Concrete Evaporation Rate Calculator
Concrete placement and finishing are highly sensitive to weather conditions. Temperature, relative humidity, wind speed, and the temperature of the concrete surface can all influence how quickly water leaves freshly placed concrete. When evaporation occurs too quickly, finishing and curing can become more difficult, and the risk of surface-related problems may increase.
The Concrete Evaporation Rate Calculator provides a convenient way to estimate evaporation from a concrete surface using five important inputs: air temperature, concrete temperature, relative humidity, wind speed, and concrete surface area. It then provides an estimated evaporation rate, equivalent metric rate, hourly water loss, and an evaporation-risk classification.
Understanding evaporation is particularly useful during outdoor concrete work such as slabs, driveways, patios, floors, sidewalks, foundations, parking areas, and other flatwork. A contractor or concrete professional can use environmental measurements to evaluate whether weather conditions may require additional planning or precautions.
This calculator is designed for estimation and planning, rather than as a substitute for field measurements, project specifications, or professional concrete-placement guidance. Actual evaporation can be affected by additional conditions such as solar radiation, cloud cover, air movement, concrete mix characteristics, surface condition, and site-specific weather changes.
What Is Concrete Evaporation Rate?
Concrete evaporation rate describes how quickly water is leaving the surface of freshly placed concrete.
It is commonly expressed as:
pounds of water per square foot per hour (lb/ft²/hr)
The calculator also provides the equivalent:
kilograms per square meter per hour (kg/m²/hr)
Fresh concrete contains water as part of its mixture. Some of that water participates in cement hydration, while some is eventually lost through evaporation. If environmental conditions cause water to leave the surface too quickly, the concrete surface may become difficult to finish and maintain in an appropriate condition.
Evaporation is not automatically a problem. Water naturally moves and leaves the concrete surface as part of the concrete's early behavior. The important consideration is the rate of evaporation and whether it is occurring faster than the surface can maintain an adequate moisture condition.
Why Concrete Evaporation Matters
Evaporation becomes especially important during concrete placement because the surface of fresh concrete needs appropriate moisture conditions while it is being finished and beginning to cure.
Rapid evaporation can contribute to a number of construction challenges, including:
- Difficult finishing
- Rapid surface drying
- Increased moisture loss
- Greater sensitivity to environmental conditions
- Potential surface cracking concerns
- Changes in finishing timing
- Increased need for appropriate curing practices
The exact consequences depend on the concrete mixture, project conditions, placement practices, and other factors.
A useful evaporation estimate can therefore help construction teams recognize weather conditions that deserve closer attention.
How the Concrete Evaporation Rate Calculator Works
The calculator asks for five inputs.
1. Air Temperature
Enter the surrounding air temperature and select either:
- °F
- °C
The calculator converts Celsius temperatures to Fahrenheit internally so that the subsequent calculations use a consistent temperature basis.
2. Concrete Temperature
Enter the temperature of the concrete itself.
This is important because concrete temperature can differ from surrounding air temperature. Fresh concrete exposed to sunlight, warm materials, or other environmental conditions may have a different temperature from the air.
3. Relative Humidity
Enter relative humidity as a percentage from 0% to 100%.
Relative humidity describes how much moisture is present in the air compared with the maximum amount of moisture the air can hold at that temperature.
Lower relative humidity generally increases the potential for evaporation because the surrounding air has greater capacity to accept additional moisture.
4. Wind Speed
Enter wind speed in:
- mph
- km/h
Moving air can increase evaporation by continuously replacing the air immediately above the concrete surface.
The calculator converts km/h to mph when necessary.
5. Concrete Surface Area
Enter the area of concrete in:
- square feet (ft²)
- square meters (m²)
The calculator converts square meters to square feet when necessary so that water loss can be calculated using a consistent area unit.
How to Use the Concrete Evaporation Rate Calculator
Using the calculator requires only a few steps.
Step 1: Measure the Air Temperature
Obtain the current air temperature near the concrete placement area.
Enter the temperature into the Air Temperature field and select °F or °C.
For example:
75°F
Step 2: Measure Concrete Temperature
Enter the temperature of the concrete.
For example:
70°F
Using the actual concrete temperature is preferable to simply assuming it is equal to the air temperature.
Step 3: Enter Relative Humidity
Enter the measured relative humidity.
For example:
50%
The calculator accepts values between 0% and 100%.
Step 4: Enter Wind Speed
Enter the current wind speed.
For example:
10 mph
If your measurement is in kilometers per hour, select km/h.
Step 5: Enter Surface Area
Enter the area of the concrete surface.
For example:
1,000 ft²
If you have measured the area in square meters, select m².
Step 6: Select Calculate
Click Calculate to generate the results.
The calculator displays:
- Evaporation rate in lb/ft²/hr
- Evaporation rate in kg/m²/hr
- Estimated water loss in lb/hr
- Estimated water loss in kg/hr
- Evaporation risk
Concrete Evaporation Rate Formula
The calculator uses a simplified evaporation model based primarily on vapor-pressure difference and wind movement.
The calculation involves several stages.
Step 1: Calculate Vapor Pressure
The calculator uses a saturation vapor pressure relationship based on temperature.
The general form used for vapor pressure is:
P = 0.6108 × e^[17.27T/(T + 237.3)]
where:
- P = saturation vapor pressure in kPa
- T = temperature in °C
- e = mathematical constant
The calculator first converts Fahrenheit temperatures to Celsius when necessary.
The conversion is:
°C = (°F − 32) × 5/9
For Celsius inputs, the value can be used directly.
Step 2: Determine Actual Air Vapor Pressure
The saturation vapor pressure represents the amount of moisture the air could hold at a given temperature.
Relative humidity is then applied:
Actual Air Vapor Pressure = Air Saturation Vapor Pressure × Relative Humidity / 100
For example, if the saturation vapor pressure is 3.17 kPa and relative humidity is 50%:
3.17 × 0.50 = 1.585 kPa
This represents the estimated actual vapor pressure of the surrounding air under the calculator's model.
Step 3: Determine Vapor-Pressure Difference
The calculator compares the vapor pressure associated with the concrete temperature to the actual vapor pressure of the surrounding air.
The formula is:
Vapor Pressure Difference = Concrete Vapor Pressure − Actual Air Vapor Pressure
A positive difference indicates conditions in which the simplified model predicts evaporation.
If the calculated difference is zero or negative, the calculator sets the evaporation rate to zero.
Wind Factor
Wind is another important part of the calculator's model.
The calculator uses:
Wind Factor = 1 + (0.10 × Effective Wind Speed)
The effective wind speed is capped at 30 mph for this calculation.
This means the calculator does not continue increasing the wind influence indefinitely beyond 30 mph.
For example, at 10 mph:
Wind Factor = 1 + (0.10 × 10)
Wind Factor = 2.0
At 20 mph:
Wind Factor = 1 + (0.10 × 20)
Wind Factor = 3.0
This simplified relationship represents the increased influence of air movement in the calculator's estimation.
Evaporation Rate Calculation
When the vapor-pressure difference is positive, the calculator applies:
Evaporation Rate = 0.18 × Vapor Pressure Difference × Wind Factor
The result is expressed as an estimated:
lb/ft²/hr
The calculator also applies a temperature adjustment under particularly cold or hot air conditions.
For air temperatures below 40°F, the model applies a factor of 0.75.
For air temperatures above 90°F, it applies a factor of 1.10.
Between these thresholds, the temperature factor remains 1.00.
These adjustments are part of this calculator's simplified estimation model.
Estimated Water Loss Formula
Evaporation rate describes water loss per unit of area. To estimate the total amount of water being lost from the entire concrete surface, the calculator multiplies the evaporation rate by surface area.
The formula is:
Water Loss = Evaporation Rate × Concrete Surface Area
The calculator performs the calculation using square feet and reports the result in pounds per hour.
It also converts the result to kilograms per hour.
Metric Conversion
The calculator provides both imperial and metric results.
For evaporation rate, it uses:
kg/m²/hr = lb/ft²/hr × 4.88243
For water loss:
kg/hr = lb/hr × 0.45359237
This allows users working with either measurement system to interpret the results conveniently.
Example Calculation
Consider a concrete slab with these conditions:
| Input | Example Value |
|---|---|
| Air Temperature | 75°F |
| Concrete Temperature | 70°F |
| Relative Humidity | 50% |
| Wind Speed | 10 mph |
| Surface Area | 1,000 ft² |
The calculator first determines the vapor pressure associated with the concrete temperature and the actual vapor pressure of the surrounding air.
It then calculates the vapor-pressure difference and applies the wind factor.
At 10 mph wind:
Wind Factor = 1 + (0.10 × 10) = 2.0
The resulting evaporation rate is then multiplied by the concrete surface area.
If the calculated evaporation rate were, for example, 0.100 lb/ft²/hr, the estimated water loss would be:
0.100 × 1,000 = 100 lb/hr
The corresponding metric evaporation rate would be:
0.100 × 4.88243 = 0.488 kg/m²/hr
And the water loss would be:
100 × 0.45359237 = 45.36 kg/hr
The actual result produced by the calculator depends on the exact vapor-pressure calculations from the temperatures and humidity entered.
Understanding the Evaporation Risk Result
The calculator categorizes the estimated evaporation rate into four levels.
| Evaporation Rate | Calculator Risk |
|---|---|
| Less than 0.05 lb/ft²/hr | Low |
| 0.05 to less than 0.10 | Moderate |
| 0.10 to less than 0.15 | High |
| 0.15 or greater | Very High |
These categories are specific to the calculator's built-in thresholds and should be interpreted as screening indicators, not as universal engineering limits.
A "Low" result does not mean that every concrete placement condition is automatically safe, and a "Very High" result does not by itself determine that placement cannot proceed. Actual project decisions should consider the concrete mixture, placement procedures, weather forecast, curing plan, specifications, and applicable professional guidance.
Factors That Increase Concrete Evaporation
Several environmental factors can increase the potential for moisture loss.
Low Relative Humidity
Dry air generally has greater capacity to receive additional moisture. As relative humidity decreases, evaporation potential can increase.
Higher Wind Speed
Wind removes moist air from directly above the concrete surface and replaces it with drier surrounding air.
Higher Concrete Temperature
A warmer concrete surface can increase vapor pressure and consequently increase the potential for moisture transfer.
High Air Temperature
Hot weather can contribute to rapid moisture loss, particularly when combined with low humidity and wind.
Large Exposed Surface
A larger concrete surface means that even a moderate evaporation rate can result in a substantial total amount of water loss per hour.
Why Concrete Temperature Should Be Measured
One common mistake in evaporation estimation is assuming that air temperature and concrete temperature are identical.
They can be different.
Fresh concrete can be affected by:
- Cement hydration
- Ambient temperature
- Sun exposure
- Aggregate temperature
- Mixing-water temperature
- Delivery conditions
- Ground temperature
- Formwork temperature
Because vapor pressure is temperature-dependent, using the concrete temperature as a separate input provides more information than relying exclusively on air temperature.
Evaporation Rate and Concrete Cracking
Rapid moisture loss is often discussed in relation to plastic shrinkage cracking.
Plastic shrinkage can occur while concrete is still in its early, relatively plastic condition. When water leaves the surface faster than moisture can migrate upward through the concrete, the surface can shrink. Under appropriate conditions, restraint within the concrete can contribute to cracking.
However, evaporation rate is only one factor.
Other considerations include:
- Concrete mixture
- Bleeding characteristics
- Finishing timing
- Reinforcement
- Surface temperature
- Ambient weather
- Curing procedures
- Concrete thickness
- Solar exposure
- Air movement
Consequently, the calculator should be used as one part of construction planning rather than as a standalone diagnosis of cracking risk.
Using the Calculator Before Concrete Placement
Weather conditions can change quickly. A useful approach is to evaluate conditions not only at the beginning of a pour but also during the expected placement and finishing period.
For example, a morning placement might begin under relatively mild conditions but encounter:
- Increasing temperature
- Falling humidity
- Increasing wind
- Greater solar exposure
These changes can alter evaporation conditions.
Checking current weather conditions and considering the forecast can therefore provide a more complete picture than using a single measurement.
Practical Tips for Reducing Moisture-Loss Problems
When evaporation conditions are challenging, construction teams may consider appropriate project-specific measures.
Depending on the project and applicable specifications, these can include:
- Planning placement during more favorable weather
- Protecting fresh concrete from excessive wind
- Using appropriate curing procedures
- Organizing finishing operations efficiently
- Monitoring weather conditions throughout placement
- Maintaining appropriate concrete temperature
- Following the concrete mix supplier's recommendations
- Following applicable project specifications and professional guidance
The appropriate response depends on the specific concrete system and job conditions.
Common Uses for a Concrete Evaporation Calculator
This calculator can be helpful for:
Concrete Slabs
Large slab placements have significant exposed surface area, making evaporation monitoring particularly useful.
Driveways
Residential driveways can be evaluated using their surface area and prevailing weather conditions.
Patios
Outdoor patios are exposed directly to environmental conditions, making temperature, wind, and humidity relevant.
Foundations
Concrete foundation work can involve substantial volumes and exposed surfaces during placement.
Commercial Floors
Large commercial floor areas can experience significant total moisture loss even when the evaporation rate per square foot is relatively modest.
Sidewalks and Pavements
Outdoor flatwork is directly exposed to changing environmental conditions.
Important Limitations
The calculator is intentionally a simplified estimation tool.
Real-world evaporation can depend on conditions that are not directly represented in the calculation, such as:
- Solar radiation
- Cloud cover
- Surface condition
- Concrete mix design
- Bleeding rate
- Air pressure
- Local microclimate
- Surface temperature variations
- Protective coverings
- Changing wind direction
- Changing weather conditions
The calculator therefore should not be treated as a laboratory measurement or a substitute for project-specific engineering analysis.
For critical concrete placements, use appropriate professional procedures, project specifications, and recognized industry guidance.
Frequently Asked Questions
1. What is a concrete evaporation rate calculator?
A concrete evaporation rate calculator estimates the rate at which moisture may leave a concrete surface based on environmental and concrete conditions such as air temperature, concrete temperature, relative humidity, and wind speed.
2. What inputs are needed for the calculator?
The calculator requires air temperature, concrete temperature, relative humidity, wind speed, and concrete surface area. Temperature, wind, and area can be entered using the available imperial or metric units.
3. Why is relative humidity important?
Relative humidity indicates how much moisture is already present in the surrounding air. Lower humidity generally increases the air's capacity to accept moisture, which can increase evaporation potential.
4. Why does wind affect concrete evaporation?
Moving air can remove moisture-rich air from directly above the concrete surface and replace it with surrounding air. This can increase moisture transfer from the concrete.
5. What does lb/ft²/hr mean?
It means pounds of water evaporated per square foot of concrete surface per hour. It describes evaporation intensity relative to surface area.
6. What does kg/m²/hr mean?
It is the metric equivalent of the evaporation rate, representing kilograms of water per square meter of concrete surface per hour.
7. What does estimated water loss mean?
Estimated water loss represents the approximate total amount of water lost from the entire concrete surface each hour based on the calculated evaporation rate and the entered surface area.
8. What does a Very High evaporation risk mean?
In this calculator, a Very High classification corresponds to an estimated evaporation rate of 0.15 lb/ft²/hr or greater. It is a calculator-specific screening category and should not be treated as a universal construction limit.
9. Can this calculator be used for outdoor concrete?
Yes. It can provide an estimate for outdoor slabs, driveways, patios, sidewalks, foundations, floors, and other concrete surfaces. Actual site conditions should still be monitored because weather can change during placement.
10. Is this calculator a substitute for professional concrete guidance?
No. It is intended for estimation and planning. For important or large-scale concrete placements, decisions should also consider project specifications, concrete mixture characteristics, weather conditions, curing requirements, and appropriate professional guidance.
Conclusion
The Concrete Evaporation Rate Calculator offers a practical way to estimate moisture loss from a concrete surface using five key inputs: air temperature, concrete temperature, relative humidity, wind speed, and surface area.
Its calculation considers the difference between the vapor pressure associated with the concrete and the actual vapor pressure of the surrounding air, then incorporates a wind factor and temperature adjustment. The final results include evaporation rate in both imperial and metric units, estimated hourly water loss, and a calculator-specific evaporation-risk category.
Understanding these results can help homeowners, contractors, builders, and concrete professionals think more carefully about weather conditions during concrete placement. In particular, combinations of warm concrete, dry air, and wind can create conditions where moisture loss deserves closer attention.
For the most useful estimate, enter accurate measurements and use the actual conditions near the concrete surface. Remember that evaporation is influenced by many variables, and the calculator does not represent every environmental or concrete-mixture factor.
Used appropriately, the tool can serve as a convenient concrete evaporation rate estimator for preliminary planning and weather-condition assessment before and during concrete placement.