Engineering and Construction

One option to control the location of shrinkage cracking is to use contraction joints. A contraction joint, also known as a control joint, is usually formed as a continuous straight slot in the top of the concrete slab. The slot forms a plane of weakness in which the shrinkage crack will develop. A common method of constructing the slot is to saw-cut the concrete after it is strong enough to resist tearing or other damage by the saw blade. Normally the concrete slab-on-grade foundation is saw-cut 4 to 12 h after the concrete begins to harden. It is often recommended that the concrete slab-on-grade be saw-cut to a depth equal to 1/4 the thickness of the slab. If the slab is not saw-cut to an appropriate depth, then the drying shrinkage crack will deviate from the saw-cut, such as shown in Fig. 12.13.

There are many other methods for installing contraction joints in concrete slab-on-grade foundations. For example, during the finishing of the top surface of the concrete, grooves can be formed in the concrete. Another possible option is to install plastic strips in the top of the slab before the concrete has set, such as shown in Fig. 12.14.



FIGURE 12.14 View of a plastic strip that was used to form a contraction joint. The plastic
strip is sometimes referred to as a zip-strip. The gaps that have opened-up on both sides of the
plastic strip were caused by drying shrinkage.





The goals of contraction joints are to force the shrinkage cracking to develop at specific areas and to minimize the width of the shrinkage cracks. By limiting the width of the shrinkage crack, vertical loads are transmitted across the contraction joint by aggregate interlock between the opposite faces of the crack. In order to increase load transfer across contraction joints, steel reinforcement or steel dowels can be placed across the joint.

Design and Control of Concrete Mixtures (PCA, 1994) provides two tables that can be useful for the design of contraction joints, the first (Table 9-1) provides dowel sizes and spacing at contraction joints and the second (Table 9-2) provides maximum spacing of contraction joints as a function of floor slab thickness, slump, and maximum-size aggregate. As this document indicates, contraction joints should not be random,  rather they should consist of a standard square pattern with the spacing of contraction joints as recommended by the text. The joint spacing should be decreased if the concrete is suspected of having high shrinkage characteristics and the resulting panels should be approximately square.

Contraction joints are commonly used for buildings such as warehouses that have large concrete slab-on-grade foundations. It should be recognized that if there is no steel reinforcement across the contraction joints, they become planes of weakness in the foundation. For example, Fig. 12.15 shows about 1 in. (25 mm) opening of a contraction joint due to slope movement that pulled apart the foundation at the contraction joints. Contraction joints placed at the living areas of slab-on-grade foundations can also result in damage to brittle floor coverings placed over the contraction joints. For example, Fig. 12.16 shows the opening of a contraction joint (lower arrow) that has resulted in the cracking of shower tiles (upper arrow) that span the contraction joint.

As previously mentioned, concrete having a low volume-to-surface ratio will produce more drying shrinkage.

This is important because excessive shrinkage is often observed for slab-on-grade foundations that are too thin.