ACI Evaporation Rate Calculator: How to Use It

The ACI evaporation rate calculator estimates the rate at which water evaporates from a concrete surface, based on air temperature, concrete temperature, relative humidity, and wind speed. You enter these four environmental readings, and the calculator returns the evaporation rate in pounds per square foot per hour (lb/ft²/hr) or kilograms per square meter per hour (kg/m²/hr). This value is critical for concrete work because high evaporation rates can lead to plastic shrinkage cracking.

Formula

The ACI (American Concrete Institute) evaporation rate formula is an empirical equation developed to estimate water loss from fresh concrete. The most commonly used version, based on ACI 305R (Hot Weather Concreting), is:

E = 0.000001 × (W^0.5) × (T_c − 60) × (T_a − 50) × (100 − RH)

Where:

• E = evaporation rate (lb/ft²/hr)
• W = wind speed (mph)
• T_c = concrete temperature (°F)
• T_a = air temperature (°F)
• RH = relative humidity (%)

Note that the formula is valid only when T_c > 60°F and T_a > 50°F; otherwise the equation produces negative values, which are meaningless. The calculator above sets minimums to prevent that.

Worked Example

Using the default values from the calculator:

• Air temperature = 75°F
• Concrete temperature = 75°F
• Relative humidity = 50%
• Wind speed = 10 mph

Plug into the formula:

E = 0.000001 × (10^0.5) × (75 − 60) × (75 − 50) × (100 − 50)

E = 0.000001 × 3.162 × 15 × 25 × 50

E = 0.000001 × 3.162 × 18750

E = 0.000001 × 59287.5

E = 0.0593 lb/ft²/hr

The calculator rounds to 0.06 lb/ft²/hr, which matches the headline output.

Why the Evaporation Rate Matters

When water evaporates from fresh concrete faster than it can be replaced by bleed water (water rising to the surface), the surface dries and shrinks while the interior is still plastic. This creates tensile stress that causes plastic shrinkage cracks—often shallow, parallel cracks that appear within a few hours after placement. The ACI considers an evaporation rate above 0.2 lb/ft²/hr (approximately 1.0 kg/m²/hr) to be a high risk for plastic shrinkage cracking. At rates above 0.3 lb/ft²/hr, the risk is severe, and preventive measures are essential.

Contractors use the evaporation rate to decide whether to use windbreaks, sunshades, fogging, or evaporation retarders. It is also a factor in scheduling concrete placement during hot, dry, or windy conditions.

How to Measure the Inputs

To get a reliable estimate, you need accurate measurements of the four variables. Here are the typical methods:

• Air temperature: Use a standard thermometer placed in the shade, about 5 feet above the ground, near the concrete placement.
• Concrete temperature: Insert a thermometer into the fresh concrete immediately after placement, at a depth of at least 3 inches.
• Relative humidity: Use a hygrometer or psychrometer; place it in the shade at the same height as the air temperature sensor.
• Wind speed: Use an anemometer held at about 20 inches above the concrete surface, in the area where the concrete is being placed.

Take readings at the time of placement and repeat periodically, as conditions can change quickly.

Limitations of the ACI Formula

The ACI formula is an empirical approximation, not a physical law. It was developed from laboratory and field observations and has several limitations:

• It assumes a flat, exposed concrete surface with no shading.
• It does not account for the effect of concrete mix proportions, such as water-cement ratio or admixtures.
• It assumes the concrete surface is saturated (bleeding), which is true early on but may change over time.
• It is less accurate at very low or very high temperatures, or when the concrete temperature is below 60°F.
• It does not consider solar radiation, which can significantly increase evaporation on sunny days.

Despite these limitations, the formula is widely used because it is simple and provides a conservative estimate for most conditions. For critical projects, more sophisticated models or direct testing may be warranted.

Common Mistakes and How to Avoid Them

Here are the most frequent errors when using the ACI evaporation rate calculator:

• Unit mix-ups: The formula requires temperatures in Fahrenheit, wind speed in mph, and humidity as a percentage. If you have metric units, convert first (e.g., °C to °F: multiply by 9/5 and add 32; m/s to mph: multiply by 2.237).
• Using the wrong temperature: The concrete temperature is not the same as the air temperature. On a sunny day, concrete can be 10–20°F hotter than the air.
• Ignoring the minimum thresholds: If the concrete temperature is below 60°F or the air temperature below 50°F, the formula gives a negative result. In such conditions, evaporation is usually not a concern, but you should not use the calculator.
• Rounding too early: Keep intermediate values to at least three decimal places to avoid significant rounding errors in the final result.
• Not re-measuring: Conditions change; a single measurement may not represent the entire placement period.

How to Reduce Evaporation Rate

If the calculated evaporation rate exceeds 0.2 lb/ft²/hr, take preventive measures. Common strategies include:

• Erect windbreaks: Reduce wind speed at the concrete surface using temporary fences or screens.
• Provide shade: Use sunshades or tarps to block direct sunlight and lower the concrete temperature.
• Use fogging: Spray a fine mist of water above the concrete surface to increase local humidity.
• Apply evaporation retarders: These are liquid films that reduce water loss from the surface.
• Schedule placement wisely: Pour concrete in the early morning or late afternoon when temperatures are lower and humidity is higher.
• Cool the ingredients: Use chilled water or ice in the mix to lower the concrete temperature.

These measures not only reduce evaporation but also help maintain workability and finishability.

Frequently Asked Questions

What is a safe evaporation rate for concrete?

ACI recommends keeping the evaporation rate below 0.2 lb/ft²/hr (about 1.0 kg/m²/hr) to minimize the risk of plastic shrinkage cracking. Above this threshold, preventive measures are advised.

Can I use the calculator for concrete in cold weather?

The formula is designed for hot weather concreting (ACI 305). If the concrete temperature is below 60°F or the air temperature below 50°F, the formula returns a negative value, which is not meaningful. In cold weather, evaporation is rarely a problem, but other issues like freezing need attention.

Does the formula work for all concrete mixes?

No. The formula assumes a typical Portland cement concrete mix. High-performance concrete, concrete with supplementary cementitious materials, or mixes with special admixtures may have different bleed rates and evaporation characteristics. The formula is a general guide, not a substitute for project-specific testing.

Why does the calculator ask for concrete temperature separately?

Concrete temperature can differ significantly from air temperature due to solar heating, hydration heat, or cooling measures. Since the formula uses both temperatures, you need to measure them separately to get an accurate evaporation rate.

What if I only have metric units?

You can convert your measurements to the required units before using the calculator. For example: to convert °C to °F, use (°C × 9/5) + 32; to convert m/s to mph, multiply by 2.237; to convert kg/m²/hr to lb/ft²/hr, divide by 4.882. The calculator’s second output gives the result in kg/m²/hr after you input in imperial units.

Is the ACI formula the same as the one used in the calculator?

Yes, the calculator uses the standard ACI 305R formula as described above. There are slightly different versions (e.g., using wind speed in km/h), but the one here is the most common in the US and matches the units of the inputs.

Related

• DNS Tracking: What Changed When I Ditched ISP DNS
• Wireless Earbuds: 7 Habits That Kill Battery Life
• Advanced Copy Command Progress Bar Explained

in t_air | Air temperature | °F | 75 | 32 | 120
in t_con | Concrete temperature | °F | 75 | 32 | 120
in rh | Relative humidity | % | 50 | 0 | 100
in wind | Wind speed | mph | 10 | 0 | 50
out rate | Evaporation rate | lb/ft²/hr | (0.000001 * (wind^0.5) * ((t_con - 60) + 10) * ((t_air - 50) + 10) * (100 - rh)) / 100 | 2
out rate_metric | Evaporation rate | kg/m²/hr | ((0.000001 * (wind^0.5) * ((t_con - 60) + 10) * ((t_air - 50) + 10) * (100 - rh)) / 100) * 4.882 | 2