Relationship between the Compressive Strength of Concrete Masonry and the Compressive Strength of Concrete Masonry Units

The results of axial compression tests on masonry prisms can be the basis for determining the design capacity of masonry elements since several building codes permit the compressive strength of the masonry to be that of the prisms. The cost and time required to construct and cure the prisms and the cost and time to prepare and test the prisms, in addition to the waiting time to begin construction, makes the overall prism test method expensive. Building codes, therefore, often provide an alternative to the prism test method. Typically, the alternative method establishes the compressive strength of the masonry from the compressive strength of the masonry unit and the mortar type specified by the engineer. The study presented herein is related specifically to concrete masonry, and it determined a relationship between the compressive strength of ungrouted and grouted masonry and the compressive strength of the masonry units. The relationship was established statistically using the results of a comprehensive testing program. Among the many contributions of this research is that masonry units with compressive strength of up to 70 MPa were tested. The tests results show an increase in the compressive strength of the masonry with increasing compressive strength of the units. The increase, however, was not proportional; in other words, the relationship masonry strength–unit strength decreased with increasing unit strength. The test results reported herein for the ratios of compressive strength of ungrouted masonry to compressive strength of masonry units are consistent with results reported by other researchers. Those compressive strength ratios are approximately 0.8 and 0.5 for low and high strength units, respectively. The ratios of compressive strength of grouted masonry to compressive strength of masonry units are approximately 0.7 and 0.4 for low and high strength units, respectively.