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Abstract:
In recent decades, fiber reinforced polymer (FRP) wrapping has become a common technique to retrofit reinforced concrete (RC) columns. Numerous research works have sought to verify analytically and experimentally its effectiveness in terms of enhancement of axial load bearing capacity and ductility. These studies highlighted that in the case of sharp-cornered sections, the maximum allowable confinement pressure is limited by premature failure at corners and, consequently, stress in the FRP, as well as the distribution of the confinement pressure, is not uniform. The prediction of this phenomenon is not straightforward, and existing theoretical studies propose complex numerical simulations, whereas technical codes provide simplified or empirical relationships for its assessment. This paper presents an analytical model for the evaluation of the effective distribution of confinement pressure in FRP confined concrete members with rounded corners. The model allows considering the interaction with the concrete core and different brittle failure modes, including FRP rupture and debonding. It leads to determining the distribution of the confinement pressure along the section. Results are compared with those achieved by finite-element (FE) analyses and with numerical and experimental data available in the literature. Good agreement is obtained in all cases, showing the reliability of the proposed model.

Keywords:
fiber reinforced polymer (FRP) wrapping, corner radius, confinement pressure, brittle failure

Abstract:
Fibre-reinforced plastic (FRP) composites are extensively used to retrofit civil structures. However, the quality and the characteristics of the bond between the FRP and the structure are critical to ensure the efficacy of the retrofit. For this reason, effective non-destructive evaluation (NDE) methods are often necessary to assess the bonding conditions. This article presents an ultrasonic technique for detecting defects at the FRP-substrate interface. The technique uses the Akaike Information Criterion, to detect automatically the onset of the ultrasonic signals, and the novel Equivalent Time Lenght (ETL) parameter, to quantify the energy of the propagating ultrasonic signals along the interface between FRP and concrete. The uniqueness of the ETL is that it is not affected by the coupling conditions between the ultrasonic probes and the structure. The proposed NDE technique has been tested numerically by performing 2D Finite-Element analysis and experimentally on reinforced concrete samples. The results show that the method is robust and cost-effective.

Keywords:
CFRP, fibre-reinforced materials, concrete, bonding, non-destructive testing, ultrasonic methods, equivalent time length

Abstract:
The seismic nonlinear response of reinforced concrete structures permits to identify critical zones of an existing structure and to better plan its rehabilitation process. It is obtained by performing finite element analysis using numerical models classifiable into two categories: lumped plasticity models and distributed plasticity models. The present work is devoted to the implementation, in a finite element environment, of an elastoplastic Euler–Bernoulli beam element showing possible slope discontinuities at any position along the beam span, in the framework of a modified lumped plasticity. The differential equation of an Euler–Bernoulli beam element under static loads in presence of multiple discontinuities in the slope function was already solved by Biondi and Caddemi (Int J Solids Struct 42(9):3027–3044, 2005, Eur J Mech A Solids 26(5):789–809, 2007), who also found solutions in closed form. These solutions are now implemented in the new beam element respecting a thermodynamical approach, from which the state equations and flow rules are derived. State equations and flow rules are rewritten in a discrete manner to match up with the Newton–Raphson iterative solutions of the discretized loading process. A classic elastic predictor phase is followed by a plastic corrector phase in the case of activation of the inelastic phenomenon. The corrector phase is based on the evaluation of return bending moments by employing the closest point projection method under the hypothesis of associated plasticity in the bending moment planes of a Bresler’s type activation domain. Shape functions and stiffness matrix for the new element are derived. Numerical examples are furnished to validate the proposed beam element.

Keywords:
Slope discontinuity, Nonlinear pushover analysis, Lumped plasticity, Plastic hinge

Abstract:
A beam/column element allowing the formation of multiple plastic hinges in columns or beams of a reinforced concrete (RC) framed structure is used in this work to show, through an application, its advantages with respect to conventional lumped plasticity models. Slope discontinuities can be located at any position of an Euler-Bernoulli beam span and not at the two extremes only. The model is in fact written in the framework of a modified lumped plasticity theory, and respectful of a thermodynamic approach. Flow rules and state equations are derived invoking the Theorem of maximum dissipation and using a Bresler’s type activation domain. The beam element has already been implemented in a researchoriented code to run nonlinear analyses on 2-D frames. The discretized loading process is separated, at each step, in two phases: a predictor and a corrector phase. Numerical examples highlight how the new finite element permits to run nonlinear analyses avoiding a mesh refinement.

Keywords:
beam element, plastic hinge, lumped plasticity, slope discontinuity, nonlinear FEM analysis

Abstract:
To study the response of cementitious joints in rock masses or masonry structures, the model of interphase is considered for which the contact stresses and strains interact with the internal stresses or strains within the joint. The frictional slip and sliding effects are then combined with the inelastic deformations of the joint. The constitutive model of the joint is analysed by assuming two interfaces separating the joint from the adjacent material. The case of a cementitious layer interposed between two rigid bodies is treated in detail.

Keywords:
Softening, Contact, Interfaces, Cohesive joint

Abstract:
An interface constitutive model is presented accounting for slip and sliding effects and also for dilatancy phenomena. The microslip effects are described by considering spherical asperity interaction with variation of contact area and generation of progressive or reverse slip zones. The incremental constitutive equations are derived with proper memory rules accounting for generation and annihilation of particular slip zones during the process of variable loading. It is further assumed that sliding of spherical contacts occurs along large asperities whose slope varies due to the wear process. The predicted shear and dilatancy curves are shown to provide close quantitative simulation of available experimental data. The strain ratchetting effect for non-symmetric cyclic loading was exhibited using the asperity wear model. The model presented could be applied to simulate rock joints, masonry, or concrete cracked interfaces, under monotonic and cyclic loading.