What Is Plastic Shrinkage Cracking in Concrete?
Plastic shrinkage cracks are the short, shallow, random cracks that appear on fresh concrete surfaces within hours of placement — caused by rapid surface moisture loss (wind, heat, low humidity) while the concrete is still plastic. They are unsightly and can compromise durability by giving water and chlorides a path in, but they rarely affect structural capacity. Prevention happens in the first hours; repair afterward is cosmetic.
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How they form
When evaporation from the fresh surface exceeds the bleed water rising from below, the surface shrinks while the concrete below has not — the surface tears in irregular map-pattern cracks, typically 1-3 inches deep. Hot, windy, low-humidity conditions are the trigger; the critical evaporation rate is about 0.2 lbs/sq ft/hour. Large exposed surfaces (slabs, pavements) are most vulnerable.
Prevention during placement
Windbreaks and sunshades, fog spraying or evaporation retarders on the surface, proper curing started immediately after finishing (not hours later), and scheduling pours for favorable conditions. Synthetic micro-fibers in the mix reduce plastic shrinkage cracking significantly — a cheap insurance many specifications now require for slabs. The concrete crew's timing on curing application is the single biggest variable.
Assessment and repair
Map-pattern surface cracks appearing within hours of placement are almost certainly plastic shrinkage — distinguish from structural cracks by their shallow depth, random pattern, and early appearance. If caught while the concrete is still plastic, re-troweling can close them. After hardening, repair is routing and sealing or cosmetic overlay; the durability concern is sealing the surface against water and chloride ingress, especially on exterior slabs.
The bottom line
Plastic shrinkage cracks come from rapid surface drying in the first hours — prevent with windbreaks, evaporation retarders, fibers, and immediate curing. Repair is sealing; prevention is timing.
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