Plastic shrinkage cracking occurs when fresh concrete loses surface moisture faster than bleed water can migrate to the surface to replace it. The cracks typically appear within the first one to six hours after placement — while the concrete is still in its plastic (unhardened) state. They usually run parallel to each other at roughly 1 to 3 foot intervals and can be anywhere from a few inches to several feet long.
Unlike structural cracks that develop under load, plastic shrinkage cracks form purely from moisture loss at the surface. They are a placement and environmental problem, not a design problem. And once they form, they create entry points for water, chlorides, and freeze-thaw damage that reduce the service life of the slab.
The mechanism is straightforward: when the rate of evaporation from the concrete surface exceeds the rate at which bleed water rises to replace it, the surface dries out and shrinks while the concrete below remains wet. This differential shrinkage creates tensile stress in the thin surface layer. Because fresh concrete has almost zero tensile strength, cracks form immediately.
ACI 305R identifies 0.15 lb/ft²/hr as the critical evaporation rate threshold. Above this rate, precautions are required to prevent plastic shrinkage cracking. With modern blended cements like Type 1L — which produce significantly less bleed water than traditional Type I/II — cracking can begin at even lower rates. This is why tracking your evaporation rate on every pour has become non-negotiable.
Five environmental variables drive evaporation rate — and they are exactly what PourDay measures on every pour:
Higher air temperatures increase the atmosphere's capacity to hold moisture, accelerating evaporation from the slab surface. Pours above 85°F require extra vigilance.
Low humidity means the air is far from saturated and will pull moisture aggressively from any wet surface — including your fresh concrete. Humidity below 40% is a red flag.
Wind strips the thin boundary layer of humid air that sits just above the concrete surface. Even moderate wind (10+ mph) at slab level dramatically increases evaporation rate.
Warmer concrete has higher vapor pressure at its surface, which drives faster evaporation. Concrete arriving above 80°F in hot conditions compounds the problem.
Solar radiation heats the slab surface faster than the interior, increasing the temperature differential and accelerating surface moisture loss.
Evaporation retarders: Spray-applied films (like Confilm or similar products) create a monomolecular layer on the concrete surface that slows moisture loss without affecting finishing. Apply immediately after screeding and reapply after each finishing pass. For more on chemical admixtures, see our guide on concrete retarders vs accelerators.
Windbreaks: Temporary barriers around the pour area reduce wind speed at slab level. Even a 50% reduction in wind speed can cut evaporation rate significantly.
Adjust pour timing: Schedule pours for early morning or late afternoon when temperature and wind are lower and humidity is higher. Avoid the 11am–3pm window on hot, dry days.
Fogging: Use fog nozzles to raise humidity in the air directly above the slab. This slows evaporation without adding water to the concrete surface.
Curing compounds: Apply immediately after final finishing to seal in moisture and support hydration. This is your last line of defense once finishing is complete.
Historically, tracking evaporation rate meant pulling out the ACI 305R nomograph — a chart that most crews do not carry and fewer know how to read. You can try the math yourself with our free evaporation rate calculator, or let PourDay do it automatically with live weather data for your exact job site. Open the app, confirm your location, and you have your GO, CAUTION, or NO GO status in seconds — no nomograph, no manual inputs.
Check your evaporation rate before your next pour. PourDay is free on iOS and Android — no subscription, no paywall.
Understand the formula behind the 0.15 threshold and what drives evaporation on the job.
Read articleThe full standard broken down — when it applies, what it requires, and how to use it in the field.
Read articleWhy conditions at placement determine whether your curing plan actually works.
Read articleReal-time evaporation rate. 16-day forecast. Pour logging. Free for contractors.