Dynamic Behavior of Reinforced Concrete Beams Strengthened with Composite Materials

The dynamic behavior of reinforced concrete (RC) beams strengthened with externally bonded composite materials is analytically investigated. The analytical model is based on dynamic equilibrium, compatibility of deformations between the structural components (RC beam, adhesive, composite material) and the concept of the high order approach. The equations of motion along with the boundary and continuity conditions are derived using Hamilton’s variational principle and the kinematic relations of small deformations. The mathematical formulation also includes the constitutive laws that are based on beam and lamination theories, and the two-dimensional elasticity representation of the adhesive layer including the closed form solution of its stress and displacement fields. The Newmark time integration method, which is directly applied to the resulting set of coupled partial differential equations, is adopted. This procedure yields a set of ordinary differential equations, which are analytically or numerically solved in every time step. The response of a strengthened beam to different dynamic loads that include impulse load, harmonic load, and seismic base excitation is numerically investigated. The numerical study highlights some of the phenomena associated with the dynamic response and explores the capabilities of the proposed model. The paper closes with a summary and conclusions.     

Behavior of FRP Strengthened Reinforced Concrete Beams under Torsion

The use of fiber reinforced plastics (FRPs) for flexural and shear strengthening of reinforced concrete beams has been scrutinized to a considerable depth by researchers worldwide. The area of torsional strengthening however has not been as popular. This paper presents the results of an experimental investigation together with a numerical study on reinforced concrete beams subjected to torsion that are strengthened with FRP wraps in a variety of configurations. In the experimental study, the increase in the ultimate torque for different strengthening configurations, failure mechanisms, crack patterns, and ductility levels are monitored and presented. Experimental results show that FRP wraps can increase the ultimate torque of fully wrapped beams considerably in addition to enhancing the ductility. The experimental results upgrade the weak archival data on torsional strengthening by application of FRP. The numerical section reports on analyses performed by the ANSYS finite element program. Predictions are compared with experimental findings and are in reasonable agreement.     

Size Effects for Reinforced Concrete Beams Strengthened in Shear with CFRP Strips

The principal motivation of this study is to obtain a clear understanding of size effects for fiber-reinforced polymer (FRP) shear-strengthened beams. The experimental program consists of seven beams of various sizes grouped in three test series. One beam of each series is used as a benchmark and its behavior is compared with a beam strengthened with a U-shaped carbon FRP (CFRP) jacket. The third test series includes an additional beam strengthened with completely wrapped external CFRP sheets. The experimental results show that the effective axial strains of the CFRP sheets are higher in the smaller specimens. Moreover, with a larger beam size, one can expect less strain in the FRPs. A nonlinear finite-element numerical analysis is developed to model the behavior of the CFRP shear-strengthened beams. The numerical model is able to simulate the characteristics of the shear-strengthened beams, including the interfacial behavior between the concrete and the CFRP sheets. Three prediction models available in current design guidelines for computing the CFRP effective strain and shear contribution to the shear capacity of the CFRP shear-strengthened beams are compared with the experimental results.     

Behavior of Reinforced Concrete Beams Strengthened with Externally Bonded Hybrid Carbon Fiber–Glass Fiber Sheets

A comparative test program including six beams was carried out. Two strengthening systems, namely hybrid carbon fiber glass fiber-reinforced polymer (H-CF/GF-RP) strengthening and CF-reinforced polymer strengthening were used. The test results showed that the H-CF/GF-RP strengthening led to a significant increase of ductility with a slight influence on stiffness of strengthened beams.     

Performance of Mechanically Fastened FRP Strengthened Concrete Beams in Flexure

The use of adhesively bonded fiber-reinforced polymer (FRP) materials has become widely accepted for use in flexural strengthening applications; however, the method of attachment presents drawbacks in application. These include extensive time and labor investments, as well as a tendency of the system to fail in a brittle manner. This paper presents a study of a series of reinforced concrete beams each strengthened in flexure with an FRP strip attached with large diameter concrete screws. The concrete screws were arranged in a variety of patterns. The effect of fastener number and spacing, as well as the effect of fastener pattern on the behavior of the beam, was investigated through the use of two groups of specimens. The beams in each group were tested to failure to verify the behavior of the strengthening system. Measured behavior was then used to determine an analytical approach for prediction of load response behavior of mechanically fastened systems. It was found that the strengthening method investigated improved the flexural capacity of the specimens 12 to 39% with little or no loss in ductility.     

Flexural Response of Reinforced Concrete Beams Strengthened with End-Anchored Partially Bonded Carbon Fiber-Reinforced Polymer Strips

This paper presents the results of experimental and analytical studies carried out to investigate the flexural behavior of reinforced concrete beams strengthened with end-anchored partially bonded carbon fiber-reinforced polymer (CFRP) strips. A total of six beams, each 2400 mm long, 150 mm wide, and 250 mm deep with a tension steel reinforcement ratio of 1.18%, were tested. One beam was left unstrengthened as the control, another beam was strengthened with a fully bonded CFRP strip, and the remaining four beams were strengthened with partially bonded CFRP strips placed on the tension face of the beam and fixed at both ends using a mechanical anchor. The influence of varying the CFRP unbonded length (250 mm, 750 mm, 2×500 mm, and 1,250 mm) on the beam flexural response was studied. The experimental results revealed that end-anchored partially bonded CFRP strips significantly enhanced the ultimate capacity of the control beam and performed better than the fully bonded strip with no end-anchorage. This observation stresses the importance of end-anchorage in such strengthening schemes, especially considering that the end-anchored partially bonded CFRP strengthened beams showed similar flexural behavior trends. Finally, an inelastic section analysis procedure that takes into consideration the incompatibility of strains was developed to verify the obtained test results. The analysis produced good predictions of the experimental results in terms of the moment-curvature response and showed the effect of CFRP unbonded length on the strain of the internal tension steel.     

Analytical Method for Evaluating Ultimate Torque of FRP Strengthened Reinforced Concrete Beams

The behavior of fiber reinforced polymer (FRP) strengthened reinforced concrete beams subjected to torsional loads has not been well understood compared to other loads. Interaction of different components of concrete, steel, and FRP in addition to the complex compatibility issues associated with torsional deformations have made it difficult to provide an accurate analytical solution. In this paper an analytical method is introduced for evaluation of the torsional capacity of FRP strengthened RC beams. In this method, the interaction of different components is allowed by fulfilling equilibrium and compatibility conditions throughout the loading regime while the ultimate torque of the beam is calculated similarly to the well-known compression field theory. It is shown that the method is capable of predicting the ultimate torque of FRP-strengthened RC beams reasonably accurately.     

Torsional Capacity of CFRP Strengthened Reinforced Concrete Beams

Many buildings and bridge elements are subjected to significant torsional moments that affect the design, and may require strengthening. Fiber-reinforced polymer (FRP) has shown great promise as a state-of-the-art material in flexural and shear strengthening as external reinforcement, but information on its applicability in torsional strengthening is limited. Furthermore, available design tools are sparse and unproven. This paper briefly recounts the experimental work in an overall investigation of torsional strengthening of solid and box-section reinforced concrete beams with externally bonded carbon fiber-reinforced polymer (CFRP). A database of previous experimental research available in literature was compiled and compared against fib Bulletin 14. Modifications consistent with the space truss model were proposed to correct the poor accuracy in predictions of CFRP contribution to strength. Subsequently, a design tool to analyze the full torsional capacity of strengthened reinforced concrete beams was validated against the experimental database.     

Energy Release in Fiber-Reinforced Plastic Reinforced Concrete Beams

A set of 30 concrete beams reinforced with carbon/epoxy FRP (fiber-reinforced plastic) and four reinforced with comparable size steel rebars were subjected to static bending tests. Adequate bond between the FRP and the concrete was obtained, due to the use of carbon fiber overwrap on the smooth pultruded FRP rods. With adequate bond, the large strain to failure (>2%) of the FRP determines the ductility and failure mode of the FRP reinforced beams. An analytical evaluation of the fracture energy in these experiments shows that there is ductility due to the large fraction of the total strain energy that is absorbed in the concrete, because of the formation of distributed cracking. Variations in overwrap configuration, addition of steel stirrups, addition of polypropylene fibers, and comparison with four beams reinforced with equivalent steel reinforcement were also analyzed.     

Mechanisms of Shear Resistance of Concrete Beams Strengthened in Shear with Externally Bonded FRP

In recent years, numerous investigations have addressed the shear strengthening of reinforced concrete (RC) beams with externally bonded fiber-reinforced polymer (FRP) composites. Despite this research effort, the mechanisms of shear resistance that are developed in such a strengthening system have not yet been fully documented and explained. This clearly inhibits the development of rational and reliable code specifications. This paper aims to contribute to the understanding of the shear resistance mechanisms involved in RC beams strengthened in shear with externally bonded FRP. It is based on results obtained from an experimental program, involving 17 tests, performed on full size T beams, and using a comprehensive and carefully optimized measuring device. The resistance mechanisms are studied by observing the evolution of the behavior of the strengthened beams as the applied loads are increased. The local behavior of the FRP and the transverse steel, in particular in the failure zones, are thoroughly examined. The operative resistance mechanisms are also studied through the load sharing among the concrete, the FRP, and the transverse steel, at increasing levels of applied load.     

Effect of Size on the Failure of Geometrically Similar Concrete Beams Strengthened in Shear with FRP Strips

Fiber reinforced plastics (FRP) are commonly used for the strengthening of concrete members. For shear strengthening of beams, FRP strips can be bonded to the sides of the member alone, to both the sides and the bottom (i.e., the U configuration), or wrapped around the whole beam. For the various strengthening configurations, empirical equations have been proposed for predicting the contribution of strips to the shear capacity of the member. However, for the same strengthened member, the equations recommended by different design guidelines (American Concrete Institute, International Federation for Structural Concrete, and Japan Society for Civil Engineers) predict different shear capacities. Moreover, as the equations were obtained through the fitting of laboratory data on relatively small beams, their applicability to beams of practical sizes have not really been assessed. In the present investigation, geometrically similar beams with depth of 180, 360, and 720?mm were retrofitted in shear with carbon FRP strips in both the U configuration and fully wrapped configuration. The retrofitted members were tested to failure to (1) provide data on beams of practical sizes for verification of design equations and (2) investigate if the strengthening effectiveness is similar for small and large beams. Measured FRP contribution to the shear capacity is also compared to predictions from equations in the various guidelines. Based on our findings, for beams retrofitted with strips in the U configuration, the strengthening effectiveness may significantly decrease with member size, and none of the available design equations can consistently provide conservative values for the shear capacity. For beams with fully wrapped strips, strengthening effectiveness is independent of member size, and the FIB equation appears to be most appropriate for practical design.     

Numerical Cracking and Debonding Analysis of RC Beams Reinforced with FRP Sheet

Bonding a fiber reinforced polymer (FRP) sheet to the tension-side surface of reinforced concrete (RC) structures is often performed to upgrade the flexural capacity and stiffness. Except for upper concrete crushing, FRP sheet reinforcing RC structure may fail in sheet rupture, sheet peeloff failure due to opening of a critical diagonal crack, or concrete cover delamination failure from the sheet end. Accompanying the occurrence of these failure modes, reinforcing effects of the FRP sheet will be lost and load-carrying capacity of the RC structures will be decreased suddenly. This study is devoted to developing a numerical analysis method by using a three-dimensional elasto-plastic finite element method to simulate the load-carrying capacity of RC beams failed in the FRP sheet peeloff mode. Here, the discrete crack approach was employed to consider geometrical discontinuities such as opening of cracks, slipping of rebar, and debonding of the FRP sheet. Comparisons between analytical and experimental results confirm that the proposed numerical analysis method is appropriate for estimating the load-carrying capacity and failure behavior of RC beams flexurally reinforced with a FRP sheet.     

Analysis of the Flexural Behavior of Partially Bonded FRP Strengthened Concrete Beams

An investigation was conducted on the flexural behavior of partially bonded fiber-reinforced polymer (FRP) strengthened concrete beams focusing on the improvement of ductility. An analytical model was developed based on the curvature approach to predict the behavior of beams strengthened with partially bonded FRP systems. The result of the analysis showed that ductility of the partially bonded system was improved while sustaining high load carrying capacity in comparison to the fully bonded system. To verify the analytical model, an experimental program was carried out with reinforced concrete beams strengthened with the externally bonded FRP system. A comparison of the analytical prediction and experimental results showed good agreement.     

Experimental Behavior of Reinforced Concrete Beams Strengthened with Prestressed CFRP Shear Straps

One promising means of increasing the capacity of existing shear-deficient beams is to strengthen the structure using external prestressed carbon fiber reinforced polymer (CFRP) straps. In this system, layers of CFRP tape are wrapped around a beam to form a strap that acts like a discrete unbonded vertical prestressing tendon. Experiments were undertaken to investigate the influence of the strap spacing, the strap stiffness, the initial strap prestress level and/or any preexisting damage on the strengthened behavior, and mode of failure. An unstrengthened control beam was tested and failed in shear. In contrast, all of the strengthened beams showed a significant increase in their ultimate load capacity with several of the strengthened beams failing in flexure. A number of different failure modes were noted and initial guidelines on the design parameters that influence the propensity for a particular failure mode were developed.     

Tension Stiffening of Reinforced Concrete Ties Strengthened with Externally Bonded Fiber-Reinforced Polymer Sheets

Externally bonded laminates of fiber reinforced polymers (FRPs) are becoming more and more common for rehabilitation and strengthening of RC structures, to solve problems either in serviceability or at ultimate conditions. This paper focuses on the behavior in the serviceability state: in particular, verifications in terms of cracks widths and crack spacing, used to warrant the functionality of FRP-RC structures, are considered. Test results on RC ties externally reinforced with FRP laminate are reported and the applicability of the Eurocode2 formulation used for RC elements is discussed. Provisions given by the International Federation for Structural Concrete are also analyzed.     

Ductility of Reinforced Concrete Members Externally Wrapped with Fiber-Reinforced Polymer Sheets

The effectiveness of external wrapping with fiber-reinforced polymer for enhancing the curvature ductility of lightly reinforced concrete members is investigated. Referring to members with circular transverse cross sections, the performances in terms of both strength and ductility capacities are analyzed, and the predictive reliability of two different recent constitutive models, available in the literature and able to take into account the softening behavior of confined concrete, is checked. A parameter characterizing the effectiveness of the confining wrapping is proposed, and characteristic values are suggested. Moreover, referring to ductility increases due to confinement effects, a comparison is made between the predictions obtained using the constitutive models and simple expressions given in recent codes. Parametric analyses carried out highlight the importance of a definition of the limits of validity of expressions given in the literature for estimation of ductility increases in order to avoid nonconservative assessment.     

Fatigue Behavior of Reinforced Concrete Beams Strengthened with Carbon Fiber Reinforced Plastic Laminates

Although there has been growing interest and field applications of poststrengthening concrete structures using carbon fiber reinforced plastic (CFRP) laminates, very little information exists regarding the flexural fatigue behavior of reinforced concrete beams strengthened with CFRP. This paper presents the results of an investigation into the fatigue behavior of reinforced concrete beams poststrengthened with CFRP laminates. The results of twenty 3 m and six 5 m beams loaded monotonically and cyclically to failure are discussed. Comparisons are made between beams without and with CFRP strengthening. The effect on fatigue life of increasing the amount of CFRP used to strengthen the beams is also examined.     

Flexural Behavior of Continuous GFRP Reinforced Concrete Beams

The results of testing two simply and three continuously supported concrete beams reinforced with glass fiber-reinforced polymer (GFRP) bars are presented. The amount of GFRP reinforcement was the main parameter investigated. Over and under GFRP reinforcements were applied for the simply supported concrete beams. Three different GFRP reinforcement combinations of over and under reinforcement ratios were used for the top and bottom layers of the continuous concrete beams tested. A concrete continuous beam reinforced with steel bars was also tested for comparison purposes. The experimental results revealed that over-reinforcing the bottom layer of either the simply or continuously supported GFRP beams is a key factor in controlling the width and propagation of cracks, enhancing the load capacity, and reducing the deflection of such beams. Comparisons between experimental results and those obtained from simplified methods proposed by the ACI 440 Committee show that ACI 440.1R-06 equations can reasonably predict the load capacity and deflection of the simply and continuously supported GFRP reinforced concrete beams tested.     

Response Prediction of RC Beams Externally Bonded with Steel-Reinforced Polymers

This paper deals with an innovative technique for strengthening reinforced concrete (RC) structures using steel-reinforced polymer (SRP) materials. The results of an experimental campaign using RC beams strengthened in flexure with carbon fiber-reinforced polymer or SRP laminates are summarized, and the experimental outcomes are compared to the predictions provided by analytical models and code formulations in terms of flexural strength, curvature of the cross section, deflections, and crack widths. Under ultimate conditions, the ACI 440.2R-02 approach provided conservative flexural strength, and a modified expression for the bond coefficient km was proposed. Under serviceability conditions, good agreement was obtained between experimental results and a theoretical model developed by the writers. Comparisons of code models in terms of both crack width and deflections highlighted the need for a calibration of code formulas to account for effects due to externally bonded reinforcement.     

Flexural Behavior of Reinforced Concrete Beams Strengthened with CFRP Sheets and Epoxy Mortar

Experiments were conducted to study the effect of using epoxy mortar patch end anchorages on the flexural behavior of reinforced concrete beams strengthened with carbon fiber-reinforced polymer (CFRP) sheets. More specifically, the effect of the end anchorage on strength, deflection, flexural strain, and interfacial shear stress was examined. The test results show that premature debonding failure of reinforced concrete beams strengthened with CFRP sheet can be delayed or prevented by using epoxy mortar patch end anchorages. A modified analytical procedure for evaluating the flexural capacity of reinforced concrete beams strengthened with CFRP sheets and epoxy mortar end anchorage is developed and provides a good prediction of test results.