Effect of polypropylene fiber on early-age properties and stress relaxation of ultra-high-performance concrete under different degrees of restraint

Abstract

This paper aimed to investigate the effect of polypropylene (PP) fibers on mechanical properties, shrinkages, cracking potential, and stress relaxation of ultra-high-performance concrete (UHPC) at early age. The proportion of PP fibers was 0%, 1%, 2%, and 3% by the volume of concrete. Two types of restrained ring specimen (DR-1 and DR-2) were also employed to consider the effect of degrees of restraint. The results indicated that the cracking age of UHPC with PP fiber proportion from 0% to 2% was postponed, but further incorporation of PP fibers showed the inconspicuous benefits. Relaxed stress decreased with the incorporation of PP fibers. Moreover, the difference of stress relaxation was less pronounced between UHPC with 1% and 2% PP fibers. The influential factors for stress relaxation were analyzed. Both DR-1 and DR-2 demonstrated a high degree of restraint. This was attributed to the dominant role of autogenous shrinkage in total free shrinkage of UHPC, which eclipsed the effect of volume-to-surface ratio of specimen.