Ultimate Response of RC Beams Strengthened with CFRP Plates

Poststrengthening and retrofitting is a growing reality, as existing structures are required to meet the demands of modern society. Apart from the need to increase load capacity, upgrading of a structure may be necessitated through deterioration of the structure by corrosion or accidental damage, a change in the structural system, or to rectify initial design and construction faults. A commonly observed mode of failure for beams strengthened using carbon-fiber-reinforced polymer composite material plates is one due to the plate peeling off prematurely and unpredictably at relatively low magnitudes of applied loading. End plate anchorages and long unanchored plate lengths, which can add significantly to the overall cost of a strengthening solution, overcome this problem. This paper presents the results of flexural tests on ten reinforced concrete beams strengthened with different plate configurations. A strain compatibility and force equilibrium method of analysis, coupled with an empirical rule derived from the test data, is demonstrated to be effective in predicting the ultimate response of simply supported beams in bending with and without end plate anchorages and irrespective of plate length.     

Flexural Response of Steel Beams Strengthened with Partial-Length CFRP Plates

This paper presents the flexural behavior of rolled steel beams that were strengthened with partial-length, adhesive-bonded carbon fiber-reinforced polymer (CFRP) plates. The hybrid beams had two types of failure mode, depending on the length of the plate: (1) plate debonding in beams with short plates;?and (2) plate rupture at midspan in beams with long plates. The flexural behavior that was investigated includes the development of tensile stresses in the plate, the moment-curvature of the strengthened section, and the load-deflection of the strengthened beam. The analytical methods used include shear lag analysis, section analysis, and application of the virtual work principle. Agreement between the experimental results and the analytical predictions is discussed.     

Flexure of Two-Way Slabs Strengthened with Prestressed or Nonprestressed CFRP Sheets

This paper presents a study on the flexural behavior of two-way reinforced concrete slabs externally strengthened with prestressed or nonprestressed carbon fiber-reinforced polymer (CFRP) sheets. Four large-scale flat plate slabs (3,000?mm×3,000?mm×90?mm) are tested and a nonlinear three-dimensional finite-element analysis is conducted to predict the flexural behaviors of the tested slabs, including the load-deflection response, strain distribution, crack propagation, and crack mouth opening displacement. An increase in the load-carrying capacity of 25 and 72% is achieved for the slabs strengthened with nonprestressed and prestressed CFRP sheets, respectively, in comparison to the unstrengthened slab. A reduction of the deflections up to 32% in service is noted for the strengthened slabs. The unstrengthened slab shows very ductile behavior, whereas, progressive failure is observed for the strengthened slabs, exhibiting pseudoductility in postpeak behavior. Stress redistribution between the internal and external reinforcement is significant in the slab strengthened with prestressed CFRP sheets.     

Debonding Strength of Steel Beams Strengthened with CFRP Plates

This paper addresses the debonding strength of partial-length, adhesively bonded carbon fiber-reinforced polymer (CFRP) plates that are used to strengthen steel beams. Bonded CFRP plates tend to debond under static and fatigue loadings because of the very high stress field at the plate end. Such failures limit the application of CFRP plates. Static and fatigue tests show that the stress intensity factor governs the debonding strength. The steel/adhesive corner was the locus of debond initiation. The effects of the following parameters on stress intensity factors are discussed: plate thickness, plate modulus, bondline thickness, adhesive modulus, and adhesive spew-fillet angle. The stress intensity factors are calculated using the Betti’s law-based reciprocal work contour integral method (RWCIM). The parametric study results indicate that the stress intensity factors cannot be used to represent the severity of the corner as the adhesive spew-fillet angle (and singularity) changes. Therefore, the use of stress intensity factors as a failure criterion for the purpose of predicting debonding strength is limited to the same spew-fillet angle.     

Fatigue Performance of Concrete Beams Strengthened with CFRP Plates

Recent increases in bridge design loading requirements have highlighted the need for fast, efficient, and durable strengthening methods. External steel plate bonding provides a satisfactory solution, but carbon fiber reinforced plastic (CFRP) offers the added advantages of resistance to corrosion, low weight, and high mechanical strength. This paper examines the fatigue performance of CFRP-strengthened concrete beams as part of a project investigating the use of CFRP as an alternative to steel. Five reinforced concrete beams were tested in fatigue; two control beams and three strengthened with externally bonded CFRP plates. Three loading options were used: (1) apply the same loads to both plated and unplated beams, (2) apply loads to give the same stress range in the rebar in both beams, and (3) apply the same percentage of the ultimate load capacity to each beam. Fatigue fracture of the internal reinforcement steel would appear to be the dominant factor governing failure, and it would appear reasonable to expect the same fatigue life for plated and unplated beams with comparable values of stress range in the steel bar.     

Fatigue Behavior of RC Beams Strengthened with NSM CFRP Rods

This paper presents experimental results of reinforced concrete beams strengthened using near-surface mounted (NSM) carbon fiber-reinforced polymer (CFRP) reinforcement. A total of nine beam specimens were tested under fatigue loads. In addition, two specimens were tested for monotonic capacity. The beams were 3,500 mm long with a cross section of 254 mm deep by 152 mm wide. Different load ranges were considered in the fatigue tests to construct the fatigue life curves. The test results showed that under monotonic loading, the beam strengthened with NSM CFRP rod exhibited increases of 26 and 50% in the yield and ultimate load over the control beam, respectively. Under cyclic loading, the fatigue life for the strengthened beams was 24% higher than that of the control unstrengthened beams. An analytical model using sectional analysis and strain-life approach was developed to estimate the fatigue life of the specimens at various cyclic load ranges. A good agreement between the experimental results and analytical prediction of the fatigue life was obtained.     

Response of RC Beams Strengthened with CFRP Laminates and Subjected to a High Rate of Loading

Results are presented of an experimental program undertaken to investigate the effects of strain rate on the behavior of reinforced concrete (RC) beams strengthened with carbon fiber-reinforced polymer (CFRP) laminates. Nine 3-m RC concrete beams, one unstrengthened, four strengthened with S-type CFRP laminates, and four strengthened with R-type laminates, were loaded under four different loading schedules. The stroke rates ranged from 0.0167 mm∕s (slow rate of loading) to 36 mm∕s (fast rate of loading). This induced a strain rate in the CFRP of 2.96 με∕s (slow rate) to 6,930 με∕s (fast rate). Some beams were subjected to either 1 or 12 cycles of loading prior to a fast rate of loading to failure. The rapidly loaded beams showed an increase of approximately 5% in capacity, stiffness, and energy absorption. Ductility and the mode of failure were not directly affected by the change in loading rate. Precycled beams performed similarly to the beams loaded monotonically to failure but showed a 10% increase in service stiffness and a 10% loss in energy absorption. A finite-element, layered analysis is presented to predict the moment-curvature response of CFRP strengthened RC beams. The model includes the effects of strain rate and correlates well with the experimental data.     

Fatigue Performance of Tension Steel Plates Strengthened with Prestressed CFRP Laminates

An experimental and analytical study was conducted to investigate the fatigue behavior of tension steel plates strengthened with prestressed carbon-fiber-reinforced polymer (CFRP) laminates. A simple fracture mechanics model was proposed to predict the fatigue life of reinforced specimens. Double-edge-notched specimens were precracked by fatigue loading and then strengthened by CFRP laminates at different prestressing levels. The effects of the applied stress range, CFRP stiffness, and prestressing level on the crack growth were investigated. Experimental results show that the increase of the prestressing level extends the fatigue life of a damaged steel plate to a large amount. The CFRP with the highest prestressing level performed best, prolonging fatigue life by as much as four times under 25% higher fatigue loading. Theoretically, predicted results were in a reasonable agreement with the experimental results. A parametric analysis was also performed to investigate the effects of the applied stress range and the prestressing level on the debonding behavior of the adhesive and on the secondary crack propagation.     

Static and Fatigue Analyses of RC Beams Strengthened with CFRP Laminates

Extensive testing has shown that externally bonded carbon fiber reinforced polymer (CFRP) laminates are particularly suited for improving the short-term behavior of deficient reinforced concrete beams. Accelerated fatigue tests conducted to date confirm that fatigue response is also improved. This paper describes an analytical model for simulating the static response and accelerated fatigue behavior of reinforced concrete beams strengthened with CFRP laminates. Static and fatigue calculations are carried out using a fiber section model that accounts for the nonlinear time-dependent behavior of concrete, steel yielding, and rupture of CFRP laminates. Analysis results are compared with experimental data from two sets of accelerated fatigue tests on CFRP strengthened beams and show good agreement. Cyclic fatigue causes a time-dependent redistribution of stresses, which leads to a mild increase in steel and CFRP laminate stresses as fatigue life is exhausted. Based on the findings, design considerations are suggested for the repair and∕or strengthening of reinforced concrete beams using CFRP laminates.     

Fatigue Performance of RC Beams Strengthened in Shear with CFRP Fabrics

In recent years, a tremendous effort has been directed toward understanding and promoting the use of externally bonded fiber-reinforced polymer (FRP) composites to strengthen concrete structures. Despite this research effort, studies on the behavior of beams strengthened with FRP under fatigue loading are relatively few, especially with regard to its shear-strengthening aspect. This study aims to examine the fatigue performance of RC beams strengthened in shear using carbon FRP (CFRP) sheets. It involves six laboratory tests performed on full-size T-beams, where the following parameters are investigated: (1) the FRP ratio and (2) the internal transverse-steel reinforcement ratio. The major finding of this study is that specimens strengthened with one layer of CFRP survived 5 million cycles, some of them with no apparent signs of damage, demonstrating thereby the effectiveness of FRP strengthening systems on extending the fatigue life of structures. Specimens strengthened with two layers of CFRP failed in fatigue well below 5 million cycles. The failure mode observed for these specimens was a combination of crushing of the concrete struts, local debonding of CFRP, and yielding of steel stirrups. This failure may be attributed to the higher load amplitude and also to the greater stiffness of the FRP which may have changed the stress distribution among the different components coming into play. Finally, comparison between the performance of specimens with transverse steel and without seems to indicate that the addition of transverse steel extends the fatigue life of RC beams.     

Slender Steel Columns Strengthened Using High-Modulus CFRP Plates for Buckling Control

This paper presents the results of an experimental investigation into the behavior of slender steel columns strengthened using high-modulus (313?GPa), carbon fiber-reinforced polymer (CFRP) plates. Eighteen slender hollow structural section square column specimens, 44×44×3.2?mm, were concentrically loaded to failure. The effectiveness of CFRP was evaluated for different slenderness ratios (kL/r), namely, 46, 70, and 93. The maximum increases in ultimate load ranged from 6 to 71% and axial stiffness ranged from 10 to 17%, respectively, depending on kL/r. As kL/r reduced, the effectiveness of CFRP plates also reduced, and failure mode changed from CFRP plate crushing after occurrence of overall buckling, to debonding prior to, or just at, buckling. A simplified analytical model is proposed to predict the ultimate axial load of FRP-strengthened slender steel columns, based on the ANSI/AISC 360-05 provisions, which were modified to account for the transformed section properties and a failure criteria of FRP derived from the experimental results. It was shown that for a given FRP reinforcement ratio, there is a critical kL/r at the low end, below which FRP may not enhance the strength of the column.     

Debonding Failures of RC Beams Strengthened with Near Surface Mounted CFRP Strips

This paper presents the results of a series of tests conducted on reinforced concrete (RC) beams strengthened in flexure with near surface mounted (NSM) carbon fiber-reinforced polymer (CFRP) strips. As the main focus of the research is on debonding failure mechanisms, the only test variable investigated was the embedment length of the NSM strip and the NSM strip was extensively strain-gauged to monitor its bond behavior. Load-deflection curves, failure modes, strain distributions in the CFRP strip, and local bond stresses at the CFRP–epoxy interface from the tests are all examined in detail and compared with the predictions of a simple analytical model where appropriate. Of the four embedment lengths investigated, all but the shortest one led to a notable increase in the load-carrying capacity and, to a lesser extent, in the postcracking stiffness of the beam. Debonding was found to be the primary failure mode in all cases except for the beam with the longest embedment length. Also reported in this paper are results from preliminary bond tests used to characterize the local bond-slip behavior of the NSM system. Apart from gaining a better understanding of debonding failures in RC beams with NSM FRP strips, the test results reported in the paper should be useful for future verification of numerical and analytical models.     

Experimental Study of Rectangular RC Columns Strengthened with CFRP Composites under Eccentric Loading

This paper presents the results of experimental studies on reinforced concrete columns strengthened with carbon fiber-reinforced polymer (CFRP) composites under the combination of axial load and bending moment. A total of seven large-scale specimens with rectangular cross section (200?mm×300?mm) were prepared and tested under eccentric compressive loading up to failure. The overall length of specimens with two haunched heads was 2,700 mm. Different FRP thicknesses of two, three, and five layers; fiber orientations of 0°, 45°, and 90°; and two eccentricities of 200 and 300 mm were investigated. The effects of these parameters on load-displacement and moment-curvature behaviors of the columns as well as the variation of longitudinal and transverse strains on different faces of the columns were studied. The results of the study demonstrated a significant enhancement on the performance of strengthened columns compared to unstrengthened columns.     

Impact Response of Externally Strengthened Unreinforced Masonry Walls Using CFRP

Research reported herein investigates the out-of-plane impact resistance of unreinforced masonry (URM) walls strengthened with carbon fiber-reinforced polymer (CFRP) composites, externally applied in sheets to one face of the wall. Two analytical methods based on energy principle and wave propagation theory and a finite-element-based numerical model have been developed, assuming a perfect bond at composite–masonry interface with an equivalent stiffness of the system. Full-scale impact tests are conducted for verification purpose, where three 1.2?m tall URM concrete walls (one unstrengthened and two strengthened with continuous unidirectional and woven CFRP sheets) are vertically tested up to cracking using a pendulum drop-weight impact tester. The test results compare reasonably well with those obtained from the analyses and simulation. It is found that the energy and finite-element methods can provide reasonable estimates for peak impact force and wall deflection, whereas the wave propagation method is rather limited by its applicability. Parametric studies are conducted to examine the effect of impactor mass, velocity, amount of CFRP reinforcement, and property of masonry material using the developed models.     

RC Slabs Strengthened with Prestressed and Gradually Anchored CFRP Strips under Monotonic and Cyclic Loading

This paper presents the test results of reinforced concrete slabs strengthened with prestressed and gradually anchored carbon fiber–reinforced polymer (CFRP) strips under monotonic and cyclic loading. To take full advantage of the externally bonded CFRP technique, it is beneficial to apply the laminates in a prestressed state, which relieves the stress in the steel reinforcement and reduces crack widths and deflection. The aim of the monotonic tests was to determine the strengthening efficiency of the new prestressing technique and to investigate serviceability and ultimate states. The cyclic tests were performed to identify the fatigue behavior of the strengthened slabs and to investigate the influence of long-term cyclic loading and elevated temperature on the bond properties of the prestressed CFRP laminates and the ductility and flexural strength of the strengthened slabs. A nonlinear analytical model of reinforced concrete members strengthened with passive and prestressed CFRP strips under static loading is proposed in the paper. A comparison of the experimental and predicted results reveals an excellent agreement in the full range of loading.     

Fatigue Performance of CFRP Strengthened RC Beams under Environmental Conditioning and Sustained Load

Carbon fiber-reinforced polymers (CFRPs) have become increasingly important in recent years in bridge rehabilitation. Significant research has been done on the static behavior of CFRP-strengthened reinforced concrete (RC) structures; however, the fatigue behavior of such structures with interface defects subjected to harsh environmental conditions still needs to be investigated. Hence, an experimental program has been carried out to investigate the fatigue behavior, under a load range, which generates service load stress levels, of RC beams strengthened with CFRP fabrics. The effect of aggressive environments was studied by subjecting the test members to freeze–thaw, extreme temperature, ultraviolet light exposure, and relative humidity cycles. All beams survived 2 million fatigue cycles without showing significant bond degradation between composite and substrate. However, significant flexural stiffness degradation was observed in the conditioned specimens. The presence of defects also affected specimen stiffness; however, limited growth in defect size was observed due to fatigue cycling.     

Experimental Investigation on Torsional Behavior of Solid and Box-Section RC Beams Strengthened with CFRP Using Photogrammetry

The construction boom over the last century has resulted in a mature infrastructure network in developed countries. Lately, the issue of maintenance and repair/upgrading of existing structures has become a major issue, particularly in the area of bridges. 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. The need for torsional strengthening in bridge box girders is highlighted by the Westgate Bridge in Melbourne, Australia, one of the largest strengthening projects in the world for externally bonded carbon FRP (CFRP) laminates. This paper reports the experimental work in an overall investigation of torsional strengthening of solid and box-section reinforced concrete beams with externally bonded carbon FRP. This was found to be a viable method of torsional strengthening. Photogrammetry was a noncontact measuring technique used in the investigation. The deformation mechanisms were found to be unchanged in the strengthened specimens. Furthermore, it was found that the crack widths were reduced and aggregate interlocking action improved with the strengthened beams.     

Reinforced Concrete T-Beams Strengthened in Shear with Fiber Reinforced Polymer Sheets

This research studies the interaction of concrete, steel stirrups, and external fiber reinforced polymer (FRP) sheets in carrying shear loads in reinforced concrete beams. A total of eight tests were conducted on four laboratory-controlled concrete T-beams. The beams were subjected to a four-point loading. Each end of each beam was tested separately. Three types of FRP, uniaxial glass fiber, uniaxial carbon fiber, and triaxial glass fiber, were applied externally to strengthen the web of the T-beams, while some ends were left without FRP. The test results show that FRP reinforcement increases the maximum shear strengths between 15.4 and 42.2% over beams with no FRP. The magnitude of the increased shear capacity is dependent not only on the type of FRP but also on the amount of internal shear reinforcement. The triaxial glass fiber reinforced beam exhibited more ductile failure than the other FRP reinforced beams. This paper also presents a test model that is based on a rational mechanism and can predict the experimental results with excellent accuracy.     

Efficient CFRP Strap Configurations for the Shear Strengthening of Reinforced Concrete T-Beams

A prestressed carbon fiber-reinforced polymer (CFRP) strap retrofitting system has been found to significantly enhance the shear capacity of existing reinforced concrete beams. In previous studies, the CFRP straps were supported on metal pads placed on the top and bottom of a beam necessitating top surface access. The goal of the current work was to develop a system where the straps were installed from underneath a slab without compromising the strengthening efficiency. A series of T-beam experiments was conducted where the CFRP straps were inserted through holes that were drilled from below the flange, thereby avoiding the need for access to the top surface. The depth of penetration of the CFRP straps into the compression flange, the concrete strength, the CFRP strap spacing, the presence of holes in the compression flange, and the size of the loading pads were all found to affect the shear performance. Using the most successful installation technique, the resulting CFRP strengthened beam failed at a load that was approximately 50% higher than that of an unretrofitted control beam.