Evaluation of minimum reinforcement ratio in FRC members and application to tunnel linings

In lightly reinforced concrete (RC) structures, the area of steel cannot be lower than a minimum value, so that the ultimate limit state can be reached under a yielding moment higher than the cracking moment. Also in the serviceability stage, a minimum amount of reinforcement should be provided in tensile zones, in order to reduce crack widths. In fiber-reinforced concrete (FRC) members, due to the presence of structural fibers in the cementitious matrix, the minimum amount of steel area can be significantly reduced. Fiber can guarantee tensile stresses in a cement-based matrix even in the presence of wide cracks. Therefore, for the same cross-section of steel, a reinforced FRC member in bending can show higher bending moments, and reduced crack widths, than those measured in classical RC beams. This is particularly true in case of massive members, like the structures of tunnel linings. For such elements, and starting from the constitutive relationships recommended by Rilem TC 162-TDF, a new approach for the evaluation of minimum reinforcement area is proposed in this paper. By means of this nonlinear model, it is possible to calculate a reinforcement area lower than that calculated according to Eurocode 2 and Rilem TC 162-TDF prescriptions.