Effects of Fiber Shape and Nanopalygorskite on the Long-Term Performance of Steel Fiber–Reinforced Ultrahigh-Performance Concrete

Abstract

Steel fibers and nanopalygorskite (NP) were introduced into ultrahigh-performance concrete (UHPC) to improve its long-term performance. Long-term mechanical properties and long-term shrinkage performance were measured to illustrate effects of fiber shape and NP on the long-term performance of fiber UHPC (FUHPC). The development of UHPC microstructure was analyzed by MIP, XRD, and SEM images. Research findings showed that steel fiber improved the long-term performance of UHPC, and hooked fiber (HF) with a dosage of 1.5% had better effect on the long-term performance than straight fiber (SF). Moreover, the addition of NP enhanced the long-term performance of FUHPC, and the long-term performance was optimal at a SF dosing of 1.0% and NP dosing of 0.2%. Specifically, nanopalygorskite-reinforced straight fiber UHPC (NPRSFUHPC) had the greatest 180-day compressive strength of 150.6 MPa, which was 10.7% higher than the control without NP (SFUHPC). And the largest 180-day flexural strength of nanopalygorskite-reinforced hooked steel fiber UHPC (NPRHFUHPC) was 26.1 MPa, which was 29.9% higher than the control (HFUHPC). In addition, adding NP also compensated for the early shrinkage of UHPC, and the combination of hooked steel fibers and NP inhibited the optimal 180-day shrinkage, which reduced by 44.5% compared with the UHPC without fibers. Moreover, NP promoted the secondary hydration reaction of FUHPC and effectively refined its internal pore structure, enhancing the macroscopic strength and microscopic compactness of FUHPC.