Effect of Member Geometry on the Modification Factor for the Degree of Restraint Before Cracking in Order to Account for the Effect of Cracking

Abstract

The scientific models for the calculation of the width of early age imposed strains cracks are based on various concepts and are constantly evolving. Due to the complexity of the matter considerable simplifying assumptions are employed. These include the member restraint factor (specified prior to its cracking) used not only for the determination of the risk of the primary crack formation, but also for the determination of subsequent cracks and the calculation of their width. Although such an assumption may lead to an approximation of the expected results, it is by no means compatible with the mechanical bases of member behavior after its primary cracking. In an attempt to address this problem, in the present paper the application of a new model for the calculation of the modification factor (Rσcr) for the degree of restraint before cracking, which takes account of the effect of various scales of cracking is proposed. The model enables a more accurate assessment of the risk of subsequent cracks formation together with the calculation of cracks width both immediately after their formation, i.e. taking account of the effect of member relaxation, and immediately prior to subsequent cracking. A parametric analysis was performed which indicated a significant impact of member geometry (i.e. cross-section and length of the member) on the change of the modification factor after cracking. Depending on the member geometry the limit values of imposed strains to the level of which reinforcement effectively restricts cracks width are also specified.