Longitudinal Carbon Fiber Reinforced Polymer Sheets Research Articles

Experimental investigation on strengthening lap joints subjected to bending in glulam timber beams using CFRP sheets

Abstract Strengthening the carpentry joints is considered a big challenge for designers to maintain the joined timber elements’ load transfer and integrity. This research work investigates different techniques to strengthen the lap joints in glulam timber beams using carbon fiber-reinforced polymer (CFRP) sheets. For this purpose, 12 specimens of glulam timber beams were tested using the four-point loading method. One of the glulam timber specimens was an intact nonlapped specimen, and the others were lapped at mid-span using half-height lap joints. The lap joints were strengthened using different configurations of the CFRP sheets. The test variables were the clamping scheme, bond length, and reinforcement ratio of the CFRP sheets. The analysis of the test results indicates that the CFRP strengthening configuration could be more effective in strengthening the lapped glulam timber beams when the reinforcement ratio of the longitudinal CFRP sheets was equal to 0.303%, and the wrapping CFRP sheets continued beyond the lapped joint by a distance equal to the beam height. Such a strengthening configuration could succeed in increasing the effective stiffness and ultimate load of the lapped timber specimen to 109 and 110%, respectively, relative to those of the intact nonlapped specimen. Moreover, the results reveal that increasing the length or the reinforcement ratio of the longitudinal CFRP sheets without increasing the width of the wrapping CFRP sheets could not succeed in achieving the required improvements in the structural behavior of the lapped timber beams.

Using grooving and corner strip‐batten techniques for seismic strengthening of square reinforced concrete columns with fiber‐reinforced polymer composites

AbstractThis study represents the results of an empirical program investigating the behavior of square reinforced concrete (RC) columns retrofitted using longitudinal carbon fiber‐reinforced polymer (CFRP) composites under simulated seismic loading. The main challenges to tackle are the premature buckling and the debonding of the longitudinal FRP composites on the tension and compression sides of the columns, respectively. To postpone both, the two versions of the grooving methods (GM), that is, the externally bonded reinforcement on grooves (EBROG) and the externally bonded reinforcement in grooves (EBRIG), were employed and the results were compared with those obtained under the traditional externally bonded reinforcement (EBR) procedure. Another aspect of the study involves investigating the influence of the recently developed corner strip‐batten (CSB) method on confining column specimens. For these purposes, seven half‐scale RC columns strengthened using FRP sheets were subjected to reversal cyclic loading. Experimental results exhibited that using the EBRIG and EBROG methods simultaneously with CSB technique could prevent altogether the debonding and buckling of longitudinal CFRP sheets. It was also found that the application of FRP strips utilizing the GM and CSB methods could produce an important increment in the performance of the RC columns in different terms such as ductility and lateral strength.

A novel anchorage system for strengthening slender RC columns with externally bonded CFRP composite sheets

Previous studies have shown that using externally bonded (EB) fiber reinforced polymer (FRP) is one of the most effective techniques for strengthening reinforced concrete (RC) structures. The current guidelines recommend using anchorage systems to prevent debonding failure. Strengthening RC columns for flexure, the use of an anchorage system is necessary to transfer the forces from the column to the foundation. In this study, three types of Carbon Fiber Reinforced Polymer (CFRP) anchors were tested under pullout loading. The best of the three anchors, based on the results of pullout tests, was used in strengthening one RC column. Three half-scale specimens of RC columns were tested under a combination of constant axial and cyclic lateral load. As expected, the results showed that strengthening RC columns with lateral confinement only increased the ductility of the columns but did not have a major effect on the moment capacity or stiffness. However, the lateral stiffness, moment capacity, and ductility of the columns confined with CFRP and strengthened with longitudinal CFRP sheets anchored to the foundation using a proposed novel anchor exhibited significant improvements.

Research on Arch Effect and CFRP Strengthening Effect of RC Beams with Corroded Stirrups

In many cases the corrosion of steel reinforcement is considered to be the main reason that leads to the deterioration of the bond between steel reinforcement and concrete and causes the failure of members. However, stirrups with smaller diameter are more likely to get rusted and it is more difficult to find corrosion expansion cracks on the surface of concrete. Then the function of stirrups to restrict longitudinal bar from slipping will be weakened, which alters the mechanical properties and failure mode of corroded Reinforced Concrete (RC) beams. In this paper, the influence of corrosion of stirrups and the effect of strengthening corroded RC beams with Carbon Fiber Reinforced Polymer (CFRP) sheets were studied. The corrosion rate for each group varied between 0% to 18% which represented mass loss of the longitudinal steel reinforcement on the tension side. The longitudinal reinforcement of beams in Group A was not wrapped by the stirrups in order to simulate the situation that the stirrups were seriously corroded and lost their function. Beams of Group B were corroded control beams. Beams of Group C were corroded and then repaired by applying longitudinal CFRP sheets at the bottom and additional U-shaped CFRP sheets around the cross-section. Test results showed that the corrosion of stirrups had a detrimental effect on strength as well as the bond stress between steel bars and concrete. The mechanical mode of experimental beams tended to act like an arch. The greater the corrosion rate was, the more obvious the arch effect was. In addition, combining longitudinal and U-shaped CFRP sheets enhanced the ultimate load by an average of 56.5% of corroded RC beams. However, because of the longitudinal cracks, the strengthening effect of CFRP was much diminished when the corrosion rate was particularly high.

Grooving methods in square RC columns strengthened with longitudinal CFRP under cyclic axial compression

The current research investigates the application of carbon-fiber reinforced polymer (CFRP) composites with longitudinally aligned fibers for enhancing the compressive strength and ductility of square reinforced concrete (RC) columns under cyclic axial compression. Global buckling of the composite described may result in the debonding of CFRP off the specimen surface, which hinders proper enhancement in the ductility and compressive strength of the strengthened specimens. To postpone the buckling of the longitudinal CFRP sheets under compression, the recently developed grooving method (GM) was employed in the current research and compared with the externally bonded reinforcement (EBR) technique using the conventional surface preparation technique. For this objective, 8 square RC columns were subjected to cyclic axial compression. The experimental parameters consisted technique employed for longitudinal strengthening of the columns and the presence (absence) of intermittent confining wraps. Two grooving techniques, the externally bonded reinforcement on grooves (EBROG) and the externally bonded reinforcement in grooves (EBRIG), were used. Experimental results indicated the considerably higher contribution of the grooving method, relative to that of the conventional EBR, to limiting the buckling of CFRP sheets and to enhancing the ductility and compressive strength of the columns. It was observed that when the GM is employed, the compressive strength of FRP is enhanced based on the exploitation of the compressive capacity of the carbon fibers in the longitudinal sheets. The results illustrated that in the specimens longitudinally strengthened using the EBROG or EBRIG techniques, fiber compressive capacity in the composite reached 43.9% and 69.3% of ultimate tensile strength of fibers in flat coupons; this amount was only 9.6% for the columns strengthened through the EBR technique.

Structural behavior of corroded RC beams with/without stirrups repaired with CFRP sheets

Strengthening/repair of existing reinforced concrete structures has become one of the important issues in the field of civil engineering. In reinforced concrete structures located in hot and humid areas, steel reinforcement is generally vulnerable to deterioration due to corrosion. Corrosion of reinforcement in many cases is considered the main cause of concrete structures deterioration which in turn requires large budgets for repair and maintenance. This paper presents the experimental results of damaged/repaired reinforced concrete beams. The experimental program consisted of testing 12 reinforced concrete rectangular beam specimens with/without shear reinforcement and exposed to accelerated corrosion. The corrosion level was varied between 5 and 7.5 % which represents mass loss of the longitudinal steel reinforcement on the tension side. Corroded beams without shear reinforcement were repaired by bonding longitudinal carbon fiber reinforced polymer (CFRP) sheets to the tension side in addition to external U-shaped CFRP sheets to restore the strength loss due to corrosion. Corroded beams with stirrups were repaired by bonding longitudinal CFRP sheets to the tension side only. The test results showed that using externally bonded U-shaped CFRP sheets restored the ductility of corroded beams without stirrups and prevented bond failure at the steel concrete interface due to the absence of internal stirrups. In addition, combining U-shaped and longitudinal CFRP sheets enhanced the ultimate load by 37 % and the stiffness by 25 % in corroded beams without stirrups. For corroded beams with stirrups ductile failure was observed. Corroded beams with stirrups strengthened with CFRP sustained higher failure loads; however, the stiffness was unchanged.

Strengthening of RC columns by longitudinal CFRP sheets: Effect of strengthening technique

In the present study, the effect of utilizing longitudinal carbon fiber reinforced polymer (CFRP), as external compression reinforcement, on the behavior of strengthened RC columns is investigated. To do so, 10 RC columns including 5 circular columns with diameter of 150mm and 5 square columns with dimension of 133mm were cast. Four different methods including conventional externally bonded reinforcement (EBR) and near surface mounted (NSM) techniques, as well as the recently introduced grooving method (GM) in the forms of externally bonded reinforcement on grooves (EBROG, to be pronounced as /ebrΛg/) and externally bonded reinforcement in grooves (EBRIG, to be pronounced as /ebrIg/) techniques were used to strengthen the specimens in compression with CFRP. The specimens were tested under monotonic uniaxial compression, and different effects of the strengthening techniques were comprehensively discussed. Furthermore, an image-based deformation measurement technique, i.e. particle image velocimetry (PIV) was used to further investigate the stress transfer between longitudinal CFRP sheet and concrete substrate. Experimental results demonstrated that utilizing grooving method (GM) for bonding CFRP to concrete surface postpones the CFRP buckling, and subsequently, a significant increase in load carrying capacity of strengthened RC columns by compression CFRP sheets was observed.

Flexural strengthening of reinforced concrete slab–column connection using CFRP sheets

The objective of this paper is to investigate experimentally the effectiveness of application of carbon fiber reinforced polymer (CFRP) sheets as a strengthening technique of a reinforced concrete (RC) slab–column connection in one-way flat-plate system to enhance its flexural strength. The experimental study was particularly conducted to examine whether there is enough anchorage with the use of CFRP wrapping to the discontinuous longitudinal CFRP sheets at column stub. Series of tests were conducted on six flat slab–column connection specimens organized in two groups of three specimens each. The first group included three control specimens with central, eccentric, and edge columns, respectively. The second group was geometrically identical to the first group, though with CFRP sheets installed on the tension side of the slab to increase flexural capacity at the negative moment region. The specimens with eccentric and edge columns are those having geometrical eccentricity and whose results are compared with those obtained for specimens with central column to study the effect of column eccentricity. The experimental work included fabrication of specimens and testing them under increasing monotonic gravity loads up to failure. Experimental results demonstrated that the flexural ultimate load carrying capacity increased by 33%, 37% and 67% for the tested specimens with central, eccentric, and edge column, respectively, when strengthened using CFRP sheets. The cross-sectional analysis was also undertaken to compare the experimental results with those obtained from Canadian Standards for FRP design for buildings.

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.

Fiber-Element Model for Slender HSS Columns Retrofitted with Bonded High-Modulus Composites

This paper presents an analytical model developed to predict the behavior of concentrically loaded cold-formed square hollow structural section (HSS) slender columns, strengthened with high-modulus carbon fiber reinforced polymer (CFRP) sheets. The model predicts the load versus axial and lateral displacements, and accounts for plasticity of steel, the built-in through-thickness residual stresses, geometric nonlinearity, initial out-of-straightness imperfection, and the contribution of CFRP sheets. The model was verified using test results. Since the gains in strength due to CFRP retrofitting of the tested columns were found to be sensitive to the inherent out-of-straightness, the model was used to uncouple the effects of out-of-straightness and CFRP reinforcement ratio. It was shown that axial strength and stiffness of HSS columns could be increased substantially when retrofitted with longitudinal CFRP sheets, due to reduction of the lateral displacement induced by buckling. It was also shown that CFRP effectiveness increases for columns with larger out-of-straightness deficiencies and columns of higher slenderness ratios.

Evaluation of Prestressed Concrete Girders Strengthened with Carbon Fiber Reinforced Polymer Sheets

This paper details the use of carbon fiber reinforced polymer (CFRP) sheets to repair and strengthen prestressed concrete bridge girders in flexure and shear. Three specimens that were removed from an overloaded bridge (Bridge No. 56) in Graham County, Kansas were tested. Two of the specimens were repaired and strengthened, and all three were tested to failure to determine flexural capacity. Test results showed that two layers of longitudinal CFRP sheets increased the flexural capacity of the strengthened specimens by 20% compared to an unstrengthened control specimen. Shear capacity was also evaluated on both ends of each specimen. Two different cases were evaluated in shear. One case allowed shear cracks to propagate inside the transfer length of the prestressing strand, allowing a bond failure to occur. The second case forced the shear cracks to remain outside of the transfer length, thereby preventing a bond failure. The test results show that transverse CFRP sheets increased the shear capacity of the specimens tested by as much as 28%, but did not prevent bond failures.