Impact of external biaxial compressive loading on the fire spalling behavior of normal-strength concrete

Explosive spalling of concrete exposed to fire consists in the expulsion of shards from the heated face during rapid heating. The phenomenon can seriously jeopardize the integrity of reinforced concrete structures due to the reduction of the cross-sectional area of the structural elements and even lead to the direct exposure of reinforcing bars to flames. The literature shows that various parameters influence the occurrence of fire spalling, such as heating rate, specimen geometry and boundary conditions, concrete grade, and external loads. In this regard, the present study aims at highlighting the role of external loading in combination with the effects of pore pressure and thermo-mechanical stresses in triggering spalling in normal-strength concrete (fc_28days ≈ 45 MPa). Unreinforced concrete slabs (size: 800 × 800 × 100 mm³) were subjected to a standard (ISO 834–1) fire curve under seven different levels of external membrane biaxial compressive load. The experimental results clearly show that compressive loading significantly increases spalling propensity and severity.