In reinforced concrete construction, efficient and reliable force transfer between reinforcement and concrete is required for optimal design. The transfer of forces from the reinforcement to the surrounding concrete occurs for a deformed bar (the following figure) by:
After initial slip of the bar, most of the force is transferred by bearing. Friction, however, especially between the concrete and the bar deformations (ribs) plays a significant role in force transfer.
it is possible to say that bond resistance is governed by:
as shown in the following figures, The cracks shown in Fig. (a), known as Goto (1971) cracks, can result in the formation of a conical failure surface for bars that project from concrete and are placed in tension. They otherwise play only a minor role in the anchorage and development of reinforcement. The transverse cracks shown in Fig. (b) form if the concrete cover or the spacing between bars is sufficiently small, leading to splitting cracks, as shown in Fig. (c). If the concrete cover, bar spacing, or transverse reinforcement is sufficient to prevent or delay a splitting failure, the system will fail by shearing along a surface at the top of the ribs around the bars, resulting in a “pullout” failure, as shown in Fig. (d). It is common, for both splitting and pullout failures, to observe crushed concrete in a region adjacent to the bearing surfaces of some of the deformations. If anchorage to the concrete is adequate, the stress in the reinforcement may become high enough to yield and even strain harden the bar. Tests have demonstrated that bond failures can occur at bar stresses up to the tensile strength of the steel.
Therefore bond, anchorage, development, and splice strength depend on the materials, spacing and geometry of the reinforcing bar and of course adequacy of the concrete cover.
The more common causes of damage to the concrete cover are as follows:
Not all damaged concrete requires immediate repair. Many factors need consideration before the decision to perform repairs can be made. Obviously, repair is required if the damage affects the safety or safe operation of the structure. Similarly, repairs should be performed if the deterioration has reached a state, or is progressing at a rate, such that future serviceability of the structure will be reduced. For more information please refer to concrete repair and structural strengthening section in this website.