Prisms Cured Under a Tarp
Three 3-block-high prisms are built with 8-in. units from the wall pallet and mortar from the field batch to represent a wall with a 1,500 psi prism requirement. The mason builds each prism with 5/8 in. bed joints, though 3/8 in. is specified, because it is easier, and stacks them on the ground beside the wall. The wall is fog-cured under plastic for seven days. The prisms sit beside it under a dark tarp that is only occasionally wetted, and surface temperatures under the tarp reach 110 degrees F. The technician caps them with gypsum four hours before the 28-day test, and one cap is visibly out of plane. Measured heights are 24.0, 24.5, and 25.5 in.; units are 7.625 in. thick.
1. Do these prisms validly represent the wall? What should the technician conclude and report?
Read the worked answer
Prisms are only meaningful if they reproduce the wall's materials, construction, and curing. Two deviations stand out. Joint thickness: 5/8 in. bed joints instead of 3/8 in. increase the mortar fraction and change how stress distributes between unit and mortar, so the prism is not the same construction as the wall. Curing: the wall is fog-cured under plastic while the prisms sit under a tarp at 110 degrees F with occasional wetting, which is a hotter, drier environment that can accelerate early gain and depress ultimate strength.
The technician should report both deviations and characterize the results as, at best, a lower bound. If prism strength governs acceptance, the proper action is to build a new set with the specified joint thickness and cure it alongside the wall, not to argue from a nonrepresentative set.
2. What must be resolved about capping and height-to-thickness ratio before the prisms are tested?
Read the worked answer
Both affect the reported number. Gypsum applied four hours before testing may not be fully set and hard, and a visibly out-of-plane cap loads the prism eccentrically, giving a low and misleading result. The affected prism should be re-capped or ground within the flatness tolerance and tested only after the cap has hardened.
For the ratio: heights of 24.0 to 25.5 in. on 7.625-in. units give a height-to-thickness ratio of roughly 3.1 to 3.3, so each prism needs its own correction factor from the standard's table applied to the computed stress. The technician should report corrected values prism by prism, not a single uncorrected average, and should state the cap material, cap age, and the measured dimensions used in the calculation.
3. What belongs in the prism test report so a reviewer can judge the results?
Read the worked answer
The report should let a reviewer reconstruct the test without calling anyone. Include prism identification and location in the wall; unit type and source; mortar type and batch; joint thickness as built versus specified; construction date; curing and storage environment with temperature records; dimensions measured, including bearing area; height-to-thickness ratio and the correction factor applied; cap material and age; test age and date; maximum load; computed stress before and after correction; the average; and every deviation, including the 5/8 in. joints, the 110 degrees F tarp, and the out-of-plane cap with its remedy.
Do not delete a low prism result; report it and explain it. If a prism is invalidated by a defect, say so and state that the set is incomplete rather than substituting a value that makes the average look acceptable.