Synergistic contribution of superabsorbent polymers and HMPE fibers to UHPC performance

Ultra-high-performance concrete (UHPC) stands out for its high mechanical strength and durability, but its low water-to-binder ratio limits cement hydration and increases autogenous shrinkage, favoring the formation of microcracks that compromise long-term durability. Superabsorbent polymers (SAPs) have been used as internal curing agents, absorbing water during the mixing process and gradually releasing it to mitigate these effects. However, the water release from SAPs generates residual pores that can reduce mechanical strength. High-modulus polyethylene (HMPE) fibers act as crack-bridging elements, improving ductility and compensating for this strength loss. This study evaluated the synergistic interaction between SAPs and HMPE fibers in the physicochemical and mechanical behavior of UHPCs in two phases. In Phase I, three types of SAPs were incorporated into cement pastes. The pastes were characterized by isothermal calorimetry, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). The SAP with the highest absorption capacity (SAP B) stood out for its greater efficiency in internal curing, promoting increased C–S–H formation and prolonged hydration. In Phase II, UHPC mixtures with isolated and combined additions of SAPs and HMPE fibers were analyzed in terms of rheological, mechanical, and microstructural properties. SAPs reduced shrinkage and mass loss but increased porosity, leading to a reduction in compressive strength. The incorporation of HMPE fibers mitigated this loss by increasing toughness and crack control. The combined action of SAP B and HMPE fiber presented the best overall performance, indicating a promising strategy for UHPCs in aggressive environments.