Diagonal Cracking and Diagonal Crack Control in Structural Concrete

A number of fundamental concepts relevant to all types of cracking are examined. A tension stiffening relationship derived from first principles indicates that traditional empirical relationships include significant residual tension stresses from uncracked concrete. Service load crack strains should not be estimated using an empirical tension stiffening expression. While primary cracks continue to form up to strains of 0.0010, due to deformation of concrete between visible cracks, the minimum strain that should be used with the stable crack spacing is 0.0005. A magnification factor must be applied to crack spacings at smaller strains, or a minimum strain of 0.0005 used to estimate crack width. Test results indicate that the 95th percentile crack width is 2.0 times the average crack width. Procedures for diagonal crack inclination, spacing and width are reviewed, and a simplified expression for estimating diagonal crack widths is presented. Diagonal crack widths are generally larger than flexural crack widths in members with orthogonal reinforcement due to diagonal strains being larger than reinforcing bar strains. Current code requirements for side-face reinforcement were developed to control flexural cracking, and may not be adequate to control diagonal cracking in certain exposure conditions. The simplified expression for diagonal cracking was used to develop an expression for the maximum spacing of side face reinforcing bars to control flexural and diagonal cracking in large members. A design example illustrates the proposal. Finally, it is shown how the proposed methodology can be used to extend the current ACI expression for spacing of reinforcement near a surface in tension to treat the case of diagonal cracking.