10.9
Types of Non-structural Cracks in Concrete
Non-structural cracks are primarily of three types: plastic, early-age thermal, and drying shrinkage cracks. Plastic cracks are further classified int…
After placing concrete, water may rise to the top of the mix, settling the aggregates.
If reinforcements within the concrete obstruct this natural settlement, plastic settlement cracks develop above the reinforcements. Increasing the top cover can mitigate these cracks.
Water evaporation from concrete's top surface induces tensile stresses in the top layers when restrained from contraction, resulting in diagonal or random patterns of plastic shrinkage cracks.
Preventing water evaporation from concrete reduces these cracks.
Drying shrinkage in externally restrained concrete develops tensile stresses at the concrete's surface, resulting in surface cracks.
Movement joints are provided to minimize these cracks.
The dissipation of heat of hydration from concrete surfaces leads to surface cooling.
If the subsequent contraction of the concrete's top surface is restrained by its interior, tensile stresses are induced on the concrete surface, developing early thermal cracks.
Insulating concrete minimizes the occurrence of these cracks.
Lastly, over-trowelling rich concrete mixes accumulate the cement paste on the surface, which, upon drying, causes discontinuous hairline surface cracks known as crazing.
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Q1: What causes plastic settlement cracks in concrete?
Plastic settlement cracks develop when water rising to the concrete's top surface is obstructed by internal reinforcements. As aggregates settle naturally, reinforcements block this movement, creating tensile stresses and cracks directly above the obstructions. These cracks commonly appear in deep sections and column tops. Increasing the top cover over reinforcements effectively mitigates their formation.
Q2: How does water evaporation lead to plastic shrinkage cracks?
Plastic shrinkage cracks form when the concrete surface dries faster than the interior, creating tensile stresses that the still-plastic concrete cannot resist. These cracks typically appear within hours after pouring in diagonal or random patterns on the surface. Preventing water evaporation through proper curing significantly reduces crack formation during this early stage.
Q3: What is the relationship between drying shrinkage and external restraint?
Drying shrinkage cracks develop when externally restrained concrete loses moisture and contracts. External restraint occurs when concrete member ends are fixed or movement is prevented. This restraint generates tensile stresses at the concrete surface, resulting in surface cracks. Movement joints are provided to minimize these cracks by allowing controlled contraction.
Q4: How do temperature gradients cause early thermal cracks in concrete?
Heat generated during cement hydration dissipates from the concrete surface, creating temperature gradients within the material. The surface cools and contracts faster than the interior, inducing tensile stresses on the surface. If this contraction is restrained by the warmer interior concrete, early thermal cracks develop. Insulating concrete minimizes heat dissipation and reduces crack occurrence.
Q5: What is crazing and how does it form on concrete surfaces?
Crazing is a network of fine, discontinuous hairline surface cracks that develop from over-trowelling rich concrete mixes. Excessive trowelling accumulates cement paste on the surface, which dries and shrinks unevenly. This creates the characteristic crazing pattern. Improper finishing or curing, especially under high humidity gradients, contributes to crazing formation.
Q6: How are plastic cracks different from drying shrinkage cracks?
Plastic cracks form within hours after concrete is poured while the material remains workable, caused by settlement obstruction or rapid surface drying. Drying shrinkage cracks develop later as mature concrete loses moisture and contracts under external restraint. Plastic cracks result from immediate post-placement conditions, while drying shrinkage cracks emerge during the concrete's aging process.
Q7: What preventive measures reduce different types of non-structural cracks?
Increasing top cover over reinforcements prevents plastic settlement cracks. Preventing water evaporation reduces plastic shrinkage cracks. Providing movement joints minimizes drying shrinkage cracks. Insulating concrete reduces early thermal cracks by slowing heat dissipation. Proper finishing and curing techniques prevent crazing. Each crack type requires targeted prevention strategies based on its formation mechanism.