Concrete evaporation rate is the speed at which moisture leaves the surface of fresh concrete, measured in pounds per square foot per hour (lb/ft²/hr). It is the single most important environmental factor in determining whether a pour will finish cleanly or develop plastic shrinkage cracks.
When evaporation outpaces the rate at which bleed water rises to the surface, the top layer of concrete dries and shrinks while the mass below remains saturated. That differential shrinkage is what causes cracking — and it happens within hours of placement, long before the concrete reaches initial set.
The evaporation rate calculation defined in ACI 305R uses four inputs that you can measure or estimate on any job site:
The ambient temperature at the job site. Higher air temperatures increase the air's capacity to absorb moisture, directly increasing evaporation from the slab.
The temperature of the concrete at placement. Warmer concrete has higher surface vapor pressure, which accelerates moisture loss. This is why concrete temperature is an independent variable in the formula — not just a proxy for air temperature.
The percentage of moisture saturation in the ambient air. Lower humidity means a larger vapor pressure gradient between the wet concrete surface and the surrounding air, driving faster evaporation.
Wind velocity at slab level (approximately 18 inches above the surface). Wind strips away the boundary layer of humid air above the concrete, exposing the surface to drier air and accelerating evaporation.
The formula combines these four variables to produce a single number — the evaporation rate — that tells you how fast moisture is leaving your slab surface right now.
ACI 305R sets 0.15 lb/ft²/hr as the rate above which precautions are necessary to prevent plastic shrinkage cracking. This threshold has been the industry standard for decades, and it remains the benchmark that most specifications reference.
With the shift to blended cements — particularly Type 1L — this threshold has become even more critical. Type 1L produces less bleed water than traditional Type I/II cement, meaning the concrete has less internal moisture to replace what evaporation removes. Many experienced contractors now treat 0.10 lb/ft²/hr as their personal caution threshold when working with blended cements.
No single variable tells the story by itself. A 95°F day at 70% humidity with no wind can have a lower evaporation rate than an 80°F day at 30% humidity with 15 mph wind. The combination matters.
Temperature sets the baseline — it determines how much moisture the air can hold. Humidity tells you how much capacity is left. Wind determines how quickly the saturated boundary layer above the slab is replaced with drier air. And concrete temperature adds its own vapor pressure contribution independent of air conditions.
This is precisely why checking a weather app is not enough. Weather apps show you temperature and humidity, but they do not combine these variables into an evaporation rate. And they have no concept of concrete temperature at all.
When the evaporation rate exceeds your threshold, you have several options depending on severity and job constraints:
Reschedule the pour to an early morning or evening window when temperature and wind are lower. This is the simplest and most effective option when the schedule allows it.
Apply evaporation retarders immediately after screeding and after each finishing pass. These spray-on films slow surface moisture loss without affecting the concrete properties. See our full guide on retarders and accelerators for product recommendations.
Erect windbreaks around the pour area. Reducing wind speed at slab level by even 50% can dramatically lower evaporation rate.
Fog the air above the slab using mist nozzles. This raises local humidity without adding water directly to the concrete surface.
Lower concrete temperature by requesting chilled water or ice in the mix from your ready-mix supplier. This reduces the vapor pressure at the concrete surface.
The ACI 305R nomograph is accurate but impractical in the field — nobody is pulling out a chart at 5am before a pour. Try the math yourself with our free evaporation rate calculator, or let PourDay do it automatically using live weather data for your exact job site. Enter your location, and you get a GO, CAUTION, or NO GO in seconds, along with the exact rate and a 16-day forecast so you can plan ahead.
Get your evaporation rate in seconds — not minutes. PourDay is free for every concrete contractor.
What happens when evaporation rate exceeds the threshold — causes, timing, and how to prevent it.
Read articleThe full standard behind the 0.15 threshold — when it applies and what it requires on the job.
Read articleHow evaporation rate and temperature determine which admixture your pour actually needs.
Read articleReal-time evaporation rate. 16-day forecast. Pour logging. Free for contractors.