Concrete shrinkage cracks are primarily caused by the material’s volume reduction during the curing process. This occurs through several specific mechanisms:
- Drying Shrinkage: This is the most common cause, where moisture evaporates from the concrete surface, leading to contraction and surface tensile stresses.
- Chemical Reactions: As concrete hydrates, it undergoes chemical changes and self-desiccation (autogenous shrinkage) that result in internal volume changes. This is particularly common in high-performance or low-water mixes.
- Restraint: Cracking occurs when the concrete attempts to contract but is prevented from doing so by surrounding elements. This creates tensile stresses that exceed the material’s strength. Restraint can be internal (aggregate interlock), external (formwork friction), or structural (rebar and ties).
- Mix Design Factors: Using a high water-cement ratio increases porosity and subsequent drying, which heightens the risk of cracking.
Environmental factors in regions like Los Angeles, such as intense sun and thermal cycling, can accelerate these processes, especially in exposed roof slabs and substrates.
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