Joints in bridges fall into two categories: deflection joints and expansion joints.
Contrary to what the name implies, deflection joints, when placed in concrete barriers and parapets, are used primarily to minimize the vertical shrinkage cracking that would otherwise occur in long, unjointed panels. Some states permit a longitudinal spacing of joints as great as 30 ft (9.1 m) in simple spans. Over piers of continuous bridges, the spacing is generally less, 7.5 ft (2.3 m) or closer. Preformed joint filler is used to form the joints and is left in place. Sometimes the placement of parapet concrete is required to be done in two stages, with placement of alternate panels only in the first stage, to facilitate placement of the joint filler.
When barriers are permitted to be slipformed, the deflection joints are sawn an inch or so deep on the periphery of the barrier, and then caulked with a joint sealer. In this case, the steel is not made discontinuous at the joints. Slipforming is a much faster way of constructing barriers, but the finished appearance, especially the straightness of the top, is sometimes rather crude compared with conventionally formed barriers.
Deflection joints can extend full depth of the barrier or parapet, or through only the top portion. Deflection joints in the New Jersey safety-shape barriers, when the concrete is placed in forms, are sometimes placed only above the curb portion of the barrier. In this case, the longitudinal reinforcing steel is continuous in the curb, but discontinuous at the joints above the curb. This usually results in reflection cracks developing in the curb below the joints.
It is also common, in spite of the joints, to see one or more vertical cracks between the joints in long panels. These cracks may be aggravated by bridge deflection but are caused primarily by shrinkage. The development of these cracks illustrates a rule of thumb applied to slabs on grade, that there will be a tendency to crack if the slab is longer than twice its width. However, the likelihood of ultimate damage to the bridge resulting from these unwanted cracks is small, and so the cost to provide more closely spaced joints is not justified.
Deflection joints are also used in the deck slab at piers or over transverse floor beams where the slab is not continuous (and sometimes when it is continuous), and at abutments where the bridge slab abuts the approach slab. Since the amount of movement is small, due only to rotation, the joint can be sealed with a small compression seal or with liquid joint sealer.
