Types of Joints

Stresses in concrete can be controlled by the proper placement of joints in the structure. We’ll discuss three basic types of joints: isolation joints, control joints, and construction joints.

Isolation Joints

Isolation joints are used to separate (isolate) adjacent structural members. An example is the joint that seperates the floor slab from a column. An isolation joint allows for differential movement in the vertical and horizontal plane and expansion/contraction due to loading conditions or uneven settlement. In this context, they allow for differential movement as a result of temperature changes, as in two adjacent slabs. All isolation joints extend completely through the member and have no load transfer devices built into them. Examples of these are shown in Figures 53, 54 and 55.

Control Joints

Movement in the plane of a concrete slab is caused by drying shrinkage and thermal contraction. Some shrinkage is expected and restrained from curling; cracking will occur wherever the restraint imposes stress greater than the tensile strength. Control joints shown in Figure 56 are cut into the concrete slab to create a plane of weakness. Cracking should occur at a designated place rather than randomly. These joints run in both directions at right angles to each other. Control joints in interior slabs are typically cut 1/3 to 1/4 of the slab thickness and then filled with joint filler. See Table 6 for suggested control joint spacings. Temperature steel (welded wire fabric) can be used to restrict crack width.

In driveways and sidewalks, space contraction joints at intervals about equal to the slab width. Drives and walks wider than 10 to 12 feet should have a longitudinal joint down the center. In patio slabs, joints should not be more than 10 ft apart in both directions. As with floor slabs, make the panels as nearly square as possible. As a general rule, the smaller the panel, the less likelihood of random cracking.

Contraction joints should also be located at re-entrant corners; otherwise cracks are likely to radiate from the corners. The part of a concrete slab within a very sharp corner is likely to crack. Avoid such sharp corners if possible, but if you cannot avoid them, make sure that the subgrade is well compacted and locate contraction joints where cracking is most likely to occur. Reinforcing steel is sometimes added to hold cracks closed tight at sharp or re-entrant corners.

Surface irregularities along the plane of the crack are usually sufficient to transfer loads across the joint in slabs on grade.

Construction Joints

Make construction joints, shown in Figures 57, 58, 59, and 60, where the concrete placement operations end for the day or where one structural element is cast against previously placed concrete. These joints allow some load to be transferred from one structural element to another through the use of keys or, for some slabs and pavement, dowels. Note that the construction joint extends entirely through the concrete element.