When a primary flexural crack forms and crosses the reinforcing steel, the concrete adjacent to the crack slips against the steel. Slip, or bond-slip, is defined as the relative movement between the steel and surrounding concrete. However, the concrete in tension does not undergo immediate and rapid unloading. Shortly after the formation of a primary crack, secondary inclined cracks form at the surface of the reinforcement. Concrete between secondary cracks is a comb-like structure which wedges behind the ribs. It offers frictional resistance to further slippage and assists the transfer of the tensile force in the concrete to the steel.