
Theoretical Bases
Failure surface and Mohr circles For reinforced concrete sections, it is well known that the concrete section resists compressive stresses while the reinforcement bars take the tensile stresses. Thus, the resistance of reinforced concrete members, especially columns, depends on the resistance of concrete to axial compression. On the other hands, the behavior of concrete elements subjected to triaxial compression is better than that of axially loaded elements. If a MohrCoulomb failure criterion is considered, yield is expressed in terms of maximum and minimum principal str. For a certain value of confining stresses σ 3 , failure takes place when the circle, with a diameter equals σ3  σ1 , touches the failure envelop as shown in Fig. Increasing the lateral confining stress, σ3 , will consequently induce an increase in the axial stress σ1 that will produce failure. However, this idea can be implemented to increase the load carrying capacity of structural members by increasing the lateral confining pressure. In reality, the load carrying capacity of reinforced concrete column can considerably increased by applying a reasonable amount of lateral external pressure. The idea of the proposed method is thus to apply a lateral confining stress on the sides of the member. As high as the value of such confining stress is the expected increase in axial load capacity of the member. If the confining of the reinforced concrete element is made along all sides, the effect of strengthening is expected to be so effective. Thus, the proposed method is aimed at applying confining forces at the corner of the section to keep the whole concrete core under confining compression. 


