Disputed Slump Flow Measurement at the Job Site
Two technicians measure slump flow on the same SCC sample. Technician A reports 23 in.; Technician B reports 25.5 in. The specification target is 24 to 26 in., so the discrepancy straddles the lower limit. The base plate is slightly out of level (about 1/8 in. across the plate), the slump cone was filled in one lift by Technician A, and Technician B measured the spread along only one axis. The concrete temperature is 88 F and the elapsed time since batching is 45 minutes. The contractor wants the higher number used for acceptance.
1. Identify the procedural errors and explain how each could bias the result.
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Errors: (1) Filling the cone in one lift rather than the specified lifts can trap air and produce an uneven, non-representative spread; proper filling in layers with rodding (per the test method) is required. (2) Measuring along only one axis ignores the requirement to average two perpendicular diameters; a single axis can over- or under-state the spread, especially if the flow is asymmetric. (3) An out-of-level base plate biases flow toward the low side, inflating one diameter and distorting the average. (4) The 45-minute elapsed time and 88 F temperature mean the concrete may have lost workability, so the two readings may also reflect different points on the slump-loss curve rather than pure measurement error. Each factor can shift the result by an inch or more, which matters at a 24-in. limit.
2. How should you resolve the discrepancy and make an acceptance decision?
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Discard both disputed readings as nonconforming measurements. Re-level the base plate, obtain a fresh representative sample, and have a single qualified technician perform the test per the standard: fill in the specified layers, strike off, lift the cone vertically, and measure two perpendicular diameters and average them. Record temperature and elapsed time. Accept or reject based on the corrected, properly performed test against the 24-26 in. target, not on which number favors the contractor. If the corrected result is below 24 in., the truck is a nonconformance regardless of the earlier higher reading. Document the original readings, the procedural deficiencies, and the corrective retest. The contractor's preference for the higher number is not a basis for acceptance.