Why dowel bar is provided in road pavement joints?

Dowel bars are short steel bars that provide a mechanical connection between slabs without restricting horizontal joint movement. They increase load transfer efficiency by allowing the leave slab to assume some of the load before the load is actually over it. This reduces joint deflection and stress in the approach and leave slabs.

Dowel bars are typically 32 to 38 mm (1.25 to 1.5 inches) in diameter, 460 mm (18 inches) long and spaced 305 mm (12 inches) apart but it depend on each country's standard code because it can be different from each other. Specific locations and numbers vary by state, however a typical arrangement might look like Figure 1. In order to prevent corrosion, dowel bars are either coated with stainless steel (Figure 2) or epoxy (Figure 3). Dowel bars are usually inserted at mid-slab depth and coated with a bond-breaking substance to prevent bonding to the PCC. Thus, the dowels help transfer load but allow adjacent slabs to expand and contract independent of one another. Figure 3 shows typical dowel bar locations at a transverse construction joint.

Figure 2. Stainless steel-clad dowel bars/ (Epoxy Coating on Ends Only)

Figure 3. Dowel bars in place at a construction joint- the green color is from the epoxy coating.

Source: http://www.pavementinteractive.org/article/dowel-bar/