Plastic Shrinkage Cracking and Evaporation Risk Before Concrete Sets
Why fresh concrete can crack within hours, how wind/sun/temperature/humidity interact, and what a placement team can do before and during the pour.
Editorial review: 2026-09-29 · How we review technical content
Key takeaways
- Why fresh concrete can crack within hours, how wind/sun/temperature/humidity interact, and what a placement team can do before and during the pour.
Plastic shrinkage occurs before hardening
Plastic shrinkage cracking develops when moisture leaves the surface faster than bleed water can replace it while the concrete is still plastic. The surface contracts and restraint produces shallow cracks that often run roughly parallel or form irregular patterns. High wind can be as important as high air temperature; low humidity, warm concrete and direct sun also increase risk.
A cool windy day can therefore be more dangerous than a hot humid day. Planning should consider combined conditions and the concrete temperature, not a single forecast number.
Reduce evaporation before changing the mixture
Schedule placements for more favorable conditions where possible, erect windbreaks/shade, cool materials as coordinated with the producer, dampen absorptive subgrade/forms where permitted, and have curing/evaporation-control materials staged. Fogging above the slab can increase local humidity when done correctly; it is not the same as spraying water into or onto the concrete surface.
Crew size and truck spacing matter because a surface left exposed while the finishing operation falls behind remains vulnerable longer.
Recognize the finishing trap
When the surface dries rapidly, it can crust while concrete below remains plastic. Workers may think the slab is ready to finish and then trap bleed water/air, increasing delamination or blistering risk. Evaporation retarders can help under appropriate product instructions, but they do not replace curing and should not be worked into the mixture as water.
Start final curing as soon as the surface and selected method permit.
If cracks appear during placement
Do not automatically trowel plastic-shrinkage cracks closed and assume the problem is solved. Evaluate depth and extent, correct the evaporation condition, and follow project guidance for any repair. Deep or structurally consequential cracking warrants professional review.
Field checklist
- Review wind, humidity, air and concrete temperature, and sun exposure.
- Stage windbreak/shade/fogging or approved evaporation-control measures.
- Match truck rate to crew capacity.
- Begin curing promptly.
Common mistakes to avoid
- Defining hot-weather risk only by air temperature.
- Spraying water on the slab to make finishing easier.
- Ignoring strong wind on a cool day.
Frequently asked questions
Can plastic shrinkage happen in cool weather?
Yes. Strong wind and low humidity can drive rapid evaporation even when air temperature is moderate.
Are plastic shrinkage cracks the same as drying-shrinkage cracks?
No. Plastic shrinkage occurs before the concrete fully hardens; drying shrinkage develops later as hardened concrete loses moisture and is restrained.
References & further reading
- ACI PRC-308-26 — Curing of Concrete—Guide — Current ACI guide to external curing practices, published in 2026.
- NRMCA Concrete In Practice (CIP) — Industry education covering common concrete field problems and practices.
- FHWA Concrete Pavement Publications — Federal technical references for concrete pavement construction, curing, joints, durability and evaluation.