Masonry Prism Construction, Curing, and Compressive Strength Testing
Prisms: build, cure, cap, test
ASTM C1314 covers masonry prism compressive strength. A prism is a small assemblage of masonry units and mortar constructed to represent the field wall. For hollow concrete masonry, a common prism is two units high with a full bed joint; for clay brick, stack-bonded prisms may contain several units. The prism must have full, uniform joints and be built by a qualified technician. After construction, prisms are cured under controlled laboratory conditions-typically moist curing followed by a period in laboratory air-and then capped with sulfur mortar or gypsum capping to provide flat, parallel bearing surfaces.
Testing requires measuring the prism dimensions, computing the net area for hollow units, applying the load uniformly, and recording the failure load and mode. C1314 includes correction factors for prism height-to-thickness ratio, so a short prism's result is adjusted before comparison with the specified compressive strength, f'm. The specified strength is a design value; prism tests verify it.
Trap: using gross area for a hollow prism, ignoring the h/t correction, or accepting a prism with a thin or missing bed joint. Another trap is testing a prism before it reaches the required age or after improper curing.
Worked example: a two-unit-high 8-in. CMU prism has a net area of 60 in², an h/t ratio of 2.0, and fails at 90,000 lb. Gross stress = 1500 psi; with the appropriate correction factor and net area, the reported prism strength may differ. Always apply the standard's net-area and correction rules, not a simple load divided by outside dimensions.