Effect of polymer fibers on pore pressure development and explosive spalling of ultra-high performance concrete at elevated temperature

This paper investigated pore pressure development of ultra-high performance concrete (UHPC) included various polymer fibers, i.e., linear low-density polyethylene (LLDPE), ultra-high molecular weight polyethylene (UHMWPE), polypropylene (PP), polyester (PET), and polyamide (PA) fibers. Temperature and pore pressure were measured simultaneously at different depths of UHPC specimens subjected to one-dimensional heating. It was found that the PP and PA fibers prevented spalling of UHPC by enhancing moisture migration, which resulted in the development of pore pressure in the deeper region of the specimens. The moisture migration in UHPC with LLDPE fibers caused spalling of a layer of concrete in a deep region of specimen. UHMWPE fibers did not affect pore pressure development and spalling resistance of UHPC significantly, while with PET fibers, the pore pressure of UHPC raised sharply due to inadequate moisture migration, leading to spalling of a whole layer. Instead of melting polymer fibers and empty channels left, microcracks created by the fibers were responsible for releasing vapor pressure and spalling prevention. Fibers with high thermal expansion between 100 and 200 °C are recommended for spalling prevention of UHPC.