Fire performance of structural concrete block masonry considering the influence of mortar and gypsum plaster coatings

Structural masonry is widely used in buildings, yet its fire performance remains insufficiently understood, particularly regarding the role of surface coatings commonly applied in practice. This paper addresses an experimental investigation into the thermal behavior and residual mechanical performance of dry-cast hollow concrete block masonry subjected to fire, considering key variables such as specimen type (blocks, prisms, and small walls), block strength and geometry, coating type, and fire exposure condition (one or both faces exposed). Specimens were subjected to the ISO 834 standard fire curve for 120 minutes. Two coatings commonly used in masonry construction were evaluated: 10mm-thick cement-lime mortar and 5mm-thick gypsum plaster. Post-fire compression tests were performed after cooling to determine residual properties. The results revealed high temperatures across the cross-section and severe degradation of masonry, especially in walls with both faces exposed, whose residual strength ranged from 5% to 24% according to type of block and coating. In contrast, separating walls retained up to 64% of their initial strength. Mortar coating provided more effective fire protection than gypsum plaster due to its higher adhesion capacity at elevated temperatures, whereas plaster exhibited intense cracking and early delamination. Coating performance was also influenced by block strength, with reduced adhesion observed in higher-strength blocks. Although coatings contributed to reducing temperature levels in the masonry, significant strength loss was observed. These findings indicate that the use of coatings as a fire protection measure must account for their adhesion capacity at elevated temperatures and adequate thickness according to the required fire resistance ratings.