Retempered Mortar on the Board
A technician arrives at a three-story CMU bearing-wall project where Type S mortar with a 1,800 psi 28-day specified strength is in use. At 10:40 a.m. the mortar board holds the remains of a batch mixed at 9:50 a.m. The tender adds about a quart of water and hoes the pile, saying it 'sets up too fast.' The technician measures cone penetration on a sample lifted from the board: 48 mm, against 34 mm measured on the same mix design two hours earlier. The mason asks the technician to take cubes now, before more water goes in, and the foreman insists the 9:50 batch is still within the time limit. The technician must decide whether the mortar is acceptable, what the penetration number actually proves, and how to sample it.
1. What does the cone penetration result establish, and can the technician reject the mortar on that number alone?
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Penetration is a consistency measurement, not a compliance test. The field method uses cone penetration to compare mortar against a reference mix established for the mix design and to track changes in workability; it does not measure water content, cementitious content, or potential strength. A 14 mm shift tells you the mortar got wetter, not that it is out of specification.
So the technician cannot reject on 48 mm alone. What he can do: compare the reading with the reference-mix consistency established for this design; note that water was added on the board after mixing, which moves water content away from the approved proportioning; and perform a water content determination if the specification or the specifier requires one. Acceptance is the specifier's or engineer's decision. The technician's job is to measure, document the deviation, and report it accurately.
2. The technician wants specimens that represent the mortar as placed. Where and when should he sample, and what must the record show?
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The sampling point determines what the cubes represent. Sample at the mixer discharge after the full mixing cycle if the question is whether the batch as proportioned meets the mix design; sample from the board if the question is what actually went into the wall. Here the board mortar has been retempered, so cubes molded now measure a wetted, non-design mix.
The technician should: remix thoroughly before sampling; record the sampling point, time since mixing, penetration at sampling, and the approximate water added; mold at least three 2-in. cubes in nonabsorptive molds, consolidate properly, moist-cure, and test at 28 days. Whether he samples the board or the mixer, the report must state clearly what the cubes represent, or the numbers will be read as a check on the approved mix when they are not.
3. Sixty blocks have already been laid with mortar from that batch. What should the technician recommend and record?
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He should neither declare the wall defective nor let the issue disappear. Adding water to stiffened mortar to restore workability is normal practice within the time limits, but water added beyond the mix design raises the water-cementitious ratio and reduces potential strength; a penetration reading cannot quantify that loss.
Practical steps: record the observation, including time, approximate water added, penetration before and after, and how many units were laid from the batch; advise the foreman that water should be adjusted at the mixer, not on the board, and that batches exceeding the time limit be discarded; notify the specifier or engineer in writing so the acceptability decision rests with the responsible party; and, if hardened-strength evidence is required, propose prisms or cores per the specification rather than relying on mortar cubes. Field testing documents conditions; it does not adjudicate them.