Introduction
Fresh-concrete evaporation rate is an estimate of how quickly water is leaving the surface. It matters when surface moisture is being lost faster than bleed water can replace it: plastic-shrinkage cracking and finishing problems can follow. Wind, direct sun, warm air, low relative humidity, and warm concrete can combine into a high-risk condition. Related guidance: Plastic Shrinkage Cracks: Wind, Sun, and Simple Prevention.
For a useful jobsite estimate, measure air temperature, relative humidity, wind speed, and concrete temperature, then use the ACI/Menzel nomograph or its project-approved equivalent. The estimate helps you decide whether to reschedule, add shade and windbreaks, use controlled fogging or an approved evaporation retarder during finishing, and begin final curing without delay.
Key takeaways
- Calculate estimated evaporation from actual air temperature, relative humidity, wind speed, and concrete temperature—not a forecast alone.
- Use the project specification first. As general guidance, about 0.5 kg/m²/h may call for precautions, while 1.0 kg/m²/h calls for protective measures; neither is a universal pass/fail limit.
- Install shade, windbreaks, and staged curing materials before placement when conditions indicate risk.
- Fogging and evaporation retarders are initial-curing aids during the plastic stage; neither replaces final curing.
- As a normal sequence, apply final curing after finishing and disappearance of bleed-water sheen, when the surface will not be marred; the governing specification and product data sheet control the timing and application conditions.
- Check curing-compound and cover compatibility before using them under later coatings, sealers, adhesives, waterproofing, or decorative finishes.
Table of Contents
- Introduction
- Key takeaways
- What Evaporation Rate Is and Why It Matters for Fresh Concrete
- Primary Environmental Drivers: Wind, Sun, Temperature, and Humidity
- How to Estimate and Measure Evaporation Rate on Site
- Relevant Standards and Specifications to Consult
- Surface Protection Methods: Wet, Membrane, and Coverings
- Practical on-Site Strategies to Mitigate Wind and Sun Effects
- Common Mistakes, Risks, and Safety Considerations
- Planning, Costs, and Seasonal Adjustments
- Conclusion
- FAQ
What Evaporation Rate Is and Why It Matters for Fresh Concrete
Evaporation rate is commonly reported as mass of water lost per area per time, such as kg/m²/h. The ACI/Menzel estimate is an estimate of evaporation from a free-water surface, not a direct measurement of a particular slab. It does not account for every factor that affects a slab, including mixture bleeding behavior, subbase absorption, finishing, and the final curing method.
Its practical purpose is to identify conditions where the surface may dry faster than bleed water arrives. That imbalance can contribute to random, irregular plastic-shrinkage cracks while the concrete is still plastic. It can also make finishing difficult. Early moisture loss is not the only factor governing setting or strength: hydration, temperature, mixture proportions, cementitious materials, and curing all matter. Proper curing retains moisture needed for hydration and strength development.
Evaporation vs. Bleeding and Setting
Bleeding is water rising through fresh concrete as solid particles settle. Evaporation is water vapor leaving the exposed surface. A changing surface sheen alone does not diagnose evaporation rate; sheen can also change because of bleeding, finishing, mixture characteristics, and setting.
Setting is the transition from plastic to stiff concrete. Do not add water to the surface or finish bleed water into the slab to make it easier to work. Instead, control the conditions above the slab and use an approved initial-curing measure if the calculated risk and project requirements call for one.
How Evaporation Influences Long-Term Durability
Rapid early surface drying can leave the surface vulnerable to cracking and poor finish quality. Once final curing can begin, maintaining the specified moisture and temperature conditions supports hydration and durable surface development. The American Cement Association’s overview of concrete applications likewise emphasizes curing as part of concrete performance, rather than treating evaporation as the sole control on strength.
Primary Environmental Drivers: Wind, Sun, Temperature, and Humidity
Evaporation risk is governed by four measurements used together: air temperature, relative humidity, wind speed, and concrete temperature. Direct sun can raise the concrete surface temperature, so direct sun exposure is an added jobsite concern even though it is not a separate input in the common calculation.
Wind: mechanisms and site behaviors to watch for
Airflow thins or removes the humid boundary layer immediately above the wet concrete. That increases vapor transfer from the surface. Edges, corners, openings, and gaps around windbreaks can have locally stronger airflow, so protection must cover the whole placement area rather than only the middle of the slab.
Sun and solar radiation: surface heating effects
Direct sunlight can heat the concrete surface and increase drying demand. Exposure varies with time of day, season, latitude, obstructions, and site orientation; do not assume one compass direction is always the highest-risk side. Use shade that does not obstruct placing, finishing, or safe access.
Temperature and relative humidity interplay
Warm air and warm concrete generally increase the moisture-driving force, while lower relative humidity leaves the air able to accept more vapor. A cool day can still be high risk when wind is strong and humidity is low. Measure at the placement area and repeat readings when wind, cloud cover, temperature, or placement conditions change.
How to Estimate and Measure Evaporation Rate on Site
Use a calibrated air-temperature/RH meter, handheld anemometer, concrete thermometer or approved temperature probe, and a job log. Measure actual conditions at the placement area before placement and again during the pour when conditions materially change. A shallow pan, damp towel, fingerprints, glare, ripples, or color changes are not validated ways to calculate a concrete evaporation rate.
The FHWA reproduces the ACI/Menzel approach and identifies the required inputs: air temperature, relative humidity, concrete temperature, and wind speed. Use the nomograph or equation required by the project; do not mix SI and U.S. customary versions. In the commonly reproduced SI form, temperatures are in °C, wind speed is in m/s, RH is percent, and the result is kg/m²/h:
ER = 4.88[0.0001(Tc + 17.8)2(2.5V + 0.01RH)(Ta + 17.8)]
Here, ER is estimated evaporation rate, Tc is concrete temperature, Ta is air temperature, V is wind speed, and RH is relative humidity. Verify the equation transcription, variable conventions, and units against the project-required chart and the FHWA discussion of the evaporation nomograph and its limits before using it for a project decision.
Step-by-Step Process
- Review the controlling plan: before concrete arrives, identify the governing specification, local or owner requirements where applicable, approved curing method, approved products, expected concrete temperature, later finishes, action limit, and the person authorized to delay or stop the pour. If no approved curing method, compatible product, action limit, or responsible authority is identified, postpone the pour or obtain qualified direction.
- Stage protection: have windbreaks, shade, approved fogging equipment or evaporation retarder, and final-curing materials ready before placement. Review current product data sheets (TDS) and safety data sheets (SDS). Wear gloves, eye protection, boots, and long sleeves; use any additional PPE required by the SDS.
- Measure actual conditions: record air temperature, RH, wind speed, and concrete temperature at the placement area before placement.
- Calculate the estimate: use the specified ACI/Menzel chart or equation and record the units, time, inputs, and result.
- Compare and decide: apply the project limit. If no limit is stated, obtain direction from the engineer, owner, ready-mix supplier, or contractor rather than treating generic values as approval. About 0.5 kg/m²/h is commonly treated as a point to take precautions and 1.0 kg/m²/h as a condition requiring protective measures, but these are guidance values, not universal failure thresholds.
- Reduce exposure before it becomes a surface problem: postpone or shift to a cooler, less windy window when feasible; ask the ready-mix supplier or follow the approved hot-weather mix plan for permitted ways to reduce concrete temperature; install windbreaks and shade; and shorten the interval between placing, finishing, and curing.
- Monitor during placement: remeasure and log conditions whenever wind, sun, clouds, or temperature changes materially. Observe whether moisture appears to be leaving the surface faster than bleed water arrives, but treat that only as a supporting observation.
- Protect in sequence: use controlled fogging or an approved evaporation retarder during the plastic stage if needed, finish without adding water, then apply final curing according to the governing specification and product TDS. The normal sequence is after finishing and disappearance of surface bleed-water sheen, when the surface can resist marring.
Quick decision checklist
- Before placement: measured conditions, calculated estimate, governing action limit checked, compatible protection selected, and materials staged.
- If the estimate is elevated: add shade and windbreaks; adjust timing or postpone if the required protection cannot be provided.
- While concrete is plastic: use only approved controlled fogging or evaporation retarder; do not spray water onto the slab or work retarder into the surface.
- After finishing: start the specified final curing at the TDS- and specification-required stage, normally after bleed-water sheen has disappeared and the surface will not be damaged.
- Stop and get direction: if wind removes covers, fogging deposits water or damages the surface, the required curing method cannot be applied promptly, plastic cracking appears, or the calculated risk exceeds the project limit.
Instruments and on-site measurements to use
Air-temperature/RH meter: records air conditions. Anemometer: records wind speed. Concrete thermometer: records concrete temperature. Use instruments with units matching the selected calculation. Keep a time-stamped log of readings, calculated rates, placement, finishing, bleed-sheen disappearance, initial-curing actions, and final-curing start.
Relevant Standards and Specifications to Consult
The project specification controls the curing method, application rate, duration, temperature limits, and any evaporation-rate action limit. Review it with the ready-mix supplier or contractor before placement, especially for structural slabs, low-w/cm or high-performance mixtures, decorative concrete, hot-weather work, and durability-critical work. Verify locally adopted specifications, owner requirements, and permit conditions where applicable.
What to look for in ACI guidance (e.g., ACI 308, ACI 318)
ACI curing guidance and the project documents should identify whether curing is prescriptive or performance-based, when final curing begins, how it must be maintained, and the acceptance or strength criteria for ending it. Do not substitute a generic online interval for the specified requirement.
AASHTO, ASTM, and agency specs: materials and test methods
Use only standards that match the work being specified. For example, ASTM C597 is an ultrasonic pulse-velocity test method; it is not a fresh-concrete evaporation-rate or curing-material test. State DOT, owner, or agency specifications may require particular curing compounds, sheets, blankets, application rates, and inspection records.