External Carbon Fiber-reinforced Polymer Research Articles

Axial compressive behavior of CFRP-confined rubber concrete-encased H-shaped steel hybrid columns

The use of rubber particles as aggregate replacement in concrete production (i.e., rubber concrete) is thought to be an efficient way to dispose of end-of-life rubber products. However, the use of rubber aggregates in concrete generally leads to reduced strength and stiffness as well as early cracking in concrete. Consequently, the practical use of rubber concrete has mainly been limited to non-structural applications. In this paper, a novel compressive component, termed carbon fiber-reinforced polymer (CFRP)-confined rubber concrete-encased H-shaped steel hybrid column (FRuCSC), is proposed for the structural application of rubber concrete. This novel hybrid column consists of an external CFRP tube, an encased H-shaped steel, and rubber concrete filled in between. The axial compressive behavior of FRuCSCs was investigated through combined axial compression tests and theoretical analyses. The test results indicate that FRuCSCs with a rubber replacement ratio of no more than 50 % possess excellent axial load-carrying capacity and ductility. Owing to the confinement offered by the external CFRP tube and encased H-shaped steel, the strength and ductility of the filled rubber concrete improved significantly. Furthermore, by modifying an existing axial stress-strain model of FRP-confined rubber concrete, the axial load-strain curves and axial-lateral strain curves of FRuCSCs with a rubber replacement ratio of no more than 50 % can be accurately predicted.

Experimental Investigation of a Novel CFRP-Steel Composite Tube-Confined Seawater-Sea Sand Concrete Intermediate Long Column

In order to fully utilize sea sand in offshore engineering, a novel carbon fiber-reinforced polymer (CFRP)-steel composite tube-confined seawater-sea sand concrete (FCTSSC) column structure has been developed. This study conducts experimental investigations on FCTSSC intermediate long columns for the first time. It investigates the failure modes and the impact of the external CFRP layers number on the axial compression performance. The results reveal that the FCTSSC intermediate long column specimens exhibit the failure modes of global buckling and localized steel tube bulging. After CFRP restriction, the ultimate load bearing capacity of the specimens increased from 12.51% to 20.87%. The ultimate load bearing capacity, its enhancement percentage, and peak lateral deformation are all positively correlated with the external CFRP layers number. Finally, a summary and assessment were conducted on the applicability and accuracy of the existing load bearing capacity models, providing a reference for subsequent research.

Rapid repair of geopolymer concrete members reinforced with polymer composites: Parametric study and analytical modeling

To minimize the carbon production from the cement industry, geopolymers are the suitable replacement cementitious materials in concrete structures. Furthermore, glass fiber-reinforced polymer (GFRP) rebars can play a vital role in replacing the corrosive steel reinforcement in structural components due to their extraordinary performance in humid and aggressive environments. The current research aims to determine the performance efficiency of the rapid repairing process of partially damaged GFRP-reinforced recycled aggregate geopolymer concrete (RGC) compressive members subjected to various types of loading. The behavior of 18 repaired steel- and GFRP-reinforced RGC compressive members was compared using experiments, analytical modeling, and finite element analysis (FEA) in ABAQUS. The rapid repair process was completed using high-strength carbon fiber reinforced polymer (CFRP) sheets. The effectiveness of external CFRP wrapping on the failure modes, strength index, stiffness index, ductility index, peak axial loading capacity, and axial shortening behavior of the repaired members was carefully evaluated. Moreover, FEA and analytical models were proposed to accurately predict the axial behavior of the repaired RGC compressive members. A parametric study is carried out to investigate the effects of various parameters of repaired columns. Both experimental and numerical outcomes discovered that the proposed rapid repair method could considerably recover the peak axial capacity and axial shortening capacity of pre-damaged RGC compressive members.

Effect of Tendon-Related Variables on the Behavior of Externally CFRP Prestressed Concrete Beams.

This work assesses the flexural performance of prestressed concrete beams with external carbon fiber-reinforced polymer (CFRP) tendons, focusing on tendon-related variables. A finite element analysis (FEA) method is verified. A numerical parametric analysis of prestressed concrete beams with external CFRP tendons is carried out. Four tendon-related variables are considered, namely, the area, initial prestress, depth and elastic modulus of tendons. The analysis shows that flexural ductility decreases as the tendon area, initial prestress or elastic modulus increases but is insensitive to the tendon depth. The ultimate tendon stress increment (Δσp) is influenced by all of the four variables investigated. JGJ 92-2016 (Chinese technical specification for concrete structures prestressed with unbonded tendons) significantly underestimates Δσp and, hence, is over-conservative for the strength design of these beams. An equation is proposed for calculating Δσp, taking into account all four variables investigated. An analytical model is then developed to estimate the flexural strength (Mu) of prestressed concrete beams with external CFRP tendons. The proposed analytical model shows good agreement with FEA, i.e., the mean discrepancy for Δσp is 0.9% with a standard deviation of 11.1%; and the mean discrepancy for Mu is -1.6% with a standard deviation of 2.1%.

Multi-objective optimization of high performance concrete columns under compressive loading with potential applications for sustainable earthquake-resilient structures and infrastructures

High performance column composites are multifunctional composites designed to improve resilience of structures and infrastructures to natural and unforeseen multiple loads during service. As a result of their superior mechanical performance, high performance column composites have been found attractive to improve resilience of medium and high-rise buildings as well as long-span structures and infrastructures in earthquake-prone regions. Scientometric analysis revealed limitations of traditional design standards and inadequate earthquake engineering to reduce collosal damage during seismic impact and the need for investment in alternative green energy to reduce anthropogenic-induced earthquakes linked to oil and gas projects. High performance CFRP (carbon fibre reinforced polymer) composite was investigated as potential laminate reinforcement and confinement for recycled concrete column using performance-based Taguchi-Response surface methodology optimization. Though columns wrapped with two layers CFRP confinement and reinforced with internal laminate CFRP reinforcement obtained higher maximum load and favourable failure mode, the optimized result, which comprises column confined with two layers of external CFRP confinement only, obtained 36.3% and 36.19% greater fracture toughness and fracture energy respectively, 1.4 times better displacement ductility index, 2.78 times strain ductility index and comparable load capacity. The superior mechanical performance of high performance recycled concrete column composite and favourable failure mode was attributed to strain hardening effect which depends on the CFRP confinement. Further studies are required to improve the mechanical performance and failure mode of high performance column composite for construction of resilient structures and infrastructures in the built environment.

Shear strengthening performance of GFRP reinforced lightweight SCC beams: Experimental and analytical study

Light weight construction using Glass Fibre-Reinforced Polymer (GFRP) bars and structural light weight concrete (LWC) have increased recently and offers a wide spectrum of advantages over traditional normal weight concrete (NWC) structures. However, the shear strengthening and rehabilitation performance of LWC beams reinforced with GFRP bars is very limited and currently there are no guidelines related to it. Therefore, this study presents the results of an experimental investigation on the shear strengthening behaviour of lightweight self-consolidating concrete (LWSCC) beams reinforced with GFRP bars. The test parameters studied in the experimental program were the longitudinal reinforcement ratio and strip configuration or orientation. A total of twelve shear deficient GFRP reinforced beams were prepared with a shear span to depth ratio of 3.12 and subjected to static four-point bending. Five beams in each series were externally strengthened with different configurations of Carbon Fibre-Reinforced Polymer (CFRP) strips and one beam served as a control specimen to compare the performance of the strengthened specimens. Crack patterns, load–displacement response curves, and strain in the external CFRP strips were recorded until the failure ultimate load. The results of the experimental study indicated that the external CFRP strips improved the nominal shear capacity of GFRP reinforced LWSCC beams, and the contribution of external CFRP strips was sensitive to the reinforcement ratio. The nominal shear capacity of the strengthened specimens increased from 33.3 to 168% compared to the control specimen. U-wraps with horizontal strips in the compression and tension zone were the most effective configuration, showing the highest increase in shear strength compared to other strengthened specimens. Moreover, the shear capacity was theoretically predicted using different Fiber-Reinforced Polymer (FRP) design standards and guidelines which were compared with the experimental results. All the FRP design standards have overestimated the shear capacity of LWSCC beams, and a modified equation with a set of recommendations was proposed based on the experimental study. In addition, a reduction factor of 0.75 is recommended for conservative design.

Performance of damaged concrete stub columns strengthened with post-tensioned CFRP under freezing-thawing environment

Reinforced concrete structure with directly bonded fiber reinforced polymer (FRP) on the surface showed obvious stress hysteresis under load, which caused that the strengthening materials cannot be fully utilized. The strengthening system formed by carbon fiber reinforced polymer (CFRP) and shrinkage compensating self-consolidating concrete (ESCC) applied posttensioning stress to the external CFRP shell as a stay-in-place strengthening method, which eliminated stress hysteresis and exerted the restraint effect immediately after being loaded. This paper explored whether this method can be applied to concrete compression members damaged by freeze–thaw environment and its effect. Experimental variables include the number of freezing-thawing cycles, FRP shell plies and types of strengthening concrete. The experimental results showed that the damage of concrete under freezing-thawing environment was an incremental process: the dynamic elastic modulus loss rate and the mass loss rate of specimens damaged by 50 freeze–thaw cycles were 5.9% and 0.4%, respectively. When the freezing-thawing cycles reached 125, the dynamic elastic modulus loss of concrete compression members reached 24.8% and the mass loss rate was 5.3%. Compared with concrete compression specimens after 125 freeze–thaw cycles, the load capacity of specimens strengthened with single-ply and double-ply CFRP were increased by 63.1% and 114.1% respectively, which fully compensated the damage caused by freeze–thaw cycles. It was indicated that when single-ply CFRP was used for strengthening, ordinary self-consolidating concrete specimens exhibited higher hoop strain and less loss of ultimate load capacity than ESCC. When two-ply CFRP was used for strengthening, concrete compression members strengthened with ESCC showed higher strength and less loss of ultimate load-bearing capacity than specimens strengthened with ordinary self-consolidating concrete (SCC). With the increase of freezing-thawing cycles, the normalized inflection stress, intercept stress as well as ductility coefficient of strengthened specimens showed decreasing trend. The theoretical value of normalized peak stress and corrected strain were in good agreement with experimental value and it shows that the maximum error were 10.74% and 24.88%, respectively

Environmental Impact on the Behavior of CFRP Sheet Attached to Concrete

Carbon fiber-reinforced polymer (CFRP) has many advantages as a construction/structural-strengthening material. However, there are still concerns regarding the long-term performance of these materials when used with reinforced concrete (RC) structures. Environmental conditions have an adverse effect on the behavior of CFRP and the bond between these sheets and concrete. Therefore, the durability of CFRP used for strengthening RC beams was evaluated under different environmental scenarios, including subjection to immersion in deicing agents, tap water, and saltwater, freeze-and-thaw cycles, and outdoor environmental changes. Laboratory tests were performed to examine the influence of these environmental scenarios on the bonding behavior between CFRP sheets and concrete in terms of deformations and modes of failure. Two types of test setups were performed in this study, namely pull-off shearing and three-point bending. Forty-two concrete prisms with CFRP were prepared and tested by using the pull-off shearing setup. It was observed that as the period of exposure increased, noticeable effects on the CFRP sheet as well as the bond stiffness were observed. Exposure to tap water had a greater impact than saltwater on the CFRP–concrete bond strength as well as the CFRP. In addition, eighteen notched concrete beams strengthened with an external CFRP were tested under three-point bending. The tap water exposure showed a 3.6% increase in the bond strength compared to the control specimen. However, the saltwater exposure showed a 10% increase.

Effect of partial replacement of E-waste as a fine aggregate on compressive behavior of concrete specimens having different geometry with and without CFRP confinement

Nowadays, the improper disposal of electronic waste (E-waste) is leading to environmental pollution and health problems majorly in developing countries. The current research examines the behavior of shredded E-waste fine aggregate concrete strengthened by the use of carbon fiber-reinforced polymer (CFRP) confinement under axial compressive load. The main objective of this study is to investigate the structural behavior of circular and non-circular concrete specimens by partial replacement of E-waste as fine aggregates and confined with CFRP sheets. Tests were performed on a total of 36 concrete specimens. The parameters considered were cross-sectional geometry of circular, rectangular and square concrete specimens, 20% partial replacement of fine aggregates by E-waste, the effect of the corner in case of non-circular specimens, and the number of CFRP sheets. These tests portrayed that the partial replacement of fine aggregates by E-waste aggregates results in decreasing axial compressive strength of concrete specimens. However, the external CFRP confinement increases the axial strength of specimens (a maximum of 71% for circular specimens, 33% for square specimens, and 25% for rectangular specimens). It was also observed that partial replacement of natural fine aggregates by E-waste increased the failure time. For the case of square and rectangular concrete specimens, the ultimate strength was lower than circular specimens for both (with and without CFRP confinement) specimens. Finally, existing strength and strain models developed for FRP-confined concrete were used for theoretical calculations. The theoretical declaration of the existing models indicates that a few of the ultimate strain models lie on the high side and few of the ultimate strain models on the low side in comparison with the experimental findings.

Shear behavior of reinforced concrete beams with web opening near supports

Deterioration of reinforced concrete beam capacity might occur due to introducing web openings to accommodate essential services such as air conditioning, and electricity. These openings might be near supporting columns at locations of maximum shear forces. In this paper, the influence of utilizing web openings near the supports on the behavior of RC beams was experimentally investigated. The enhancement in these beams ‘behavior due to strengthening the openings with external Carbon Fibre Reinforced Polymer (CFRP) sheets was estimated. Six RC beams with different opening sizes located near the supports were cast and tested under four-point loading. Crack pattern, deflections, and strains of the six beams were compared to those of a reference solid RC beam without an opening. Experimental load–deflection and shear strength of the tested beams were compared to those predicted from analytical and code equations. The experimental results revealed that introducing large openings reduced the shear capacity of RC beams by up to 35% compared to that of a solid beam. Also, it was concluded that strengthening the web opening with CFRP sheets increased the shear capacity by 21% ∼ 28% compared to similar beams without CFRP.

Shear behavior of ultra-high-performance concrete beams prestressed with external carbon fiber-reinforced polymer tendons

Shear behavior of ultra-high-performance concrete beams prestressed with external carbon fiber-reinforced polymer tendons

Flexural Behavior of UHPC Beams Prestressed with External CFRP Tendons

This paper presents an experimental investigation on the flexural behavior of ultra-high-performance concrete (UHPC) beams prestressed with external carbon fiber-reinforced polymer (CFRP) tendons. A total of eight T-shaped beam specimens were fabricated and tested, and the effects of the effective prestressing stress, partial prestressing ratio, deviated angle, and loading condition on the flexural behavior were analyzed. The experimental results indicate that the fully prestressed beams experienced a brittle failure, and the shear capacity of these beams was mainly controlled by the effective prestressing stress in CFRP tendons and the ultimate tensile strength of UHPC, whereas the partially prestressed beams failed in a ductile manner. The presence of internal steel reinforcement could significantly improve the flexural capacity and deformation ability. Thus, internal reinforcements should not be omitted from UHPC beams with CFRP tendons. A higher effective prestressing stress resulted in enhanced cracking load and flexural capacity. The deviated angle enhanced the utilization efficiency of high strength CFRP tendons. The loading condition exerted a slight influence on the flexural behavior of the specimens. Moreover, a method considering the effect of steel fibers was proposed and verified to predict the flexural capacity of UHPC beams prestressed with external CFRP tendons.

Behavior and modeling of CFRP nonuniformly wrapped circular seawater sea-sand concrete (SSC) columns under axial compression

An experimental study was performed in this paper to investigate the compressive behavior of carbon fiber-reinforced polymer (CFRP) nonuniformly wrapped seawater sea-sand concrete (SSC) cylinders. The effect of test variables on the mechanical performance was comprehensively discussed, including the clear spacing and thickness of external CFRP strips. The test results indicate that the failure process of tested specimens is less brittle compared with that of the fully or partially FRP-confined columns. Besides, the average ultimate stresses and strains of the specimens nonuniformly wrapped with two-layer external CFRP strips increase by 6.7%-27.4% and 5.9–41.6% respectively with the reduction of clear spacing ratio. Moreover, when the thickness of external CFRP strips double increases, the maximum increment of strength and strain at ultimate condition becomes 42.8% and 63.8% respectively. It is also shown that the FRP efficiency factor of confined specimens with nonuniform wrapping scheme is the highest among various strengthening strategies under the same FRP volumetric ratio. Finally, a new nonuniform confinement effective coefficient, as well as a analytical stress–strain model, is developed based on the available test results. Compared with several existing predicted models, the proposed model can provide more accurate and reliable predictions.

COMPARATIVE ANALYSIS OF FLEXURAL STIFFNESS OF CONCRETE ELEMENTS WITH DIFFERENT TYPES OF COMPOSITE REINFORCEMENT SYSTEMS

Various materials and reinforcement technologies have been created for concrete structures. However, there is no uniform methodology to compare the mechanical characteristics of different reinforcement systems. In structural systems, residual stiffness can estimate the efficiency of the reinforcement. This study introduces a simplified approach for the flexural stiffness analysis. It employs a new testing layout designed with the purpose to form multiple cracks in a small laboratory specimen. The achieved solution requires neither iterative calculations nor a description of the loading history. Several composite reinforcement schemes, including internal glass fibre reinforced polymer (GFRP) bars, carbon fibre reinforced polymer (CFRP) sheets and near-surface mounted (NSM) strips are considered. The analysis of the test results reveals a substantial efficiency of the external CFRP reinforcement systems.

Behavior of FRP-confined sea-sand concrete columns with a prefabricated concrete-filled FRP-steel core

Adopting fiber-reinforced polymer (FRP) composites as a confining device and a protective skin not only allows usage of sea-sand concrete which is generally forbidden in conventional steel reinforced concrete structures, but also leads to enhancement of concrete strength and deformation capacity. This paper presents a pilot study on axial compression tests on hybrid FRP-confined sea-sand concrete columns with a prefabricated concrete-filled FRP-steel tube core (simply referred to as FRP-Confined Sea-sand Concrete-Prefabricated Core Columns or FCSC-PCCs). The prefabricated concrete-filled glass FRP (GFRP)-steel core, serving as reinforcing core in FRP-confined concrete columns, is expected to enhance the FRP-confined concrete columns and enable to make use of sea-sand in the concrete between the two tubes. Effects of the steel tube diameter, the GFRP tube thickness and the thickness of the external Carbon FRP (CFRP) tube are carefully investigated. The axial load–axial strain curve of FCSC-PCCs as well as the confined concrete in FCSC-PCCs generally exhibit a monotonically ascending behavior owning to the confinement from the external CFRP tube and the internal FRP-steel composite tube. A new model for average ultimate axial stress and axial strain of confined concrete in FCSC-PCCs is established and the comparisons demonstrate that the proposed model is of satisfactory accuracy.

Determination of flexural capacity for GFRP-reinforced concrete beams retrofitted using external CFRP sheet

This paper investigates the flexural capacities and deflection characteristics of ordinary portland cement-based concrete (OPCC) and geopolymer concrete (GPC) beams reinforced with glass-fiber-reinforced polymer bars (GFRP). The applicability of carbon-fiber-reinforced polymer (CFRP) sheets as a strengthening technique for such systems is also studied. A total of 15 beams were tested under a four-point quasi-static load. Eight beams served as a reference, while the remaining seven were damaged by loading up to 50% of their load capacities and subsequently strengthened with CFRP sheets. Moment capacities, strain development, load-deflection relationships, and failure modes are reported. The ACI 440.1R-06 guideline was found to be appropriate to give conservative estimates of flexural capacities of control beams for both OPCC and GPC. The previously proposed deflection models for OPC beams reinforced with FRP bars may be used to determine the deflection of GPC composite beams provided that the modulus of elasticity is adequately adjusted for such concretes. For the strengthened beams, however, the ACI 440.2R17 process does not account for the dissimilarity between the CFRP and GFRP modulus values and therefore overestimates the capacity of these unique composites. Finally, a factor (λ) is proposed to consider the contribution to the flexural capacity of GFRP bars in such strengthened system.

Reinforced concrete beam behavior with strengthening of CFRP lamination in the seawater environment

The strength of concrete structures in the seawater environment decreases with the duration of exposure seawater due to high content of chloride compounds. Repairing structures are difficult in marine environments requires creating new methods that are effective, easy to work and economical. One of them is by using CFRP (Carbon Fiber Reinforced Polymer) material which is proven to corrosion resistant. Strengthening is done with external bond CFRP on the area that you want to strengthen. This research was conducted with an experimental method, namely reinforced load concrete beams were given a monothonic load with two points to determine the ultimate load capacity of the beams and then compared with the increase in the CFRP variable layer which the ends were anchored with CFRP U-wrap and given sea water immersion behavior. The results of the flexural test showed that along with the time of sea water immersion there was a decreases in the capacity of the ultimate load from the normal beam and with CFRP reinforcement are on normal test specimens without reinforcement of 3.12%, concrete with CFRP layers 1,2 and 3 in sequence are 2.92%, 2.84, and 2.75%. The largest ultimate load capacity with 3 layers of CFRP is 36.23%.

Combined effects of Carbon Fiber Reinforced Polymer flexural reinforcements and post installed shear dowels on the performance of flat slabs

The sudden punching shear failure of flat slabs is a critical issue. The combined effects of externally strengthened Carbon Fiber Reinforced Polymer (CFRP) flexural reinforcements and post-installed CFRP shear reinforcements which explain the real behavior in the strengthening phase of a shear deficient slab, on the punching shear performance are not fully explained. In this study, the flat slab panels of 1200 mm × 1200 mm × 100 mm were retrofitted with seven alternative bonding arrangements of CFRP to investigate the combined effects of shear dowels and flexural reinforcements. A transient load was applied using a stub column, size 100 mm × 100 mm located at the center of the slab. The non-strengthened specimens failed in punching shear. The strengthened specimens showed a maximum punching shear capacity enhancement of 120%. A numerical model was also developed to identify the load transfer behavior of post-installed CFRP shear dowels. Further, the effects from the end anchorage of external CFRP and the refilling material at drilled locations to install CFRP shear strengthening scheme were also determined.

Performance and failure analysis of carbon fiber-reinforced polymer (CFRP) strengthened reinforced concrete (RC) beams

At present, the strengthening and retrofitting of reinforced concrete (RC) beams using carbon fiber-reinforced polymer (CFRP) are receiving extensive attention. The use of strengthening technique for RC beams vary according to purpose. In general, this technique aims to increase load-carrying capacity, either through flexural or shear strengthening. The present study aims to investigate the load–deflection response of CFRP-strengthened RC beams through experimentation with five various strengthening configurations and two different types of surface treatment. The RC beams that were tested in this work were small scale. Results indicate that compared with control beams, CFRP-strengthened beams notably reinforced ultimate load and deflection, as well as ductility, because of the stiffness and grip provided by CFRP. Among the five configurations, the continuous U-wrapping and strip wrapping systems were proven effective for strengthening. A close interaction between internal stirrups and external CFRP strips, which will negatively affect the overall strength of the beams if the reinforcements were placed simultaneously along a similar section of the beam. The failure pattern of the strengthened beams considerably depends on strengthening configuration and internal steel reinforcement, which control the ultimate effectiveness of strengthening. In addition, the ultimate capacity of the beams was predicted in accordance with the ACI 440.2R-17 guidelines and was finally compared with the experimental ultimate loads of the CFRP-strengthened beams. Experimental results show that the predicted values were extremely conservative.

Numerical assessment of continuous prestressed NSC and HSC members with external CFRP tendons

Carbon fiber reinforced polymer (CFRP) is a promising material for prestressing tendons. This study assesses the structural performance of continuous prestressed members with external CFRP tendons, focusing on the effect of using high-strength concrete (HSC) instead of normal-strength concrete (NSC). A nonlinear model is introduced and numerical tests are then carried out. Three concrete cylinder compressive strengths of 40, 60 and 90 MPa are used. The area of external CFRP tendons varies from 200 to 1700 mm2. The results show that the effect of using HSC instead of NSC heavily depends on the area of external CFRP tendons. In general, HSC members with external CFRP tendons exhibits considerably different response characteristics from those of NSC ones, including cracking, deformations, tendon and rebar stresses, neutral axis depths and so on. The concrete grade influence on moment redistribution is found to be negligible, provided that the members demonstrate favorable ductility. The rules in ACI 318-14 concerning the unbonded tendon stress and the redistribution of moments are evaluated.