Hot Weather Concreting per ACI 305
Why this matters
Concrete placed in hot weather is a different material than concrete placed in moderate weather. The same mix design that gives a flat 6-bag slab in 70 degree weather can give a network of plastic shrinkage cracks before it sets in 95 degree weather with low humidity and a 15 mph wind. The cracks are not aesthetic, they create paths for water infiltration, freeze-thaw damage, and reduced load capacity. ACI 305 (Hot Weather Concreting) is the consensus standard for the practices that prevent these failures: when to schedule, how to cool the mix, what admixtures to use, and how aggressively to wet-cure. A contractor working in any climate with summer highs above 85 degrees is doing hot weather concreting whether they call it that or not.
When ACI 305 applies
ACI 305.1 defines hot weather as any combination of air temperature, concrete temperature, relative humidity, and wind velocity that tends to impair the quality of fresh or hardened concrete or otherwise result in abnormal properties. The standard's actionable threshold is when the rate of evaporation from the concrete surface exceeds approximately 0.2 lb per square foot per hour (1.0 kg/m2/h). Above that rate, plastic shrinkage cracking is highly likely and intervention is required.
The evaporation rate depends on:
Air temperature (higher = more evaporation).
Concrete temperature (higher = more evaporation from the surface AND faster hydration heat).
Relative humidity (lower = more evaporation).
Wind velocity (higher = more evaporation).
ACI Nomograph (in ACI 305R) plots these four variables and gives the predicted evaporation rate. Carry a copy on the truck. A 90 degree air temperature, 80 degree concrete, 50 percent humidity, 10 mph wind reads as approximately 0.2 lb/ft2/hr on the nomograph and is the threshold.
A weather station app (or a basic anemometer + thermometer + hygrometer at the pour site) gives you the numbers in real time. The nomograph reading tells you whether you have a hot-weather pour on your hands today.
Concrete temperature at placement
The primary hot-weather risk is concrete temperature at placement, not air temperature. ACI 305 recommends concrete placement temperature not exceed 95 degrees F (35 degrees C) for general construction, with lower limits for mass concrete and architectural finishes.
Concrete temperature at placement is influenced by:
Aggregate temperature (the largest mass in the mix; sun-heated aggregate stockpiles can be 110 to 130 degrees F).
Cement temperature (cement straight from the silo in hot weather can exceed 150 degrees F).
Mix water temperature (water from above-ground storage tanks heats quickly).
Time in transit (mixer drum heats up; hydration starts and generates heat).
Cooling strategies, in order of cost-effectiveness:
Cold mix water. Chilled water (50 degrees F) replaces ambient water (80 to 100 degrees F). Each 10 degree drop in mix water temperature drops concrete temperature roughly 1 degree.
Ice substitution for mix water. Ice absorbs heat as it melts (latent heat of fusion is 144 BTU/lb, vs water's 1 BTU/lb-deg). Each 10 percent of mix water replaced with ice drops concrete temperature roughly 4 degrees.
Wet aggregate stockpiles. Sprinkler systems on coarse aggregate piles cool the aggregate by evaporation. Concrete temperature drops roughly 1 degree per 4 degree drop in aggregate temperature.
Liquid nitrogen injection into the mixer. Industrial-scale; not typical for residential work but standard for hot-weather mass concrete (bridge decks, large slabs).
Coordinate cooling strategy with the ready-mix supplier when booking the pour. A "hot weather mix" specification commits the supplier to deliver chilled product.
Plastic shrinkage cracking
Plastic shrinkage cracks form in fresh concrete before final set, while the surface is losing water faster than bleed water can replace it. The surface dries, shrinks, and cracks while the concrete below is still plastic.
Characteristic appearance: random shallow cracks, often parallel or in chicken-wire patterns, typically 1 to 4 inches deep, opening 1/16 to 1/8 inch. Develop within 30 minutes to 4 hours after placement.
These cracks are not load-related; they are evaporation-related. Once formed they cannot be tooled out; they are permanent.
Prevention:
Reduce evaporation rate below 0.2 lb/ft2/hr at placement. The four variables on the nomograph all push in the same direction; addressing two or three drops the rate enough.
Apply evaporation retarder (Sika Antisol, Euclid Eucobar, BASF Confilm) immediately after strike-off. The retarder is a film-forming chemical that floats on the concrete surface and reduces evaporation by roughly 80 percent for 1 to 4 hours. Re-apply if surface dries before final set. Evaporation retarder is NOT a curing compound; it is a placement aid only.
Wind breaks (snow fence, temporary plywood walls, parked vehicles upwind). A 6 foot wind break reduces wind velocity at the surface by 50 to 70 percent for a distance of roughly 8 to 10 times the height downwind.
Fog spray. Hand-held or boom-mounted spray nozzles producing fine mist over the surface raise local humidity and reduce evaporation. Fog spray DOES NOT add water to the concrete itself; the mist evaporates above the surface.
Shade. Tarps or shade cloth over the pour reduce solar gain and surface temperature.
Adding water to the concrete on site to "fix" stiffening from hot weather destroys the mix's water-cement ratio and reduces ultimate strength. A typical 4000 psi mix re-tempered with an extra 5 gallons per yard can lose 1000+ psi of strength and develop interconnected porosity that fails ASTM C1543 chloride penetration testing. If the concrete is stiffening too fast to finish, address the placement temperature, not the slump on site. NEVER add water at the pour without the ready-mix supplier's written approval and a documented adjustment to the mix water already in the load.
Set time and finishing window
Hot weather shortens the finishing window. A mix that gives 4 hours of finish time at 70 degrees may give 90 minutes at 95 degrees. The crew must scale up to finish faster, or use a retarding admixture to extend the window.
References
- ACI 305.1 Specification for Hot Weather Concreting current edition
- ACI 305R Guide to Hot Weather Concreting current edition (includes the evaporation rate nomograph)
- ACI 308R Guide to External Curing of Concrete current edition
- ASTM C309 Standard Specification for Liquid Membrane-Forming Compounds for Curing Concrete
- ASTM C494 Standard Specification for Chemical Admixtures for Concrete (Types A through G)
- ASTM C1064 Standard Test Method for Temperature of Freshly Mixed Hydraulic-Cement Concrete
- Portland Cement Association EB001 Design and Control of Concrete Mixtures current edition