Shear Strengthening of RC Beams with EB FRP: Influencing Factors and Conceptual Debonding Model

This paper deals with the shear strengthening of RC beams using externally bonded (EB) fiber-reinforced polymers (FRP). Current code provisions and design guidelines related to shear strengthening of RC beams with FRP are discussed in this paper. The findings of research studies, including recent work, have been collected and analyzed. The parameters that have the greatest influence on the shear behavior of RC members strengthened with EB FRP and the role of these parameters in current design codes are reviewed. This study reveals that the effect of transverse steel on the shear contribution of FRP is important and yet is not considered by any existing codes or guidelines. Therefore, a new design method is proposed to consider the effect of transverse steel in addition to other influencing factors on the shear contribution of FRP (Vf). Separate design equations are proposed for U-wrap and side-bonded FRP configurations. The accuracy of the proposed equations has been verified by predicting the shear strength of experimentally tested RC beams using data collected from the literature. Finally, comparison with current design guidelines has shown that the proposed model achieves a better correlation with experimental results than current design guidelines.     

Engineering Properties of Sand-Fiber Mixtures for Road Construction

The purpose of this investigation was to identify and quantify the effect of numerous variables on the performance of fiber-stabilized sand specimens. Laboratory unconfined compression tests were conducted on sand specimens reinforced with randomly oriented discrete fibers to isolate the effect of each variable on the performance of the fiber-reinforced material. Five primary conclusions were obtained from this investigation. First, the inclusion of randomly oriented discrete fibers significantly improved the unconfined compressive strength of sands. Second, an optimum fiber length of 51 mm (2 in.) was identified for the reinforcement of sand specimens. Third, a maximum performance was achieved at a fiber dosage rate between 0.6 and 1.0% dry weight. Fourth, specimen performance was enhanced in both wet and dry of optimum conditions. Finally, the inclusion of up to 8% of silt does not affect the performance of the fiber reinforcement.     

High-Pressure Isotropic Compression Tests on Fiber-Reinforced Cemented Sand

High-pressure isotropic compression tests were carried out on reconstituted sand samples that were reinforced with cement, randomly distributed fibers, or both, making comparisons with the unreinforced sand and conducting tests from a variety of initial specific volumes. The results indicated changes in the isotropic compression behavior of the sand due to the inclusion of fibers and/or cement. Cementitious bonds are sufficiently strong relative to the particles to allow the cemented samples to reach states outside the normal compression line (NCL) of the uncemented soil, but the effectiveness of cemented fiber-reinforced specimens is even larger due to the control of crack propagation in the cemented sand after the inclusion of fibers. Distinct NCLs were observed for the sand, fiber-reinforced sand, cemented sand, and fiber-reinforced cemented sand. Both fiber breakage and fiber extension were observed in fibers measured after testing indicating that fibers individually have worked under tension, even though in the macroscopic scale, isotropic compressive stresses were applied. Fiber reinforcement was found to reduce the particle breakage of both the uncemented and cemented sands.     

Shear Strengthening of Interior Slab–Column Connections Using Carbon Fiber-Reinforced Polymer Sheets

This paper presents the results of an experimental investigation undertaken to evaluate the punching shear capacity of interior slab–column connections, strengthened using flexible carbon fiber-reinforced polymer (CFRP) sheets. Sixteen square (670×670?mm) slab–column connections with different slab thicknesses (55 and 75 mm) and reinforcement ratios (1 and 1.5%) were tested. Twelve specimens were strengthened using CFRP sheets and the remaining four specimens were kept as controls. Without strengthening, all specimens were designed to experience punching shear failure. The CFRP sheets were bonded to the tension face of the specimens in two perpendicular directions parallel to the internal ordinary steel reinforcement. The test results clearly demonstrate that using CFRP leads to significant improvements in the flexural stiffness, flexural strength, and shear capacity of beam–column connections. Depending on the content of the ordinary reinforcement, thickness of the slab, and area of CFRP sheet, the flexural strength increased between 26 and 73% and the shear capacity increased between 17 and 45%. The measured stress in the CFRP sheets at nominal strength varied between 22 and 69% of the ultimate tensile strength of the fibers. Comparison with available prediction equations showed that the punching shear capacity can be predicted with reasonable accuracy if the contribution of CFRP reinforcement to the increase in flexural strength is accounted for. On the other hand, the code design expressions were conservative in predicting the capacity observed in the tests.     

Behavior of a Fiber-Reinforced Bentonite at Large Shear Displacements

The behavior of a polypropylene fiber-reinforced bentonite was evaluated at large shear displacements by a series of ring shear tests carried out at normal stresses varying between 20 and 400?kPa. Bentonite/polypropylene fiber composites were molded at an initial moisture content of 170%, with fiber lengths of 12 or 24?mm. The fiber thickness was 0.023?mm and the fiber content was either 1.5 or 3% by dry weight. The inclusion of randomly distributed fibers increased the peak shear strength of the bentonite, but the increase in strength deteriorated at large displacements and the residual strengths of both the nonreinforced and fiber-reinforced bentonite were similar. The peak shear strength was found to increase both with increasing fiber length and content. The fibers were exhumed after testing and it was found that the fibers had both extended and broken, with a predominance of broken fibers.     

Seismic Behavior of RC Columns with Bond-Critical Regions: Criteria for Bond Strengthening Using External FRP Jackets

In 2003, an experimental research program was initiated at the American University of Beirut with the objectives of (1) evaluating the effectiveness of external fiber-reinforced polymer (FRP) confinement in improving the bond strength of spliced reinforcement in reinforced-concrete (RC) columns and its implications on the lateral load capacity and ductility of the columns under seismic loading; and (2) establishing rational design criteria for bond strengthening of spliced reinforcement using external FRP jackets. This paper presents a discussion of recent experimental results dealing with rectangular columns and the results of a pilot study conducted on circular columns with particular emphasis on aspects related to the bond strength of the spliced column reinforcement. A nonlinear analysis model is developed for predicting the envelope load–drift response, taking into account the effect of FRP confinement on the stress–strain behavior of concrete in compression. Results predicted by the model showed excellent agreement with the test results. Design expressions of the bond strength of spliced bars in FRP-confined concrete were assessed against the current experimental data, and a criterion for seismic FRP strengthening of bond-critical regions in RC members is proposed.     

Shear Failure of Pultruded Fiber-Reinforced Polymer Composites under Axial Compression

When structural elements are subjected to compressive loads, the shear forces and stresses induced by second-order effects may lead to shear failure prior to compressive failure. This is particularly likely to occur in the case of pultruded glass fiber-reinforced polymer profiles, which normally exhibit low shear strength in relation to compressive strength. This paper analyzes the effects of initial imperfection, slenderness, shear-to-compressive strength ratio, shear coefficient, and type of shear failure criterion on ultimate load and failure mode (shear or compressive failure). A formulation for predicting ultimate load based on shear failure and second-order deformation is proposed. The results obtained compare well with similar results obtained using other methods and experimental data available in literature. The proposed method is based strictly on mechanics and thus requires no fitting to experimental data.     

Full Torsional Behavior of RC Beams Wrapped with FRP: Analytical Model

Torsion failure is an undesirable brittle form of failure. Although previous experimental studies have shown that using fiber-reinforced polymer (FRP) sheets for torsion strengthening of reinforced concrete (RC) beams is an effective solution in many situations, very few analytical models are available for predicting the section capacity. None of these models predicted the full behavior of RC beams wrapped with FRP, account for the fact that the FRP is not bonded to all beam faces, or predicted the ultimate FRP strain using equations developed based on testing FRP strengthened beams in torsion. In this paper, an analytical model was developed for the case of the RC beams strengthened in torsion. The model is based on the basics of the modified compression field theory, the hollow tube analogy, and the compatibility at the corner of the cross section. Several modifications were implemented to be able to take into account the effect of various parameters including various strengthening schemes where the FRP is not bonded to all beam faces, FRP contribution, and different failure modes. The model showed good agreement with the experimental results. The model predicted the strength more accurately than a previous model, which will be discussed later. The model predicted the FRP strain and the failure mode.     

Closed-Form Model and Parametric Study on Connection of Concrete-Filled FRP Tubes to Concrete Footings by Direct Embedment

Concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) have been introduced as a new system for piles, columns, and poles. A simple moment connection based on direct embedment of the CFFT into concrete footings or pile caps, without using dowel-bar reinforcement, has been proposed by the authors. Robust analytical models to predict the critical embedment length (Xcr) were also developed and experimentally validated. In this paper, a comprehensive parametric study is carried out using the models developed earlier along with a newly developed closed-form model for the general case of axial loading, bending, and shear applied to the CFFT member. The parameters studied are the diameter (D), thickness (t), length outside the footing (L), and laminate structure of the FRP tube, as well as the tube-concrete interface bond strength (τmax?), concrete compressive strength in the CFFT (fct′) and footing (fc′), and the magnitude and eccentricity of axial compressive or tensile loads. It was shown that increasing D, L/D, τmax?, and fc′ of the footing, or the axial compression load, reduces (X/D)cr, whereas increasing t and fct′ of the CFFT, the fraction of longitudinal fibers in the tube, or the axial tension load, increases Xcr. As the axial load eccentricity increases, Xcr reduces for tension loads and increases for compression loads until both cases converge asymptotically to the same Xcr value, essentially that of pure bending.     

Shear Strength and Stiffness of Silty Sand

The properties of clean sands pertaining to shear strength and stiffness have been studied extensively. However, natural sands generally contain significant amounts of silt and∕or clay. The mechanical response of such soils is different from that of clean sands. This paper addresses the effects of nonplastic fines on the small-strain stiffness and shear strength of sands. A series of laboratory tests was performed on samples of Ottawa sand with fines content in the range of 5–20% by weight. The samples were prepared at different relative densities and were subjected to various levels of mean effective consolidation stress. Most of the triaxial tests were conducted to axial strains in excess of 30%. The stress-strain responses were recorded, and the shear strength and dilatancy parameters were obtained for each fines percentage. Bender element tests performed in triaxial test samples allowed assessment of the effect of fines content on small-strain mechanical stiffness.     

State of Research on Seismic Retrofit of RC Beam-Column Joints with Externally Bonded FRP

A considerable amount of research has been directed recently toward understanding and promoting the use of externally applied fiber-reinforced polymer (FRP) for the seismic retrofit of reinforced concrete (RC) structures. In this paper, a comprehensive review and synthesis of published experimental studies on the seismic rehabilitation of RC frame beam-column joints with FRP is presented, and the issues that need to be addressed for further research are discussed. In addition, the paper presents a simple design model for predicting the contribution of the FRP to the shear strength of retrofitted joints. The key element in the model is the derivation of an expression for the effective FRP strain, based on the calibration of test data reported in the literature. A total of 54 tests carried out worldwide were considered in the review, and a database of the published studies, encompassing all relevant design parameters, was assembled. The reported test results confirm the structural effectiveness of the FRP strengthening technique for the seismic retrofit of RC joints. However, there are some gaps which need to be addressed. For instance, there is a lack of a rationale explanation of the resistance mechanisms involved in the beam-column joints retrofitted with FRP. Such a rational explanation is a prerequisite for the development of more comprehensive and rigorous design procedure.     

Modeling of an Unbonded CFRP Strap Shear Retrofitting System for Reinforced Concrete Beams

A retrofitting technique has been developed that uses carbon fiber-reinforced polymer (CFRP) straps to increase the shear capacity of reinforced concrete beams. The vertical straps are not bonded to the beam but are instead anchored against the beam, which makes this technique potentially more effective than bonded FRP retrofitting techniques. However, it also means that models for bonded FRPs are not appropriate for use with the straps. Instead, a model based on a shear friction approach has been developed where the strain in the straps is calculated based on a term that accounts for the effects of prestress and additional strain in the strap due to shear crack opening. The model can either consider the shear reinforcement to be smeared along the length of the beam or discrete elements. The “smeared” model was checked against an experimental database consisting of rectangular, T-, and deep beams, both in terms of predicted capacity and predicted strain in the straps. Overall the smeared model predicted the capacity of the specimens and, with some adjustments, the strains quite accurately. There were, however, cases when it was more appropriate to use the “discrete” model such as when the transverse reinforcement ratio was low or when the transverse reinforcement spacing was high. Further experimental data are required to fully validate the models and to determine appropriate limits on the use of the smeared model and the discrete model. However, the initial results are promising.     

Pullout Strength Models for FRP Anchors in Uncracked Concrete

Mechanical anchorage can delay or even prevent premature debonding failure in externally bonded fiber-reinforced polymer (FRP) composite strengthening systems. A promising type of anchor made from FRP, which is known as a FRP spike anchor or FRP anchor among other names, is noncorrosive and can be applied to a wide range of structural elements and externally bonded FRP strengthening schemes. Experimental investigations have shown FRP anchors to be effective under tension (pullout) and shear loading, however, few analytical models exist to date. This paper in turn presents analytical models to quantify the pullout strength of FRP anchors. As existing research on the pullout behavior of metallic anchors is partially relevant to FRP anchors, this paper first presents a review of current pullout strength models for metallic anchors. These models are then assessed with experimental data of FRP anchors and modified and recalibrated where appropriate. As a result, simple and rational pullout strength models for FRP anchors are proposed which can also be used in design. Finally, parametric studies are undertaken and the influence of key variables is identified.     

Evaluating and Proposing Models of Predicting IC Debonding Failure

Debonding failure due to intermediate crack-induced (IC) fracture is one of the most dominant failure modes associated with the fiber-reinforced polymer (FRP) bonding technique. To date, extensive efforts have been paid by many researchers worldwide to study the debonding phenomenon for effective applications of FRP composites and rational design of FRP-strengthened structures. Based on these efforts and various relevant field applications, different models and code provisions have been proposed to predict IC debonding failure. Out of all the existing code provisions and models, five typical ones are investigated in the current paper. A comprehensive comparison among these code provisions and models is carried out in order to evaluate their performance and accuracy. Test results of 200 flexural specimens with IC debonding failures collected from the existing literature are used in the current comparison. The effectiveness and accuracy of each model have been evaluated based on these experimental results. Finally, based on a statistical analysis, a simple and more effective model for predicting the load-carrying capacity of FRP-strengthened flexural members due to IC debonding failure is proposed.     

Simplified Impedance Model for Adhesively Bonded Piezo-Impedance Transducers

The electromechanical impedance technique employs surface-bonded lead zirconate titanate piezoelectric ceramic patches as impedance transducers for structural health monitoring and nondestructive evaluation. The patches are bonded to the monitored structures using finitely thick adhesive bond layer, which introduces shear lag effect, thus invariably influencing the electromechanical admittance signatures. This paper presents a new simplified impedance model to incorporate shear lag effect into electromechanical admittance formulations, both one-dimensional and two-dimensional. This provides a closed-form analytical solution of the inverse problem, i.e. to derive the true structural impedance from the measured conductance and susceptance signatures, thus an improvement over the existing models. The influence of various parameters (associated with the bond layer) on admittance signatures is investigated using the proposed model and the results compared with existing models. The results show that the new model, which is far simpler than the existing models, models the shear lag phenomenon reasonably well besides providing direct solution of a complex inverse problem.     

Damage Approach for the Prediction of Debonding Failure on Concrete Elements Strengthened with FRP

In this study, experimental and numerical procedures are proposed to predict the debonding failure of concrete elements strengthened with fiber-reinforced polymers (FRPs). Such debonding is modeled as a damage process, which takes place in a band along the bond line (crack band). Three-point bending tests were designed to obtain the softening curve of the crack band. The numerical simulations are conducted using a plastic-damage model. In this approach, the damage resulting in debonding is defined using the softening curve of the crack band. Numerical results are validated against experimental results obtained from single-lap shear tests. The numerical models were capable of predicting the experimentally observed load versus strain behavior, failure load, and failure mechanism of the single-lap shear specimens. The predictive capabilities of the numerical approach presented here were further investigated by means of a parametric study of the single-lap shear test. Results from this study indicate the applicability of the crack band approach to predict the behavior of concrete–FRP joints; they also indicate that the failure load determined from a single-lap shear test is geometry dependent.     

Prediction of Punching Shear Strength of Two-Way Slabs Strengthened Externally with FRP Sheets

Strengthening two-way slabs by using fiber-reinforced polymer (FRP) is experimentally and analytically evaluated. Results show that the punching capacity of two-way slabs can increase to up to 40% greater than that of a reference specimen. A three-dimensional FEM program called 3D CAMUI, which was developed at Hokkaido University, was used to simulate the experimental slabs. Very good agreement is obtained in load-carrying capacity and modes of failure. An analytical model based on the numerical simulation, which discloses the mechanism of punching shear strength enhancement by FRP strengthening, is proposed to predict the punching shear strength of two-way slabs externally strengthened with FRP sheets.     

Nonlinear Finite Element Analysis of a FRP-Strengthened Reinforced Concrete Bridge

Three-dimensional nonlinear finite element (FE) models are developed to examine the structural behavior of the Horsetail Creek Bridge strengthened by fiber-reinforced polymers (FRPs). A sensitivity study is performed varying bridge geometry, precracking load, strength of concrete, and stiffness of the soil foundation to establish a FE model that best represents the actual bridge. Truck loadings are applied to the FE bridge model at different locations, as in an actual bridge test. Comparisons between FE model predictions and field data are made in terms of strains in the beams for various truck load locations. It is found that all the parameters examined can potentially influence the bridge response and are needed for selection of the optimal model which predicts the magnitudes and trends in the strains accurately. Then, using the optimal model, performance evaluation of the bridge based on scaled truck and mass-proportional loadings is conducted. Each loading type is gradually increased until failure occurs. Structural responses are compared for strengthened and unstrengthened bridge models to evaluate the FRP retrofit. The models predict a significant improvement in structural performance due to the FRP retrofit.     

Nonlinear Earthquake Response Modeling of a Large-Scale Two-Span Concrete Bridge

A quarter scale model of a two-span RC bridge was tested using the Network for Earthquake Engineering Simulation (NEES) multiple shake table system at the University of Nevada, Reno, Nev. The project was funded through a National Science Foundation-NEES demonstration grant. The bridge system was tested from a preyield state until column failure. In depth analytical modeling was conducted to determine the effectiveness of current structural analysis software and methodology in predicting the bridge model response. Both SAP2000 v.9 and Drain-3DX were used for this purpose. Both models produced reasonable results up to column failure, however, the Drain-3DX model was determined to be most effective to predict the nonlinear bridge model response. Parametric studies were conducted to investigate optimal element discretization and integration parameters. Existing equations for pre and postyield column shear stiffness showed good correlation when compared with the measured data.     

Behavior of Brick Masonry Vaults Strengthened by FRP Laminates

The results of experimental research on brick masonry vaults strengthened at their extrados or at their intrados by fiber-reinforced polymer (FRP) strips is presented here. The presence of the fibers prevents the typical brittle collapse that occurs in a plain arch because of the formation of four hinges; therefore, depending on the position and amount of the reinforcement in the strengthened vaults, three mechanisms are possible: (1) masonry crushing, (2) detachment of the fibers; and (3) sliding along a mortar joint due to the shear stresses. Some first theoretical approaches describing some of these mechanisms are discussed, and the formulation of further models based on the local interaction among the constituent materials is proposed. Six masonry vaults strengthened by glass FRPs or carbon FRPs have been tested. The results have pointed out the enhancement in strength and ductility of the strengthened vaults and the influence in the ultimate strength of the width of the strips and of the bond between the laminate and the masonry.