How do polypropylene fibers improve the spalling behavior of in-situ concrete?

The results of a recently finished research project have shown a great influence of the amount of polypropylene (PP) fibers on the spalling behavior of concrete under fire loading. Starting from the identification of the permeability as the parameter with the greatest influence on spalling, results of permeability tests on normal-strength in-situ concrete without and with PP-fibers (1.5 kg/m3) are presented in this paper. The values for the permeability, which are obtained for concrete pre-heated to different temperature levels, are related to the pore structure, accessible by mercury-intrusion-porosimetry (MIP) tests. The considered concrete was prepared under on-site conditions, accounting for the workability and densification when casting at the construction site. In order to illustrate the effect of the permeability of concrete with and without PP-fibers on spalling, which was experienced during the aforementioned research project, a finite-element analysis, taking the coupling between heat and mass transport into account, is performed. The so-obtained results provide insight into the risk of spalling of concrete with varying amount of PP-fibers.