Plastic Shrinkage Cracks in Concrete: Causes and How to Prevent Them

Plastic shrinkage cracks are among the most common issues encountered during concrete placement. These cracks occur while concrete is still in its plastic state, typically within hours of pouring, due to the rapid loss of surface moisture. Although they may not compromise structural integrity, they can impact appearance and performance, especially in exposed or polished applications.

What Causes Plastic Shrinkage Cracks?

Plastic shrinkage cracks form due to a combination of environmental and application factors:

• Wind

• High temperatures

• Low humidity

• Rapid evaporation of mix water

These conditions accelerate the drying of the concrete surface, causing it to shrink and crack before it has a chance to properly set. Cracks often appear parallel to the longitudinal axis of the slab or pour.

How to Prevent Plastic Shrinkage Cracks

According to ACI 302.1R-15, Section 13.2.2.1, several preventive measures can significantly reduce the risk of shrinkage cracking:

• Plan Placement Around Weather Conditions
  Always check the weather forecast. Delay or adjust the scope of the pour during hot, windy, or dry conditions to avoid risk-prone setups.

• Use SINAK AntiVap and Finishing Aid
  These products are formulated to slow evaporation and retain surface moisture during critical phases of finishing.

• Cover the Slab with Plastic Sheets
  Protecting the slab with plastic sheeting helps reduce evaporation before the final finish.

• Use Water Fog Upwind of the Pour
  Introduce fine mist into the air—not onto the slab—to increase ambient humidity and stabilize surface moisture loss.

• Tamp the Slab Before Final Finishing
  Proper tamping helps reduce air voids and supports better consolidation, minimizing cracking risk.

Conclusion

Plastic shrinkage cracks can be effectively prevented with the right techniques and tools. By incorporating preventive best practices, including those from ACI guidelines, you can protect slab integrity and avoid costly repairs.

Ready to build smarter with better slab performance?

Download the Architect’s Guide to Concrete