Plastic Shrinkage Cracking: Causes and Mechanism

Plastic shrinkage cracking occurs when the rate of water evaporation from the surface of freshly placed concrete exceeds the rate at which bleed water can rise to the surface. This creates a volume deficit at the top layer, leading to tensile stresses that the "plastic" (unhardened) concrete cannot resist.

1. The Core Mechanism

When concrete is placed, the heavy aggregate and cement particles begin to settle, displacing water upwards—a process known as bleeding.

• Normal Condition: Bleed water reaches the surface and forms a thin sheen, protecting the concrete from drying out.

• Cracking Condition: If environmental factors accelerate evaporation, this sheen disappears. The water begins to evaporate from the "capillary pores" inside the concrete. This creates negative capillary pressure, which pulls the solid particles together, causing the surface to shrink while the concrete below remains stable. 

2. Environmental "Perfect Storm" Factors

The risk of cracking increases dramatically when the evaporation rate exceeds 0.2 lb/ft²/hr (1.0 kg/m²/hr).

Factor	Impact on Concrete
High Temperature	Increases the vapor pressure of the water in the concrete, making it "want" to escape into the air faster.
High Wind Speed	Removes the saturated air layer directly above the concrete surface, constantly replacing it with dry air that pulls more moisture out.
Low Humidity	The drier the air, the higher the "moisture gradient" between the wet concrete and the atmosphere, accelerating evaporation.
Large Exposed Surface	Slabs and pavements have a high surface-area-to-volume ratio, leaving a massive area vulnerable to the elements.

3. Visual Characteristics

• Timing: Usually appears 1 to 6 hours after casting (before final set).

• Pattern: Often parallel to each other or in a "crow's foot" / "map cracking" pattern.

• Depth: Usually shallow (25-50mm), but can sometimes penetrate the full depth of thin slabs.

4. Immediate Prevention Strategies

To stop these cracks during or soon after casting:

1. Fogging: Using a fine mist spray to keep the humidity above the slab at 100%.

2. Windbreaks: Erecting temporary barriers to reduce wind velocity across the surface.

3. Sunshades: Providing shade to lower the surface temperature.

4. Evaporation Retarders: Spraying a thin chemical film that prevents water molecules from escaping.

5. Early Curing: Applying wet burlap or plastic sheeting as soon as the surface can support it without damage.