Impact of Unit Web Height on the Structural Performance of Concrete Masonry Prisms

Abstract

Webs are responsible for the transfer of shear stress between the face shells of concrete masonry units (CMUs) and provide structural stability. Any changes to their geometry (i.e., their height and/or thickness) might therefore impact the structural performance of masonry members. While CAN/CSA S304-14 stipulates minimum web thickness requirements for each CMU size, there are no provisions for web height other than full-height web and so that would limit the range of CMU configurations. Nonetheless, Canadian block suppliers typically limit knock-out webs to heights of about 120 mm. In contrast, U.S. code ASTM C90-11 stipulates the web configurations of CMUs by minimum normalized web area requirements which allow for varying web heights including, but not limited to, web heights that are shorter than those typically used for Canadian knock-out web units. Shorter web heights reduce structural self-weight, and potentially minimize worker fatigue and injuries, and so would increase construction productivity. An extensive literature review did not, however, reveal the existence of published studies related to the impact of web height of CMUs on the structural performance of masonry assemblages.

An experimental investigation of three-course tall by one block wide masonry prisms was therefore conducted to evaluate the impact of CMU web height on the masonry assemblage strength and failure modes of prisms. Prisms constructed in running bond with face-shell mortar bedding were constructed and the following parameters were investigated: CMU size, web height, and the use of grout. A total of 18 unique prism configurations with 7 replicates of each resulted in a total of 126 prisms. The actuators available in the laboratory were used for compression testing; in addition, a digital image correlation (DIC) system was used to capture the deformation properties. The analysis of compression test results revealed that CMU web height was an influential parameter on the masonry assemblage strength of hollow prisms but not of grouted prisms, irrespective of CMU size. Failure of both hollow and grouted prisms, obtained from the DIC system results, was primarily governed by the tensile splitting of the webs, but the crack patterns varied with web height. Finite element modeling (FEM) of prisms within selected test series was conducted to predict the potential crack locations within the webs and results were in a good agreement with those obtained from the DIC system. It was found that CMUs with web heights shorter than typically used in Canadian masonry construction could be used without significantly compromising the structural performance of masonry members.