Cracks in Early Concrete: A Comprehensive Guide to Identification, Prevention, and Repair
Early-age cracking in concrete can compromise its durability, aesthetics, and structural integrity. Understanding the types of cracks that occur during or within a few hours of casting is crucial for effective prevention and remediation. This guide details the causes, identification, prevention strategies, and repair guidance for common early concrete cracks, with a focus on plastic shrinkage and plastic settlement cracks.
Why Does Concrete Crack So Soon?
Concrete is a complex material, and its behavior in the fresh (plastic) state is highly influenced by environmental conditions and mix properties. The primary reason for early cracking is the concrete's inability to resist tensile stresses while it is still soft and unhardened.
1. 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.
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.
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).
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.
Visual Characteristics of Plastic Shrinkage Cracks:
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.
2. Plastic Settlement Cracking: Causes and Mechanism
Plastic settlement cracks occur due to the downward settlement of concrete while it is still in its plastic state. This happens when the concrete mix has a high water content, inadequate vibration, or when obstructions (like rebar or formwork) prevent uniform settlement.
The Core Mechanism:
As concrete bleeds, the solid particles settle. If this settlement is hindered or uneven, it can lead to cracking.
Obstruction: Rebar, embedded items, or changes in slab thickness can create zones where settlement is restricted. The concrete above these obstructions is "hung up" while the surrounding concrete settles, leading to tensile stresses.
Insufficient Cover: If rebar or other embedded elements are too close to the surface, the differential settlement above and around them can cause cracks.
Poor Vibration: Inadequate or excessive vibration can lead to localized areas of poor compaction and differential settlement.
Visual Characteristics of Plastic Settlement Cracks:
Timing: Often appears immediately after placing and vibrating (during bleeding).
Pattern: Tends to occur over reinforcing bars, embedded items, or changes in section. They are typically straight or slightly curved.
Depth: Can be deeper and wider at the surface compared to plastic shrinkage cracks, sometimes extending deeper than 50mm.
Comprehensive Identification Guide: Plastic Shrinkage vs. Plastic Settlement
| Feature | Plastic Shrinkage Cracks | Plastic Settlement Cracks |
| Timing | 1-6 hours after casting (before final set) | Immediately after placing & vibrating (during bleeding) |
| Appearance | Crow's foot / Map pattern, sometimes parallel | Often follows rebar/obstacles, linear |
| Depth | Shallow (25-50mm), can be full depth in thin slabs | Deeper, wider at surface |
| Primary Causes | Rapid surface evaporation, high winds/temp/low humidity, insufficient bleed water | Concrete settles, obstacles hinder settling, insufficient cover over rebar, poor vibration |
| Finishing | Occurs before final finishing | Can occur before or during initial finishing |
Methods to Prevent Plastic Settlement Cracks
Preventing plastic settlement cracks requires careful attention to mix design, placement, and vibration practices.
4. Immediate Prevention Strategies for Plastic Shrinkage Cracks
To stop these cracks during or soon after casting:
Fogging: Using a fine mist spray to keep the humidity above the slab at 100%.
Windbreaks: Erecting temporary barriers to reduce wind velocity across the surface.
Sunshades: Providing shade to lower the surface temperature.
Evaporation Retarders: Spraying a thin chemical film that prevents water molecules from escaping.
Early Curing: Applying wet burlap or plastic sheeting as soon as the surface can support it without damage.
