Reevaluation of fm′ for the Unit-Strength Method with Application to Lightweight Concrete Block Masonry and Face Shell–Bedded Mortar Joints

Abstract

The compressive strength of masonry is an important consideration in the design of masonry structures. Compressive strength is principally determined in one of two ways specified in The Masonry Society (TMS) 602-16 “Specification for Masonry Structures.” These are respectively the prism test method (PTM), in which masonry prisms are constructed and subject to compression tests as a prediction of the strength of masonry structures, and the unit-strength method (USM), in which concrete masonry unit strength is correlated to mortar strength. Depending on the requirements of the project one method may be preferred over the other. The PTM typically reports greater counmpressive strengths but takes more time and cost for testing compared to the USM, which requires less time and cost but also estimates a lower masonry strength, and therefore recommends a less conservative design. The USM for concrete masonry is based on an existing strength table that correlates the strength of concrete blocks or concrete masonry units (CMUs) and mortar type to masonry strength. However, the USM as currently practiced does not specify block weight or bed joint type (i.e., full bedding or face shell bedding). The present research seeks to determine the best way to apply the USM to lightweight concrete block masonry and to face shell–bedded mortar joints. To investigate the effects of weight and bedding type a total of 48 prisms were constructed; 24 normal-weight and 24 lightweight. For each weight category 12 were fully bedded and 12 were face shell–bedded. All prisms were compared to the USM estimates that are based on TMS 602-16. Results indicate that the USM is not applicable and should not be used for lightweight block or face shell–bedded masonry because it is not sufficiently conservative. Additionally, fully and face shell–bedded prisms were compared and strength ratios were developed.