Hot, Windy Patio Pour: Evaporation Rate Exceeds the Threshold
You are the lead finisher on a 20 ft x 30 ft exterior patio slab, 5 in thick, placed over a prepared base with a 6 mil vapor retarder and #4 bar at 18 in on center. The day is 94°F, 25% relative humidity, with an 18 mph wind gusting to 25. Concrete arrives at 88°F with a retarder and a mid-range water reducer; slump is 5 in. On site you have curing compound and two rolls of 6 mil plastic - no evaporation retarder, no fog sprayer, no wind screen. The plant is 25 minutes away and the second truck is already 40 minutes behind. Working the Menzer relationship, the evaporation rate comes out at roughly 1.1 kg/m²/h (about 0.23 lb/ft²/h). The general contractor wants the slab struck off, floated, and steel troweled to a hard, slick surface before lunch, and a laborer is standing by with a hose to "keep the surface workable."
1. What does the calculated evaporation rate tell you about this pour, and what must be on site before the first truck backs in?
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The 1.0 kg/m²/h (about 0.2 lb/ft²/h) line is the threshold at which ACI 305 says precautions are needed. At 1.1 you are over it, meaning the surface loses bleed water faster than it can rise - plastic shrinkage cracking can start within the first hour, and 18 mph wind with 25% RH is exactly the condition that produces it. Concrete at 88°F makes it worse: warm concrete sets faster and bleeds less, so the window to finish is short.
Before the first truck: (1) evaporation retarder and a fine-mist fog sprayer - a hose that wets the surface is worse than nothing; (2) a wind screen or shade on the windward side; (3) plastic to cover the slab between trucks; (4) tell the plant to cool the mix with ice or chilled water and reject anything over 90°F; (5) two extra finishers so every placement is struck off and bull floated within minutes; (6) curing compound and burlap or plastic staged before placement; (7) no water on the surface, and no extra retarder dumped in at the site. If none of that can be assembled, the pour should move to evening.
2. Two hours in, the upwind edge has crusted while the middle is still plastic. What is your sequence, and what do you do with the crusted edge?
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Treat the two zones separately rather than trying to float the whole slab on one schedule.
Middle (still plastic): leave it alone and cover it with plastic or spray evaporation retarder to hold moisture until the crew can get to it.
Crusted edge: a crust is a set skin. Troweling it tears the surface and pulls flakes, and adding water creates a weak, dusty layer that will fail later. Spray a fine fog mist - mist only, no puddling - to slow further drying, then finish from the downwind side toward the windward edge so nobody is standing in the dry zone working it. Use a magnesium float and a light touch, keeping the leading edge moist.
If the float no longer leaves a smooth surface, stop. Saw or cut the edge back to sound concrete in a straight line and plan a full-depth replacement strip with a bonded, doweled joint per the engineer. That is a structural repair, not a cosmetic patch. For the next placement, work the windward edge first and keep it covered - this slab's problem was finishing order, not mix design.
3. The GC insists on a hard steel-troweled finish before lunch. How do you respond, and what is the correct criterion for starting the trowel?
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Refuse two things: troweling over bleed water, and steel troweling an exterior slab.
The start criterion is not the clock. Start when the bleed sheen has disappeared and a footprint sinks no more than about 1/8 in. With a retarder on a 94°F windy day that may be quick, but the sheen has to go first - troweling water back into the surface produces dusting, low wear resistance, and blisters.
Sequence: strike off, bull float immediately to push aggregate down and seal the surface, spray evaporation retarder or cover, wait for bleed, magnesium float, edge and cut joints, then only a light steel pass if the blade is not drawing water. For an exterior patio, a light broom or swirl is the correct finish for slip resistance; a hard-troweled slab is slick when wet and the GC's lunch schedule is not a reason to trade away durability.
Cure immediately when finishing ends - curing compound at the specified coverage or 7 days wet - because the same sun and wind that dried the surface will dry the curing compound off it.