Recent research has cast new light on the mechanism of cracking in reinforced concrete members under load. Much of the work has been of a theoretical nature, often involving higher mathematics and advanced computer techniques. This question and answer discussion has been chosen as the most effective way of presenting the findings. What causes a reinforced concrete member to crack under load? Cracking occurs under two conditions: a) when the member is loaded in axial tension and the tensile strength of the concrete is exceeded. And b) when the member is loaded in bending and the modulus of rupture is exceeded. How does the phenomenon proceed? As load increases, secondary crack develop between the primary cracks. The spacing of these cracks proceeds to a minimum value. This spacing is reached when the tensile force necessary to from an additional crack between two existing cracks can no longer be transmitted by bond between the steel and the concrete. Does the development of secondary cracks mean that failure is imminent? No. The minimum crack spacing is the minimum that can be tolerated under safe working load. Failure is preceded by the development of one or more horizontal cracks. The length of these cracks increases rapidly with increasing load. Such compression failure, resulting in final collapse, usually occurs on a plane at about 30 degrees to the horizontal can located at one or both ends of a horizontal crack. Does failure always take place in this manner? No. The mechanism described refers to a state of pure bending. The actual pattern of failure will depend on the type of loading. Also when there is a shear load involved, diagonal tension cracks will also appear.