Instantly determine if fogging is required for your fresh concrete pour based on evaporation rate per ACI 305R — enter air temperature, concrete temperature, relative humidity, and wind speed to get an instant fogging decision.
Enter your job-site conditions to calculate evaporation rate and determine if concrete fogging is required per ACI 305R.
Shade temperature at slab level — not direct sun reading
ACI 305R max: 95°F at point of placement
Use a psychrometer or digital hygrometer on site
Measure at 20 in above slab surface (ACI 305R)
Direct sun adds approx. 10–20°F to concrete surface temp
Concrete fogging is the practice of applying a very fine water mist — not a direct water spray — above the surface of freshly placed concrete to increase the relative humidity directly above the slab and reduce the rate of surface moisture evaporation. Per ACI 305R-10 (Hot Weather Concreting), fogging is required whenever the calculated evaporation rate at the concrete surface exceeds 0.20 lb/ft²/hr. At this threshold, moisture is leaving the concrete surface faster than bleed water can rise to replace it — creating the conditions for plastic shrinkage cracking, which can occur within 30–60 minutes of finishing.
The 0.20 lb/ft²/hr evaporation rate is the critical threshold established by the American Concrete Institute in ACI 305R-10. Below 0.10, no special precautions are typically needed. Between 0.10 and 0.20, contractors should monitor conditions closely and have fogging equipment on standby. Above 0.20, fogging must be initiated immediately — before and during finishing operations. Many US DOT specifications for bridge decks and highway pavement use 0.15 lb/ft²/hr as their threshold, requiring fogging at an even lower rate.
ACI 305R defines hot weather concreting as any combination of conditions that accelerates moisture loss or cement hydration — including air temperature above 90°F, concrete temperature above 95°F at placement, low relative humidity, high wind speed, or direct solar radiation. All four factors combine in the evaporation rate formula to determine fogging need.
Fogging is a pre-finishing protective measure — not a curing method. It raises the humidity directly above the slab surface to slow evaporation while finishers are still working. Curing (wet burlap, curing compound per ASTM C309, or plastic sheeting) begins after final finishing. Fogging fills the gap between placement and the start of curing on hot US job sites.
Fogging systems used on US construction sites include portable fog cannons, oscillating mist fans, and hose-attached fog nozzles (10–50 micron droplet size). The mist must be fine enough not to add free water to the slab surface — water droplets landing on fresh concrete can cause surface blemishes, scaling, and localized strength reduction. Equipment rental runs $75–$250/day at US equipment suppliers.
The concrete fogging requirement calculator uses the ACI 305R evaporation rate equation, derived from the original Menzel (1954) nomograph adapted for US customary units. The formula takes four on-site measurements and outputs the evaporation rate in pounds per square foot per hour — the industry-standard unit used by US concrete engineers and DOT inspectors.
The single most underestimated variable in field evaporation calculations is solar radiation heating of the concrete surface. In direct summer sun across the US Sun Belt states (Texas, Arizona, Florida, California), a concrete slab surface can reach 110–120°F even when the air temperature is 90°F. ACI 305R recommends measuring concrete surface temperature directly with a contact thermometer — not estimating from air temperature alone — and erecting temporary sunshades over slabs during placement when solar radiation is high.
The table below provides the full ACI 305R evaporation rate action level classification used by US engineers, concrete contractors, and DOT inspectors to determine what protective measures are required at each evaporation rate level during hot weather concreting operations.
| Evaporation Rate (lb/ft²/hr) | Risk Level | Fogging Required? | ACI 305R Action | Plastic Cracking Risk | Status |
|---|---|---|---|---|---|
| Below 0.10 | Low | No | Normal concreting — standard precautions | Very Low | ✔ Safe |
| 0.10 – 0.15 | Moderate | Monitor | Have fog equipment on standby; shade slab | Low – Moderate | ⚙ Monitor |
| 0.15 – 0.20 | Elevated | Recommended | Begin fogging; reduce mix water temp; shade | Moderate | ⚠ Caution |
| 0.20 – 0.30 | High | ✔ YES — Required | Fog immediately; wind barriers; cool mix water | High | ❌ Fog Now |
| 0.30 – 0.50 | Very High | ✔ YES — Critical | Fog + delay pour or reschedule to cooler time | Very High | ❌ Critical |
| Above 0.50 | Extreme | ✔ YES — Halt Pour | Stop placement — conditions exceed ACI 305R limits | Extreme | 🚫 Stop Pour |
The table below provides a seasonal fogging risk guide by US climate region, helping contractors plan hot weather concreting precautions in advance. Evaporation risk is rated based on typical summer temperature, humidity, and wind conditions in each region — not absolute values, which must always be calculated for each specific pour.
| US Climate Region | States Included | Peak Risk Season | Typical Summer Evap. Rate | Fogging Frequency | Risk Level |
|---|---|---|---|---|---|
| Desert Southwest | AZ, NM, NV, S. CA | May – Oct | 0.35 – 0.70+ lb/ft²/hr | Almost Every Pour | Extreme |
| South / Gulf Coast | TX, LA, MS, AL, FL | Jun – Sep | 0.20 – 0.45 lb/ft²/hr | Most Summer Pours | Very High |
| Inland Southeast | GA, SC, NC, AR, TN | Jun – Aug | 0.15 – 0.35 lb/ft²/hr | Frequent Summer | High |
| Great Plains | KS, OK, NE, SD, ND | Jun – Aug | 0.20 – 0.40 lb/ft²/hr | Most Summer Pours | Very High |
| Mid-Atlantic / Northeast | NY, PA, NJ, MD, VA | Jul – Aug | 0.10 – 0.25 lb/ft²/hr | Hot Days Only | Moderate |
| Pacific Northwest | WA, OR, N. CA | Jul – Sep | 0.10 – 0.20 lb/ft²/hr | Occasional | Low – Moderate |
| Upper Midwest / Great Lakes | MN, WI, MI, IL, OH | Jul – Aug | 0.10 – 0.22 lb/ft²/hr | Occasional Hot Days | Moderate |
The ACI 305R concrete fogging requirement is driven by four measurable on-site conditions that work together — not independently. Understanding how each factor contributes helps US contractors make real-time adjustments to bring the evaporation rate below the critical 0.20 lb/ft²/hr threshold without halting production.
Higher air temperature increases the moisture-holding capacity of air, driving more evaporation from the concrete surface. Per ACI 305R, hot weather conditions begin at 90°F air temperature. Reducing air temp influence requires scheduling pours during cooler parts of the day — early morning or evening pours are common on US summer projects.
The most impactful variable in the ACI 305R formula. Concrete surface temperature is driven by mix temperature at delivery, solar radiation, and subgrade temperature. ACI 305R limits concrete temperature at placement to 95°F max. Ice water substitution (replacing up to 75% of mix water with crushed ice) is the most effective way to reduce this value on hot US sites.
Low relative humidity (below 50%) dramatically increases evaporation rate. The US Desert Southwest regularly sees RH of 10–20% in summer, making fogging mandatory on virtually every pour. Fogging itself raises the localized RH above the slab from ambient levels to 80–95%, directly reducing the driving force for evaporation.
Wind is a multiplier in the ACI 305R formula — it removes humid air from the slab surface and replaces it with drier ambient air, continuously driving evaporation. Even a 10 mph wind can double the evaporation rate compared to still air. Wind barriers (tarps, plastic sheeting, temporary fencing) around the pour area are a highly effective and low-cost solution used on US job sites.
Direct sun on a concrete slab surface adds 10–20°F above the measured air temperature. ACI 305R recommends using temporary sunshades or scheduling placements to avoid peak solar radiation hours (10 AM – 3 PM in most US locations). White-colored curing compounds (ASTM C309 Type 2 — white pigmented) also reduce solar heat gain after finishing.
Plastic shrinkage cracking can develop within 30–90 minutes of placement if evaporation exceeds bleed water supply. The critical window on US hot weather pours is from screeding through bull-floating and final finishing. Fogging must remain active throughout this entire period — stopping too early can still result in surface cracking even if conditions appeared safe initially.
Before every summer pour: ① Calculate evaporation rate using air temp, concrete temp, RH, and wind speed. ② If E ≥ 0.15, pre-position fogging equipment. ③ Pre-cool mix water or use ice (document w/c ratio). ④ Erect wind barriers if wind >10 mph. ⑤ Schedule pour for early morning to avoid peak heat. ⑥ Apply white-pigmented curing compound (ASTM C309) immediately after final finishing. ⑦ Cover with wet burlap + plastic sheeting for minimum 7 days per ACI 308R on Type I/II cement mixes.
Authoritative references for concrete fogging, hot weather concreting, and curing standards used across the United States.
The primary US standard for hot weather concrete operations. Covers the evaporation rate nomograph, fogging requirements, mix temperature limits (95°F max at placement), precooling methods, and scheduling guidance for summer pours across all US climate regions.
View ACI 305RThe companion standard to ACI 305R, covering curing methods after concrete finishing — including wet curing, curing compounds (ASTM C309), insulating blankets, and minimum curing durations by cement type and temperature. Required reading for any US contractor working in hot or cold weather conditions.
View ACI 308RThe Federal Highway Administration's technical guidance on concrete pavement curing and hot weather protection for US highway and bridge deck projects. Covers fogging requirements, evaporative retarders, and state DOT special provisions — used as the baseline for most US public works concrete specs.
View FHWA Guide