Grout Specimens from a Sun-Baked Pallet
A contractor is grouting 8-in. CMU walls in 8-ft lifts with a 2,000 psi coarse grout pumped from a mixer about 90 ft away. The technician must cast grout specimens. He builds three molds from units pulled off the top of a pallet that has sat uncovered in 95 degree F sun for two weeks. Grout is sampled from the pump hopper after the line has stood 25 minutes without pumping; it looks stiff, so the operator adds a bucket of water at the hopper. The technician consolidates each mold with a small pencil vibrator run about 30 seconds per lift, and bleed water rises to the top of each mold. The foreman wants the specimens cast now because the lift will be finished by noon.
1. Where and when should the grout be sampled, and what does the water addition at the hopper do to the test?
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Specimens must represent the grout that enters the wall. Sampling at the hopper is defensible only if the material there is what is being placed; after 25 minutes of standing in the line plus a bucket of water, it is not. Water added at the pump raises the water-cementitious ratio of what goes into the wall, so a specimen cast after the addition understates the intended design grout, while one cast before it overstates what was actually placed.
The technician should hold the sample until the line is recirculated or the grout remixed, then sample at the point of discharge into the wall or at the hopper immediately after remixing. He should record the time, the standing period, and the water addition, and cast specimens from material that matches the wall. If the contractor refuses to allow resampling, cast from what is available and document the dispute.
2. How does the condition of the mold units affect the measured strength?
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Grout is a fluid mix and will give up water to dry masonry. Units that have baked in the sun for two weeks will absorb water from the grout, stiffening it, lowering the effective water-cementitious ratio, and biasing the measured strength upward. That is a nonconservative error when the purpose is to show the grout in the wall is adequate, and it also makes consolidation harder and increases the chance of honeycomb in the specimen.
Molds should be built from units of the same type and in a moisture condition consistent with the standard and with the wall, and the technician should record the unit source and condition. He should also verify that each mold is tight, aligned, and supported, because grout leaking through joints carries paste away and lowers strength independently of the mix itself.
3. Is 30 seconds of pencil-vibrator consolidation per lift acceptable, and what should the technician do about the bleed water?
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Thirty seconds with a pencil vibrator in a small grout mold is over-consolidation. It drives bleed water and fines to the surface, promotes segregation, and can leave a specimen weaker and more variable than the grout in the wall, so the test would misrepresent the material in either direction. Consolidation should follow the specified procedure, typically rodding in layers with a specified number of blows, or limited vibration where permitted.
Bleed water should not be worked back into the specimen or used to top off the mold; the surface is struck off as the method requires. The technician should finish casting with the correct procedure, record that the first attempt was over-vibrated, and, if the specimens show segregation when stripped, document them and cast replacements at the next grout placement rather than testing specimens known to be unrepresentative.