A Re-evaluation of f'm - Unit Strength Method, Face Shell and Fully Bedded Mortar Joints

The compressive strength of masonry is an important consideration in the design of masonry structures and is principally determined using one of two methods specified in The Masonry Society, TMS 602-13 “Specification for Masonry Structures” the Prism Test Method (PTM) and the Unit Strength Method (USM). The USM is inherently conservative and should be used in combination with fully bedded masonry. However, what if face shell bedding were used, what is this compressive strength and would the USM still produce conservative estimates? A means to attain f’m for face shell bedded masonry using the USM would be to utilize a strength ratio between full and face shell bedding types. TMS 602-11 specified a strength ratio, however, this was removed beginning with the 2013 edition. This is problematic because the USM does not specify the use of fully bedded joints, thus, creating a potential interpretation that it is permissible to utilize face shell bedding and that it would achieve the USM f’m compressive strength estimate. This interpretation though would be erroneous and un-conservative. Hence, there is a need to flush out, clarify and substantiate f’m for face shell bedded masonry; further, to validate the USM compression strength estimates of fully bedded masonry. A total of 48 compression tests of various prism configurations for both face shell (24) and fully bedded (24) prisms were conducted; from the results, a new data base was generated to substantiate f’m for face shell masonry as well as recommendations for amending the TMS 602 for the upcoming code cycle.