Sadashi NAKAGAWA

Crack Width Control Design Taking into Account of Initial Cracking State

Masatugu NAGAI, Eiji IWASAKI

In the design of continuous composite girder bridges, at intermediate supports, the concrete slab is subjected to tensile force, and there is possibility of cracking in the concrete. Since cracks cause the damage or deterioration of the concrete, crack initiation has to be prevented or crack width has to be below an acceptable limit, such as 0.2 or 0.3 millimeters. In order to prevent crack initiation, the countermeasures such as pre-stressing are inevitable, and the construction cost is increased due to their procedure. >:From a viewpoint of economy, recently the crack width control design has been adopted. In Japan, two methods for evaluating the crack width have been proofed and used. The difference is in evaluating the strain in reinforcing bars. One calculates the strain in a section consisting of steel girder & reinforcing bars and another takes into account of tension stiffening effect and uses mean strains. However, both assume the cracking state in the concrete to be stabilized state (fully cracked state) and not depend on the magnitude of hogging bending moment. It is supposed to be invalid when the bending moment is small. Hence, our research group proposed new design method taking into account of the initial cracking state.
In order to confirm the validity of the proposed method, the experimental study was carried out. Three types of concrete, such as plain concrete, expansion concrete and concrete with fibers were dealt with.
The followings are main results obtained from this study. 1)The experimental results using plain concrete well agree with those of the proposed method, 2)The sagging bending moment at initial crack of the expansion concrete became larger than that of the plain concrete and that of concrete with fibers, 3)The effect of fibers on decreasing the crack width was not clear.