Investigating residual properties of masonry units at elevated temperatures

Abstract

Masonry structures are renowned globally for their strength, durability, affordability, and thermal and sound insulation properties. However, there is limited information on the residual properties of masonry units after exposure to elevated temperatures. This research addresses the aforementioned gap by investigating the residual properties of four representative masonry units; clay bricks, concrete blocks, compressed stabilized earth blocks and lightweight foam blocks. The study aimed at determining variations in physical and mechanical properties, including visual appearance, density, and compressive strength of these masonry units after exposure to elevated temperatures. Additionally, conventional cement-sand mortar (1:6) was also tested both in their ambient state and after exposure to elevated temperatures. The study involved an experimental approach where four types of masonry units named clay bricks, concrete blocks, compressed stabilized earth blocks (CSEB), and lightweight foam blocks were subjected to controlled elevated temperatures up to 1200°C using a muffle furnace. Physical and mechanical properties, including density and compressive strength were measured both prior and post stages of exposure to these temperatures. Variations in these properties were then analyzed to assess the residual performance of each masonry unit. Visual observations and Scanning Electron Microscopy (SEM) were examined to document surface alterations and microstructural changes after exposure to elevated temperatures. Further, characteristic compressive strength values of the masonry assembly were also calculated using an empirical equation in the ambient and residual states. Finally, a comparative analysis between ambient and elevated temperature conditions was conducted to assess the impact of elevated temperatures on the masonry units and mortar. The results indicate that the compressive strength values of masonry units and mortar decrease after exposure to temperatures up to 1200°C. The reduction factor in compressive strength of each unit after full heating process were observed as 0.57, 1, 0.68, 0.88 for clay brick, CSEB, lightweight foam block and CMU respectively. Clay bricks exhibited better resistance than other types, retaining most of their initial strength after exposure to elevated temperatures. Compressed stabilized earth block was observed to fail into a brittle failure after exposure to 1200°C. Dry density of all four types decreased significantly after exposure to elevated temperature conditions. Additionally, residual compressive strength of generally used mortar (1:6) exhibited a clear reduction after exposure to elevated temperature conditions. The study's primary contributions include the investigation of the residual state behaviour of masonry structures after exposure to elevated temperatures which simulates the close behavior of a masonry structure at post-fire condition. This understanding aids in selecting appropriate masonry materials in a fire-prone area in future masonry construction.