, tensile and flexural properties, and autogenous shrinkage. The plastic viscosity increases with the steel fiber content. At a fiber content of 2%, increasing the content of PVA or HF increases the plastic viscosity. Compared with the reference UHPC mixture made with 2% SF, the incorporation of 1% SF and 1% HF increases the flexural strength, toughness, and tensile strength by approximately 25, 30, and 20%, respectively, and reduces the autogenous shrinkage by 25%. The addition of 1.5% SF and 0.5% PVA increases the flexural strength and toughness by 10 and 15%, respectively, and decreases autogenous shrinkage by 40%. Increasing the SF content from 2 to 5% does not significantly improve the flexural properties, but notably reduces autogenous shrinkage.