Cracking analysis of FRC beams under sustained long-term loading

Crack formation within concrete members undergoing flexural loading is a complex mechanism, which governs the serviceability and durability of concrete structures. As for reinforced concrete (RC) members, a number of works based on empirical or theoretical approaches are published in the scientific literature. All the models propose a formulation for the estimation of crack spacing and crack width taking into account several parameters. Mechanical properties of concrete matrix, reinforcement ratio, concrete cover, bar diameter and size effect are the most influencing parameters on the cracking pattern of RC members, while tension stiffening can be influential as well. In Fiber Reinforced Concrete (FRC) elements the presence of short fibers modifies the crack pattern within the members due to the development of a residual tensile stress and greater toughness. Normally the number of cracks within the length of FRC members is higher while the mean crack spacing and the crack width are lower. In fact the crack bridging effect of fibers consists in post-cracking stresses between the crack faces. Such mechanism is mainly governed by the interface bond between fiber and concrete matrix. Therefore, the volume fraction and the geometrical properties of fibers strongly influence the overall contribution in the cracking phenomena. A limited number of design codes have taken into account the modified behaviour of FRC members (especially in the case of steel fibers) by providing specific equations for crack width. This work presents the results of an experimental campaign on RC beams subjected to sustained service loads and environmental exposure for 72 months. In some beams, short steel or polyester fibers were added to the concrete matrix. The results presented in the paper show that the addition of fibres in concrete reduces both flexural displacements and crack widths, by modifying also the long-term behaviour of FRC members.