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Any concrete structure would eventually see cracks and demand proper repair. Nonetheless, one needs to understand the nature of the crack to repair them properly. Generally, cracks are either static or dynamic. The former is primarily aesthetic and may or may not need repair, whereas the latter endangers the structure's integrity and requires adequate repair.
“Cracking in concrete is influenced by the long-term conditions to which the concrete member is subjected. Generally, long-term exposure and long-term loading extend the magnitude of cracks, especially their width, in both reinforced and plain concrete." (ACI 22R-01 2001).
Static cracks develop due to one-time events such as thermal and shrinkage stresses during concrete curing. However, dynamic cracks are due to recurring forces like rapid freezing and thawing cycles, expansive soils, loading of concrete, and thermal changes.
Dynamic Cracks vs. Static Cracks
Environmental conditions and the nature of imposed loads control whether a crack is static or dynamic.
- A static crack is a thin or hairline surface crack that commonly occurs in concrete and propagates very slowly. It seems to be unchanging and considered to be primarily aesthetic. A static crack may need little or no structural crack repair.
- Dynamic crack is a term used to describe the growing process of cracks in concrete structures due to rapidly occurring forces like changing loads imposed on the structure.
- Static cracks can be eyesores and allow water penetration, whereas dynamic cracks compromise the structure's integrity.
- Static cracks may turn to dynamic cracks if they suffer from repeated or increased loading, which at this stage may endanger the integrity of the concrete.
- Static cracks result from a one-time event like thermal or shrinkage stress during concrete curing or damage from a car hitting concrete.
- Dynamic cracks result from recurring forces such as rapid thermal changes, freezing and thawing cycles, expansive soils, loading of concrete, etc.
- Corrosion of steel bars can increase the speed of dynamic crack growth.
- Shrinkage cracks are a common type of static cracks in the concrete. When concrete is exposed to temperature fluctuation, it expands and contracts accordingly. The thermal influences initiate thin cracks on the concrete surface because the material tries to breathe. Commonly, the width and length of shrinkage cracks do not increase.
- Static cracks are commonly vertical or diagonal in direction, whereas dynamic cracks are horizontal in orientation.
- Epoxies are suitable to repair static cracks if they do not leak. However, flexible polyurethane resin can permanently seal active leaks.
The best strategy to specify whether a crack is dynamic or static is to observe the crack over a period of six months. If the width of cracks increases, they are likely to be active, and the repair work should involve both structural reinforcement and sealing. Static cracks can be repaired with injection and rarely exceed 0.6 mm in width.
Dynamic crack is a term employed to define cracks that are growing over time. It results from recurring forces such as rapid thermal changes, freezing and thawing cycles, expansive soils, loading of concrete, etc.
A static crack is a thin or hairline surface crack that commonly occurs in concrete and propagates very slowly. It seems to be unchanging and considered to be primarily aesthetic.
The acceptable width of cracks in a concrete structure is 0.3 cm.
Cracks in concrete commonly start within 12 hours of the finishing process. This time period may vary based on the weather conditions.