Determination of Strain and Stress Field in Screening Test for Concrete Fire Spalling—Passive Restraint Effect

The paper examines the impact of passive restraint on fire-induced spalling in concrete, utilizing a concrete mixture to minimize compositional variability. A variety of specimen geometries was prepared, including standard cubes and cylinders for the determination of mechanical properties and slabs of different dimensions for fire spalling tests conducted under controlled conditions. A top-opening Dragon furnace, which applies ISO 834-1 fire curves, was used to evaluate the influence of "cold rim" boundaries, where slab edges were insulated to create thermal restraint. The cold rims were categorized as 0 cm, 10 cm, and 20 cm, with each modification representing a different degree of thermal expansion restraint. Digital image correlation (DIC) was utilized to monitor the strain fields on the unheated slab surfaces. The findings demonstrated that increasing the cold rim width implies a rise in compressive stress and strain in the central zone, thereby precipitating a more pronounced spalling behaviour. The unrestrained slabs (cold rim 0 cm) exhibited minimal spalling, whereas the restrained slabs (cold rim 20 cm) demonstrated significant spalling depths and volumes. The study confirms that thermal dilation restraint intensifies the severity of spalling and provides a quantitative framework that links stress evolution, strain distribution, and spalling depth. The findings emphasize the necessity of managing thermal restraint to properly assess fire-induced concrete spalling in material screening tests.