The influence of pore pressure on the apparent tensile strength of concrete

The rise of pore pressure is generally recognised as one of the governing factors of explosive spalling in concrete structures exposed to fire. However, to what extent spalling is related to concrete fracture properties is still an open issue. In order to clarify this point a special setup has been designed, aimed at performing simple indirect-tension tests (split-cube tests) under different levels of sustained pore pressure. These levels are achieved by preliminarily heating two opposite faces of cubic specimens, whereas the lateral sides are sealed and thermally insulated, so as to create a mono-dimensional thermo-hygral transient field. Fracture is induced in the symmetry plane, where both temperature and pressure are monitored by means of a customized probe. The results show that pore pressure and the concurrent reduction of the apparent tensile strength have comparable values. Simple considerations based on linear elastic fracture mechanics corroborate this evidence. The conclusion is that pore pressure may be, in itself, a sufficient driving force to trigger explosive spalling in concrete members exposed to fire.