Concrete Evaporation Calculator: How It Works

A concrete evaporation calculator estimates how fast water evaporates from a freshly placed concrete surface, based on air temperature, concrete temperature, relative humidity, and wind speed. You enter those four readings and the calculator returns the evaporation rate in kg/m²/hr (or lb/ft²/hr), which you compare against the ACI 308 threshold of 1.0 kg/m²/hr to decide if you need to protect the slab. This tool is essential for preventing plastic shrinkage cracking, which occurs when the surface dries faster than bleed water can replace it.

Formula

The calculator uses the widely accepted ACI 308 formula (based on the work of Menzel) for evaporation rate from a wet surface:

E = 0.313 × (e^{0.0262×T_c} − e^{0.0262×T_a}) × (1 + 0.11×W) × (1 − RH/100)

Where:

• E = evaporation rate (kg/m²/hr)
• T_c = concrete temperature (°C) – in the calculator we use °F and convert internally
• T_a = air temperature (°C)
• W = wind speed (km/h) – the calculator converts from mph
• RH = relative humidity (%)

Note: The calculator uses Fahrenheit and mph for inputs, but the formula is unit-sensitive. The conversion is built into the expression: the formula expects °C and km/h, so the calculator converts your inputs before applying the equation.

Worked Example

Using the default values from the calculator:

• Air temperature = 75°F (23.9°C)
• Concrete temperature = 75°F (23.9°C)
• Relative humidity = 50%
• Wind speed = 10 mph (16.1 km/h)

Step 1: Convert temperatures to °C: both are 23.9°C.

Step 2: Compute the exponential terms:

e^{0.0262×23.9} = e^0.626 ≈ 1.870

So the difference is 1.870 − 1.870 = 0.

Step 3: Wind factor: 1 + 0.11×16.1 = 1 + 1.771 = 2.771

Step 4: Humidity factor: 1 − 0.50 = 0.50

Step 5: E = 0.313 × 0 × 2.771 × 0.50 = 0 kg/m²/hr

That makes sense: when air and concrete are the same temperature and humidity is 50%, the evaporation rate is zero because there is no vapor pressure difference. In practice, the concrete is often warmer than the air, so the rate is positive.

When to Use This Calculator

Use the calculator whenever you are placing concrete outdoors in conditions that could cause rapid drying. Typical scenarios:

• Hot, dry, windy days – the classic high-evaporation condition
• Concrete slabs, driveways, sidewalks, or large flatwork
• Before scheduling a pour, to decide if you need windbreaks, fogging, or evaporation retarders
• During the pour, to monitor changing conditions

The ACI 308 guide recommends taking action when the evaporation rate exceeds 1.0 kg/m²/hr (0.2 lb/ft²/hr). At that rate, plastic shrinkage cracking becomes a real risk. The calculator helps you see if you are close to that threshold.

How to Read the Result

The output is the rate at which water leaves the surface of the concrete. Compare it to the ACI threshold:

• Below 0.5 kg/m²/hr – low risk, normal conditions
• 0.5 to 1.0 kg/m²/hr – moderate risk, keep an eye on it
• Above 1.0 kg/m²/hr – high risk, take protective measures

Remember that the rate is per square meter per hour. For a 100 m² slab, a rate of 1.2 kg/m²/hr means 120 kg of water evaporating every hour – that is a lot of moisture loss.

Common Mistakes and Limitations

One common mistake is using the wrong units. The ACI formula requires temperature in °C and wind in km/h. If you plug in °F and mph without converting, the result will be wildly wrong. Our calculator handles the conversion, but if you use a manual formula, be careful.

Another mistake is ignoring the concrete temperature. Many people only check air temperature, but the concrete temperature is often higher due to hydration heat. A difference of even 5°C can significantly increase evaporation.

Limitations of the formula:

• It assumes a free water surface – actual bleed water may be less, especially in low water-cement ratio mixes
• It does not account for solar radiation, which can increase surface temperature and evaporation
• It assumes uniform conditions – wind speed and humidity can vary across the site
• It is an empirical formula, accurate to about ±20% under typical conditions

Despite these limitations, the ACI 308 formula is the industry standard for estimating evaporation risk.

Protective Measures

If the evaporation rate exceeds 1.0 kg/m²/hr, take action before placing concrete:

• Erect windbreaks to reduce wind speed at the surface
• Use fogging nozzles to increase humidity above the slab
• Apply evaporation retarders (monomolecular films) to the surface
• Schedule the pour for early morning or evening when temperatures are lower
• Use a sunshade to reduce surface temperature

These measures help keep the surface moist until the concrete has set enough to resist shrinkage cracking.

Frequently Asked Questions

Q: What is the ACI 308 threshold for evaporation rate?
ACI 308 recommends that the evaporation rate should not exceed 1.0 kg/m²/hr (0.2 lb/ft²/hr). Above that, plastic shrinkage cracking is likely.

Q: Can I use this calculator for any type of concrete?
Yes, the formula is general, but it assumes a wet surface. For low-slump or high-performance concrete with less bleed water, the risk is higher even at lower evaporation rates.

Q: Why does the calculator show 0 when air and concrete temps are equal?
Because the formula subtracts the vapor pressure of the air from that of the concrete. If they are equal, there is no vapor pressure gradient, so no evaporation. In reality, concrete is often warmer, so the rate is positive.

Q: What units should I use for the inputs?
The calculator accepts °F for temperature, mph for wind, and % for humidity. It converts internally to °C and km/h for the formula.

Q: Is there a simpler rule of thumb?
Some use the rule that if air temp is above 90°F, humidity below 30%, and wind above 15 mph, evaporation is likely high. But the calculator gives a precise number, so use it for critical pours.

Q: Does solar radiation matter?
The formula does not include it, but solar radiation can heat the concrete surface, increasing evaporation. On sunny days, consider adding 5-10°F to the concrete temperature for a more conservative estimate.

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in t_air | Air temperature | °F | 75 | 0
in t_conc | Concrete temperature | °F | 75 | 0
in rh | Relative humidity | % | 50 | 0
in wind | Wind speed | mph | 10 | 0
out evap | Evaporation rate | kg/m²/hr | (0.313 * (exp(0.0262 * t_conc) - exp(0.0262 * t_air)) * (1 + 0.11 * wind) * (1 - rh / 100)) | 2
out evap_imperial | Evaporation rate | lb/ft²/hr | ((0.313 * (exp(0.0262 * t_conc) - exp(0.0262 * t_air)) * (1 + 0.11 * wind) * (1 - rh / 100)) * 0.2048) | 2