Sampling Location and an Invalid Slump Test
A 9 yd³ truck mixer arrives at 2:10 p.m., 45 minutes after batching. The specified concrete is 3 to 5 in. slump, 5 to 7% air, and 4,000 psi at 28 days. The ambient temperature is 88°F. The contractor asks the technician to sample immediately because another truck is waiting. The first 0.5 yd³ discharged looks watery. The technician collects one 0.5 ft³ portion from that first discharge, then performs a slump test using three equal layers, 25 rod strokes per layer, and strike-off. The cone is lifted in about 1 second. The slump reads 4¾ in., but the concrete shears to one side. Six 6 by 12 in. cylinders are still required.
1. Identify the sampling errors and describe how the sample should have been obtained for all required tests.
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Sampling errors:
- The sample came from the first discharge, which is not representative; the first and last portions of a truck load are typically wetter or richer.
- A single portion is not a composite sample.
- Only 0.5 ft³ was taken, but six 6 by 12 in. cylinders plus slump, air, and temperature require at least 1.0 ft³ (28 L).
Correct procedure (ASTM C172): After all water and admixtures are added and mixing is complete, discharge a small amount and then collect at least three increments from the middle portion of the load, spaced across the discharge. Combine them into one composite sample of at least 1.0 ft³. Protect it from contamination and evaporation, and start the required tests within 15 minutes. Because strength specimens are needed, do not use the smaller sample size that might be permitted for slump and air content alone.
2. Is the slump result valid? Explain the effects of the 1-second cone lift and the sheared specimen, then state the next step.
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The 4¾ in. result should not be reported. ASTM C143 requires the cone to be lifted vertically in 5 ± 2 seconds. A 1-second lift is too fast and can reduce or distort the measured slump. More importantly, a specimen that shears to one side is invalid; the test must be discarded, not averaged or rounded. The entire slump test should also be completed within about 2.5 minutes of starting the fill.
Next step: Since the sample itself was not representative, obtain a new composite sample from the middle of the discharge. Perform a new slump test on a fresh portion, using a 5 ± 2 second lift. If the retest shears again, discard it and repeat with another portion. Do not adjust the truck or report acceptance based on the invalid 4¾ in. value.
3. The contractor wants to add 5 gal of water and remix to make the next sample easier to test. What should the technician consider before allowing this?
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Adding water is not a routine fix. It increases the water-cementitious materials ratio, reduces strength, changes yield, and can affect air content and durability. The technician should first check whether the specification permits jobsite water addition and whether the maximum w/cm is still satisfied. If water is allowed, it must be measured and recorded, and the load must be remixed thoroughly before a new representative sample is taken. The original sample and any tests on it remain invalid.
If the added water would exceed the maximum w/cm, or if the specification prohibits jobsite additions, the load should not be watered. The technician should also consider that adding water does not correct the original sampling error; a new sample from the middle of the properly remixed load is still required.