Bond Behavior of Near-Surface Mounted CFRP Laminate Strips under Monotonic and Cyclic Loading

Near-surface mounted (NSM) carbon fiber reinforced polymer (CFRP) laminate strips are used to increase the load-carrying capacity of concrete structures. This is done by inserting the CFRP strips into slits made in the concrete cover of the elements to be strengthened and gluing the strips to the concrete with an epoxy adhesive. In several cases the NSM technique has substantial advantages when compared with externally bonded laminates. To assess the bond behavior between the CFRP and concrete under monotonic and cyclic loading, an experimental program was carried out based on a series of pullout-bending tests. The influence of the bond length and loading history on the bond behavior was investigated. In this work the details of the tests are described and the obtained results discussed. Using the experimental data and an analytical-numerical strategy, a local bond stress-slip relationship was determined. A finite-element analysis was performed to evaluate the influence of the adhesive on the global response observed in the pullout-bending tests.     

Strengthening of Reinforced Concrete Bridge Decks Using Carbon Fiber-Reinforced Polymer Composite Materials

This paper presents the results of an investigation of the monotonic and fatigue behavior of one-way and two-way reinforced concrete slabs strengthened with carbon fiber-reinforced polymer (CFRP) materials. The five one-way slab specimens were removed from a decommissioned bridge in South Carolina. Three of the slabs were retrofitted with CFRP strips bonded to their soffits and the other two served as unretrofit, control specimens. Of the five one-way slab specimens, one unretrofit and two retrofit slabs were tested monotonically until failure. The remaining two specimens, one unretrofit and one retrofit, were tested under cyclic (fatigue) loading until failure. In addition, six half-scale, two-way slab specimens were constructed to represent a full-scale prototype of a highway bridge deck designed using the empirical requirements of the AASHTO LRFD Bridge Design Manual. Of the six square slabs, two were unretrofitted and served as the control specimens, two were retrofitted using CFRP strips bonded to their soffits making a grid pattern, and two were retrofitted with a preformed CFRP grid material bonded to their soffit. Three slabs, one unretrofit, one CFRP strip, and one CFRP grid retrofitted, were tested monotonically until failure and the remaining three slabs were tested under cyclic (fatigue) loading until failure.     

Behavior of Prestressed Concrete Strengthened with Various CFRP Systems Subjected to Fatigue Loading

Many prestressed concrete bridges are in need of upgrades to increase their posted capacities. The use of carbon fiber-reinforced polymer (CFRP) materials is gaining credibility as a strengthening option for reinforced concrete, yet few studies have been undertaken to determine their effectiveness for strengthening prestressed concrete. The effect of the CFRP strengthening on the induced fatigue stress ratio in the prestressing strand during service loading conditions is not well defined. This paper explores the fatigue behavior of prestressed concrete bridge girders strengthened with CFRP through examining the behavior of seven decommissioned 9.14?m (30?ft) girders strengthened with various CFRP systems including near-surface-mounted bars and strips, and externally bonded strips and sheets. Various levels of strengthening, prestressing configurations, and fatigue loading range are examined. The experimental results are used to provide recommendations on the effectiveness of each strengthening configuration. Test results show that CFRP strengthening can reduce crack widths, crack spacing, and the induced stress ratio in the prestressing strands under service loading conditions. It is recommended to keep the prestressing strand stress ratio under the increased service loading below the value of 5% for straight prestressing strands, and 3% for harped prestressing strands. A design example is presented to illustrate the proposed design guidelines in determining the level of CFRP strengthening. The design considers the behavior of the strengthened girder at various service and ultimate limit states.     

Flexural Strength of RC Beams with GFRP Laminates

The results of an experimental and numerical study of the flexural behavior of reinforced concrete beams strengthened with glass-fiber-reinforced-polymer (GFRP) laminates are presented in this paper. In the experimental program, ten strengthened beams and two unstrengthened beams are tested to failure under monotonic loading. A number of external GFRP laminate layers and bond length of GFRP laminates in shear span are taken as the test variables. Longitudinal GFRP strain development and interfacial shear stress distribution from the tests are examined. The experimental results generally showed that both flexural strength and stiffness of reinforced concrete beams could be increased by such a bonding technique. In the numerical study, an eight-node interface element is developed to simulate the interface behavior between the concrete and GFRP laminates. This element is implemented into the MARC software package for the finite-element analyses of GFRP laminate strengthened reinforced concrete beams. Reasonably good correlations between experimental and numerical results are achieved.     

Efficient Strengthening Technique to Increase the Flexural Resistance of Existing RC Slabs

Composite materials are being used with notable effectiveness to increase and upgrade the flexural load carrying capacity of reinforced concrete (RC) members. Near-surface mounted (NSM) is one of the most promising strengthening techniques, based on the use of carbon fiber-reinforced polymer (CFRP) laminates. According to NSM, the laminates are fixed with epoxy based adhesive into slits opened into the concrete cover on the tension face of the elements to strength. Laboratory tests have shown that the NSM technique is an adequate strengthening strategy to increase the flexural resistance of RC slabs. However, in RC slabs of low concrete strength, the increase of the flexural resistance that NSM can provide is limited by the maximum allowable compressive strain in the compressed part of the slab, in order to avoid concrete crushing. This restriction reduces the effectiveness of the strengthening, thus limiting the use of the NSM technique. A new thin layer of concrete bonded to the existing concrete at the compressed region is suitable to overcome this limitation. Volumetric contraction due to shrinkage and thermal effects can induce uncontrolled cracking in the concrete of this thin layer. Adding steel fibers to concrete [steel fiber-reinforced concrete (SFRC)], the postcracking residual stress can be increased in order to prevent the formation of uncontrolled crack patterns. In the present work, the combined strengthening strategy, a SFRC overlay and NSM CFRP laminates, was applied to significantly increase the flexural resistance of existing RC slabs. Experimental results of four-point bending tests, carried out in unstrengthened and strengthened concrete slab strips, are presented and analyzed.     

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.     

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.     

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.     

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.     

Investigation of Bond in Concrete Structures Strengthened with Near Surface Mounted Carbon Fiber Reinforced Polymer Strips

Fiber reinforced polymer (FRP) materials are currently produced in different configurations and are widely used for the strengthening and retrofitting of concrete structures and bridges. Recently, considerable research has been directed to characterize the use of FRP bars and strips as near surface mounted reinforcement, primarily for strengthening applications. Nevertheless, in-depth understanding of the bond mechanism is still a challenging issue. This paper presents both experimental and analytical investigations undertaken to evaluate bond characteristics of near surface mounted carbon FRP (CFRP) strips. A total of nine concrete beams, strengthened with near surface mounted CFRP strips were constructed and tested under monotonic static loading. Different embedment lengths were used to evaluate the development length needed for effective use of near surface mounted CFRP strips. A closed-form analytical solution is proposed to predict the interfacial shear stresses. The model is validated by comparing the predicted values with test results as well as nonlinear finite element modeling. A quantitative criterion governing the debonding failure of near surface mounted CFRP strips is established. The influence of various parameters including internal steel reinforcement ratio, concrete compressive strength, and groove width is discussed.     

Mechanism of Bond Behavior of Concrete Beams Strengthened with Near-Surface-Mounted CFRP Rods

Bond tests were conducted on concrete beams strengthened with near-surface-mounted (NSM) nonprestressed and prestressed carbon fiber-reinforced polymer (CFRP) rods under static loading. In the NSM technique, the CFRP rods are placed inside precut grooves and bonded to the concrete with epoxy adhesive. Six concrete beams were tested. The test variables included presence of internal tension steel reinforcement (unreinforced and reinforced), use of NSM CFRP strengthening (nonprestressed and prestressed), and type of CFRP rod (spirally wound and sand blasted). The beams were tested statically in four-point bending. Based on the test results, the transfer length for the prestressed CFRP rod in epoxied groove was 150 and 210 mm for the sand blasted and spirally wound rods, respectively. The main failure mode was debonding between the CFRP rod and the epoxy that starts at sections close to the midspan then, as the load increases, it propagates toward the supports. At failure, the beams strengthened with a given rod type showed the same CFRP strain at sections close to the support (29% of ultimate strain for spirally wound bars and 39% of ultimate strain for sand blasted bars). A cracked section analysis was carried out and compared well with the measured results.     

Moving-Wheel Fatigue for Bridge Decks Strengthened with CFRP Strips Subject to Negative Bending

This paper presents the negative bending of reinforced concrete slabs strengthened with near-surface mounted (NSM) carbon fiber-reinforced polymer (CFRP) strips. Six slab specimens, three of which are strengthened with CFRP strips, are tested in static and fatigue loads. A wheel-running fatigue test machine is used to simulate vehicular loads on a bridge deck. The effectiveness of CFRP strengthening for bridge decks in cantilever and pseudonegative bending is examined based on moment-carrying capacity and cyclic behavior under the wheel-running fatigue loads, including crack patterns and damage accumulation. The moment-carrying capacity (static) of the cantilever slab strengthened with the NSM CFRP strips is improved by 68.4% when compared to that of an unstrengthened slab. The damage accumulation rate of the strengthened cantilever slab owing to the fatigue load is significantly lower than that of the unstrengthened slab. The damage accumulation of the strengthened slab gradually increases and is irreversible when the fatigue cycles increase. The fatigue-induced flexural cracks of the slabs develop along the wheel-running direction. A simple predictive model is presented to estimate the fatigue life of the test slabs.     

CFRP-Based Strengthening Technique to Increase the Flexural and Energy Dissipation Capacities of RC Columns

A strengthening technique, combining carbon fiber-reinforced polymer (CFRP) laminates and strips of wet layup CFRP sheet, is used to increase both the flexural and the energy dissipation capacities of reinforced concrete (RC) columns of square cross section of low to moderate concrete strength class, subjected to constant axial compressive load and increasing lateral cyclic loading. The laminates were applied according to the near surface mounted technique to increase the flexural resistance of the columns, while the strips of CFRP sheet were installed according to the externally bonded reinforcement technique to enhance the concrete confinement, particularly in the plastic hinge zone where they also offer resistance to the buckling and debonding of the laminates and longitudinal steel bars. The performance of this strengthening technique is assessed in undamaged RC columns and in columns that were subjected to intense damage. The influence of the concrete strength and percentage of longitudinal steel bars on the strengthening effectiveness is assessed. In the groups of RC columns of 8 MPa concrete compressive strength, this technique provided an increase of about 67% and 46% in terms of column’s load carrying capacity, when applied to undamaged and damaged columns, respectively. In terms of energy dissipation capacity, the increase ranged from 40%–87% in the undamaged columns, while a significant increase of about 39% was only observed in one of the damaged columns. In the column of moderate concrete compressive strength (29 MPa), the technique was even much more effective, since, when compared to the maximum load and energy dissipation capacity of the corresponding strengthened column of 8 MPa of average compressive strength, it provided an increase of 39% and 109%, respectively, showing its appropriateness for RC columns of buildings requiring upgrading against seismic events.     

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.     

Structural Performance of CFRP-Strengthened RC Slabs in a Corrosive Environment

This study was undertaken to address the effect of the main steel corrosion on the structural performance of RC slabs strengthened with carbon-fiber-reinforced polymer (CFRP) strips and exposed to a corrosive environment. A total of eight specimens (500×100×1,500?mm) were constructed and tested under monotonic static loading. Three specimens were CFRP-strengthened and corroded, three specimens were CFRP-strengthened and kept at room temperature, one specimen was unstrengthened and corroded, and one specimen was neither strengthened nor corroded. Three different strengthening schemes were applied: (1) externally bonded CFRP strips; (2) externally bonded CFRP strips provided with CFRP anchors; and (3) near surface mounted (NSM) CFRP strips. During the corrosion process, the specimens were placed in a small tank filled with sodium chloride (NaCl) solution concentration (3%) which covered only the slabs’ bottom third, and corrosion was induced by means of an impressed current. The corrosion process lasted for 20 days, and the average mass loss of the main steel reinforcement due to corrosion was 9%. Following corrosion, the specimens were tested under four-point bending. The experimental results showed that the increase in flexural capacity achieved using the three strengthening schemes were significantly reduced due to corrosion of the main steel. The recorded reductions in flexural strength gains for the CFRP-strengthened corroded slabs relative to the gains for the strengthened uncorroded slabs were about 55, 38, and 41% for the externally bonded CFRP system without anchors, externally bonded CFRP with anchors, and NSM-CFRP system, respectively.     

Effects of Gradually Anchored Prestressed CFRP Strips Bonded on Prestressed Concrete Beams

Four prestressed concrete beams were constructed and tested to investigate the effectiveness of flexural post-strengthening with prestressed carbon fiber-reinforced polymer (CFRP) strips. One of the beams served as a reference beam, another was bonded with an unstressed CFRP strip, and the remaining two specimens were strengthened with prestressed CFRP strips at two prestressing levels. The gradient method was used for the anchorage of the prestressed CFRP strips. Experimental and analytical calculations are compared with the test results. Further, different failure modes are explained. On the basis of this investigation, recommendations for the use of prestressed CFRP strips anchored with the gradient method are given.     

Prestressed Carbon Fiber Reinforced Polymer Sheets for Strengthening Concrete Beams at Room and Low Temperatures

A technique for strengthening damaged concrete beams using prestressed carbon fiber reinforced polymer (CFRP) sheets was developed at Queen’s University and the Royal Military College of Canada. As part of this study, an anchorage system was developed to directly prestress the CFRP sheets by jacking and reacting against the strengthened concrete beam itself. The feasibility and effectiveness of using bonded prestressed CFRP sheets to strengthen precracked concrete beams at both room (+22°C,+72°F) and low (?28°C,?20°F) temperatures have been investigated experimentally. Materials and prestress changes due to temperature variations that would affect and cause changes in flexural behavior were studied. The strengthened beams showed significant increases in flexural stiffness and ultimate capacity as compared to the control-unstrengthened beams. The flexural behavior of the strengthened beams was not adversely affected by short-term exposure to reduced temperature (?28°C,?20°F). In addition to the experimental investigation, analytical models were developed to predict the overall flexural behavior of the strengthened beams during prestressing of the CFRP sheets and under external loading at both room and low temperatures. The model accurately predicted the flexural beam behavior. Improved serviceability behavior and higher strength were predicted for beams strengthened with the bonded prestressed CFRP sheets.     

Concrete Beams Strengthened with Prestressed Near Surface Mounted CFRP

Retrofitting concrete structures with fiber reinforced polymer (FRP) has today grown to be a widely used method throughout most parts of the world. The main reason for this is that it is possible to obtain a good strengthening effect with a relatively small work effort. It is also possible to carry out strengthening work without changing the appearance or dimensions of the structure. Nevertheless, when strengthening a structure with external FRP, it is often not possible to make full use of the FRP. The reason for this depends mainly on the fact that a strain distribution exists over the section due to dead load or other loads that cannot be removed during strengthening. This implies that steel yielding in the reinforcement may already be occurring in the service limit state or that compressive failure in the concrete is occurring. By prestressing, a higher utilization of the FRP material is made possible. It is extremely important to ensure that, if external prestressing is used, the force is properly transferred to the structure. Most of the research conducted with prestressing carbon fiber reinforced polymer (CFRP) for strengthening has been on surface bonded laminates. However, this paper presents research on prestressed CFRP quadratic rods bonded in sawed grooves in the concrete cover. This method has proven to be an advantageous means of bonding CFRP to concrete, and in comparison to surface bonded laminates, the shear and normal stress between the CFRP and the concrete are more efficiently transferred to the structure. In the presented test, no mechanical device has been used to maintain the prestress during testing, which means that the adhesive must transfer all shear stresses to the concrete. Fifteen beams with a length of 4?m have been tested. The tests show that the prestressed beams exhibited a higher first-crack load as well as a higher steel-yielding load as compared to nonprestressed strengthened beams. The ultimate load at failure was also higher, as compared to nonprestressed beams, but in relation not as large as for the cracking and yielding. In addition, the beams strengthened with prestressed FRP had a smaller midpoint deflection. All strengthened beams failed due to fiber rupture of the FRP.     

Interfacial Behavior and Debonding Failures in FRP-Strengthened Concrete Slabs

It has been demonstrated, through laboratory investigations and various field projects, that the external bonding of fiber- reinforced polymer (FRP) laminates is an effective technique for the structural enhancement of reinforced concrete slabs. In such applications, failure is generally governed by debonding of the FRP laminate. Nevertheless, numerical simulations to date of FRP-strengthened slabs have usually been based on the assumption of full bond between the concrete and FRP. In this study, the interfacial behavior between the FRP laminates and the concrete substrate is accounted for by introducing appropriate bond-slip models for the interface in a nonlinear finite-element analysis of FRP-strengthened two-way slabs. The numerical model is capable of simulating slabs strengthened in shear or in flexure; it can be applied to arbitrary FRP configurations, and can also accommodate both passive as well as prestressed FRP strengthening schemes. Results are presented in terms of load-deflection relationships, ultimate load capacities, failure modes, and interfacial slip and stress distributions. When compared to test results reported in the literature, the analysis is shown to lead to excellent predictions in that, for the entire set of FRP-strengthened specimens considered, the average of the numerical-to-experimental load capacity ratios is 0.966, with a standard deviation of 0.066. Furthermore, in all cases when FRP debonding was observed experimentally, the analysis correctly predicted the mode of failure.     

Impact Loading of Concrete Beams Externally Strengthened with CFRP Laminates

The quasi-static behavior of flexural members strengthened with carbon fiber reinforced polymer (CFRP) laminates has been well documented in recent years, but there is a gap in knowledge regarding the effects of dynamic and impact loading of such members. This paper presents the test results of four 8 m beams externally strengthened for flexure, two with CFRP laminates and two with steel plates. Impact loading was induced by lifting one end of the simply supported beams and dropping it from given heights. The strain rates induced in the CFRP laminates were at least three orders of magnitude greater than the strain rates used for testing CFRP laminate coupons in tension. Comparisons are made between the dynamic impact behavior of the beams strengthened with CFRP laminates and steel plates, and the behavior of both beam types is modeled using an equation of motion. The beams externally strengthened with CFRP laminates performed well under impact loading, although they could not provide the same energy absorption as the beams externally strengthened with steel plates. Additional anchoring, at least at the ends of the CFRP laminates, would improve the impact resistance of these beams. Good predictions were made with the derived equation of motion by using the flexural stiffness of the beams at their ultimate limit state.