Hot Weather Pour Standard

Purpose

Hot weather does not damage concrete through the air temperature on the truck radio. It works through two measurable quantities: the temperature of the concrete, and the rate water leaves the surface faster than bleed water replaces it. Miss the first and the mix stiffens early, loses slump, gets retempered with a hose and ends up weaker and more permeable than the one you sold. Miss the second and plastic shrinkage cracks open before the slab sets, which is not cosmetic: they reach through the cover and become the freeze-thaw and corrosion path for its whole life. Both are prevented by numbers taken on site, and the crew is at risk from the same weather, so their heat plan belongs here too.

Scope

Covers residential and light commercial flatwork, footings and walls in hot, dry or windy conditions, from the go/no-go call the day before through the start of a continuous cure.

It does not cover mix design, which belongs to the supplier and engineer beyond the cooling and admixture requests here; the pre-pour subgrade and steel checks, owned by their own SOPs; or diagnosis of surface defects after the fact.

Roles and responsibilities

Procedure

1. Set the placement window the day before. Pull the hourly forecast for air temperature, relative humidity and wind and pick the window with the lowest combination, which in summer is early morning and sometimes a night pour. Acceptance: a start time on the work order with the forecast values that justified it, plus a plan B. Wrong looks like a 10:00 start booked for crew convenience into a rising-temperature, falling-humidity afternoon; stop rule, if no window clears the step 5 threshold with the protection available, move the pour. Hazard: a night pour needs lighting set before dark at every working face and a spotter for every truck backing in.

2. Stage the kit and the crew's heat plan before the truck. Put foggers, evaporation retardant, wind screens, curing materials at the specified coverage rate and a shaded rest area on site before placement, with drinking water at the work. Acceptance: all present and counted, water at roughly one quart per worker per hour, shade within a short walk, and a stated rest cycle. Wrong looks like a curing compound sprayer nobody unclogged since the last job; stop rule, no placement until the kit passes. Hazard: heat illness is the real hazard here, and with no specific federal OSHA heat standard OSHA enforces it under the General Duty Clause at section 5(a)(1) of the OSH Act, while several states, California and Washington among them, bind employers there to their own outdoor heat rules; either way new or returning workers are acclimatized over several shifts rather than put straight onto a full day in the sun.

3. Order the mix to a temperature, and check the arithmetic. Ask the plant for chilled mixing water, ice substituted for part of the water, shaded or sprinkled stockpiles, and a retarder where the haul is long. Acceptance: the plant states an as-mixed temperature clearing the ACI 305 ceiling of 95 F for general construction, and it holds up against the mixture temperature relationship, in which temperature is the mass-weighted average of the components with solids weighted by 0.22, the ratio of the specific heat of solids to that of water, ignoring free moisture on the aggregate. Wrong looks like a plant promising a cooler load with nothing changed but the promise; stop rule, no order until a cooling measure is agreed. Hazard: none, a phone call.

4. Measure temperature at the chute and enforce the discharge clock. Take concrete temperature per ASTM C1064 on the first load and every fourth after, and check batch time against arrival. Acceptance: measured temperature at or below 95 F, and discharge complete within the ASTM C94 limit of 90 minutes from introduction of the mixing water or 300 drum revolutions, whichever comes first, unless the specification approves otherwise. Wrong looks like a load discharged at 110 minutes after a pump line plugged; stop rule, reject a load over the ceiling or past the limit, and never add water on site, because retempering buys slump with strength. Hazard: fresh concrete is strongly alkaline and burns without pain warning, so eye protection and waterproof gloves and boots stay on at the chute, sleeve and boot cuffs are taped, any splash is flushed with clean water at length, and anything holding concrete against skin - a boot, a glove, a kneepad - comes off immediately rather than at the end of the pour, because the burn depth is set by how long it sits.

5. Read the evaporation rate on site, then hourly. Take air temperature, relative humidity, wind speed at about 20 in above the surface and concrete temperature, and read the four against the ACI 305R evaporation nomograph. Acceptance: predicted evaporation below approximately 0.2 lb per square foot per hour, the rate above which ACI 305 treats plastic shrinkage cracking as highly likely. Wrong looks like reading air temperature alone, when wind is usually what pushes a comfortable morning over the line; stop rule, at or above the threshold do not place unless fogging and screens are running with someone assigned to them. Hazard: these readings are taken at the slab edge, so stay clear of the pump boom swing and the discharge arc.

6. Protect the surface between screed and cure. Fog upwind to raise humidity over the surface without landing water on it, run wind screens, and where the specification allows apply an evaporation retardant film per its label. Acceptance: no visible drying or dulling between screed and float, retardant as a film only. Wrong looks like a crew fogging onto the slab and floating that water in, or working the retardant in as a finishing aid, which raises the surface water to cement ratio and leaves a soft skin that dusts by the second summer; stop rule, add screens or shorten the placement front rather than push the crew faster. Hazard: foggers throw mist near powered finishing tools, so cords stay out of the wetted area and every circuit is GFCI protected.

7. Start the cure the moment the surface will take it. Begin as soon as the surface will not be marred, with continuous wet cure, wet burlap under plastic, or a curing compound at the manufacturer's coverage rate in two passes at right angles. Acceptance: cure started inside the specification's window, coverage calculated against slab area and containers emptied, and the cure held without a dry interval for the specified duration, commonly seven days. Wrong looks like a wet cure allowed to dry out over a weekend, which is worse than never wetting it because the cycling drives surface cracking; stop rule, a broken cure is logged and the duration extended. Hazard: curing compounds carry solvents, so read the SDS for the product in use, use the respiratory protection it specifies in any enclosed or partially enclosed area, and keep ignition sources away from solvent-based products.

8. Verify the cure held, then release the slab. Walk the slab at the end of the cure period against what the finish should be, plus any specified test. Acceptance: no plastic shrinkage cracking, no chalking under a thumb, no dull patches where the cure lifted, and a log showing continuous coverage. Wrong looks like a release on the calendar day with no walk, leaving a dry patch that becomes a customer call in two seasons; stop rule, hold the release and photograph the location and cue before anything is applied over it. Hazard: a cured slab in full sun reflects heat and light, so the walk happens with eye protection.

The record this produces

One hot weather placement log per pour. Fields: job and pour identifier, forecast values and chosen window, kit staged and counted, the heat plan with water, rest cycle and buddy-check owner, batch tickets with batch, arrival and discharge-complete times and measured temperature per checked load, the four evaporation inputs and nomograph result each hour, protection running, cure start and method with coverage rate and containers used, any cure break, and the release walk with photographs.

It sits on the job record beside the batch tickets. The PM reads it the day a customer reports fine cracking, and an hourly log either shows the crew inside the threshold or shows exactly when they were not. The estimator reads it across a season, because a shop whose July pours all start at 09:30 is scheduling crew convenience into its own warranty work.

One pass, with a step that failed

A 1,900 sq ft commercial walkway and apron, 5 in, mid-July, window set for 06:00 after step 1 showed the forecast crossing on wind by 09:00. Step 2 staged two foggers, screens, retardant, burlap and plastic, a shade canopy, water at the work and a 15 minute hourly rest for the four-person crew.

Step 4 failed on the first load: 97 F at the chute against the 95 F ceiling. The crew leader rejected it and called the plant, and the ticket values run through the step 3 relationship explain it. Aggregate 3,000 lb at 100 F, cement 564 lb at 145 F, mixing water 282 lb at 72 F give a numerator of 0.22 x (3,000 x 100 + 564 x 145) + 282 x 72, which is 83,992 plus 20,304, or 104,296, over a denominator of 0.22 x 3,564 + 282, which is 1,066. That is 97.8 F, within a degree of the meter, so arithmetic and instrument agree.

The fix came from the same relationship. Chilling the mixing water from 72 to 40 F removes 282 x 32 divided by 1,066, or 8.5 F. Shading and sprinkling the coarse aggregate from 100 to 90 F removes 0.22 x 3,000 x 10 divided by 1,066, or 6.2 F. Together that is 14.7 F, predicting 83 F as mixed. Holding everything else unchanged at these proportions, where water is about 7 percent of batch mass, that is roughly 1 F off the concrete per 3.8 F off the water and per 1.6 F off the aggregate; different proportions shift both ratios, so recompute rather than carry those numbers to another job.

The next three loads measured 86, 87 and 86 F, all clear of the ceiling, the gain over the predicted 83 F explained by a 40 minute haul in full sun. Step 5 read 96 F air, 88 F concrete, 22 percent relative humidity and 11 mph wind, well above the 0.2 lb per square foot per hour threshold, so both foggers and the screens ran from the first yard. Step 7 started the cure at 08:55, 25 minutes after the last float.

Placing that first load rather than rejecting it produces no visible fault that morning. It produces a run of walkway from concrete that stiffened early, was harder to finish and carried whatever water the crew added to move it, with the evidence arriving years later as a surface that scaled while the rest of the walk did not.

When the day gets away from you

A crew member shows confusion, stumbling or has stopped sweating: this is a medical emergency, so move them to shade, cool them actively with water and airflow, do not leave them alone, and call 911 before deciding whether it looks serious enough. Conditions cross mid-pour: keep placing only if fogging and screens are already running and someone owns them, and stop at a planned construction joint rather than wherever the concrete runs out. A customer pushes for an afternoon slot: the answer is the evaporation reading and what it does to the surface, with the early slot that works.

References

• ACI 305R and ACI 305.1, hot weather concreting, for the evaporation nomograph and its 0.2 lb per square foot per hour threshold, the 95 F placement ceiling and the mixture temperature relationship, in the edition your specification names.
• ASTM C94/C94M for the 90 minute or 300 revolution discharge limit, and ASTM C1064 for concrete temperature.
• OSH Act section 5(a)(1), the General Duty Clause, under which OSHA enforces heat hazards absent a federal heat standard, plus your state's outdoor heat rule where adopted.
• NIOSH criteria for occupational exposure to heat and hot environments, for water, rest, shade and acclimatization.
• See related: the cold weather pour standard and the cylinder sampling SOP.