Instantly calculate the moisture evaporation rate from fresh concrete surfaces using the official ACI 305R formula. Know your plastic shrinkage cracking risk before you pour — based on air temp, concrete temp, humidity & wind speed.
A concrete evaporation rate calculator for the USA uses the official ACI 305R hot weather concreting formula to determine how fast moisture is evaporating from the surface of fresh concrete based on four site conditions: air temperature, concrete temperature, relative humidity, and wind speed. When the evaporation rate exceeds 0.20 lb/ft²/hr, the risk of plastic shrinkage cracking increases significantly and precautionary measures — such as windbreaks, evaporation retarders, or fogging — must be implemented immediately to protect the slab surface.
💧 Concrete Evaporation Rate Calculator — USA (ACI 305R)
Fresh Concrete Surface — Bleed Water Evaporates Evaporation Rate depends on: Air Temp · Concrete Temp · Humidity · Wind Speed
Concrete Body — Internal Moisture (Concrete Temperature °F)
0.20
lb/ft²/hr ACI 305R action threshold
+10°F
Air temp rise doubles evaporation risk
+10 mph
Wind increase = major evaporation spike
Air / Wind LayerFresh Concrete SurfaceConcrete BodyCritical Threshold
What Is Concrete Evaporation Rate & Why Does It Matter in the USA?
The concrete evaporation rate is the speed at which water evaporates from the surface of freshly placed concrete before it has set. When this rate is too high, the concrete surface dries faster than bleed water can rise to replace lost moisture — causing the surface to shrink and crack while the concrete body is still plastic (unhardened). This is known as plastic shrinkage cracking and is one of the most common and preventable concrete defects on US job sites. According to the American Concrete Institute (ACI), any evaporation rate above 0.20 lb/ft²/hr requires immediate protective action.
🔵 The ACI 305R Critical Threshold — 0.20 lb/ft²/hr
The ACI 305R standard for hot weather concreting establishes 0.20 lb/ft²/hr as the action threshold. Below this rate, normal curing methods are sufficient. At or above this rate, contractors must implement protective measures — fogging, windbreaks, evaporation retarder sprays, or scheduling the pour during cooler hours — before and immediately after finishing the concrete surface.
🌡️ Air Temperature Effect
Every 10°F increase in air temperature approximately doubles the evaporation rate. Pouring concrete when air temperatures exceed 90°F — common in Texas, Arizona, Florida, and Southern California summers — dramatically increases plastic shrinkage cracking risk without proper precautions.
🌬️ Wind Speed Effect
Wind is the single most powerful evaporation accelerator. A 10 mph wind on a warm day can increase the evaporation rate by 3–5× compared to still air. Wind breaks — temporary plywood panels or commercial wind screens — are the most effective and affordable countermeasure on US job sites.
💧 Relative Humidity Effect
Low humidity supercharges evaporation. In arid regions like Nevada, Arizona, New Mexico, and West Texas, relative humidity routinely drops below 20–30% during summer afternoons, pushing evaporation rates to critical levels even at moderate temperatures and wind speeds.
ACI 305R Concrete Evaporation Rate Formula Explained
The ACI 305R evaporation rate formula is the industry-standard method used by engineers and concrete contractors across the United States to predict plastic shrinkage cracking risk before and during a pour. It takes four measurable site conditions and produces an evaporation rate in pounds of water lost per square foot per hour (lb/ft²/hr).
📐 ACI 305R Evaporation Rate Formula (US Imperial Units)
E = [5 × (Tc^2.5 − RH × Ta^2.5) × (V + 4)] × 10⁻⁶
Where: E = Evaporation Rate (lb/ft²/hr)
Tc = Concrete Temperature (°F) | Ta = Air Temperature (°F)
RH = Relative Humidity (decimal: e.g. 40% = 0.40)
V = Wind Speed at surface level (mph)
Critical: E ≥ 0.20 lb/ft²/hr → Take immediate protective action (ACI 305R)
By the time plastic shrinkage cracks appear on a concrete surface — typically 1–6 hours after placement — the damage is done and cannot be repaired by troweling or wetting the surface. The concrete evaporation rate must be calculated before the pour begins, ideally the night before using the next-day weather forecast. If the rate is predicted to exceed 0.20 lb/ft²/hr, adjust your pour time, pre-wet the sub-base, erect wind breaks, or apply a concrete evaporation retarder spray immediately after screeding.
Concrete Evaporation Rate Reference Table — USA Conditions (2026)
The table below shows calculated evaporation rates under typical US weather scenarios using the ACI 305R formula. Use this as a quick field reference to assess risk without running the full calculation. All values assume a concrete temperature 5°F above air temperature, which is typical for summer morning pours.
Scenario
Air Temp (°F)
Humidity (%)
Wind (mph)
Evaporation Rate
Risk Level
Cool Morning Pour
65°F
70%
5 mph
0.04 lb/ft²/hr
✅ Low Risk
Mild Spring Day
75°F
55%
8 mph
0.10 lb/ft²/hr
✅ Low–Moderate
Warm Summer Afternoon
85°F
45%
10 mph
0.19 lb/ft²/hr
⚠️ Moderate — Monitor
Hot Summer Day (South USA)
90°F
40%
15 mph
0.31 lb/ft²/hr
🔴 HIGH — Action Required
Desert Afternoon (AZ / NV)
100°F
20%
15 mph
0.60 lb/ft²/hr
🔴 CRITICAL — Reschedule
Windy Hot Day
88°F
35%
25 mph
0.55 lb/ft²/hr
🔴 CRITICAL — Wind Break Needed
Overcast Humid Day
78°F
80%
5 mph
0.03 lb/ft²/hr
✅ Very Low Risk
✅ Cool Morning Pour
Air Temp / RH / Wind65°F · 70% · 5 mph
Evaporation Rate0.04 lb/ft²/hr
Risk Level✅ Low Risk
⚠️ Warm Summer Afternoon
Air Temp / RH / Wind85°F · 45% · 10 mph
Evaporation Rate0.19 lb/ft²/hr
Risk Level⚠️ Moderate — Monitor
🔴 Hot Summer Day (South USA)
Air Temp / RH / Wind90°F · 40% · 15 mph
Evaporation Rate0.31 lb/ft²/hr
Risk Level🔴 HIGH — Action Required
🔴 Desert Afternoon (AZ / NV)
Air Temp / RH / Wind100°F · 20% · 15 mph
Evaporation Rate0.60 lb/ft²/hr
Risk Level🔴 CRITICAL — Reschedule
🔴 Windy Hot Day
Air Temp / RH / Wind88°F · 35% · 25 mph
Evaporation Rate0.55 lb/ft²/hr
Risk Level🔴 CRITICAL — Wind Break Needed
✅ Overcast Humid Day
Air Temp / RH / Wind78°F · 80% · 5 mph
Evaporation Rate0.03 lb/ft²/hr
Risk Level✅ Very Low Risk
How to Reduce Concrete Evaporation Rate on US Job Sites
When your concrete evaporation rate calculation shows a moderate or high risk, these field-proven precautions — recommended by ACI 305R and used by professional concrete contractors across the USA — can significantly reduce plastic shrinkage cracking risk and protect your slab finish.
🌬️ Erect Wind Breaks
Temporary plywood panels or commercial wind screens around the pour area can reduce wind speed at slab level by 60–80%. This is the single most cost-effective precaution when wind speed exceeds 10 mph on a warm day. Stakes and plywood cost under $50 for a typical residential slab.
💦 Apply Evaporation Retarder
Concrete evaporation retarder sprays (such as Confilm by BASF or Eucobar) form a monomolecular film on the freshly screeded surface that reduces evaporation by 60–80%. Apply immediately after screeding and before final finishing. Cost is approximately $30–$60 per gallon covering 500–800 sq ft.
🌅 Schedule Pours for Early Morning
In Southern and desert US states, scheduling pours to begin at dawn (5–7 AM) and finish before noon takes advantage of cooler temperatures, higher morning humidity, and lower wind speeds — typically reducing evaporation rates by 40–70% compared to midday pours in the same conditions.
🔵 Pre-Cool the Concrete Before Delivery
In hot weather, request that your ready-mix supplier use chilled mix water or ice to lower the concrete delivery temperature. ACI 305R recommends keeping fresh concrete at or below 85–90°F at the point of placement. Every 10°F reduction in concrete temperature reduces the evaporation rate by approximately 20–25%. Major US ready-mix suppliers including Quikrete and regional batch plants can accommodate this request with 24–48 hours advance notice.
What is the ACI 305R critical evaporation rate threshold for concrete?
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The ACI 305R standard establishes 0.20 lb/ft²/hr as the critical evaporation rate threshold for fresh concrete surfaces. At or above this rate, the risk of plastic shrinkage cracking becomes significant and protective measures are required. The four risk levels used by US concrete engineers are:
Low (<0.10 lb/ft²/hr): Normal curing practices — no special precautions needed
Moderate (0.10–0.20 lb/ft²/hr): Monitor conditions, have precautions ready
High (0.20–0.50 lb/ft²/hr): Immediate action required — wind breaks, retarder spray, or fogging
Critical (>0.50 lb/ft²/hr): Consider rescheduling — pour only with full precaution suite in place
How does wind speed affect the concrete evaporation rate?
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Wind speed has a dramatic effect on the concrete evaporation rate. In the ACI 305R formula, wind speed appears as (V + 4) — meaning even zero wind still has an equivalent evaporation effect of 4 mph due to natural air circulation. Increasing wind speed from 0 to 15 mph on a typical warm day (85°F, 50% RH) can increase the evaporation rate by 3–4 times. At 25+ mph, evaporation rates can exceed the critical threshold even under mild temperature and humidity conditions. Erecting wind breaks is always the first line of defense on windy days.
What is plastic shrinkage cracking in concrete and how do I prevent it?
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Plastic shrinkage cracking occurs when the concrete surface loses moisture faster than bleed water can rise to replace it — causing the surface layer to shrink and crack while the concrete beneath is still plastic (not yet set). Cracks typically appear within 1–6 hours of placement and range from hairline surface cracks to deep structural cracks 1–3 mm wide. Prevention steps include:
Calculate evaporation rate before the pour using ACI 305R formula
Apply an evaporation retarder spray immediately after screeding
Erect wind breaks when wind exceeds 10 mph
Use fogging or misting near (not on) the concrete surface
Schedule pours in early morning during hot weather months
Add polypropylene or steel fiber reinforcement to concrete mix
At what temperature is it too hot to pour concrete in the USA?
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ACI 305R does not set a hard maximum air temperature for pouring concrete, but most US concrete engineers and contractors follow these guidelines:
Below 90°F air temp: Generally safe with standard hot weather precautions
90–100°F air temp: Higher risk — requires evaporation retarder, wind breaks, and pre-cooled mix water
Above 100°F air temp: Very high risk — reschedule to early morning or evening; use ice in mix water and chilled aggregates
In practice, experienced US contractors in Southern states and desert regions pour at night or before sunrise during peak summer months to avoid dangerous evaporation conditions.
How do I measure site conditions for an accurate concrete evaporation rate calculation?
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For the most accurate evaporation rate calculation using the ACI 305R formula, measure conditions as close to pour time as possible:
Air temperature: Use a digital thermometer at pour site level (not the truck cab or shaded area)
Concrete temperature: Insert a calibrated concrete thermometer 3–4 inches into the fresh mix immediately after discharge
Relative humidity: Use a digital hygrometer or psychrometer at the job site — do not rely solely on weather apps which measure conditions at the weather station
Wind speed: Use a pocket anemometer held at slab surface height — wind is typically 30–50% lower at ground level than at the 10-meter weather station height
All four instruments can be purchased at US hardware stores or online for under $80 total.
Does the concrete evaporation rate affect curing time?
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Yes — significantly. A high evaporation rate accelerates surface drying and can cause the concrete to appear to set faster than normal, but this is deceptive. The surface is losing moisture (drying out) rather than hydrating properly. True concrete strength development (hydration) requires adequate moisture for 7–28 days. High evaporation conditions require earlier application of curing compound or wet curing methods (burlap + water, curing blankets) to seal in moisture and ensure the concrete achieves its full design strength. ACI 308R covers proper curing requirements for hot weather concrete in the USA.
📚 Trusted US Concrete & Hot Weather Concreting Resources
Official standards and guides used by US concrete engineers and contractors for evaporation rate management.
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ACI 305R — Hot Weather Concreting
Official US Standard
ACI 305R is the official American Concrete Institute guide for hot weather concreting, covering the evaporation rate formula, plastic shrinkage cracking prevention, mix design adjustments, and field precautions for high-temperature and high-wind conditions across the USA.
The Portland Cement Association (PCA) provides detailed guidance on hot weather concreting practices, evaporation rate charts, mix water temperature control, and curing best practices for residential and commercial concrete construction across US climates.
Quikrete's hot weather concreting guide covers practical steps for homeowners and contractors to protect fresh concrete during high-temperature pours — including evaporation retarders, proper curing timing, and scheduling recommendations for summer projects across the USA.