Long-term drying shrinkage cracks are formed when drying shrinkage is restrained and typical locations are thin slabs and walls, as illustrated in Fig. 3.19. The main causes of shrinkage cracking are inefficient or insufficient joints, inadequate reinforcement, poor curing and too high a water content in the original concrete mix. Remedies are to address those causes at the design stage and to consider the use of a shrinkage reducing admixture as well as the factors affecting drying shrinkage discussed in Section 3.2.3. As in the case of early-age thermal cracking, the form of restraint can either be internal or external. An example of external restraint is that of an unreinforced concrete built-in slab drying from the surface as shown in Fig. 3.23. If, after being relieved by creep, the induced tensile stress exceeds the strength, the result would be a single crack through the slab. For cracking to occur, the tensile stress is given by:
Of course, in practice, steel reinforcement would be provided for structural reasons. Reinforcement does not prevent cracking but instead of a single crack, much smaller cracks are produced that are distributed over the surface of the slab according to the amount and spacing of the reinforcement (Beeby, 1979; Carino and Clifton, 1995). In concrete members having thicker sections, drying from the surface results in a moisture gradient since the inner layers have a greater moisture content than the surface layers. This moisture gradient causes internal restraint and can also lead to surface cracking. In fact, drying shrinkage is not a `true’ shrinkage in the sense that it is simply proportional to the loss of water but is the combination of `true’ shrinkage and strain induced by the moisture gradient. `True’ shrinkage cannot really be measured although researchers have attempted to determine it using very thin cement paste sections. Figure 3.24 demonstrates what would happen in the case of a long member drying from the surface whose end is sealed. `True’ shrinkage would result in the end developing a parabolic shape (Fig. 3.24(b)) but, because of the moisture gradient, the contracting surface layers are restrained by the inner layers so that tension develops in the surface and this is balanced by compression in the interior (Fig. 3.24(c)). The net result is a restrained (observed) shrinkage (Fig. 3.24(d)) and the induced tensile stress at the surface is responsible for potential drying shrinkage cracks. The condition for cracking is:
