Eva Oller

Shear-flexural strength mechanical model for the design and assessment of reinforced concrete beams

A conceptual model for the prediction of the shear-flexural strength of slender reinforced concrete beams with and without transverse reinforcement is presented. The model incorporates the shear transferred by the un-cracked concrete chord, along the cracks length, by the stirrups, if they are, and, in that case by the longitudinal reinforcement. After the development of the first branch of the critical shear crack, failure is considered to occur when the stresses at any point of the concrete compression chord reach the assumed biaxial stress failure envelope. A physical explanation is provided for the evolution of the shear transfer mechanisms, and the contribution of each one at ultimate limit state is formulated accordingly. Simple equations are derived for shear strength verification and for designing transverse reinforcement. The method is validated by comparing its predictions with the results of 1131 shear tests, obtaining very good results in terms of mean value and coefficient of variation. Because of its accuracy, simplicity and theoretical consistency, the proposed method is considered to be very useful for the practical design and assessment of concrete structures subjected to combined shear and bending.

Shear Design and Assessment of Reinforced and Prestressed Concrete Beams Based on a Mechanical Model

Safe and economical design and assessment of reinforced (RC) and prestressed concrete (PC) beams requires the availability of accurate but simple formulations which adequately capture the structural response. In this paper, a mechanical model for the prediction of the shear-flexural strength of PC and RC members with rectangular, I, or T sections, with and without shear reinforcement, is presented. The model is based on the principles of concrete mechanics and on assumptions supported by the observed experimental behavior and by the results of refined numerical models. Compact, simple, and accurate expressions are derived for design and verification of the shear strength, which incorporate the most relevant shear transfer actions. Excellent agreement between the predictions of the model and the results of the recently published ACI-DAfStb databases, including more than 1,287 tests on RC and PC beams with and without stirrups, has been observed. The theory behind the model provides consistent explanations for many aspects related to the shear response that are not clearly explained by current code formulations, making it a very helpful tool for daily engineering practice.

Pilot Experiences in the Application of Shape Memory Alloys in Structural Concrete

The interesting properties of shape memory alloys (SMA) have increased the investigation, study, and development of possible applications of these alloys in the field of civil engineering. This paper presents a state of the knowledge of existing studies and applications of SMAs in distinguishing their properties: superelasticity, shape memory, and damping. The main objective of this paper is to discuss the advantages of concrete structures and improvements in their behavior when using SMAs and to collect critical unsolved aspects of SMAs that could be a starting point for the development of future research in this field.

Numerical Analysis of Shear Critical RC Beams Strengthened in Shear with FRP Sheets

AbstractThe objective of this paper is to contribute to the understanding of the shear resisting mechanisms in RC beams shear-strengthened by externally bonded fiber-reinforced polymer (FRP) sheets. For this purpose, a fiber beam model of RC frames subjected to combined normal and shear forces, previously developed by the authors, has been extended to include the response of externally bonded FRP shear reinforcement in a wrapped configuration. No FRP delamination phenomena or tensile strength reductions in the corner zones are taken into account in the model. The numerical results have been compared with eight existing experimental results and the influence of the FRP sheets on the shear strength of the beam has been studied. The effects of the contribution of FRP ratio on the concrete, on the transversal steel strains and stresses, on the longitudinal tensile steel stresses, and on the diagonal compression struts have been analyzed. It is concluded that the presence of FRP reinforcement modifies the incl...

Predicting the Response of FRP-Strengthened Reinforced-Concrete Flexural Members with Nonlinear Evolutive Analysis Models

To design efficient and economical strengthening solutions, the structural performance before and after the intervention must be accurately evaluated. In the case of statically indeterminate structures or when the structure has suffered damage or deterioration, linear elastic analysis methods are not adequate to obtain the residual capacity and the structural effects of the intervention because of the nonlinear response of the structure. In such cases, refined analytical models able to capture the structural nonlinear behavior, the effects of previous damage, and those produced by any intervention are required to design safe and economical strengthening solutions. In this paper, a nonlinear and time-dependent evolutive analysis model, previously developed by the authors, is applied to the prediction of the response of fiber-reinforced polymer (FRP)-strengthened concrete structures in flexure. The model can take into account the structural effects of changes in geometry, structural scheme, material properties, and applied loads that may occur along the structure service life, including those attributable to strengthening. A criterion to predict peeling failure in FRP-strengthened beams on the basis of nonlinear fracture mechanics consisting in evaluating the maximum shear force that can be transmitted to the concrete by the FRP laminate between cracks or at the laminate end is incorporated in the model. Two previous experimental programs have been used to validate the model. First, four RC continuous beams, three of them strengthened with FRP laminates and tested to study the influence of the FRP arrangement, are analyzed. Second, two beams previously precracked owing to service loads and strengthened with FRP are analyzed under increasing load up to failure. In all cases, very good agreement between the theoretical and the experimental results is obtained in terms of deflections, strains, reactions, internal forces, and failure mode, showing the capabilities of the model to evaluate the efficiency of proposed strengthening solutions.

Laminate debonding process of FRP-strengthened beams

Despite the significant enhancement in service and ultimate conditions that can be achieved by bonding a fibre-reinforced polymer (FRP) laminate to a beam, the existing experimental research has shown the appearance of some types of brittle failures that involve the laminate debonding, before the design load is reached and a classical failure mode (concrete crushing or FRP rupture) occurs. The laminate debonding is generally initiated from within the concrete substrate between the externally bonded laminate and the internal reinforcement. The debonding initiation point can be found either at the laminate end if debonding is due to a high stress concentration at the cut-off point, or along the span when debonding is caused by the effect of bending moments and/or shear forces. The design procedure to obtain the laminate area to strengthen a reinforced concrete element should avoid these premature peeling failures. Therefore, there is a need to understand the mechanics of the laminate debonding process in order to prevent it. The propagation process of an interfacial crack can be described through the evolution of different stages by using non-linear fracture mechanics (NLFM) theory and assuming a bilinear constitutive law for the interface between the concrete and the laminate. For each stage, it is possible to obtain the interfacial shear stress distribution and the transferred force between the laminate and the support. Since the reliability of the FRP reinforcement depends mainly on a proper stress transfer between concrete and laminate through the interface, the transferred force should be limited to a maximum value in order to prevent peeling failure. This paper provides this limit value for the transferred force along the interface. In addition, the stress distributions are obtained for a particular case of a beam in a three-point bending configuration.

Analysis of FRP shear strengthening solutions for reinforced concrete beams considering debonding failure

AbstractIn this paper, a fiber beam model previously developed by the authors for the nonlinear analysis of strengthened elements, including the effects of shear, is used to predict the response of reinforced concrete (RC) beams strengthened in shear with fiber reinforced polymers (FRP) sheets. In the previous version of the model, debonding failure of FRP was not included; hence, its application was limited to the simulation of wrapped configurations. The model is now extended to account for debonding failure in order to allow for its application to beams strengthened with U-shaped and side-bonded configurations. Existing experimental tests on RC beams strengthened in shear by FRP sheets in both wrapped and U-shaped configurations were numerically simulated. The model reproduces, with reasonable accuracy, the experimental failure loads, the load-deflection behavior, and the strains in FRP and stirrups with increasing load. The advantages of this proposal are related with the simplicity and straightforwar...

Assessment of the Existing Formulations to Evaluate Shear-Punching Strength in RC Slabs with FRP Bars Without Transverse Reinforcement

Nowadays, the real application of fibre reinforced polymer (FRP) bars in slabs or in bridge decks submitted to environments susceptible of corrosion is very limited. In spite of its cost, one of the reasons might be the limited knowledge about the punching-shear behaviour of this type of elements in their connection slab-column or under point loads. The punching-shear strength of slabs reinforced with FRP bars without transverse reinforcement is a complex phenomenon. The main difference with the conventional reinforced concrete is that the passive reinforcement has a linear elastic behaviour up to failure. There is a limited number of experimental and analytical studies about this subject. According to the existing experimental programs, it seems that the basic perimeter at failure is lower in an FRP reinforced concrete (RC) slab than in a conventional RC slab, strains are higher due to the lower modulus of elasticity of the FRP bars, and the cracks are wider. In this paper, a comparative analysis of the reliability of the existing formulations to evaluate the punching strength of FRP RC slabs without transverse reinforcement is performed. A database of 92 tests compiled from 21 experimental programs has been assembled. Most of the existing formulations to predict punching of FRP RC slabs without transverse reinforcement are an adaption of the existing formulations for conventional RC slabs, which take into account the lower stiffness of the FRP bars. Based on the unified model for shear and punching developed by Mari et al. (2015), a preliminary proposal has been made and its reliability has also been evaluated by means of the assembled database.