Effect of construction joints on the performance of reinforced concrete structures

Abstract

1. Construction joints cause reduction of the loading required to induce failure by 15% to 20% below the computed value for the subject section. Compared with a monolithic beam, the actual strength of beams having a construction joint was found to be 15% lower. On the basis of the results, obtained and until more detailed investigations are carried out, for structures provided with block joints and the usual reinforcement, it can be recommended that the strength loss be accounted for by applying a coefficient of 0.85. 2. For a beam having a construction joint and subjected to a bending moment and a transverse force, the strength is determined by the strength of the concrete compression zone. A low bond strength across the construction joint initiates early crack formation at this section, a wider opening of the joint, and generally larger beam deformations. After the crack has formed, the frictional forces transferred to the compressed zone and the normal component of both the reinforcement shear and that of the remaining part of the compressed concrete generate an oblique crack which is directed from the joint toward the support, and along which the beam failure occurs. 3. The strength of oblique sections in beams with construction joints can be computed by the equations presented in this article. The values of inclination angle α of oblique cracks, for different ratios of M and Q, classes of concrete, layouts of joints, and bond strengths, must be established by tests. 4. To ensure that beams with construction joints will have adequate strength, the concrete compression zone in the joint region must be provided with oblique reinforcement.