Effect of lateral restraint and inclusion of polypropylene and steel fibers on spalling behavior, pore pressure, and thermal stress in ultra-high-performance concrete (UHPC) at elevated temperature

Abstract

This paper investigates effects of lateral restraint and inclusion of polypropylene (PP) and steel fibers on pore pressure, thermal stress, and explosive spalling in ultra-high-performance concrete (UHPC) at high temperature. Temperature and pore pressure were measured simultaneously at different depths of six cylindrical UHPC specimens exposed to one-dimensional heating following ISO 834 heating curve. Numerical models were built to reveal thermal stress development and damage propagation of the UHPC specimens. The significance of pore pressures and thermally stresses for explosive spalling was discussed. The results found that addition of PP and steel fibers fully suppressed spalling. But the maximum pore pressures measured in the spalled specimens were well below the tensile strength of the concrete. The thermal gradient induces compressive stresses to the heated surface and tensile stress to the interior regions. The steel ring restraint increased the compressive stress as the lateral loading were superimposed upon the thermal stresses and thus induced more damage in the UHPC specimens. The thermally stress is considered to be the primary factor in initiating concrete cracking, while the pore pressures are the driving force for increasing kinetics of the spalled concrete pieces.