Effects of geometry and fraction of polypropylene fibers on permeability of ultra-high performance concrete after heat exposure

This paper investigated the effect of geometry and fraction of polypropylene (PP) fiber and the induced microcracks on permeability of ultra-high performance concrete (UHPC) subjected to elevated temperature. Residual permeability and microcrack networks of fifteen UHPC mixes were characterized and an analytical model correlating residual permeability of UHPC with fiber fraction and geometry was proposed. Results showed that increasing fiber length and dosage had much stronger effect than increasing fiber diameter on enhancing permeability due to greater enhancement on percolation of fiber tunnels. It was found that permeability of UHPC is positively correlated with both the aspect ratio and dosage of PP fibers. However, at low fiber aspect ratio, increased fiber dosage does not increase the permeability of UHPC. Similarly, at low fiber dosage, solely increasing fiber aspect ratio does not contribute much to increasing permeability of UHPC. The proposed model thus provides insight for PP fiber selection and optimization to prevent explosive spalling of concrete.