Comparative study on the effect of steel and polyoxymethylene fibers on the characteristics of Ultra-High Performance Concrete (UHPC)

This paper presents a comparative study on the effect of steel and polyoxymethylene fibers on the characteristics of Ultra-High Performance Concrete (UHPC). Firstly, based on the modified Andreasen & Andersen packing model, a UHPC with ultra-low cement content is produced. Then, the steel fibers and polyoxymethylene (POM) fibers are added into this UHPC matrix separately and together. The obtained experimental results show that the addition of POM fiber has limited contribution to the compressive strength of UHPC, while it has a positive influence on its flexural strength and high temperature resistance. The results of nanoindentation indicate that POM fiber can slightly disturb the packing skeleton of UHPC and enlarge the ITZ between matrix and fibers. Based on the nanoindentation results, it can be found that the interfacial transition zone (ITZ) is about 50 μm in the case of UHPC with only steel fibers, while this value gradually increase to about 60 μm when POM fiber is added individually. This results further demonstrate that the POM fiber can slightly disturb the packing skeleton of UHPC and enlarge the ITZ between matrix and fibers. Therefore, to efficiently apply the POM fiber in UHPC, it is suggested to combine it with steel fibers and its optimal content should be around 1% (vol.). It can be concluded that it is difficult for POM fiber to fully replace steel fiber in referencing UHPC, while the hybrid fibers should be a good choice to produce excellent UHPC composite.