FRP Plates Research Articles

Experimental investigation on shear behavior of double-row perforated GFRP rib connectors in FRP-concrete hybrid beams

Perforated GFRP rib (PFR) connectors have been used in FRP-concrete hybrid beams due to durability and ease of construction. PFR connectors are parallel FRP plates with predrilled holes positioned in the flange of FRP beams. The optimal plate spacing needs to be determined because it affects the shear performance of PFR connectors. 18 push-out tests were conducted to investigate the effect of plate spacing ( Sl), penetrating GFRP bar diameter ( d), and concrete strength ( fc) on the failure mode, capacity, and shear load-slip (P-S) curves of double-row PFR connectors. Results showed that PFR connectors suffered plate shear failure with the concrete dowel undamaged. Typical P-S curves consisted of micro-slipping and significant-slipping phases. The shear capacity and stiffness of PFR connectors were improved by 33.3% and 45.1%, respectively, by increasing the plate spacing from 1.2 h (where h denotes the plate height) to 3.2 h. The effect of plate spacing on shear capacity and stiffness could be neglected if the ratio ( Sl/ h) was more than 3.2. Specimens with a larger diameter of penetrating bar and higher concrete strength demonstrated higher capacity and stiffness. An empirical equation based on the maximum stress failure criterion was proposed to estimate the capacity of PFR connectors, considering the plate spacing effect, and verified by available data. Additionally, a description of the P-S curve was developed and calibrated by the experimental results.

Seismic Retrofit of Concrete Buildings Damaged by Corrosion: A Case Study in Southern Italy

A case study of a building in southern Italy, subjected to high degradation by corrosion and waiting to be assessed for retrofit interventions, is presented. The owner required modifications to the building configuration, including a new layout of the floors and retrofitting for a high level of seismic load. A double strategy of an assessment and retrofit was carried out: dynamic linear and static non-linear analyses were performed, and the results were compared. Afterwards, a global strategy of mass and stiffness redistribution was implemented together with different retrofit interventions on the foundations, columns, and beams of the framed structure, such as reinforced concrete and steel jacketing, the application of FRP plates and fabrics, new steel elements, and steel–concrete composite floors. The results of the intervention are discussed and the implications of corrosion on the structures are explored. From the results obtained, it is possible to see how the use of different techniques for strengthening and passive seismic protection strategy can allow engineers to obtain the result of structural adaptation to earthquakes with low-cost interventions. The widespread adoption of steel jackets, coupled with the construction of floors using a steel–concrete composite structure, grants a good confinement of the beam–column r.c. joints, together with the overall strengthening of the existing structure. The adoption of CFRP wrapping at the lower edge of the beams implies a limited increase in thickness and the limited interventions of partial demolition from the existing structural members. A critical review of the steel jacketing aspects in terms of bending and shear strengthening is reported by considering this technique in the form of a steel exoskeleton containing the damaged concrete structure, by confining concrete elements, and by increasing the performance for both gravitational and seismic loads.

Mechanical Behavior of Large Symmetric Fiber Reinforced Polymer-Reactive Powder Concrete Composite Tanks with Floating Tops

In order to investigate the mechanical behavior of FRP-reactive powder concrete composite tanks with floating tops (FRPCTs) subjected to gravity, twenty-two full-scale FRPCTs were designed with varying parameters for the inner diameter of the storage tank (D) and the thickness of the reactive powder concrete (tc). Based on nonlinear constitutive models and the contact of the materials, and considering tank–liquid coupling, three-dimensional finite element refined models of FRPCTs were established under gravity with ADINA8.5 finite element software, and finite element models of FRPCTs under gravity were verified based on theoretical frequency formulae and existing static tests. Then, the influence of the regularity of different parameters on the equivalent stress, hoop stress, radial stress, and axial stress of the FRPCTs was obtained, and the stress distributions of FRPCTs were clearly described. The results show that the natural frequency of FRPCTs increases gradually with an increase in the height of the tank liquid (Hw); however, the natural frequency of FRPCTs reduces with an increase in D. The equivalent stress, hoop stress, radial stress, and axial stress of the FRP plate and RPC decrease slowly with an increase in tc. The axial stress of the inner RPC increases with an increase in D. The equivalent stress of the inner FRP plate subjected to gravity is distributed in a W shape, the hoop stress, and the axial stress of the FRPCTs are distributed in a U shape, and the radial stress of the inner FRP plate is distributed in an I shape. The maximum displacement occurs in the middle of the FRPCTs, and the bonding between the FRP plate and the concrete is better. Finally, a calculation formula for the variation in the regularity of the tc is developed with different D, and design and construction suggestions for FRPCTs are given, which can provide technical support for the application of the FRPCTs in practical engineering.

Repairing Behaviors of Cracked Steel Plates Based on Bolted Fiber-Reinforced Polymer Plates.

The use of FRP materials to repair cracked/damaged steel structures has gradually been adopted by researchers. This paper investigates the repairing effect of bolted FRP plates for cracked steel plates based on experimental and numerical simulation methods. In the experimental investigation, the tensile strengths of six specimens, including three repaired specimens and three pure cracked steel specimens, were evaluated. The test outcomes indicated that the bolt repairing method significantly enhanced the tensile strengths of the cracked steel plates. As an example, the failure of a pure steel plate with a 1 mm width crack occurred at 813 N, whereas after being repaired, a tensile strength of 1298 N was observed. Based on finite element (FE) analysis, the influence of bolt preloads and interfacial friction coefficients were verified. The stress-relative ratio for specimens was contingent on the bolt preload magnitude and gradually decreased as the preload was augmented. By exploring the repairing effect for varied friction coefficients, it was concluded that using a higher bolt preload can aid in eliminating the performance discrepancy of the overall component caused by interface treatment errors.

Repair and Retrofit of Bridge structural elements: New Cementitious Materials

AbstractThe condition of infrastructure, the constant need to repair and maintain them and the increasing costs associated with the maintenance has led to the search for replacements for steel in new concrete infrastructure around the world. Further, alternatives for repair and strengthening of existing concrete structures ranging from the use of FRP as plates or fabrics wraps to the use of ultra‐high‐performance concrete (UHPC) elements as external reinforcements to achieve requisite strength and serviceability requirements are being attempted. The present study evaluates the performance of repaired/retrofitted structural concrete elements and sub‐assemblages damaged in flexure or shear using different repair schemes including those based on use of FRP plates or wraps, UHPC elements, etc., to restore the capacity of damaged structural concrete elements. An ANSYS based finite element model that accounts for both the constitutive properties of the primary structure in its damaged state and the repair material is used in this study to assess the different repair schemes. The analysis is shown to predict the test results very well. The importance of considering bond slip between rebar and surrounding concrete in the analysis is highlighted. Likewise, the consequence of bond between repair material and the primary structure on the overall structural performance is clearly seen. Performance of normal and high performance of concrete under high temperature resulting in spalling and significant damage from a fire and its rehabilitation with geopolymer concrete will be shown in lab scale tests and simulations. Finally, a field application of load testing of a defunct masonry arch bridge, its retrofit strategy and subsequent deployment with cementitious mixes as a repair to operationalise the bridge with significant cost saving will be illustrated.

Performance Test of 20-Stacked Two-Tapes-Bundled REBCO Pancake Coils for Upgrading of 25-T Cryogen-Free Superconducting Magnet to 30 T

For next generation of high field cryogen-free superconducting user magnets, we adopt a robust coil concept, which consists of two tapes co-wound in a face-to-back configuration, a fluorine-coated polyimide tape as the inter-bundle insulation and thin FRP plates glued on the coil edges. To validate our coil design, we investigated electromagnetic and current-voltage properties of 20-stacked REBCO pancake coils wound with Fujikura EuBCO tapes with the robust coil concept at 4.2 K under a background field of 14 T. The inner and outer diameters and total height of the coils are 68 mm, 266 mm, and 101 mm, respectively. The average of measured hoop and axial strain on the outermost winding at the mid plane of the 20-stacked coils were 0.23% and -0.15% respectively at a hoop stress of 370 MPa estimated by the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">BJR</i> relation. The <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> - <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> properties of the coils showed no anomalous behavior up to 25 T totally generated by the coils and background field, suggesting that our coil structure has an advantage against large electromagnetic stress.

Investigation of the effect of pre-tightening forces on bolted connection for FRP-steel joints

This paper experimentally and numerically investigates the effect of pre-tightening forces on bolted connection for double-lap FRP-steel joints, considering varied levels of pre-tightening forces. In the experimental phase of this study, six bolted double-lap pull-pull specimens with varying pre-tightening degrees (preloaded of 0, 3, 6, 7, 8, and 9 N·m, respectively) were examined. Test results indicated that all specimens exhibited bearing failure of FRP plates and the load capacity was significantly influenced by the pre-tightening forces applied through bolts. The highest loading capacity (27.1 kN) was recorded for the specimen with a preloaded of 6 N·m. In order to study the stress distribution of FRP plate for the bolted specimens, three-dimensional finite element (FE) models were conducted. Based on the FE analyzing results, the pre-tightening coefficient Pc (the coefficient of the preload increment per pre-tightening torque of bolts) of 750 m−1 was concluded and further proved by the analysis of the stress in contact zones for specimens preloaded by 0, 3, and 6 N·m.

The Effects of Heat on the Cyclic Behavior of Retrofitted Concrete Columns (by Steel Plates and CFRP)

In this research, in addition to ensuring the accuracy of the numerical simulations, a 3D reinforced concrete column is numerically modeled in three conditions, including a non-retrofitted state, as well as two states retrofitted with steel plates and CFRP. The coupled temperature-displacement analysis of these columns is conducted under cyclic lateral loading in various heat levels, and the effects of both the retrofits and heat on the cyclic lateral behavior of the columns are studied and compared. According to the results of the study, with temperature rise, both lateral forces and energies increase in the non-retrofitted column, the column retrofitted with steel plates, and the column retrofitted with FRP. This is even more significant at 500°C, particularly in retrofitted columns with steel plates. Besides, retrofitting the columns with steel plates (compared to FRP plates) causes a more substantial increase in the heat sensitivity of the column during cyclic lateral loading.

A binary resin system of epoxy and phenol-formaldehyde for improving the thermo-mechanical behavior of FRP composites

A binary resin system combing epoxy and phenol–formaldehyde is proposed for improving the thermo-mechanical behavior of FRP composites. Five types of FRP plates with varying resin recipes were fabricated through compression molding process. Thermo-mechanical tests were conducted at various temperatures to evaluate the glass transition temperature and the mechanical properties of FRP plates. Owing to the cross-linking occurred between epoxy and phenol–formaldehyde, proposed binary resin system is able to increase the glass transition temperature as well as to improve the mechanical properties of FRPs. Predictive models are recommended to assist the practical design of FRPs with binary resin system.

Utilizing XFEM model to predict the flexural strength of woven fabric Kenaf FRP plate strengthened on plain concrete beam

The evolution of advancing computing technology has encouraged the use of finite element analysis to require constitutive models, mostly adopted extensive experimental datasets. Nevertheless, fewer material properties are required within the bilinear traction-separation relationship incorporated with the XFEM technique and require fewer computation efforts due to the energetic approach simulation. This study used ABAQUS CAE 2021 to predict the flexural strength of plain concrete beams strengthened with Kenaf Fibre Reinforced Polymer (KFRP) plates under a four-point bending test, later validated with experimental datasets. The varying parameters, including woven fabric architectures, lengths, widths, and plate thickness, were considered. The results demonstrated that the consistency proposed modelling technique in this study well-captured failure modes and crack development as experimental observations. All models demonstrated an increase in peak load, and a comparison of load-deflection profiles was made between the numerical FEA modelling with the experimental works. The peak load discrepancies between the numerical outputs and the experimental datasets were found to be less than 12% in all testing series. Despite promising findings, stress analysis on peel and shear stresses associated with failure modes exhibited was performed. It was found that KFRP ruptures occurred due to peel stress peak at plate mid-span, while shear mode failure demonstrates concentrated peel stress (to lesser extent shear stress) at KFRP edge tip. Hence, a more unified approach is promoted to require only two material properties (preferably independently measured values). This approach enables a designer to choose the optimum FRPs size using the current modelling tool, substantially reducing laborious and expensive experimental setup.

Investigation of dynamic behavior of prestressed FRP plate intended for strengthening prestressed RC beam

Strengthening concrete beams by prestressed or not prestressed composite plates has become one of the widely used techniques in civil engineering with the aim of improving the durability of infrastructures, specifically against seismic solicitations. However, this type of reinforcement is quite new for prestressed concrete beams. Before using a composite plate for strengthening prestressed concrete beams, it is important to know its dynamic characteristics. In this paper, an analytical model based on the high-order theory and the laminate theory is proposed in order to well determine the free vibration characteristics of presetressed composite plate. The presence of prestressing tension effort in the composite plate, which was not taken into consideration in earlier studies for slender composite plates, is the main innovation of the current research. Parametric studies varying prestressing effort and fiber orientations are carried out in aim to find the best way to use the composite plate in the reinforcing of prestressed concrete beams. For comparative purposes, a finite element model is carried out in order to explore the behavior of the composite plate under several boundary conditions (B.Cs). As an important result, this research reveals that prestressing effort stiffens the composite plate and makes it a strong choice for reinforcing prestressed concrete beams by increasing natural frequencies and reducing deflexion.

Numerical study on flexural and bond-slip behaviours of GFRP profiled-concrete composite beams with groove shear connector

GFRP profiled-concrete composite beams with groove shear connectors are analyzed using finite the element (FE) analysis. The concrete damaged plasticity (CDP) model was adopted for normal strength concrete (NSC) and reactive powder concrete (RPC). The orthotropic behaviour of GFRP profile was taken into consideration, and the bi-linear traction-separation model was used to investigate the bond-slip behavior between GFRP profile and concrete. Furthermore, parametric studies were conducted to investigate the effects of strength and the cross-sectional dimensions of concrete, strength (orthotropy), and the cross-sectional dimensions (the web height and the thickness of FRP plate). Numerical analysis results correlate well with experimental results. Based on numerical analysis, the composite beam with shear connectors spacing at 100 mm has a deflection-limit load of 21.4 % higher than the specimens with 150 mm spacing. It is possible to improve the bonding behavior of interfaces by using groove shear connectors. The ultimate load and deformation, and pseudo-ductility were significantly improved by using RPC with high strength and toughness (ultimate compressive strain). GFRP profiles with greater orthotropy coefficients provide fully utilized concrete's compressive strength, preventing premature crushing and enhancing composite structure stiffness. Flexural performance of the composite beams can be improved efficiently by choosing the appropriate sectional size during design and construction.

Experimental Study on the Blast Resistance Performance of FRP Grid & Mortar Reinforced Concrete Arch Structure.

In order to verify the feasibility of using FRP grid and mortar reinforcement technology to enhance the blast resistance of concrete arch structures, this paper designed and fabricated FRP grid and mortar reinforced concrete arch structures and conducted blast resistance tests in the field. A detailed design of anti-explosion scheme was carried out before the experiment. The tests were conducted to observe the structural cracking, concrete collapse, and reinforcement peeling of FRP grid and mortar reinforced concrete arch under the explosion. In order to compare the anti-explosion performance with the protective arch structures in other literature, the explosion of 2 kg TNT with a blast distance of 600 mm was selected. After the explosion, it was found that the blast resistance of the FRP grid and mortar reinforced concrete arch was significantly higher than that of the unreinforced arch, and the concrete arch reinforced with FRP grid and mortar has a better damage patterns and improved blast resistance performance than that of the FRP and steel plate reinforced arch. According to the research results, the FRP grid and mortar composite reinforcement technology can be used to enhance the blast resistance of arch structures in protection projects.

A New Bond-Slip Model of Hybrid Bonded FRP-to-Concrete Joints

The dowel effect in hybrid bonded fiber reinforced plastic (HB-FRP) to concrete joints is difficult to quantify and make the anchoring joint instability. A new HB-FRP joint was built, which eliminates bonding between steel and FRP plates. Based on an empirical model of external bonding FRP (EB-FRP) to concrete joints and the new model of slip at debonding, the new unified bond-slip model of the HB-FRP joints is proposed. The parameters that affect the bond-slip model are analyzed. Pull-off tests are conducted for eight specimens with different numbers of mechanical fasteners, which has verified the proposed model. The load-slip relationship at the loaded end of the FRP plate was well predicted by the partial interaction numerical calculation and further verify the theoretical model.

The effects of woven fabric Kenaf FRP plates flexural strengthened on plain concrete beam under a four-point bending test

Kenaf fibre possesses excellent tensile strength and elongation at break and is suitable to replace synthetic fibres as reinforcing materials in FRPs composites. This research is aimed to develop an experimental programme to study the behaviour of strengthened concrete beams externally bonded woven fabric Kenaf FRP (KFRP) plates. A four-point bending test was conducted to explore the effects of KFRP plate variation, such as woven fabric architectures, KFRP lengths, KFRP widths, and plate thickness (with cross-ply layers) on plain concrete beams. KFRP plate significantly improved the ultimate load, deflection at peak, stiffness, and performance factor of the strengthened plain beams when kenaf volume fraction and KFRP plate dimensions increased. The best effect to increase plain concrete beam performance was seen in specimen D-PX4 (Series D) with ultimate load, deflection and stiffness are 84 %, 50 % and 36 % with an average performance factor is 103 % compared to control beam. Thus, the flexural strengthening of a plain concrete beam using a KFRP plate is found to be a good alternative technique.

Mechanical Properties of Four-Stacked Two Tape Bundled REBCO Pancake Coils

There is an ongoing plan to upgrade the 25 T cryogen-free superconducting magnet (25T-CSM), at the High Field Laboratory for Superconducting Materials (HFLSM), IMR, Tohoku University, to a 30T-CSM by replacing the existing Bi2223 insert coils with REBCO coils. For the REBCO coils of the 30T-CSM, we will adopt a robust coil concept, which consists of two tapes co-wound in a face-to-back configuration with thin FRP plates glued on the edges. To confirm the effectiveness of this concept, we performed hoop stress tests on full-scale-size four stacked pancake coils fabricated with Fujikura EuBCO tapes at conduction-cooled 20 K under a background field of 11 T. The inner and outer diameters of the pancake coils were 68 and 268 mm, respectively. The averages of the measured strains on the outermost winding were 0.17–0.24% at a hoop stress of 400 MPa estimated by the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">BJR</i> relation. The <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I-V</i> properties of the coils indicated no anomalous voltage, implying that our coil structure was robust against large electromagnetic stress.

Cohesive Crack Models and Finite Fracture Mechanics analytical solutions for FRP-concrete single-lap shear test: An overview

In the present paper we review and compare several analytical models describing the single-lap shear test, which is the most common test to determine the bonding behaviour between a strengthening FRP plates and the concrete substrate. The models are one-dimensional and formulated under the assumption that debonding occurs as a pure mode II cracking process throughout a zero-thickness interface between the FRP strip and the brittle substrate. As such, they are all amenable to an analytical treatment. The FRP-concrete interface is described by at most three parameters among the interfacial fracture energy, the tensile strength and the elastic stiffness. Particularly, we compare the effective bond length estimates provided by different models and compare them with the ones present in Design Codes. Finally, a comparison with experimental data sets available in the Scientific Literature is also given.

Numerical analysis of delamination degradation of epoxy-impregnated superconducting coils wound with REBCO tapes caused by thermal stress

REBa2CuOy (REBCO) superconducting wires are expected to be used in coil applications owing to their excellent magnetic field properties. However, their weak delamination strength may deteriorate the superconducting characteristics of an epoxy-impregnated REBCO coil during cooling. In general, degradation is unlikely to occur if the outer-to-inner diameter ratio is less than 1.4; however, degradation has been reported in single pancake coils with an inner diameter of 560 mm and an outer diameter of 670 mm. A thermal stress analysis study is conducted to investigate the cause of this problem. Considering the non-uniformity of the epoxy resin thickness at the top and bottom of the coil, coil warping during cooling is simulated. When an FRP reinforcement plate is attached to the coil to suppress warpage, a high radial stress generated in the coil owing to the thermal contraction difference between the coil and FRP plate. The maximum local radial stress exerting in the delamination direction is 30 MPa, which is believed to degrade the superconducting characteristics of the REBCO single pancake coils.

Adhesive Layer Modeling in Concrete Beam Strengthened with FRP in an EFG Framework

The main challenging issue in concrete beam strengthening with FRP plates is the insurance of their joint performance. As been reported, the external load is mainly the primary reason for plate separation. The applied force corresponding to the moment of failure is called the debonding load. This load is of great importance. Besides, the determination of shear stress in the adhesive layer and tensile strain of the FRP plate is of the topics raised in empirical research. This study recommends using numerical methods to avoid the high cost of empirical studies. Here, failure concepts of the cohesive element are utilized. Horizontal and vertical springs are used to represent the properties of this element in which springs’ stiffness deputizes cohesive element characteristics and can be adjusted in three linear, nonlinear, and zero-stiffness steps. Springs’ stiffness is related to point displacements. So in an element-free Galerkin (EFG) method formulation, a set of springs, beam, and FRP is used to determine these displacements. Using these estimated nodes’ displacement values and utilizing springs’ stiffness, the unknown parameters inclouded debonding load, glue shear stress, and the tensile strain produced in the FRP plate can be determined. Whole calculations are coded in Matlab software. The results have been compared with the experimental outcomes of Kim and her colleagues’ work. These results indicate glue performance can be investigated through spring modeling based on the cohesive element in an EFG framework with adequate accuracy. So, the significant parts of an experimental setup can be avoided.

Effect of temperature variation on the plate-end debonding of FRP-strengthened beams: A theoretical study

Steel/concrete structures strengthened with externally bonded FRP plates may be subjected to significant temperature variations during their service time. Such temperature variation (i.e., thermal loading) may significantly influence the debonding mechanism in FRP-strengthened structures due to the thermal incompatibility between the FRP plate and the substrate as well as the temperature-induced bond degradation at the FRP-to-steel/concrete interface. However, limited information is available on the effect of temperature variation on the debonding failure in FRP-strengthened beams. This paper presents a new and closed-form solution to investigate the plate-end debonding failure of the FRP-strengthened beam subjected to combined thermal and mechanical (i.e., flexural) loading. A bilinear bond-slip model is used to describe the bond behavior of the FRP-to-substrate interface. The analytical solution is validated through comparisons with finite element analysis results regarding the distributions of the interfacial shear stresses, the interfacial slips and the axial stresses of the FRP plate. Given that a constant bond-slip relationship is adopted, it is observed that an increase in service temperature will lead to an increased interfacial slip at the plate end and consequently a reduced plate-end debonding load, and vice versa. Further parametric studies have indicated that the thermal loading effects become more significant when shorter and stiffer FRP plates are applied for strengthening.