Conference Agenda

The present paper deals with the brittle behaviours of high-performance reinforced concrete beams for rather low or high reinforcement percentages. In the former case, the loading drop is due to tensile concrete cracking, whereas in the latter it is due to compression concrete crushing at the opposite beam edge. For the former case, an analytical model is introduced (the Bridged Crack Model) that is able, through a peculiar rotational compatibility condition, to deduce the redundant closing forces applied by the longitudinal reinforcement to the crack faces. This model is conceptually relevant, since it permits to find the minimum reinforcement condition. The linear elasticity of the matrix and the LEFM stress-singularity at the crack tip provide a power-law for the reinforcement percentage as a function of the beam depth raised to –1/2. On the other hand, introducing a numerical model where concrete is considered as a cohesive softening material both in tension and compression, we can obtain a double size-scale brittle-to-ductile-to-brittle transition. By applying Dimensional Analysis and a best-fitting procedure, both in tension and compression, it is possible to find the scaling laws for minimum and maximum reinforcement percentages, respectively. The two exponents become equal to –0.15 and –0.25, respectively. The absolute values of both these exponents are lower than the absolute value of the reference LEFM exponent 0.50 (scaling of extreme severity) and agree with the available experimental results very well. The first has recently been assumed as the reference value in the AASHTO Guidelines for the minimum flexural reinforcement. Unfortunately, we can not affirm the same for the most well-known National and International Standard Codes.

The structural behaviour of prestressed concrete beams is considerably affected by different nonlinear phenomena occurring in the post-cracking and crushing regimes, such as snap-back or snap-through instabilities. Design procedures included in current technical Standards are not able to take into account the actual flexural crushing regime, since the adopted constitutive laws overlook the strain-softening and strain-localization behaviour of the concrete matrix. Moreover, design provisions are usually based on Plasticity Theory, leading to completely disregard size-scale effects and ductile-to-brittle transitions as functions of the beam depth. The present work intends to outline a comprehensive theoretical framework for prestressed concrete structural behavior by means of a Fracture Mechanics approach. The Cohesive/Overlapping Crack Model (COCM) is able to simulate the strain-softening and strain-loaclization behaviour of concrete both in tension and compression, predicting the nonlinear crushing behaviour of prestressed concrete beams. As a matter of fact, the correct estimation of scale effects on maximum reinforcement percentage requires a thorough knowledge of the complex phenomena characterizing the compression crushing failure, leading to define the field in which prestressed concrete structures can develop a safe ductile behaviour. New standard requirements for an effective structural design are formulated.

The Cohesive/Overlapping Crack Model (COCM) is able to describe the transition between flexural cracking and flexural crushing failures occurring in high-performance GFRP-bar reinforced concrete (GFRP-RC) beams by increasing beam depth and/or GFRP reinforcement percentage. In this framework, tensile and compression ultimate behaviours of the concrete matrix are modeled through two different process zones that advance independently one of another. The application of this nonlinear fracture mechanics model to GFRP-RC highlights that the ductility, which is represented by the plastic rotation capacity of the beam, occurs for high-strength concrete matrix only when the internal reinforcement can slip. Thus, the slippage of the GFRP-bar inside of the concrete matrix becoming a basic new requirement for this type of reinforcement, a comprehensive preliminary campaign is devoted to optimize the surface roughness of this internal reinforcement, to obtain a suitable pull-out behaviour. In particular, the GFRP-bar roughness is varied following a geometrical progression in rib spacing, to investigate the slippage behaviour, thanks to which the pseudo-plastic rotation capacity of the composite beam is guaranteed.