EK118

A reinforced concrete beam will initially behave elastically throughout, as the applied uniform moment is increased. When the limiting tensile strain for the concrete is reached a crack will form, and the adjacent tensile zone will no longer be acted upon by tension forces. The curvature of the beam, however, causes further direct tension stresses to develop at some distance form the original crack to maintain internal equilibrium. This in turn causes further cracks to form, and the process continues until the distance between cracks does not permit sufficient tensile stresses to develop to cause further cracking. These initial cracks are called “primary cracks“, and the average spacing in a region of constant moment will be largely independent of reinforcement detailing. As the applied moment is increased beyond this point, the development of cracks governed to a large extent by the reinforcement. Tensile stresses in the concrete surrounding reinforcing bars are caused by bond as the strain in the reinforcement increases. These stresses increase with distance from the primary cracks and may eventually cause further cracks to form approximately midway between the primary cracks. This action may continue with increasing moment until the bond between concrete and steel is incapable of developing sufficient tension in the concrete to cause further cracking in the length between existing cracks. Since the development of the tensile stresses is caused directly by the presence of the reinforcing bars, the spacing of cracks will be influenced by the spacing of the reinforcement. If bars are sufficiently close for their zones of influence to overlap then secondary cracks will join up across the member, while otherwise they will form only adjacent to the individual bars.