Carbon Fiber Reinforced Polymer Confinement Research Articles

Effect of Sulfate Attack on Reinforced Concrete Columns Confined with CFRP Sheets under Axial Compression

Sulfate ions in saltwater can considerably deteriorate the strength and durability of reinforced concrete (RC). To date, there has been no study that systematically investigates the effects of a sulfate attack on the capacity of mid-/large-scale RC columns confined with carbon fiber–reinforced polymer (CFRP). The capacity of CFRP-confined concrete columns subjected to sulfate attacks with various schemes and durations (up to 210 days) was examined in this study. The primary objectives of this study were to (1) examine the effect of a sulfate attack on the structural properties of midscale RC columns confined with CFRP; (2) quantify the contribution of the sulfate-damaged concrete core and CFRP confinement to the load-carrying capacity of the columns; and (3) propose a new formula to estimate the compressive strength of FRP-confined sulfate-damaged concrete considering the exposing time, number of FRP layers, and concrete strength. The experimental results showed that the sulfate attack significantly reduced the initial axial stiffness of the column up to 36% and increased its ultimate axial displacement up to about 40%. These influences in the CFRP-strengthened columns were slightly lower than those of unstrengthened ones. The sulfate attack considerably reduced the strength of the unstrengthened columns (up to 29.4%), but it only slightly affected the capacity of the strengthened columns with a 3.7% reduction. The strength reduction increased with the number of wet–dry cycles. CFRP confinement effectively mitigated the penetration of sulfate ions into the concrete core and, thus, slowed the degradation of the strength up to 90%. An empirical formula was proposed to estimate the strength of the CFRP-confined concrete subjected to a sulfate attack with close predictions regarding the experimental results.

Behavior of CFRP-confined Concrete-filled square steel tube columns with section circularized under axial compression

Eight carbon fiber-reinforced polymer (CFRP)-confined concrete-filled square steel tube (CFSST) specimens and twelve CFRP-confined CFSST specimens with section circularized were tested under axial compression to investigate the effect of different loading conditions and CFRP layers on axial compressive behavior of CFRP-confined CFSST columns with section circularized. Experimental results indicated that section modification method (circularization) improves the CFRP confinement and increases bearing capacity of CFSST columns. Meanwhile, the effect of CFRP layers on the bearing capacity of CFSST columns is also evident. Combination models of Han–Lam & Teng and Mander–Pham & Hadi are suggested in this study for calculating the bearing capacity under different loading conditions. Results showed that predictions are consistent with the experimental results.

Impact Behavior of Fiber-Reinforced Concrete with Polypropylene Fibers and Carbon Fiber–Reinforced Polymers

Abstract In this study, the influences of bidirectional carbon fiber–reinforced polymer (CFRP) confinement along with polypropylene fibers on the impact resistance of concrete samples were experimentally studied. Forty-nine concrete cylindrical samples (with compressive strengths of 20, 30, and 40 MPa) with polypropylene fibers (with content ratios of 0 %, 1 %, 1.5 %, and 2 %) were subjected to weight (46.7 and 66.8 kg) dropping at a height of 1.6 m above the cross-section of the sample. Moreover, half of the samples were tested with CFRP confinement. The number of weight releases required to destroy the samples and the weights of the samples after each release were recorded. The results indicated that the concrete without fibers and fiber-reinforced polymers (with compressive strengths of 20, 30, and 40 MPa) did not have sufficient impact resistance and were swiftly destructed. However, as the content ratios of polypropylene fibers increased, the number of cracks and weight droppings (to reach 70 % of the initial sample weight) increased. Moreover, it was observed that the concrete samples wrapped with the CFRP sheets also withstood impact loading better than fiber-reinforced concrete samples without the confinements. Besides, it was indicated that the prevailing failure mode of the samples with CFRPs was CFRP rupturing. Finally, it was observed that the more the compressive strength of concrete, the more the impact resistance.

Bond characteristics of CFRP strengthened concrete members bonded using Modified Engineered Cementitious Composite

This study focuses on the development of an insulating cementitious adhesive for bonding Carbon Fibre Reinforced Polymer (CFRP) fabrics to a concrete surface. The epoxy adhesive which is the generally used adhesive for CFRP shows a low thermal performance. As a potential solution, Engineered Cementitious Composite (ECC) mortar was modified as a cementitious adhesive with improved thermal insulation. The ECC adhesive was developed using locally available class F fly ash, silica sand and Polyethylene terephthalate (PET) fibres other than the generally used cement and water (PET-ECC). The selected mix proportion for PET-ECC achieved a low thermal conductivity of 0.145–0.180 W/mK which limited the heat transfer through the adhesive layer. The effective bond length of the developed PET-ECC adhesive was 125 mm and the corresponding bond strength was 224.9 MPa. The respective bond strength was further enhanced up to 570.28 MPa by sticking river sand on top of CFRP fabric using epoxy adhesive before bonding it to the concrete surface using the PET-ECC adhesive. The use of PET-ECC as the bonding agent to strengthen compression members with CFRP confinement yielded an average strength enhancement of 34%.

Finite element analysis of square concrete columns strengthened with carbon fiber composites in different configurations

This paper offers the findings of numerical study carried out to evaluate the behavior of low strength concrete columns in square section strengthen by Carbon Fiber Reinforced Polymer (CFRP) composites and subjected to axial compression load. In this study, a three dimensional non-linear finite element formulation model has been presented using ANSYS-16.1 (software computer program) to analyse low strength concrete columns in square section strengthen by CFRP composites in different configurations, to evaluate the improvement in the ultimate load capacities of columns as a result of CFRP confinement, and to study the impact of the most important parameters such as: compressive strength of concrete, thickness of the CFRP layers jacket/strips and the presence of steel reinforcement on the performance of th modelled columns subjected to compression load. A comparison has been made between the results obtained from the Finite Element Analysis (FEA) and experimental data. The results of the numerical FEA study showed that, external strengthening with CFRP composites are very effective in upgrading the ultimate load of square concrete columns. In addition, the findings revealed good agreement between the ANSYS and the experimental test results. The results obtained from the parametric study revealed that when the CFRP layers increased from 2 to 3 layers and from 2 to 4 layers the ultimate loads of the columns increased about (2%-10%) and (7%-15%) respectively. While the compressive strength showed significant effect on the upgrade of the ultimate load, when increase the compressive strength to 42MPa and to 60 MPa, the ultimate load increased to about 220% and to about 300% respectively. The presence of steel reinforcing did not show reasonable effect on the ultimate load.

Axial compressive performance of CFRP confined rectangular CFST columns using high-strength materials with moderate slenderness

The bearing capacity of concrete-filled steel tube (CFST) columns can be significantly increased by using high-strength materials. However, the ductility of high-strength CFST columns is reduced due to the brittleness of high-strength concrete and the buckling of thin-walled high-strength steel tubes, especially for slender columns. By wrapping the carbon fiber reinforced polymer (CFRP) to increase the confinement on the steel tube, the ductility can be improved. This paper presents experimental and theoretical studies on the axial compressive performance of CFRP confined rectangular CFST columns using high-strength steel and concrete with moderate slenderness. The influence of the section aspect ratio, concrete strength, and CFRP confinement on the axial behavior was analyzed. The experimental results indicate that the local buckling of CFRP-confined specimens is delayed compared with the unconfined specimens. The horizontal CFRP confinement increases the yield load by 1.5–25.6%, and the peak load by 2.5–11.2%. The bearing capacity and ductility of specimens are improved under the confinement of CFRP. The calculation method for axial compression bearing capacity considering the stability of moderate slender specimens was proposed. The ratio of the experimental results to the calculated value is between 0.86 and 1.09, which verified the accuracy of the calculation method.

Carbon fibre-reinforced polymer confinement of corroded circular concrete columns

Corrosion is a major cause of deterioration and cracking in reinforced concrete (RC) members and leads to a significant reduction in their structural capacity and service life. The rehabilitation of damaged RC members using carbon fibre-reinforced polymer (CFRP) is a viable and cost-effective method. This study evaluates the effectiveness of CFRP wrapping of corroded circular RC columns. Four sets of scaled columns were fabricated and subjected to different levels of accelerated corrosion to simulate no, mild, moderate, and severe corrosion. A total of 14 CFRP-confined columns and 10 unconfined columns were axially tested. The load–strain capacity, stress–strain relationship, and failure modes were analysed. The results showed that CFRP wrapping of corroded columns improved their stiffness properties as well as the ductility and deformation behaviour at their ultimate capacities. Ruptures due to failure were largely located in the upper half of the columns, which suggests that the region near the supports must be strengthened to prevent extreme stress concentration. Furthermore, the compressive performance of the tested columns demonstrated their sensitivity to the severity of corrosion. Hence, at higher corrosion levels, CFRP confinement with more layers is recommended.

Axial behavior of slender reactive powder concrete-filled steel tubular columns confined by CFRP

Reactive powder concrete-filled steel tubes (RPCFSTs) have become the focus of research in recent years owing to their ultra-high strength and good ductility. However, the steel tube in this type of member often exhibits an inelastic outward local buckling. The use of carbon-fiber-reinforced polymer (CFRP) can effectively suppress this outward local buckling and has shown great advantages in strengthening a new alternative structure. In this work, we conducted axial compression experiments to study the effect of slenderness ratio on the performance of circular RPCFST columns strengthened by CFRP. A total of 12 specimens were manufactured, and compression tests were conducted by varying the length of the members (800, 1200, and 1600 mm) and CFRP layers (0–3). The results showed that sufficient CFRP confinement can significantly improve the bearing capacity and ductility of the RPCFST columns. However, the confinement effect decreased with the increase in the slenderness ratio. We suggest a slenderness ratio of 40 and a three-layer CFRP-confined slender RPCFST column for engineering applications. Based on Euler’s theory for unified materials, a theoretical model for the elastic and elastoplastic instabilities of slender CFRP-confined RPCFST columns was proposed, and the calculation and experimental results were in good agreement.

Compression Behaviour of Concrete Cylinder with Carbon Fibre Reinforced Polymer (CFRP) Confinement

Carbon fibre reinforced polymer (CFRP) confinement has always been one of the strengthening methods available for a vulnerable concrete column. This paper presents the compressive behaviour of nine circular concrete cylinders with CFRP confinement. Three different specimen conditions considered; full CFRP confinement, partial CFRP confinement and unconfined (control specimen). Nine concrete cylinders with 100 mm x 200 mm were tested under compression load. It is discovered that full and partial CFRP confinement had improved concrete cylinder ultimate load capacity by 300% and 150% respectively when compared to the unconfined concrete cylinder. With 150% strength enhancement achieved by partial CFRP confined specimen, it is proven that partial CFRP confinement does provide sufficient confinement in enhancing concrete column strength as full CFRP confinement. This finding has led to remarkable discoveries which with lesser CFRP used the functionality of CFRP as strengthening material can still be utilized. Therefore, could contribute significant input to the construction industry in using lesser CFRP for more sustainable material approach.

Performance evaluation of hybrid fiber reinforced low strength concrete cylinders confined with CFRP wraps

The improvement in the compressive strength (CS) and ductility of plain concrete always remained an active area for advanced research. The present study aims to enhance the strength of low strength hybrid fiber reinforced concrete (HFRC) cylinders confined with different number of carbon fiber reinforced polymer (CFRP) sheets by examining the axial CS, axial compressive strain, and compressive stress–strain behavior of low strength HFRC cylinders confined with CFRP sheets. The HFRC consisted of steel fibers and polypropylene fibers. Two groups of low strength HFRC specimens (12.5 MPa and 16.5 MPa) were fabricated to investigate the effect of CFRP confinement on the different CS of HFRC. The experimental outcomes depicted that the lateral CFRP confinement of concrete significantly improved the CS, axial compressive strain, and axial stiffness of low strength HFRC specimens. The improvements of 115.7% and 130.7% occurred in the CS of 12.5 MPa group for single and double CFRP layers, respectively. Similarly, the improvements of 37.4% and 112.6% occurred in the CS of 16.5 MPa group for single and double CFRP layers, respectively. Therefore, the CFRP confinement is more effective for low strength HFRC as compared with high strength HFRC in terms of axial CS and axial compressive strain.

Axial stress–strain behavior of carbon FRP-confined seawater sea-sand recycled aggregate concrete square columns with different corner radii

This paper presents an experimental study on the axial stress–strain behavior of carbon fiber-reinforced polymer (CFRP)-confined seawater sea-sand recycled aggregate concrete (SSRAC) columns. Forty-two circular and square specimens were tested under axial compression, and the effects of the aggregate replacement ratio, CFRP thickness, and corner radius were investigated. The results indicated that, by replacing natural aggregates with recycled aggregates (RAs), the CFRP-confinement effectiveness can be enhanced, showing a higher enhancement ratio in the ultimate strength and strain. The effect of the corner radius ratio on the ultimate condition of CFRP-confined SSRAC is also more pronounced compared with CFRP confined natural aggregate concrete (NAC). This conclusion was also validated by the nonuniform hoop strain distribution for specimens with varying corner radius ratios. The difference in CFRP confinement efficiency caused by the recycled aggregate replacement and corner radius is accurately captured by the DIC (Digital image correlation) measurement system. By reasonably taking the coupling effect of RAs and the corner radius ratio into account for the confinement efficiency parameter, the proposed ultimate strength and strain model for CFRP-confined SSRAC demonstrates a satisfactory performance when compared with test results.

Infiltration of H2SO4 through Concrete with and without Carbon Fiber-Reinforced Polymer Confinement

Infiltration of H2SO4 through Concrete with and without Carbon Fiber-Reinforced Polymer Confinement

Experimental study on cyclic behavior of post-tensioned segmental retaining walls (PSRWs)

This paper reports on an experimental study on nine Post-tensioned Segmental Retaining Walls (PSRWs). All the walls were post-tensioned and tested under incrementally increasing cyclic loading. Each test wall was constructed using four T-shaped precast concrete blocks, assembled on top of each other with dry joints. The integrity of the system was maintained using post-tensioning (PT). Different levels of PT forces were applied to investigate the effect of it. Different wall-footing interface materials were used including steel plate, concrete and neoprene pad. Conventional concrete and crumb rubberized concrete was used to investigate the effect of concrete properties on the structural behavior of retaining walls. Three walls with confinement (confined with either Carbon Fiber Reinforced Polymer (CFRP), steel reinforcement or steel face plate) were tested. The force–displacement responses, energy dissipation, post-tensioning stresses, gap rotation, lateral stiffness, and normal strains were observed and compared. The results from this study showed that the level of axial stress ratio, eccentricity of pre-stressing as well as the properties of the wall-footing interface material is critical to provide enough lateral strength and stiffness for engineering design of the PSRW. The wall with steel plate on compression face had the largest ultimate strength. The specimen having largest PT force generated largest initial and secant stiffness. Additional reinforcement of the bottom most segment is also required to avoid sudden failure and increase ductility in T-shaped blocks. Lateral stiffness increases in conventional concrete was much higher than rubberized concrete when PT force was increased. None of concrete confinement methods applied (CFRP confinement, steel reinforcement and steel face plate) affected energy dissipation characteristics of the tested walls. Using neoprene as the interface material for wall-footing significantly increased energy dissipation and concrete wall-footing interface showed more energy dissipation in comparison to steel interface material. Stiffness degradation of all walls had similar rate against wall lateral displacement. This paper concluded that the proposed segmental retaining wall system is potentially a practical alternative to accelerate the construction of retaining walls using precast concrete blocks.

Design-Oriented Stress-Strain Model for FRP-Confined Lightweight Aggregate Concrete

Despite the advantages of lightweight aggregate concrete (LWAC), its brittleness in compression has limited its vast application. Jacketing by fiber-reinforced polymer (FRP) tends to be one method to overcome this negative point. The current study attempted to recognize the FRP confinement impact on the LWAC compressive monotonic behavior. Both fine and coarse aggregates were Scoria aggregate. The variables include the type and number of FRP layers; 150 × 300 mm cylindrical specimens were wrapped with 1, 2, 3, and 4 layers of glass FRP (GFRP) and carbon FRP (CFRP) strips. An investigation was performed on the monotonic stress-strain curve belonging to LWAC confined with FRP, and key points, including ultimate stress and strain on the curve, were explored. The results revealed that the key points would significantly improve by increasing FRP layers. Although the influence of GFRP confinement on enhancing the ultimate strain appears to be more considerable than the CFRP confinement, CFRP jacketing of LWAC upgrades the ultimate strength to the higher level compared to wrapping with GFRP. Based on the interpretation of experimental results, a design-oriented stress-strain model was established to predict the monotonic compressive response of Scoria-LWAC circular specimens confined with CFRP and GFRP. The validity of the model was examined. The predicted output was appropriately in line with the experimental findings.

An experimental study on axial behavior of CFRP-strengthened RC rectangular columns with variable slenderness ratio

In the construction industry, carbon fiber-reinforced polymer (CFRP) composites are widely used for retrofitting and rehabilitation of reinforced concrete (RC) rectangular columns to enrich the service life of structures. The CFRP-confined RC rectangular column with a variable slenderness ratio has limited design provisions in available design codes. The aim of the current experimental work is to explore the efficacy of slenderness ratio on the axial performance of CFRP composite-confined rectangular columns having a constant cross-sectional area and steel reinforcement ratio. The experimental results clearly demonstrated that the yield strength, ultimate strength, and ductility of the RC column after CFRP confinement are significantly influenced by the variation in the slenderness ratio. The response of CFRP wrap, transverse and longitudinal reinforcements in the strengthening process of the RC rectangular column also remarkably varied for different slenderness ratios.

Seismic strengthening of improperly repaired reinforced concrete columns using CFRP confinement

In this study, lack of strength, stiffness, energy dissipation and ductility characteristics of the improperly repaired RC square columns were examined and the recoverability of seismic performance with the external Carbon Fiber Reinforced Polymer (CFRP) confinement was investigated. A total of six RC columns were tested for three times with different test stages; as-built, improperly repaired with Normal Strength Concrete (NSC) and retrofitted with CFRP confinement. In the first tests, relatively high axial load (0.40Acfc) and cyclic lateral load up to 15% lateral strength loss were applied to expose heavy damage on test columns. After that, the damaged columns repaired by using NSC and retested under the same loading conditions. In the first two test stages, the drift ratio at the end of tests was approximately 5%. The lateral strength, stiffness, ductility and energy dissipation characteristics of the improperly repaired columns dramatically decreased. At the last stage tests, the stiffness of strengthened columns was considerably reduced due to the damage that occurred in the concrete core after the two pre-damage phases. The lateral strengths of the CFRP retrofitted columns did not change much compared to the as-built columns because of the quality of the repairing concrete (26 MPa and 54 MPa) and the strain hardening behavior of steel reinforcement. Compared to the as-built columns, the cumulative energy dissipation capacities of columns were significantly increased, however, ductility increase was not achieved for all specimens because of the dramatic loss of stiffness.

Mechanical performance of CFRP-confined sustainable geopolymeric recycled concrete under axial compression

Sustainable geopolymeric recycled aggregate concrete (RAC) by utilizing environmentally-friendly binder-geopolymer and constructional solid waste-recycled aggregate (RA) will facilitate the sustainability in concrete industry. This study investigated the compressive behavior of sustainable geopolymeric RAC confined by carbon fiber-reinforced polymer (CFRP) jackets. A total of 72 cylindrical fly ash/slag-based geopolymeric concrete specimens, including 48 CFRP-confined specimens and 24 unconfined specimens were fabricated and tested. The testing variables included: coarse aggregate type (i.e., natural aggregate and RA), thickness of CFRP jackets (i.e., 1, 2, and 3 layers) and (iii) slag content (i.e., 0, 10%, 20% and 30% of the total binder by mass). The results indicate that the CFRP confinement remarkably enhances the compressive strength and ultimate strain of geopolymeric concrete, and the enhancement is more pronounced with the increase of CFRP jacket thickness. Moreover, the RA replacement and the inclusion of slag have minor influences on the CFRP confinement performance for the compressive strength, but have obvious effects on the CFRP confinement performance for the ultimate axial strain. Based on the test results, empirical stress and strain models were proposed to predict the ultimate condition of the CFRP-confined geopolymeric concrete.

Influence of CFRP confinement on bond behavior of steel deformed bar embedded in concrete exposed to high temperature

The effectiveness of retrofitting with carbon-fiber-reinforced polymer (CFRP) wrapping for maintaining and improving the bond between steel rebars and heat-damaged concrete was extensively studied. Here, the effect of temperature (room temperature as the reference, 200, 400, 600, and 800 °C), concrete cover (25 mm and 35 mm), concrete compressive strength (30 and 40 MPa), and CFRP wrapping had on the concrete-rebar bond were investigated by conducting experiments. The results show that retrofitting with CFRP can not only significantly increase the bond strength of the specimens subjected to elevated temperatures, but also change the failure mode from brittle to a more ductile one, i.e., rebar pull-out failure. Moreover, the mechanical properties degraded as temperature increased, especially after 400 °C, such that at 800 °C, the compressive strength was reduced by about 80%. Exposure to heat reduced the concrete-rebar bond strength in the pull-out test specimens, and the level to bond strength reduction was less pronounced in the specimens with greater compressive strength and concrete cover. However, confinement with CFRP sheets significantly improved the bond strength, which was more considerable for the specimens exposed to higher temperatures. The effectiveness of CFRP wrapping was more significant for the specimens with a lower compressive strength and thinner concrete cover.

A Compressive Peak Strength Model for CFRP-Confined Thermal Insulation Materials under Elevated Temperature.

In this paper, a compressive peak strength model for CFRP-confined thermal insulation materials under elevated temperature was proposed. The thermal insulation material was made by Portland cement with different portions of perlite. The compressive strengths of four different perlite ratios in weight, such as 0%, 10%, 20%, and 30% of thermal insulation materials, confined by one-layer, two-layer, and three-layer carbon fiber-reinforced polymer (CFRP) composite materials, were obtained. The test results indicated that the specimen’s compressive strength decreased with an increase in the amount of perlite replacement and increased with an increase in the number of CFRP wrapping layers. Based on the test results, a theoretical compressive peak strength model with some parameters was proposed. In the meantime, the compressive strengths of the above four different perlite ratios of thermal insulation materials under elevated temperature, such as ambient temperature, 100 °C, 150 °C, 200 °C, 250 °C, and 300 °C, were obtained. For compression tests of specimens with a fixed amount of perlite, the test results indicated that the specimen’s compressive strength decreased with an increase in temperature, highlighting a thermal softening phenomenon. Based on the test results, a compressive peak strength model with a thermal softening parameter was proposed to predict the peak strength under elevated temperature. Finally, a compressive peak strength model for thermal insulation material with CFRP confinement under different elevated temperature was derived, and it achieved acceptable results in comparison to the experimental results.

Experimental study of circular hollow reinforced concrete column strengthened with partial Carbon Fibre Reinforced Polymer (CFRP) confinement

Structural strengthening with Carbon Fibre Reinforced Polymer (CFRP) confinement has been widely used in civil engineering applications. However, the use of CFRP has always triggered environmental and sustainable issues due to its hazardous manufacturing and disposal process. By using partial CFRP confinement, the use of CFRP as the strengthening material could be reduced. The circular hollow reinforced concrete column is commonly used in seismic zones due to its self-weight reduction and better structural efficiency of strength/mass and stiffness/mass ratios properties. However, up until today, the understanding of circular hollow reinforced concrete column behaviour still lacks, especially with CFRP partial strengthening confinement. Therefore, this paper is testing a 6 full-scale circular hollow reinforced concrete columns with 2 m height, 250 mm of outer diameter, and 110 mm of inner diameter. The behaviour of the unconfined circular hollow column will be compared with partial CFRP confinement under axial load. The circular hollow reinforced concrete column with partial CFRP confinement is proven to be able to strengthen circular hollow column effectively through this study.