Non-structural cracks are primarily of three types: plastic, early-age thermal, and drying shrinkage cracks. Plastic cracks are further classified into plastic shrinkage cracks and plastic settlement cracks.
Plastic shrinkage cracks typically form within hours after the concrete is poured. The concrete's surface dries faster than the bottom, creating tensile stress that the still-plastic concrete cannot withstand, leading to diagonal or randomly patterned cracks on the concrete surface.
Plastic settlement cracks occur when the settlement in concrete is obstructed by internal elements such as reinforcements, resulting in cracks above these obstructions, often seen in deep sections and column tops.
As concrete matures and loses moisture, drying shrinkage cracks can form if the concrete member is externally restrained, such as when the concrete's ends are fixed. This generates tensile stresses that lead to surface cracks. The heat generated during cement hydration and its dissipation from the concrete's surface create temperature gradients within the concrete, causing thermal strains. When concrete is prevented from moving due to this strain, thermal cracks develop. Crazing, another form of non-structural cracking, creates a network of fine cracks on the concrete surface. This often results from improper finishing or curing, especially under high humidity gradients within the concrete.
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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