Uniaxial compressive tests on ancient brick masonry from heritage buildings under unsaturated freeze-thaw conditions

Abstract

Freeze-thaw action, as an atmospheric impact, affects masonry by altering its original microstructures, which causes severe damage by decreasing the compressive strength of the masonry. This process, studied mostly under saturated conditions, in reality usually occurs under unsaturated conditions, which are characterized by more complex modes and have not been studied in depth before. To simulate and investigate processes and basic trends of these modes and examine the post-freeze-thaw compressive behaviors, we place our specimens (including ancient bricks and the mortar created and rebuilt masonry specimens) in an environmental chamber before performing compressive tests. We classify all specimens into eight scenarios, varying the numbers of freeze-thaw cycles and corresponding degrees of saturation. We discuss the impacts of these two factors on the compressive behaviors of the test specimens. Our results suggest that freeze-thaw cycles and degrees of saturation have combined effects on both ancient bricks and the mortar created. Under conditions of low saturation degree, freeze-thaw, as the primary factor, improves the strengths of both materials, although to a limited extent, while under conditions of high saturation degree, the degree of saturation becomes the predominant factor that results in the cracking and even failure of the materials, powered by freeze-thaw processes. Using these two factors (i.e., freeze-thaw action and degree of saturation), we summarize three different stages (or classified groups) in terms of stress-strain curves and crack development in masonry specimens based on the slopes of stress-strain curves and the rates of crack development. After studying all test results, we recommend an optimal saturation degree (approximately 50%) that is considered beneficial for ancient masonry in terms of freeze-thaw resistance.