Plastic Shrinkage
Cracking that occurs when concrete surface dries faster than water bleeds to the surface
Plastic shrinkage is cracking that occurs when concrete surface dries faster than water bleeds to the surface. These early cracks appear within hours of placement—sometimes while finishing is still underway—and typically run parallel to each other in random patterns across the slab.
Why It Matters
Plastic shrinkage cracks are preventable but often overlooked until too late. Hot weather, low humidity, wind, and direct sun all accelerate surface drying. When surface drying outpaces bleeding, the surface shrinks while the interior doesn't, causing surface tensile stresses that exceed the strength of very young concrete. The result: cracks that penetrate 1-3 inches deep across the entire slab.
These cracks don't necessarily indicate structural problems, but they're ugly, allow water infiltration, and are difficult to repair effectively. Prevention requires managing environmental conditions and timing operations properly—much easier than fixing the cracks later.
Technical Details
High-risk conditions for plastic shrinkage:
- Temperature above 70°F
- Low humidity (under 50%)
- Wind speed over 10 mph
- Direct sun exposure
- High concrete temperature
The combination of these factors matters more than any single factor. A sunny, hot, dry, windy day is worst-case. Concrete evaporation rate exceeding 0.2 lbs/sq ft/hour indicates high risk.
Prevention strategies:
- Windbreaks: Block wind during placement and finishing
- Fog spray: Light mist above surface to increase humidity (not directly on concrete)
- Sunshades: Protect slab from direct sun
- Evaporation retarders: Spray-on films that slow surface drying
- Fiber reinforcement: Synthetic fibers significantly reduce plastic shrinkage cracking
- Timing: Pour early morning or evening when conditions are cooler
- Wet curing: Begin immediately after finishing
Once plastic shrinkage cracks appear, they can't be prevented—only repaired. Routing and sealing is the typical fix, but appearance remains compromised.
Related Terms
- Bleed Water - Process competing with evaporation
- Curing - Proper moisture retention prevents plastic shrinkage
- Crack Control - General strategies for minimizing cracks
Learn More
- How to Repair Cracks - Fixing plastic shrinkage cracks
- Best Time to Pour Concrete - Avoiding high-risk conditions
- Concrete Calculator - Calculate your project volume