Carbon Fiber Reinforced Polymer Strain Research Articles

Behavior of reinforced concrete beams strengthened with CFRP rod panels CRP 195

This paper investigates the behavior of reinforced concrete beams strengthened in flexure using externally bonded carbon fiber reinforced polymer (CFRP) rod panels (CRPs) fabricated using 4 mm rods spaced at 9.5 mm. The experimental program consisted of a control beam and three beams strengthened with either a continuous CRP, a spliced CRP with two half-length CRPs spliced through a 150 mm finger joint, or a spliced and end-anchored CRP. Spliced panel performed similar to the continuous one, with no debonding observed at finger joint. They resulted in a comparable strength increase relative to the non-strengthened control beam of 95 and 104%, respectively, and failed by concrete cover separation (CCS) at CRP end. When the spliced CRP was end-anchored, the increase in strength was 195% relative to the control beam, with the finger-joint still intact, and the failure mode changed to intermediate crack-induced debonding (ICID). The analytical model, developed from sectional analysis and strength models for debonding failures, is capable of predicting the load-deflection response and ultimate loads, including the two observed debonding failures. For CCS failure, two models are presented, one is based on the concrete tooth model, and the other on the shear capacity model. The ICID failure was based on limiting the CFRP strain in accordance with ACI 440.2R-17 design guide. Results showed a good correlation between analytical predictions and test data.

Behavior of plain concrete beams with DSSF strengthened in flexure with anchored CFRP sheets—Effects of DSSF content on the bonding length of CFRP sheets

In this study, the behavior of the anchored carbon fiber reinforced polymer (CFRP) plain concrete beams with various dosages of macro discontinuous structural synthetic fiber (DSSF) were investigated. Seventy two plain concrete beams were casted and tested. The investigated parameters in this study are the dosages of DSSF, length of CFRP sheets, and with and without CFRP anchorage sheets. Therefore, the feasibility of using macro synthetic fibers and anchored CFRP sheets for enhancing the behavior of plain concrete beams is of enormous interest. The behavior characteristic of each tested beam was evaluated in terms of failure mode, load versus mid-span deflection curve, ultimate load capacity, ultimate deflection, stiffness, energy absorption, performance factor, displacement ductility index, energy absorption ductility index, and CFRP and concrete strain. The tested results reveal a considerable enhancement on the majority of the investigated parameters and show the efficiency of using a combination macro synthetic fibers and anchored CFRP sheets in terms of the behavior characteristic of tested beams. The enhancements were recognized primarily to the efficiency of the used anchored CFRP and macro synthetic fibers in providing external and internal, respectively, cracks arresting mechanism after cracking and even after reaching the ultimate load capacity.

Experimental Investigation of the Shear Strength of RC Beams Extracted from an Old Structure and Strengthened by Carbon FRP U-Strips

In recent years, old reinforced concrete (RC) buildings have increasingly been protected as historic symbols in Italy. The conversion of these buildings can resolve the increasing need for new structures, reducing much social, economic, and environmental impact. Retrofitting solutions by carbon fiber reinforced polymer (CFRP) reinforcements are very common nowadays. Code predictions present uncertainties due to the brittle behavior and the debonding of CFRP strips resulting from concrete cracking. Therefore, especially in the case of old beams, experimental validation is necessary. This paper deals the experimental evaluation of the shear strength of two beams extracted from an old RC building in Rome built in 1929 with modest-quality concrete. Preliminary tests were carried out to evaluate the beam elastic response and the material characteristics. These beams were tested until failure (three-point load shear/bending tests) after strengthening for shear by CFRP U-jacketing and for bending by new steel rebars. The results obtained, together with the extensive data taken from the literature, were compared with the predictions by design code equations. The influence of construction details on beam shear strength and the experimental maximum CFRP strain were also analyzed. Code predictions can be effective to estimate the shear strength of the retrofitted beams.

Torsional behavior of reinforced concrete T-beams strengthened with CFRP strips

Torsion is resisted well by locked system of reinforcement, due to the blood nature of the torsion induced-shear flow stresses in a beam. Therefore, it will be more effective to have strengthening schemes, which are wrapped in closed form around the cross section. But strengthening with U wrap (three sides) or strengthening in the web and flange unconnectedly is more useful because of the unreachability of the entire cross section due to addition of flange in uniform beam – slab construction (T section of interior beam).This work aims at studying the strengthening of T beams in torsion by using carbon fiber reinforced polymer (CFRP). The CFRP laminates will be used for strengthening previous to loading. The experimental work includes investigation of twelve reinforced concrete T beams tested under pure torsion. The first beam was the reference beam, without CFRP. The second beam was a simple concrete beam. The remaining ten beams. Variables considered in the test program include; effect of flange strengthening, effect of fiber orientation (90° or 45° CFRP strips with respect to the longitudinal axis), effect of using additional longitudinal CFRP strips with transverse CFRP strips, effect of bolt anchoring CFRP strips in the web and flange and effect of continuous CFRP strips between web and flange (isolated T section). Test results were discussed based on torque – twist behavior, beam elongations, CFRP strain, concrete strain and influence of CFRP on cracking torque, ultimate torque and failure modes.

Determination of the bond–slip behavior of CFRP-to-steel bonded interfaces using digital image correlation

The bond–slip behavior between carbon fiber reinforced polymer (CFRP) plates and steel is crucial for predicting the structural behavior of CFRP-strengthened steel members. In this paper, the CFRP-to-steel bonded joints with different bond lengths and adhesive thicknesses were tested to investigate the bond–slip behavior of CFRP-to-steel bonded interfaces. The digital image correlation (DIC) technique was used to measure the displacement and strain data. The test results, including the failure mode, ultimate load, displacement distribution, and CFRP strain distribution, are presented and discussed. The non-linear bond–slip curves of CFRP-to-steel bonded interfaces are determined by a fitting procedure for CFRP strain distributions. A bilinear bond–slip relationship is further proposed to simplify the non-linear bond–slip curve. The test results indicate that the adhesive thickness has an influence on the failure mode, ultimate load, and bond–slip behavior. Similar to a CFRP-to-concrete bonded interface, an effective bond length also exists for a CFRP-to-steel bonded interface. The development of the interfacial shear stress is analyzed based on the determined non-linear and bilinear bond–slip curves. Finally, two theoretical models are employed for the prediction of the ultimate load, effective bond length and bond–slip curve. This study shows that the DIC method is suitable for investigating the bond–slip behavior of CFRP-to-steel bonded joints.

Slender RC columns strengthened with combined CFRP and steel jacket under axial load

This paper presents an experimental study on the effectiveness of simultaneous application of carbon fiber-reinforced polymer (CFRP) and steel jacket in strengthening slender reinforced concrete (RC) column. The columns were 200 mm square cross section with lengths ranging from 1600 to 3000 mm. Ten columns were tested under axial load. The effects of the strengthening technique, slenderness ratio, cross-section area of steel angle and CFRP layer number were examined in terms of axial load-axial strain curve, CFRP strain, steel strip strain and steel angle strain. The experiments indicate that strengthening RC columns with combined CFRP and steel jacket is effective in enhancing the load capacity, ductility and energy dissipation capacity of RC column. Based on the existing models for RC columns strengthened with CFRP and with steel jacket, a design formula considering a slenderness reduction factor is proposed to predict the load capacity of the RC columns strengthened with combined CFRP and steel jacket. The predictions agree well with the experimental results.

Modeling of anchored CFRP strips bonded to concrete

A computational model was developed for simulating the load transfer mechanisms from Carbon Fiber Reinforced Polymer (CFRP) strips to CFRP anchors in reinforced concrete strengthening applications. Of particular interest was a model capable of reproducing the strength and strain distributions in anchored CFRP strips up to fracture. Computational results were compared with experimental results from concrete beams strengthened using various anchor and strip details. The numerical results showed good correlation with the experimental results both at the beam response level and the CFRP strain level. The sensitivity of computational results to key modeling assumptions was also examined.

Repair of shear-deficient normal weight concrete beams damaged by thermal shock using advanced composite materials

The use of advanced composite materials such as Fiber Reinforced Polymers (FRPs) in repairing and strengthening reinforced concrete structural elements has been increased in the last two decades. Repairing and strengthening damage structures is a relatively new technique. The aims of this study was to investigate the efficiency and effectiveness of using Carbon Fiber Reinforced Polymer (CFRP) to regain shear capacity of shear-deficient normal weight high strength RC beams after being damaged by thermal shock. Sixteen high strength normal weight RC beams (100×150×1400mm) were cast, heated at 500°C for 2h and then cooled rapidly by immersion in water, repaired, and then tested under four-point loading until failure. The composite materials used are carbon fiber reinforced polymer plates and sheets. The experimental results indicated that upon heating then cooling rapidly, the reinforced concrete (RC) beams exhibited extensive map cracking without spalling. Load carrying capacity and stiffness of RC beams decreased about 68% and 64%, respectively, as compared with reference beams. Repairing the thermal damaged RC beams allowed recovering the original load carrying without achieving the original stiffness. Repaired beams with CFRP plates with 90° and 45° regained from 90% to 99% of the original load capacity with a corresponding stiffness from 79% to 95%, whereas those repaired with CFRP sheet on the web sides and a combination of CFRP plates and sheet regained from 102% to 107% of the original load capacity with a corresponding stiffness from 81% to 93%, respectively. Finally, finite element analysis model is developed and validated with the experimental results. The finite element analysis showed good agreement as compared with the experimental results in terms of load–deflection and load–CFRP strain curves.

Flexural strengthening of RC beams by using a near surface mounted T-section profiles

The paper presents test results of an experimental program of four full-scale RC beams, strengthened in flexure with T-section carbon fiber reinforced polymer (CFRP) profiles and two reference beams subjected to a six point bending. The novel shape of CFRP profile combines both the near surface mounted (NSM) and externally bonded (EB) strengthening systems. The application of the CFRP profiles consisted of gluing both the web and the flange of the profile to the concrete surface. RC beams made of the same concrete class were differed by the internal steel reinforcement ratio and in a number of applied profiles. Efficiency of this new strengthening product was determined by comparison of the strengthened and non-strengthened RC beams. An increase of the CFRP-concrete bond area and high stiffness of the T-section profiles significantly improved the strengthening ratio (up to 130% of the reference beam) and reduced the maximum mid-span deflection (ranged of 70-80%) of the non–strengthened beams. The CFRP strain utilisation equal to 67% of the tensile strain corresponded to the maximum CFRP strains equal to 0,73%. The promising test results exhibit this system as a very attractive proposal of new strengthening technique used for field applications of the existing structures.

Response of Concrete Corbels Reinforced with Internal Steel Rebars and External Composite Sheets: Experimental Testing and Finite Element Modeling

This paper aims to investigate the structural response of concrete corbels reinforced internally with steel rebars and externally with carbon fiber-reinforced polymer (CFRP) composite sheets. Nine specimens were constructed and tested. Test parameters included the amount of internal longitudinal steel rebars and configuration of the external composite sheets. Nine two-dimensional finite-element (FE) models were developed, assuming a perfect bond between the CFRP and concrete. Three additional FE models were developed in which an interfacial bond stress-slip model was adopted between the diagonal CFRP reinforcement and the concrete. The external CFRP composite reinforcement resulted in up to 40% increase in the load capacity. The contribution of the external CFRP reinforcement to the load capacity decreased with an increased amount of internal steel rebars. The addition of primary longitudinal CFRP sheets in a direction parallel to the primary steel rebars reduced the steel strains and increased the yield and ultimate loads. The inclusion of secondary CFRP longitudinal reinforcement at the midheight of the corbels did not result in additional strength gain. The diagonal CFRP reinforcement restricted growth and widening of the shear cracks, and hence, increased the gain in the load capacity. The numerical load capacity was in the range of 10% error band. The numerical crack pattern, deflection response, and steel and CFRP strain responses were in good agreement with those measured experimentally. The integration of the interfacial bond stress-slip model in the FE analysis between the diagonal CFRP reinforcement and concrete resulted in more conservative/accurate predictions for the structural response.

The Finite-Element Analysis for CFRP and Steel Bar Strain in the Strengthened Concrete Slab

This paper studies the mechanical behavior of Carbon-fiber-reinforced polymer (CFRP) in one-way reinforced concrete slabs, and the relation of CFRP and steel bar strain under the uniformly distributed load. Based on the theoretical analysis of the constitutive relationship, the analysis calculation model was built by using the ABAQUS software. Data analysis demonstrates that under the same conditions the load-strain curves of CFRP and steel bar are similar, which indicates that the mechanical cooperation performance of CFRP and steel is mutually beneficial. The load-strain curve of CFRP and steel bar is linear before the steel bended, and the curve is approximately linear after the steel bended. In the reinforced stage, the curve is again linear and the strain rate of the CFRP and steel bar increases constantly with the increase of load. This result is valuable for related studies.

Experimental Study on Confinement Effectiveness of Different CFRP Hybridized Methods in Retrofitting Shear Deficient HSC Columns

The effectiveness of four different carbon fiber reinforced polymer (CFRP) hybridized methods to retrofit the high strength concrete columns with shear capacity deficiency for improving the shear behavior is described in this paper. These retrofitting methods include the wrapped jacketing, the L-shaped jacketing combined with wrapped jacketing, the clamp plates plus wrapped jacketing and the L-shaped jacketing, clamp plates and wrapped jacketing. Five specimens were tested under low reversed cyclic loading. The influence of retrofitting on the failure mode, ductility, strength, stiffness degradation, and distribution of CFRP strain is analyzed. The results indicate that the retrofitted specimens exhibit high levels of ductility superior to the reference specimen. Among the four retrofitting methods, the effectiveness of method of only the wrapped jacketing is the worst, the method of the clamp plates plus wrapped jacketing has better performance compared to the method of the L-shaped jacketing combined with wrapped jacketing, and the method of the L-shaped jacketing, clamp plates and wrapped jacketing performs best.

Performance of RC Beams Strengthened Using Prestressed NSM-CFRP Strips Subjected to Fatigue Loading

AbstractThe fatigue performance of Reinforced Concrete (RC) beams strengthened using prestressed Near Surface Mounted (NSM) Carbon Fiber Reinforced Polymer (CFRP) strips was evaluated in this paper. Five full-scale RC beams were tested under fatigue loading simulating in-service loading conditions: one control unstrengthened beam and four beams strengthened using NSM-CFRP strips prestressed to 0, 20, 40, and 60% of the CFRP ultimate tensile strength. All beams were subjected to fatigue loading for 3 million cycles at a frequency of 2.0 Hz such that the stress range induced in the tension steel, during the initial cycle, is 125 MPa as specified by the Canadian Highway Bridge Design Code. Experimental test results show that the percentage deflection increase at the end of fatigue loading was almost the same for all beams, which implies that damage accumulation is independent from the prestress level. CFRP strain variation at midspan indicated the occurrence of deboning during the initial cycling of beams pr...

Mechanics of Bond Fatigue Behavior of Concrete Beams Strengthened with NSM CFRP Rods

Bond tests were conducted on 28 concrete beams strengthened with near-surface mounted (NSM) carbon-fiber-reinforced polymer (CFRP) rods under different fatigue load levels. In the NSM technique, the CFRP rods are placed inside precut grooves and bonded to the concrete with epoxy adhesive. The test variables included: presence of internal tension steel reinforcement (unreinforced or reinforced), type of CFRP rod (spirally wound or sandblasted), and the fatigue load level. The beams were tested in four-point bending. Bond failure was by debonding between the CFRP rod and the epoxy that started at sections close to the beam midspan. As the load was cycled further, debonding propagated toward the supports. At failure, the beams strengthened with a given rod type (sandblasted or spirally wound) showed the same CFRP strain at sections close to the support. The sandblasted rods showed a better bond fatigue performance than the spirally wound rods, which, at a given load level, failed at a higher number of cycles.

Full-Scale Experimental Investigation of Repair of Reinforced Concrete Interstate Bridge Using CFRP Materials

An experimental study is presented of the behavior of eight reinforced concrete bridge girders taken from a decommissioned Interstate bridge and retrofitted with three different carbon-fiber-reinforced polymer (CFRP) systems. Specimens were subjected to monotonic loading to failure with and without significant fatigue conditioning. Experimental observations indicated that intermediate crack-induced debonding was the dominant failure mode for monotonically loaded beams and that degradation of the CFRP-to-concrete interface was caused by fatigue conditioning. Conventional adhesive applied and near-surface mounted (NSM) CFRP systems behaved well under monotonic loads, with the NSM system exhibiting significantly greater ductility. Powder actuated fastener applied retrofit was observed to be less efficient, requiring a relative slip of the CFRP in order to engage the shear transfer mechanism of the fasteners. The application of current accepted design guidelines for FRP retrofit indicated that guidelines aimed at mitigating debonding failure appear to be appropriately conservative under monotonic loading conditions; however, a significant additional reduction in CFRP strain limits is required to account for even small levels of fatigue loading.

Performance of Reinforced Concrete T-Girders Strengthened in Shear with Carbon Fiber-Reinforced Polymer Fabric

Results are presented from an experimental investigation into the performance of 20-ft-long reinforced concrete (RC) T-girders strengthened in shear using epoxy-bonded bidirectional carbon fiber reinforced polymer (CFRP) fabric. The aim was to evaluate and gain insight into the effectiveness of shear strengthening of large-scale girders with externally bonded CFRP under a low shear span condition. Four series of tests, corresponding to stirrup spacings of 5.5, 8, 16, and 24 in, were considered. Each series of girders included control specimens with no CFRP wrap and specimens retrofitted in shear with 1, 2, and 3 layers of CFRP wrap. Results indicate that for unwrapped specimens, values for nominal shear predicted by ACI underestimated, by 40-80%, the shear resistance of beams developing arch action, such as those considered herein. For wrapped specimens, the maximum shear force as well as the midspan deflection generally increased with the number of CFRP layers. The optimum number of layers to achieve the maximum gain in shear resistance was found to depend on the internal shear steel reinforcement provided. The effective CFRP strain used to calculate the contribution of the CFRP to the shear capacity was correlated to the total shear reinforcement ratio consisting of steel stirrups and CFRP wrap. Retrofitting RC girders in shear with CFRP wrap also increased the ductility. Experimental evidence shows an optimum combination of CFRP layers and steel stirrups exists for a maximum increase in ductility.