Five Cubes, One Broken Corner
A technician molds 2-in. mortar cubes for a 28-day acceptance set on a project 40 minutes from the laboratory. Because of the distance he molds five cubes instead of three, strips them the next morning, and transports them in a cooler whose thermometer reads 96 degrees F, the ice having run out the previous afternoon. In transit one cube loses a corner and a 3/8 in. edge spall. The remaining cubes show a small void on the face that will bear load. The laboratory asks whether the damaged cube should be tested or discarded and whether the hot storage invalidates the set. The technician must decide which specimens are usable and what to report if fewer than three valid cubes survive.
1. Which cubes are usable, and how should the damaged and voided specimens be handled?
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Judge each specimen by whether the defect lies in a load-bearing or measuring surface. A 3/8 in. edge spall on a 2-in. cube removes bearing area, so the recorded strength would be artificially low and failure would be eccentric: discard it, and record why. Small voids are more subtle. If the void leaves the bearing surface incomplete so the cap cannot seat fully, the specimen should not be tested as a valid result; if the void is minor and the cap fully seats, the cube can be tested with a note in the record.
Inspect every cube against the flatness and parallelism requirement before testing. Never patch a spalled corner with mortar or grind it to shape; that changes the specimen and makes the result meaningless.
2. Does the 96 degrees F storage invalidate the set, and how should the technician handle it?
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Yes, it materially undermines the set, and it cannot be corrected after the fact. Mortar strength develops as a function of time, temperature, and moisture. Storage at 96 degrees F in a cooler with no ice means hot, probably drying conditions, which accelerate early hydration and can produce a 28-day result higher or more variable than a standard moist cure, while also risking plastic shrinkage cracking.
There is no valid temperature correction for field mortar cubes, so the technician must not adjust results to compensate. He should report the storage history, the temperature record, and the duration as a deviation, state that the results may not be comparable to standard-cured specimens, and let the specifier decide what weight to give them. If acceptance depends on the number, the correct remedy is a new set from a fresh sample under controlled initial curing.
3. If only two valid cubes remain, what is the proper result and report?
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Report what exists and label it honestly. If the applicable method permits discarding a manifestly defective specimen, the average of the two remaining cubes may be reported, with the discard and the reason stated; many laboratories handle it this way. But if only one usable cube remains, there is no set: do not report a single cube as a test result, and do not pair it with a damaged cube to fabricate a set.
Instead, the report should state the molding date, batch and sampling point, storage conditions, number molded, and the disposition of each specimen; declare that no valid strength result exists; and recommend resampling a new batch under controlled conditions. A report that conceals a lost or damaged specimen is worse than no result at all, because it removes the reader's ability to judge the data.