Postrepair Fatigue Performance of FRP-Repaired Corroded RC Beams: Experimental and Analytical Investigation

This paper presents the results of an experimental and analytical study of the fatigue performance of corroded reinforced concrete (RC) beams repaired with fiber-reinforced polymer (FRP) sheets. Ten RC beam specimens (152×254×3,200?mm) were constructed. One specimen was neither strengthened nor corroded to serve as a reference; three specimens were corroded and not repaired; another three specimens were corroded and repaired with U-shaped glass FRP sheets that wrapped the cross section of the specimen; and the remaining three specimens were corroded and repaired with U-shaped glass FRP sheets for wrapping and carbon-fiber-reinforced polymer (CFRP) sheets for flexural strengthening. The FRP sheets were applied after the main reinforcing bars were corroded to an average mass loss of 5.5%. Following FRP repair, some specimens were tested immediately to failure, while the other repaired specimens were subjected to further corrosion before being tested to failure to investigate their postrepair (long-term) performance. Reinforcement steel pitting due to corrosion reduced the fatigue life significantly. The FRP wrapping had no significant effect on the fatigue performance, while using CFRP sheets for flexural strengthening enhanced the fatigue performance significantly. The fatigue results were compared to smooth specimen fatigue data to estimate an equivalent fatigue notch factor for the main reinforcing bars of the tested specimens.     

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.     

Strengthening RC Beams in Flexure Using New Hybrid FRP Sheet/Ductile Anchor System

This paper explores a new hybrid fiber-reinforced polymer (FRP) sheet/ductile anchor system for rehabilitation of reinforced concrete (RC) beams. The advantages of the proposed strengthening method is that it overcomes the problem of low ductility that is associated with brittle failure mode in conventional methods of strengthening beams using epoxy-bonded FRP sheets. The proposed system leads to a ductile failure mode by triggering yielding to occur in a steel anchor system (steel links) rather than by rupture or debonding of FRP sheets, which is sudden in nature. Four half-scale RC T-beams were tested under four-point bending. Three retrofitted beams were strengthened using one layer of carbon FRP sheet. The results of the two beams that were strengthened with the new hybrid FRP sheet/ductile anchor system were compared with the results from the beam strengthened with conventional FRP bonding method and the control beam. The results show the effectiveness of the proposed strengthening system in increasing flexural capacity and ductility of RC beams.     

Fiber Reinforced Polymer Shear Strengthening of Reinforced Concrete Beams with Transverse Steel Reinforcement

The paper aims to contribute to a better understanding and modeling of the shear behavior of reinforced-concrete (RC) beams strengthened with carbon fiber reinforced polymer (FRP) sheets. The study is based on an experimental program carried out on 11 beams with and without transverse steel reinforcement, and with different amounts of FRP shear strengthening. The test results provide some new insights into the complex failure mechanisms that characterize the ultimate shear capacity of RC members with transverse steel reinforcement and FRP sheets. After the discussion of the above topics, a new upper bound of the shear strength is introduced. It should be capable of taking into account how the cracking pattern in the web failing under shear is modified by the presence of FRP sheets, and how such a modified cracking pattern actually modifies the anchorage conditions of the sheets and their effective contribution to the ultimate shear strength of the beams.     

Behavioral Trends of RC Beams Strengthened with Externally Bonded FRP

The performance of conventionally reinforced concrete (RC) beams strengthened in flexure with externally bonded fiber-reinforced polymers (EB-FRP) was studied by compiling and analyzing an experimental database. A total of 127 specimens from 23 separate studies were included in the database. A profile of specimens in the database is given, followed by an analysis of trends in failure mode, strength gain, and deformability. Failure by debonding of FRP was prevalent among specimens in the database. One-third of the specimens with external reinforcement added showed strength increases of 50% or more in combination with considerable deflection capacity. It was clear from the experimental studies that the procedures followed were most representative of member strengthening rather than repair. Most of the specimens in the database were not subjected to sustained loading or damage causing loss of original capacity before external reinforcement was added. To assess the real potential of using FRP for expedient and economical field repair and strengthening of RC members, it was concluded that future research on the application of FRP to RC members should focus on conditions that are similar to what is observed in the field, including the effects of sustained load during repair∕strengthening as well as corrosion- and load-induced damage.     

Tests on Seismically Damaged Reinforced Concrete Structural Walls Repaired Using Fiber-Reinforced Polymers

This paper presents the results of an experimental study on the seismic performance of axially loaded reinforced concrete (RC) walls with boundary elements confined by limited transverse reinforcement. These specimens were initially subjected to axial compression loading and cyclic lateral loading to failure, and subsequently repaired and subjected to loading again. The test specimens include two low-rise walls of aspect ratio 1.125 and two medium-rise walls of aspect ratio 1.625. Results show that significant drift capacities were achieved from the strengthened walls. The performance of the repaired walls was similar to the original walls before repair in terms of the flexural behavior, shear strength, and ductility capacities. While the fiber-reinforced polymer (FRP) anchorage may undergo premature failure, it however failed only after the peak lateral strength of the repaired wall was attained. This paper demonstrates that repair of damaged RC walls using FRP is able to restore the performance of damaged RC walls while also serving as repair method of relative ease.     

Behavior of Concrete Beams Strengthened in Shear with Carbon-Fiber Sheets

This paper presents the results of a test program for shear strengthening characteristics of continuous unidirectional flexible carbon-fiber polymer sheets bonded to reinforced concrete (RC) beams. A total of eight 150?mm×200?mm×2,600?mm concrete beams were tested. Various sheet configurations and layouts were studied to determine their effects on ultimate shear strength of the beams. From the tests, it was found that the externally adhesive bonded flexible carbon-fiber sheets are effective in strengthening RC beams in shear. Further, it was observed that the strength increases with the number of sheet layers and the depth of sheets across the beam section. Among the various schemes of wrapping studied, vertical U-wrap of sheet provided the most effective strengthening for concrete beam. Beam strengthened using this scheme showed 119% increase in shear capacity as compared to the control beam without any strengthening. Two prediction models available in literature for computing the shear contribution of carbon-fiber tow sheets to the shear capacity of fiber reinforced polymers bonded beams were compared with the experimental results.     

Empirical Approach for Determining Ultimate FRP Strain in FRP-Strengthened Concrete Beams

The flexural capacity of concrete beams can be efficiently and effectively improved through bonding fiber-reinforced plastic (FRP) plates to the tensile side. Failure of the strengthened member often occurs through debonding of the FRP from the concrete substrate. If the ultimate FRP strain at debonding failure is known, the moment capacity of the member can be obtained through a simple section analysis. In the American Concrete Institute (ACI) Design Guideline, simple empirical equations are proposed to find the ultimate FRP strain in terms of the FRP stiffness alone. However, when the proposed equations are compared to experimental data, a very large scatter is observed, indicating that the effect of other parameters cannot be neglected. In the present investigation, a new empirical approach to obtain the FRP debonding strain is developed. With a comprehensive experimental database of 143 tests, a neural network relating the ultimate FRP strain to various geometric and material parameters is trained and validated. Using the validated network, an empirical design curve and several correction equations are generated to provide a simple means to find the debonding strain in practical design. Through use of the chart and equations, the calculated ultimate failure moments for the 143 tests in our database are found to be in good agreement with experimental results. The applicability of the new empirical approach to the failure prediction of strengthened members is thus demonstrated.     

Strengthening of Short Shear Span Reinforced Concrete T Joists with Fiber-Reinforced Plasic Composites

In this work, the results of an experimental study conducted in a 1964-vintage building are presented. Twelve reinforced concrete (RC) T-joists strengthened with fiber-reinforced plastic (FRP) composites were loaded until failure in a short shear span configuration. Different strengthening schemes, including different FRP materials and a new FRP anchorage system, were adopted in order to compare the performance of the different installations. Carbon FRP and aramid FRP sheets in an epoxy matrix were bonded to the RC joists using the wet layup technique. All of the joists were loaded close to one end support and showed similar cracking patterns at failure. The design calculations were based on experimental results. All of the unanchored FRP strengthened beams showed failure due to peeling, while the anchored FRP strengthened members showed failure due to anchor pullout at higher load values. It was found that an increase in the amount of FRP did not result in a proportional increase in the shear capacity, as expected by design equations, but all of the beams showed a considerable increase in stiffness. The experimental results are compared with the results expected by analytical models in order to discuss the structural behavior of FRP strengthened beams tested in a real building with a short shear span. It was found that theoretical calculations resulted in nonconservative results for the tested specimens.     

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.     

Tests on Seismically Damaged Reinforced Concrete Walls Repaired and Strengthened Using Fiber-Reinforced Polymers

The behavior of six 1:2.5-scale reinforced concrete cantilever wall specimens having an aspect ratio of 1.5, tested to failure and subsequently repaired and strengthened using fiber-reinforced polymer (FRP) sheets is investigated. Specimens were first repaired by removing heavily cracked concrete, lap splicing the fractured steel bars by welding new short bars, placing new hoops and horizontal web reinforcement, and finally casting nonshrink high-strength repair mortar. The specimens were then strengthened using FRP sheets and strips, with a view to increasing flexural as well as shear strength and ductility. In addition to different arrangements of steel and FRP reinforcement in the walls, a key parameter was the way carbon-FRP strips added for flexural strengthening were anchored; steel plates and steel angles were used to this effect. Steel plates were anchored using U-shaped glass-FRP (GFRP) strips or bonded metal anchors. Test results have shown that by using FRP reinforcement, the flexural and shear strength of the specimens can be increased. From the anchorage systems tested, metal plates combined with FRP strips appear to be quite efficient. The effectiveness of the bonded metal anchors used was generally less than that of the combination of plates and GFRP strips. In all cases, final failure of the FRP anchorage is brittle, but only occurs after the peak strength is attained and typically follows the fracture of steel reinforcement in critical areas, hence the overall behavior of the strengthened walls is moderately ductile.     

Flexural Fatigue Behavior of Reinforced Concrete Beams Strengthened with FRP Fabric and Precured Laminate Systems

Rehabilitation of existing structures with carbon fiber reinforced polymers (CFRP) has been growing in popularity because they offer resistance to corrosion and a high stiffness-to-weight ratio. This paper presents the flexural strengthening of seven reinforced concrete (RC) beams with two FRP systems. Two beams were maintained as unstrengthened control samples. Three of the RC beams were strengthened with CFRP fabrics, whereas the remaining two were strengthened using FRP precured laminates. Glass fiber anchor spikes were applied in one of the CFRP fabric strengthened beams. One of the FRP precured laminate strengthened beams was bonded with epoxy adhesive and the other one was attached by using mechanical fasteners. Five of the beams were tested under fatigue loading for two million cycles. All of the beams survived fatigue testing. The results showed that use of anchor spikes in fabric strengthening increase ultimate strength, and mechanical fasteners can be an alternative to epoxy bonded precured laminate systems.     

Flexural Strengthening of Reinforced Concrete Beams by Mechanically Attaching Fiber-Reinforced Polymer Strips

The current method of bonding fiber-reinforced polymer (FRP) strengthening strips to concrete structures requires extensive time and semiskilled labor. An alternative method is to use a commercial off-the-shelf powder-actuated fastening system to attach FRP strips to concrete. A series of flexural tests were conducted on 15 304.8×304.8×3,657.6?mm (12×12×144?in.) reinforced concrete beams. Two beams were tested unstrengthened, 12 were strengthened with mechanically fastened FRP strips, and one was strengthened with a bonded FRP strip. The effects of three different strip moduli, different fastener lengths and layouts, and predrilling were examined. Three of the beams strengthened with mechanically attached FRP strips showed strengthening comparable to the beam strengthened with a bonded FRP strip. The same three beams strengthened with mechanically attached FRP strips also showed a greater ductility than the beam strengthened with a bonded FRP strip.     

Tensile Behavior of FRP Anchors in Concrete

Strengthening of concrete structures using fiber-reinforced polymer (FRP) systems has become a widely accepted technology in the construction industry over the past decade. Externally bonded FRP sheets are proven to be a feasible alternative to traditional methods for strengthening and stiffening deficient reinforced or prestressed concrete members. However, the delamination of FRP sheets from the concrete surface poses major concerns, as it usually leads to a brittle member failure. This paper reports on the development of FRP anchors to overcome delamination problems encountered in surface bonded FRP sheets. An experimental investigation was conducted on the performance of carbon FRP anchors that were embedded in normal- and high-strength concrete test specimens. A total of 81 anchors were tested under monotonic uniaxial loading. Test parameters included the length, diameter, and angle of inclination of the anchors and the compressive strength of the concrete. The experimental results indicate that FRP anchors can be designed to achieve high pullout capacities and hence can be used effectively to prevent or delay the delamination of externally bonded FRP sheets. The results also indicate that the diameter, length, and the angle of inclination of the anchors have a significant influence on the pullout capacity of FRP anchors.     

Strengthening of Interior Slab–Column Connections Using a Combination of FRP Sheets and Steel Bolts

The results of an experimental investigation undertaken to evaluate a new technique for strengthening interior slab–column connection in combined flexural and shear modes are presented. The technique consists of using a combination of shear bolts inserted into holes and prestressed against the concrete surface for improving the punching shear capacity, and external [fiber-reinforced polymer (FRP)] reinforcement bonded to the tension face of the slabs in two perpendicular directions for increasing the flexural strength of the slabs. Square slab specimens of 670×670?mm dimensions were tested and the main test variables included the ratio of steel reinforcement (1.0 and 1.5%), span–depth ratio or thickness (55 and 75?mm) of the slabs, area, and configuration of steel bolts, and area of FRP reinforcement. It was found that the use of shear bolts alone improves the punching shear strength and increases the ductility of failure by changing the failure mode from punching to flexural. However, the use of a combination of shear bolts and a moderate amount of FRP reinforcement increased the flexural strength and resulted in a substantial improvement of the punching shear capacity of the slabs. The corresponding increases attained levels between 34 and 77%. A design approach is presented for evaluating the ultimate capacity of the slab–column connections when strengthened using the proposed strengthening technique. Strength results predicted by the proposed approach were in good agreement with the experimental results.     

Debonding in RC Beams Shear Strengthened with Complete FRP Wraps

Substantial research has been conducted on the shear strengthening of reinforced concrete (RC) beams with bonded fiber reinforced polymer (FRP) strips. The beams may be strengthened in various ways: complete FRP wraps covering the whole cross section (i.e., complete wrapping), FRP U jackets covering the two sides and the tension face (i.e., U jacketing), and FRP strips bonded to the sides only (i.e., side bonding). Shear failure of such strengthened beams is generally in one of two modes: FRP rupture and debonding. The former mode governs in almost all beams with complete FRP wraps and some beams with U jackets, while the latter mode governs in all beams with side strips and U jackets. In RC beams strengthened with complete wraps, referred to as FRP wrapped beams, the shear failure process usually starts with the debonding of FRP from the sides of the beam near the critical shear crack, but ultimate failure is by rupture of the FRP. Most previous research has been concerned with the ultimate failure of FRP wrapped beams when FRP ruptures. However, debonding of FRP from the sides is at least a serviceability limit state and may also be taken as the ultimate limit state. This paper presents an experimental study on this debonding failure state in which a total of 18 beams were tested. The paper focuses on the distribution of strains in the FRP strips intersected by the critical shear crack, and the shear capacity at debonding. A simple model is proposed to predict the contribution of FRP to the shear capacity of the beam at the complete debonding of the critical FRP strip.     

CFRP-Strengthened and Corroded RC Beams under Monotonic and Fatigue Loads

An experimental program has been carried out to investigate the structural behavior of RC beams strengthened by carbon-fiber–reinforced polymer (CFRP) sheets and exposed to a corrosive environment. A total of eight specimens (120 × 175 × 2,000 mm) were tested. Six specimens were CFRP strengthened and corroded, one specimen was unstrengthened and corroded, and one specimen was neither strengthened nor corroded. Two different strengthening schemes were applied: (1) wrapping the specimen with CFRP sheets; and (2) both specimen wrapping and flexural strengthening. Three specimens were tested under monotonic loading and five specimens were tested in fatigue. The results showed that the use of CFRP sheets for strengthening RC beams that are experiencing steel reinforcement corrosion is an efficient technique that can maintain the structural integrity and enhance the structural behavior of such beams. The ultimate monotonic strength of the CFRP strengthened-corroded specimens increased to a level between 37 and 87% above the predicted strength of a similar unstrengthened-uncorroded (virgin) specimen. The fatigue life of the CFRP strengthened-corroded specimens was increased within a range of 2.5–6.0 times that of a similar unstrengthened-corroded specimen but was lower than that of the uncorroded (virgin) specimen.     

Shear Strengthening of RC Beams with Externally Bonded FRP Composites: Effect of Strip-Width-to-Strip-Spacing Ratio

The results of an experimental and analytical investigation of shear strengthening of reinforced concrete (RC) beams with externally bonded (EB) fiber-reinforced polymer (FRP) strips and sheets are presented, with emphasis on the effect of the strip-width-to-strip-spacing ratio on the contribution of FRP (Vf). In all, 14 tests were performed on 4,520-mm-long T-beams. RC beams strengthened in shear using carbon FRP (CFRP) strips with different width-to-spacing ratios were considered, and their performance was investigated. In addition, these results are compared with those obtained for RC beams strengthened with various numbers of layers of continuous CFRP sheet. Moreover, various existing equations that express the effect of FRP strip width and concrete-member width and that have been proposed based on single or double FRP-to-concrete direct pullout tests are checked for RC beams strengthened in shear with CFRP strips. The objectives of this study are to investigate the following: (1)?the effectiveness of EB discontinuous FRP sheets (FRP strips) compared with that of EB continuous FRP sheets; (2)?the optimum strip-width-to-strip-spacing ratio for FRP (i.e., the optimum FRP rigidity); (3)?the effect of FRP strip location with respect to internal transverse-steel location; (4)?the effect of FRP strip width; and (5)?the effect of internal transverse-steel reinforcement on the CFRP shear contribution.     

Residual Behavior of Fire-Exposed Reinforced Concrete Beams Prestrengthened in Flexure with Fiber-Reinforced Polymer Sheets

Numerous research studies have shown externally bonded fiber-reinforced polymer (FRP) materials can be used efficiently and economically to repair and retrofit deteriorated or understrength concrete structures. FRP materials are being widely applied in the rehabilitation of deteriorated bridges, however, their use in buildings has been limited, partly because of insufficient knowledge about the performance of FRP materials in fire. To enable further applications of FRPs in buildings, this paper presents a study on the residual performance after fire of four reinforced-concrete (RC) T-beams that were prestrengthened with externally bonded FRP sheets and provided with a supplemental fire protection system. Results from this study suggest that the RC beams strengthened with FRPs prior to fire exposure retained most of their initial unstrengthened flexural capacity after fire. This is attributed to the fact that the temperature of the internal concrete and reinforcing steel was kept to below 200 and 593°C, respectively.     

Behavior of Gravity Load-Designed Rectangular Concrete Columns Confined with Fiber Reinforced Polymer Sheets

This study concentrates on analytical evaluation of the effect of external confinement using fiber reinforced polymers (FRP) sheets on the response of concrete rectangular columns designed for gravity load only and having spliced longitudinal reinforcement at the column base. A general analytical scheme for evaluating the strength capacity and ductility of the columns under combined flexural–axial loads was developed. The analysis takes into account the bond strength degradation of the spliced reinforcement with increase in lateral load by incorporating a generalized bond stress–slip law, and considers the effect of FRP confinement on the stress–strain response of concrete material. Particular emphasis is placed in the analysis on the slip response of the spliced bars and the consequent fixed end rotation that develops at the column base. Results predicted by the analysis showed very good agreement with limited experimental data. A parametric evaluation was carried out to evaluate the effect of different design and strength parameters on the column response under lateral load. Without confinement, the columns suffered premature bond failure and, consequently, low flexural strength capacity. Confining the concrete in the columns end zone at the splice location with FRP sheets enhanced the bond strength capacity of the spliced reinforcement, increased the steel stress that can be mobilized before bond failure occurs, and consequently improved the flexural strength capacity and ductility of the columns. A general design equation, expressed as a function of the main parameters that influence the bond strength capacity between spliced steel bars and FRP confined concrete, is proposed to calculate the area of FRP sheets needed for strengthening of the subject columns.