Prism Test Fails While Unit Strength Method Passes
An 8-inch hollow CMU bearing wall is specified with f'm = 1,500 psi, Type S mortar, and 2,000-psi grout in cells at 32 inches on center. The contractor proposes the unit strength method: the CMU units have a net-area compressive strength of 2,150 psi, and the mortar and grout mix designs meet the specified requirements. The structural engineer had specified prism testing, and three prisms tested at 28 days average 1,320 psi. The contractor wants to disregard the prisms because the units exceed the unit-strength table values and because the prisms were stored outdoors at the site and capped with gypsum cement. The special inspector notes that the prisms were built ungrouted even though the wall has grouted cells. The masonry is partially grouted and will carry floor loads.
1. Can the contractor switch to the unit strength method after the specified prism tests fail? Explain.
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Not unilaterally. The code may permit either prism testing or the unit strength method as a basis for f'm, but the contract documents govern which method is required. If the engineer specified prism testing, a failed prism result does not give the contractor the right to switch methods after the fact. The contractor can submit a formal substitution request to the engineer, supported by the unit strength calculations and mix designs, but the engineer must approve it. The unit strength method is also based on unit strength and mortar type; it is not a direct substitute for a specified prism test, especially where grout is part of the wall. The contractor's argument that the units exceed the table values may be relevant to an approval request, but it does not override the specified test method.
2. What is wrong with using the 1,320-psi prism result as a direct measure of the wall's capacity?
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The prisms were ungrouted, while the wall is partially grouted. An ungrouted prism is not representative of the grouted assemblage, and its strength may be lower or higher depending on how the grout contributes. The outdoor storage and gypsum capping also need to be checked against ASTM C1314 requirements; curing and capping can affect results. The 1,320-psi value therefore should not be treated as a precise measure of the wall's capacity. That does not mean the failure can be ignored. It means the first step is to build and test prisms that represent the actual wall, including the grouted cells, and to verify curing and capping. The engineer should then evaluate the results.
3. What course of action protects both safety and the contractor's cost?
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The contractor should stop work in the affected area and submit a plan to the engineer. The plan should include representative prism tests built with the same CMU, mortar, and grout as the wall, cured and capped per ASTM C1314. If those prisms meet f'm = 1,500 psi, the engineer can accept the wall based on the new data. If they fail, the engineer should evaluate the wall's capacity and require repair or redesign. The unit strength method can be submitted as a supplemental check, but only with the engineer's approval. This path avoids unnecessary demolition if the original prisms were unrepresentative, while protecting structural safety if the wall is actually deficient. Documentation of the mix designs, sampling, and testing is essential.