CFRP Strips Research Articles

Axial behaviour of CFRP strips confined lithium-slag concrete filled square steel tube stub columns

10 square columns with different lithium slag (LS) content were designed and tested under axial load condition to investigate the compressive behavior of lithium slag concrete filled steel tube (LSCFST) short columns externally confined by CFRP strips. It is found in this paper that the failure mode of CFRP strips confined square LSCFST stub column is CFRP strip fracture and local buckling of square steel tube. The confinement of CFRP strips is able to limit dilation tendency of columns, more layers and smaller spacing allow their bearing capacity to obtain a greater enhancement rate. And columns with LS replacement rate of 20% have the best bearing capacity, ductility and initial stiffness. A calculation method for the ultimate strength of this kind of columns was put forward eventually.

Shear strengthening performance of fiber reinforced lightweight SCC beams

In recent years, the demand for fiber reinforced lightweight concrete (FLWC) has increased rapidly to make high-strength, durable and lightweight structures. The strengthening behaviour of FLWC has never been studied in the literature and currently there are no standard guidelines for strengthened FLWC beams. Therefore, the main aim and objective of the current experimental study is to investigate the shear behaviour of fiber reinforced lightweight self-compacting concrete (FLWSCC) beams externally strengthened with CFRP strips. Six full-scale reinforced concrete (RC) beams were tested under four-point bending with varying CFRP strip spacing, number of layers, and configuration. One additional beam served as the control specimen to compare the performance of the strengthened specimens. The experimental results indicated that the external CFRP strips enhanced the strength and stiffness of the strengthened FLWSCC specimens. The strength enhancement ranged from 12% to 91% and pull out of steel fiber and debonding of CFRP strips was the predominant mode of failure in the control and strengthened specimens. The presence of steel fibers in the concrete has increased the shear strength of concrete and limits the external contribution of CFRP strips. Furthermore, all the experimental results were compared with theoretically predicted shear capacities using various FRP design standards for external strengthening. All the design standards have overestimated the shear capacity of FLWSCC specimens and CNR-DT (2013) design standard was found to be the most conservative as compared to other design standards. In addition, a lightweight modification factor has been recommended for strengthened FLWSCC beams

Behavior of reinforced recycled aggregate concrete beams and slabs strengthened in flexure and punching with CFRP composites

Recycling of concrete generated from construction and demolition waste as coarse aggregates in concrete has been a focus of several studies for the evolution of the green concrete and conservation of natural resources. This investigation, therefore, focusses on the study of the behavior of RC beams strengthened with externally bonded CFRP wraps, cast with both natural and recycled aggregates concrete. Furthermore, one-way slabs were strengthened with Near Surface Mounted (NSM) bars, while two-way slabs were strengthened with CFRP strips. One-way and two-way slabs used in the study were cast with both natural and recycled aggregate concrete (RAC). In RAC, natural aggregates were replaced by 30% recycled concrete aggregates. All beams and slabs were cast with concrete mix design of 1:1.24:2.6 and w/c ratio of 0.43. Beams and one-way slabs, control and strengthened, were tested under four-point bending. Whereas, two-way slabs were tested in punching. A significant increase in the load-carrying capacity was observed for both natural and recycled aggregates concrete beams without shear reinforcement and two-way slabs, indicating the effectiveness of strengthening scheme. Increase of 20.5% and 12.6%, respectively, was found in the load-carrying capacities for the cases of both and natural and RAC beams with natural aggregate concrete (NAC) and RAC beams when results were compared for the case of shear deficient control and strengthened RC beams with CFRP wraps. Furthermore, increase of 27 − 37% was noted in load-carrying capacities of both natural and RAC two-way slabs when results were compared for the case of control and strengthened NAC and RAC slabs.

Regaining Structural Performance of Thermally Damaged Beams Using NSM CFRP Strips: A NLFE Study

Regaining Structural Performance of Thermally Damaged Beams Using NSM CFRP Strips: A NLFE Study

Monotonic and Fatigue Response of RC Beams Strengthened with Near-End Enhanced Embedment Prestressed CFRP Strips

Monotonic and Fatigue Response of RC Beams Strengthened with Near-End Enhanced Embedment Prestressed CFRP Strips

Seismic behavior of self-restrained grid shear wall with diagonal CFRP-steel composite strips

This paper introduces an innovative structure for a self-restrained grid shear wall with diagonal CFRP-steel composite strips. Steel and CFRP strips are interwoven to enhance the structural integrity of the shear wall. Quasi-static tests are initially conducted on two shear wall specimens with and without diagonal CFRP-steel composite strips. Subsequently, a finite element model is established to represent the self-restrained grid shear wall with diagonal CFRP-steel composite strips. This model is helpful for exploring the influence of composite strips arrangement on the performance of shear walls. Furthermore, a prediction formula is developed to estimate the load-bearing capacity of the CFRP-steel composite strips grid shear wall. The test results show that the yield bearing capacity, ultimate bearing capacity, stiffness and energy dissipation capacity of the specimens increase by 46.6%, 58.2%, 43.6%, and 16.3% respectively after the replacement of composite strips. In addition, the results of finite element simulation indicate that the diagonal arrangement exhibits the highest efficiency in the use of CFRP materials among three reinforced structural forms-diagonal, spaced, and fully arranged composite strips. This arrangement is also the most economical and has the least adverse influence on the internal forces of the frame columns.

Improvement of lateral property of unidirectional-strengthened CFRP laminates using recycled carbon fiber

The unidirectional carbon fiber reinforced polymer (UD-CFRP) lacks the modulus of elasticity and strength in the lateral direction. This study investigates whether matrix resin with CFRP waste, recycled carbon fiber (rCF), can improve the lateral properties of CFRP. In total, twelve CFRP strips specimen were prefabricated of unidirectional carbon fiber (CF) sheet by hand lay-up (HLU) method and were tested by tensile test and X-ray computed tomography (X-ray CT). Factors such as fiber direction and void distribution significantly affecting its mechanical properties are assessed by X-ray CT inspection. It can be seen that rCF is mixed in a random direction at the position filled with matrix resin without rCF. However, a similar frequency of unimpregnation and voids can be observed in both specimens. Test results showed that experimental values of CFRP laminates with rCF-mixed matrix resin increased compared to the CFRP laminates without rCF. The percentage increase in the lateral tensile strength and modulus of elasticity of the rCFRP compared to the control specimen without rCF is 27.36% and 10.62%, respectively. This study proved that rCF can increase the lateral properties of unidirectional CFRP and shows the effective use of rCF for strengthening material in construction applications.

Nonlinear FE analysis of RC slabs strengthened with prestressed EB CFRP strips under high-cyclic fatigue loading

A significant issue in structural health monitoring is identifying fatigue damage in concrete structures that have been retrofitted with externally bonded Carbon Fiber Reinforced Polymer (EB-CFRP). Although the finite element method has significantly contributed to modeling the damage mechanisms of concrete, reinforcement, and CFRP, further research is still needed in this area. For instance, RC slabs reinforced with EB-CFRP technology must be evaluated and predicted for high-cyclic fatigue and progressive damage. Therefore, this research aimed to develop intricate numerical models that could be utilized to analyze the dynamics of fatigued RC slabs and find damage in slabs that have prestressed EB-CFRP reinforcement. It was discovered that prestressed EB-CFRP-reinforced RC slabs can significantly affect the vibration response of specimens. When the prestressing and fatigue loads were high, the prestressing altered the damage distribution at the CFRP-concrete interface, leading to debonding of the CFRP near the ends of the RC slabs. Additionally, for high-cyclic fatigue progressive damage, the mixed damage mode—steel yielding and CFRP debonding—was the primary failure mechanism of RC slabs reinforced with prestressed EB-CFRP.

Experimental and analytical evaluation of externally bonded BFRP and CFRP strips on the load-bearing behaviour of reinforced concrete structures using distributed fibre optic sensing

Recently, the implementation of strengthening systems for concrete structures as externally bonded carbon fibre-reinforced polymer (EBR-CFRP) has been used more frequently than the system with an externally bonded basalt fibre-reinforced polymer (EBR-BFRP). Through many years of an international investigation, a safe design model for CFRP strips has been developed, unfortunately, it could not be used for the calculation of reinforced concrete strengthened by lower stiffness strips such as BFRP, without previous validation. The differences appear in the bond shear stress – slip relationship, which is required for a more accurate design. The research on bending behaviour of strengthened concrete structures is necessary to offer a practical engineering and design model for a new retrofitting system. This paper presents the analytical evaluation of externally bonded BFRP and CFRP strips on the load-bearing behaviour of reinforced concrete structures and its verification on the experimental results. The validity of this study was completed based on the basis of the strain measurement of steel- and FRP-reinforcement using distributed fibre optic sensing.

A REVIEW ON FLEXURAL STRENGTHENING OF REINFORCED CONCRETE BEAMS WITH PRESTRESSED NEAR SURFACE MOUNTED CFRP LAMINATE

Reinforced structures are subjected to tremendous loading, which causes early deterioration, and replacing RC structures on before the end of intended life is prohibitively expensive. The application of fiber- reinforced polymer (FRP) composites to strengthen damaged structural components is obtaining favour in the construction industry because to their better strength, durability, and compatibility with reinforced concrete structures during usage. The main purpose of this research is to examine the existing performance, problems, and further potential of CFRP-strengthened reinforced concrete structural member under various loading circumstances. FRP reinforcement outperforms in static, dynamic, and harsh environmental criteria. The most typical type of failure in reinforced beams are debonding, separation of concrete cover, and FRP rupture. Acceptance and usage of FRP in RC structure strengthening will increase further as new techniques for utilising the full strength of FRP, reducing brittleness, fire risk and damages due to accident, lowering energy consumption and during production emission of carbon, and finally reducing the initial cost are developed. This report identifies knowledge gaps as well as promising research topics for FRP- strengthened structures. Keywords: Flexural strengthening, NSM, CFRP strip, Prestressed, Failures.

STRUCTURAL PERFORMANCE OF COMPOSITE BOX BEAMS WITH CORRODED BOTTOM FLANGE UNDER MONOTONIC AND REPEATED LOADS

The current research is dedicated to studying the flexural behavior of composite box beams with corroded bottom flanges. Six simply supported beam specimens were manufactured and tested. The composite box beams were fabricated with steel box beams of 1100mm, 100mm, and 100mm for length, width, and height, respectively, while the concrete deck slabs were made with dimensions of 1100mm, 400mm, and 50mm for length, width, and thickness, respectively. The composite sections were formed by connecting the RC deck slabs with the steel box beams by using a headed shear connector. Three main variables were considered; the load type (monotonic or repeated), bottom flange thickness (with or without corrosion), and whether to consider or ignore the strengthening by CFRP strip. Experimental results indicated that the ultimate loads were decreased by 29-33% for the corroded bottom flange beam specimens; while the ultimate loads for the strengthened beam specimens were increased by 60-67% as compared with the un-strengthened corroded corresponding beam specimens. Also, the change of the applied loads from monotonic to repeated leads to a reduction of the ultimate load by 17-22%.

Effects of the embedding depth on the interfacial bond performance of horizontally NSM CFRP strips to concrete structures

Horizontally NSM CFRP strip reinforcing concrete structures are a new type of reinforcing method, as they require a small groove depth compared with that of the traditional NSM strengthening method. In order to explore the effect and mechanism of the embedding depth of horizontally NSM CFRP strip on interfacial bond performance, the single shear pull-out test was used to compare and analyze the bond properties of concrete specimens reinforced with horizontally NSM CFRP strips, EB CFRP strips, and NSM CFRP strips, and explore the effect of horizontally NSM reinforcement methods. Then, the influence of the embedding depth on interface bonding was studied. Finally, the finite element software was used to establish a finite element model for numerical simulation to verify the test results of horizontally NSM CFRP strips. The results show that the specimens reinforced with horizontally NSM CFRP strips have better bonding capacity and interface bonding stiffness. The embedding depth of the CFRP strip affects the bond efficiency between the CFRP strip and concrete, and the interfacial bonding performance is the best when the embedding depth is 12 mm. The maximum bond capacity, strain distribution, and load-end slip curve of CFRP strip under finite element numerical simulation are in good agreement with the experimental results. This indicates that the bonding performance between the horizontally NSM CFRP strip and concrete can be analyzed by the finite element method. This provides a theoretical basis for the application of horizontally NSM CFRP strip reinforcement technology in practical engineering.

Experimental investigation on the behavior of fly-ash based geopolymer reinforced concrete beams strengthened with CFRP

Recently, the demand for strengthening and rehabilitation of existing RC structures has increased due to the corrosion of internal steel reinforcement, variations in temperature, and increasing loading. As a result, several experimental studies have been performed to investigate the structural behaviour of strengthening RC beams with CFRP sheets, but few for GPC beams; therefore, this investigation focuses on the behaviour of strengthening GPC beams with CFRP sheets. In this experimental work, a set of ten specimen beams with the same cross section of 100 × 250 mm and 850 mm length with a 750 mm clear span were cast in two groups of five beams each. First group (flexural group) to study the flexural behavior, and the second one for the shear behaviour (shear group). In each group, the first beam was carried out as an RC control beam and the second as a GPC control beam without strengthening, while the other three beams were cast as GPC beams and strengthened with various schemes of CFRP sheets. All specimens were tested up to failure under two-sided static loading (four-point bending). The first cracking, yielding, and ultimate failure loads, the deflection values at midspan, the longitudinal bar strain, and the concrete strain were recorded for all tested specimens. The experimental results indicated that the Flextural Strengthening of GPC with CFRP sheet increased the First Cracking, yield and ultimate load capacity by 25.33%, 15.3% and 15% respectively, as well as, deflection was decreased by 16% on average while ductility and toughness have improved by 10% and 12% on average compared to R.C Beam.On the other side, the Shear Strengthening of GPC with CFRP strips increased the First Cracking, yield and ultimate load by 43%, 70% and 68% respectively, as well as, shear ductility has improved by 8% on average compared to R.C Beam. Overall, the different schemes of externally bound CFRP sheets have improved the flexural and shear behaviour of GPC beams.

Dynamic contribution of CFRP strips to CFRP-strengthened RC shear walls

For accurately simulating the dynamic mechanical behavior of CFRP-strengthened RC shear wall, a three-dimensional numerical model was proposed in the present study to capture the effect of strain rate on the constitutive relations of concrete, steel bar, CFRP and CFRP-concrete interface. The influences of strain rate, CFRP wrapping method and the number of CFRP layer on the dynamic shear properties and dynamic CFRP shear contribution of CFRP-strengthened RC shear wall were investigated. The results indicated that: 1) as the strain rate increases, the damage of the CFRP-strengthened shear wall became more severe, as evidenced by the increase in the damage area of concrete and CFRP strips; 2) the failure pattern and shear bearing capacity of shear wall was affected by the CFRP layout configurations; 3) the ductility coefficient and energy dissipation capacity increase with an increasing strain rate and CFRP layout configurations. It was to emphasize that, the shear contribution provided by CFRP strips was substantially enhanced subjected to the dynamic loads, especially with the strain rate exceeding 10−3 /s, therefore, yielding a significant increase in the overall bearing capacity of CFRP-strengthened shear wall. Moreover, it was found that the degree of enhancement was extensively affected by the CFRP wrapping methods and the number of CFRP layer bundled on the RC shear wall. Based on numerical results, a strain-rate-dependent formula predicting the dynamic shear contribution of CFRP strips was proposed to quantitatively describe influences of CFRP wrapping method and bundled layers.

Shear capacity estimation of core steel tube reinforced PCCC column-beam interior joint under quasi-static load

In this study, the quasi-static tests were conducted on 13 core steel tube (CST) reinforced PVC-CFRP confined concrete (PCCC) column-beam interior joints. Distinct shear failure occurred in all joint specimens. The load-displacement hysteresis curves appeared pinching phenomenon, and the shape of hysteresis loops gradually deformed from shuttle to arch and finally changed to reverse S shape. The overall shape of hysteresis curves were relatively full, demonstrating that the joints had favorable seismic performances and energy dissipation capacity. The strain development law of materials in the joint area in the process of stress was clarified. The increment of joint stirrup ratio ([Formula: see text]), axial compression ratio ([Formula: see text]), column reinforcement ratio ([Formula: see text]) or beam reinforcement ratio ([Formula: see text]) improved the calculated shear capacity of the joints. The calculated shear capacity increased first and then decreased slightly as the diameter-thickness ratio of CST ([Formula: see text]) increased. In comparison, spacing of CFRP strip had little impact on the calculated shear capacity. Additionally, the concrete strength enhancement coefficient ([Formula: see text]) and joint stirrup utilization coefficient ([Formula: see text]) were introduced respectively to modify the shear capacity of concrete and joint stirrups, and a simplified formula for predicting the shear capacity of CST reinforced PCCC column-beam interior joints under quasi-static load was proposed. The predicted results agreed well with the test data.

Experimental and numerical investigation of punching behavior of two-way RC slab with different opening locations and sizes strengthened with CFRP strip

Flat slabs are widely used due to their advantages. However, punching collapse is a situation that should not be ignored during the design as a risk in this type of slab. The openings left around the column or opened later for various reasons negatively affect the punching strength of the slabs. Within the scope of the study, a total of nineteen flat slabs were produced to investigate this negative effect and to eliminate it with an appropriate strengthening method. In the study, the size and position of the two square-section openings adjacent to the column (around the column) were chosen as variables. In addition, identically produced slab test specimens with openings were strengthened with fan-type anchors and CFRP strips. Applied strengthening technique aimed to catch the punching strength of the reference test specimen without opening and, if possible, exceed this value by preventing the punching behavior that occurs in the slabs with openings, as mentioned in the first part of the study. It was determined that the anchored CFRP strips increased the load-carrying capacity of the opening test specimens by 50% on average. Energy dissipation capacity, initial stiffness, maximum bearing capacity, strain behavior of CFRP strips, failure modes, and crack distributions were interpreted using obtained experimental data. The specimens were modeled with the ABAQUS and numerically analyzed, and in the last step, the numerical results were interpreted by comparing them with experimental results.

Shear strengthening of precast prestressed I‐girders using carbon fiber reinforced polymers and in‐fill concrete blocks

AbstractDue to the rapid increase in traffic volumes, existing infrastructure demands proper assessment and in some cases rehabilitation is needed. Prestressed concrete (PC) I‐shaped girders are widely used in prefabricated girder bridges. With sequel generations of shear design provisions their shear capacity might need to be increased in the framework of assessing safety of aging bridges. Earlier attempts in shear strengthening this specific type of girder with fiber reinforced polymers (FRP) has shown early debonding along the concrete surface at the web‐flange corner so that the increase in shear strength was very limited. This study presents the results of full scale PC I‐girders strengthened with CFRP through epoxy bonded concrete in‐fill blocks that locally transforms the I‐section to a rectangular section at the position of CFRP strips. This transformation eliminated the early debonding of CFRP shear reinforcement and for the studied configurations a 30%–40% increase in shear capacity was obtained with respect to the unstrengthened control specimen, without and with additional CFRP spike anchors, respectively. In comparison, similar configurations without in‐fill blocks reached 5% and 12% increase in shear capacity, respectively. As such, the use of in‐fill blocks appeared more effective than local anchorage of the CFRP at the web‐flange corner. Analytical verification of the experimental results confirmed that existing formulations lack ability to predict the shear capacity of strengthened I‐girders, and further work is needed in this respect.

Low Velocity Impact Response of Reinforced Concrete Flat Slabs

Background Dimensions of the reference concrete slab are 1950x1950x100 mm subjected to drop-weight impact loading. Objective A comprehensive parametric study was performed to examine the influence of many parameters on the RC slabs. Method From the viewpoint of cost and time savings, a three-dimensional finite element is a very good tool to predict the real behavior of the structural elements. Result and Discussion Results showed that the use of CFRP strips enhance the impact behavior of the slab. Contrarily, the existence of opening led to a dramatic decrease in the dynamic capacity of RC slabs with stress concentration around the openings. Furthermore, changing the shape of the impactor showed a significant effect on the peak impact load as well as the ultimate deflection at impact instant. Conclusion In the scope of this paper, the response of RC slab with top and bottom reinforcements exposed to drop-weight impact loading was inspected. Time histories of impact loads and deflections were presented in detail based on ABAQUS/ Explicit analysis. The findings presented in this paper can be presented as follows: 1. The FE models show a good correlation with the experimental data. Consequently, the proposed finite element models are efficient and economical tool to explorer the effect of many parameters on the performance of RC slabs subjected to drop-weight impact load. 2. The numerical simulation confirmed that using externally bonded CFRP strips has more influence on the peak deflection of the reinforced concrete slab than the recorded impact force. 3. Comparing to the flat shape of the impactor, the hemispherical and curved shape impactor can produce large penetration depth at the impact zone with higher plastic deformations in the concrete slab. However, the flat impactor produced higher deflection at the impact instant. 4. As the radius of the impactor increases, both the duration time and the peak impact force are increased. This is because of the higher contact area was obtained when the flat impactor (infinity radius of curvature) was used as compared to other impactors. 5. Due to decreases in RC slab stiffness, the presence of openings (regardless of their shape) has considerably increased deformations in concrete especially around perimeter of the openings extended to the nearby support. 6. It has been found that the eccentric impact loading causes higher plastic deformations than the concentric one.

Shear Strengthening of Reinforced Concrete Beams Using CFRP Strips

Shear Strengthening of Reinforced Concrete Beams Using CFRP Strips

Impact response prediction of square RC slab of normal strength concrete strengthened with (1) laminates of (i) mild-steel and (ii) C-FRP, and (2) strips of C-FRP under falling-weight load

Reinforced concrete (RC) slab is an indispensable component of all kinds of structures to carry gravity loads in general. Quite often than not, the slabs are subjected to impact loads from accidental mishaps, falling construction equipment, and falling boulders and rocks which are not considered in the structural design. Design codes of practice for RC structures permit the structural design based on the experimental investigation which is not viable because of constraints such as experimental facilities, materials, and time requirements. However, current advancements in the computational techniques available make it possible to obtain the comprehensive response of the slabs subjected to impact loadings. In the present research work, different strength-enhancing techniques such as steel/C-FRP sheet laminates and C-FRP strips are considered for impact resistance enhancement of two-way normal strength concrete slab, 1000 mm × 1000 mm × 75 mm, with 0.88 % conventional tension steel reinforcement subjected to a free-falling concentrated load of 1035 N. For this purpose, a 3-D finite element model of impacting test setup is formed in ABAQUS-v.6.15 explicit commercial computer code. Computational analyses and simulations of free-fall impact are carried out on the validated ABAQUS model by allowing a 105 kg steel weight (impactor) from a height of 2500 mm with an initial impacting velocity of 7 m/ sec on the top surface of the slab at its centroid. The Concrete Damage Plasticity model is implored for nonlinear elastic and inelastic behaviors, degradation of stiffness, and loading rate effect on concrete. The nonlinear behavior of the reinforcing bars is taken into account. Steel is idealized with the Johnson-Cook model but the C-FRP laminate is defined or modeled with ABAQUS Hashin’s criterion. It is found that the used strengthening techniques improve the response of the slab in terms of displacement and damage severity, and alter the mode of failure.