Freeze-Thaw resistance and mechanical properties of UHPC reinforced with a lower amount of hybrid fibers

Sustainable development is becoming critical in many industrial fields, including building materials. The rapid development of the construction industry in Kazakhstan requires construction materials having high-quality, low maintenance, and good durability. The main durability issue of construction material associated with Kazakhstan’s severe climate is the volume change of water in concrete caused by the freezing and thawing (F-T) cycles, which threatens the material’s strength and microstructure. Therefore, ultra-high-performance concrete (UHPC), a relatively new material with high compressive and tensile strength and improved microstructural properties, is suggested. Nevertheless, a limited number of researches have been done regarding the F-T resistance of UHPC. This research focuses on the effect of combining two types of fibers (steel and polypropylene (PP) fibers) on the compressive, flexural, and direct tensile strength and F-T resistance of UHPC. Test results indicated higher dosages of PP fiber in UHPC increased strength before exposure to F-T cycles. Adding PP fiber to UHPC led to more prolonged strains for each UHPC mixture. However, adding more steel fibers was beneficial for UHPC exposed to F-T cycles. Although mixtures with high steel fiber dosage presented a more brittle type of failure and lower strain values than mixtures with higher PP fiber dosage, the compressive, flexural, and tensile strength of UHPC mixtures containing higher steel fiber dosages increased after 90F-T cycles. Relative dynamic modulus of elasticity (RDME), which is a common indicator of F-T resistance of concrete, tends to increase for all tested mixtures, with the highest values belonging to ones with higher steel fiber dosage. Hence, it is suggested that UHPC with higher steel fiber dosage is more beneficial in terms of the F-T resistance of UHPC.