Cracking processes

The design of reinforced concrete structures is based on the assumption that concrete does not withstand tension stresses induced by bending, and steel reinforcement is provided to resist these stresses. The concrete in the tension zone will crack and measures are taken to provide sufficient reinforcement to ensure that if cracks do occur they are acceptable both functionally and visually. This type of cracking, along with any cracking due to external loading, is called structural cracking. Concrete is also liable to crack due to internal tensile stresses induced by some form of restraint to free movement. The heterogeneous composite nature of concrete induces differential stresses and strains due to differing properties of the ingredients, as well as when there are moisture and thermal gradients present. Since concrete is weak in tension, induced tensile stress can often lead to cracking but the problem is compounded by creep. Such cracks are known as intrinsic cracks or non-structural cracks. The types of cracks are wide-ranging and their diagnosis is often difficult so that the decisions as to what remedial action to take and when to apply it are correspondingly difficult. The causes and remedies of common types of intrinsic cracks occurring in concrete construction are discussed in this chapter together with the study of stress and strain in the vicinity of cracks, viz. fracture mechanics. Techniques for the repair of cracks and materials used are specialised subjects; this topic is dealt with in detail elsewhere: Concrete Society Technical Report No. 22, 1992 and ACI 224R, 1996.

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