Mid-Slab Set Time Shift on a Warm, Windy Placement
You are finishing a 6,000 ft² interior slab on grade placed at 9:00 a.m. The mix is a 4,000 psi air-entrained mix with a water reducer and a normal-set retarder dosed for a 70°F day. Actual conditions: ambient 88°F, 20 mph wind, 35% RH, and the slab is on a vapor barrier with no shade. By 11:00 a.m. bleed water has disappeared from the leading third of the pour, and the surface there is stiffening while the trailing third placed 40 minutes later is still plastic. The spec requires a medium broom finish and a saw-cut joint schedule starting at 12 hours. The general contractor wants the whole slab broomed before lunch.
1. What is happening to the leading edge, and what finishing risk does the split set time create for the broom finish?
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The leading third is undergoing rapid surface moisture loss from wind plus high ambient temperature, so it is crusting (plastic shrinkage drying) rather than truly setting. The retarder was dosed for 70°F, but the actual concrete temperature and evaporation rate are well above that design basis, so the field set time no longer matches the mix design.
Risk: if you broom the stiff leading edge on the GC's schedule, the broom will tear and leave a ragged, non-uniform texture; if you wait for the trailing third, the leading edge may be too hard to broom at all. The slab can no longer be finished as one continuous operation on one timing. You must treat the leading and trailing zones as separate finishing windows and control evaporation on the leading edge (wind screen, fog mist, evaporation retarder) rather than forcing the schedule.
2. The GC insists on brooming everything before lunch. Give your response and the specific actions you take.
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Push back with the technical basis: the leading edge is crusting, not set, and brooming it now produces a finish that will fail the medium-broom requirement. Propose and execute:
- Apply a monomolecular evaporation retarder or fog mist to the leading edge to hold the surface while the trailing third catches up.
- Erect a windbreak on the upwind side to cut the 20 mph wind, the dominant evaporation driver.
- Finish in placement sequence, brooming each zone as it reaches the correct degree of set (finger pressure leaves a slight imprint, no bleed water sheen).
- Document the ambient conditions, evaporation rate, and the mix's actual behavior, and notify the ready-mix supplier that the retarder dose is undersized for these conditions on the next placement.
Note that the 12-hour saw-cut window may also be affected: crusted surfaces and differential set can cause random cracking before the saw is on the slab, so saw timing should be judged by the leading edge, not the average.