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 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.