Slump Flow vs. T50 Conflict on a Bridge Deck Pour
You are the field technician on a bridge deck placement using SCC from a local ready-mix supplier. The mix design specifies a slump flow target of 24 to 26 in. and a T50 of 2 to 4 seconds. On the first truck, you measure a slump flow of 25.5 in. (average of two perpendicular diameters) and a T50 of 6.5 seconds. The concrete visually looks cohesive with no visible bleed water or segregation. The contractor is pushing to start discharge because the deck is scheduled to be finished before an afternoon rain forecast. The pump operator notes the concrete 'feels thick' in the hopper. You have two more trucks staged behind the first.
1. How should you interpret the slump flow and T50 results together, and what does the combination tell you about the mix?
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Slump flow is within the 24-26 in. target, so the mix has adequate filling ability by that single metric. However, T50 of 6.5 s exceeds the 2-4 s band, indicating high viscosity / low flow rate. A high slump flow with a long T50 is a classic signature of a mix that is over-stabilized (excess VMA or high fines) rather than a true high-flow SCC. The 'thick in the hopper' observation corroborates elevated viscosity. This combination signals potential problems with passing ability and with the rate of rise in the forms, even though the spread diameter alone looks acceptable. The two tests must be evaluated as a pair; neither alone governs acceptance.
2. The contractor wants to discharge. What actions are defensible before you allow placement, and what is your acceptance recommendation for this truck?
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Do not release the truck on slump flow alone. Recommended actions: (1) run the passing-ability test appropriate to the element (e.g., J-Ring or L-box) to confirm the elevated viscosity does not create blocking at the congested deck reinforcement; (2) check the visual stability index (VSI) and confirm no segregation; (3) contact the supplier to determine whether a dosage or batching error caused the high T50; (4) evaluate whether the remaining staged trucks show the same pattern. Acceptance recommendation: reject or hold this truck pending passing-ability results, because T50 outside specification is a nonconformance independent of slump flow. If J-Ring/L-box results are also marginal, reject. Document the readings, the time, truck number, and the decision. Do not let the rain deadline override a specification nonconformance.