Joints, dowels and tie bars

Concrete shrinks as it dries and expands and contracts with temperature. A long slab restrained by friction on its base would crack at random. Joints are deliberate, planned weak lines that let cracks form where they can be sealed, maintained and kept tight. Getting joints right is as important as getting the thickness right; most distress in concrete roads appears at joints.

Transverse contraction joints are sawn to about a quarter or a third of the slab depth, early enough to beat random cracking but late enough that the saw does not ravel the edges; spacing is typically about 4.5 m in plain jointed pavements. Longitudinal joints divide lanes and control cracking across the width. Expansion joints allow for expansion, mainly at bridges and fixed structures; isolation joints separate slabs from manholes and buildings. Construction joints occur at the end of a day's work and are designed like contraction joints.

Dowel bars are smooth round steel bars placed across transverse joints at mid-depth, parallel to the road and to the surface, and debonded on one side so the slab can move lengthwise. They transfer wheel load from one slab to the next, preventing faulting and pumping. Tie bars are deformed bars across longitudinal joints that hold adjacent lanes together but are not meant to transfer much load. Dowel misalignment, from poor baskets or inserters, locks joints and causes cracking. Joint sealant keeps water and grit out.