Fiber Reinforced Polymer Concrete Interface Research Articles

Experimental and numerical study on the torsional behavior of rectangular hollow reinforced concrete columns strengthened By CFRP

This paper reported a series of hysteretic torsion experiment to investigate the torsional behavior of rectangular hollow reinforced concrete (RHRC) column strengthened by fiber reinforced polymer (FRP). Six RHRC column specimens with different number of longitudinal reinforcements, spacing of stirrup and strengthening method using FRP were designed. One was not strengthened, four were strengthened with CFRP, one was strengthened with CFRP and GFRP. The experimental results showed that the primary failure modes of specimens were the spalling of surface concrete with the detachment of FRP. In details, under the hysteretic torsional load, the interaction between adhesive and concrete caused the intersecting diagonal cracks in the internal concrete. Compared with the hysteretic curve of specimen without FRP strengthening, FRP strengthening can significantly improve the initial stiffness by 50 % and peak torsional strength by 70 %. For RHRC column without strengthening, the fullness was poor because of the weak torsional energy dissipation. The FRP strengthening can also enhance the torsional energy dissipation and seismic behavior of RHRC column. To predict the complex torsional behavior of RHRC column strengthened by FRP, a finite element (FE) model and a constitutive model were developed. The FE model considered potential cracks in concrete and FRP–concrete interface based on the application of the cohesive zone model (CZM), whereas the constitutive model accounted for interface damage and plasticity. The results of the performed simulations indicated that the proposed model can effectively represent the hysteretic mechanical behavior of columns under torsional load, which cannot be achieved using conventional FE methods.

Bond Strength Evaluation of FRP–Concrete Interfaces Affected by Hygrothermal and Salt Attack Using Improved Meta-Learning Neural Network

Fiber-reinforced polymer (FRP) laminates are popular in the strengthening of concrete structures, but the durability of the strengthened structures is of great concern. Due to the susceptibility of the epoxy resin used for bonding and the deterioration of materials, the bond performance of the FRP–concrete interface could be degraded due to environmental exposure. This paper aimed to establish a data-driven method for bond strength prediction using existing test results. Therefore, a method composed of a Back Prorogation Net (BPNN) and Meta-learning Net was proposed, which can be used to solve the implicit regression problems in few-shot learning and can obtain the deteriorated bond strength and the impact weight of each parameter. First, the pretraining database Meta1, a database of material strength degradation, was established from the existing results and used in the meta-learning network. Then, the database Meta2 was built and used in the meta-learning network for model fine-tuning. Finally, combining all prior knowledge, not only the degradation of the FRP–concrete bond’s strength was predicted, but the respective weights of the environment parameters were also obtained. This method can accurately predict the degradation of the bond performance of FRP–concrete interfaces in complex environments, thus facilitating the further assessment of the remaining service life of FRP-reinforced structures.

A CNN-integrated percussion method for detection of FRP–concrete interfacial damage with FEM reconstruction

Reinforced concrete (RC) structures are commonly strengthened using externally bonded fiber-reinforced polymer (FRP) sheets. The bond between the FRP and concrete is a crucial factor affecting the strengthening effect, and debonding along the FRP–concrete interface is usually accompanied by the fracture of the underlying concrete. Therefore, it is necessary to identify the interface damage of FRP-to-concrete joints and conduct mechanical analysis. However, debonding is invisible damage that occurs inside the underlying FRP layer, which makes damage detection more difficult. To this end, this study fuses a percussion method with a deep learning framework to address the detection of such invisible lesions. Meanwhile, the visualization study provides guidance for later maintenance work. To further illustrate the hazard of the identified lesions, three-dimensional reconstruction for finite element modeling (FEM) with detected damage information based on percussion is proposed to elucidate the mechanical degradation caused by the fracture of underlying concrete. Lastly, the results of this study demonstrate that the detection, visualization, and FEM reconstruction of FRP–concrete interface damage using percussion signals has considerable application potential and is worthy of further study.

Ensemble Machine Learning-Based Approach for Predicting of FRP–Concrete Interfacial Bonding

Developments in fiber-reinforced polymer (FRP) composite materials have created a huge impact on civil engineering techniques. Bonding properties of FRP led to its wide usage with concrete structures for interfacial bonding. FRP materials show great promise for rehabilitation of existing infrastructure by strengthening concrete structures. Existing machine learning-based models for predicting the FRP–concrete bond strength have not attained maximum performance in evaluating the bond strength. This paper presents an ensemble machine learning approach capable of predicting the FRP–concrete interfacial bond strength. In this work, a dataset holding details of 855 single-lap shear tests on FRP–concrete interfacial bonds extracted from the literature is used to build a bond strength prediction model. Test results hold data of different material properties and geometrical parameters influencing the FRP–concrete interfacial bond. This study employs CatBoost algorithm, an improved ensemble machine learning approach used to accurately predict bond strength of FRP–concrete interface. The algorithm performance is compared with those of other ensemble methods (i.e., histogram gradient boosting algorithm, extreme gradient boosting algorithm, and random forest). The CatBoost algorithm outperforms other ensemble methods with various performance metrics (i.e., lower root mean square error (2.310), lower covariance (21.8%), lower integral absolute error (8.8%), and higher R-square (96.1%)). A comparative study is performed between the proposed model and best performing bond strength prediction models in the literature. The results show that FRP–concrete interfacial bonding can be effectively predicted using proposed ensemble method.

Generalized Evaluation of Bond Behavior of the Externally Bonded FRP Reinforcement to Concrete

This paper presents a generalized evaluation method for determining the bond slip relationship of an external reinforcement bonded to concrete based on the experimental global force versus the displacement curve. The proposed method also provides several advantages, including predicting the global force versus displacement curve with any bond slip relationship, deriving the bond strength model, predicting the strain distribution along the fiber reinforced polymer (FRP)–concrete interface, and evaluating the effective bond length. An introductory example shows detailed implementation of the proposed method on an FRP–concrete interface reported in the literature. Since no prior assumption is made on the detailed configuration of the bond slip relationship using the proposed method, the obtained bond slip relationship can be seen more generalized and representative. The obtained bond slip relationship differs from the available bond slip models and leads to predictions that best approximate the experimental results. In addition, the reliability of the proposed method in determining the effective bond length is also verified against the experimental and analytical results.

Experimental evaluation of the tensile bonding strength of the basalt fiber-reinforced polymer–concrete interface

The bonding performance of basalt fiber-reinforced polymer and concrete substrate has a significant effect on the reliability of externally strengthened existing concrete structure, due to being the most vulnerable element to failure in this fiber-reinforced polymer–concrete strengthening system. Its failure can result in the failure of the whole structure. Although many previous researchers have been interested in the tensile bonding strength of carbon fiber-reinforced polymer and glass fiber-reinforced polymer–concrete interface, that of basalt fiber-reinforced polymer–concrete interface has been very limited. Thus, the objective of this study is to experimentally assess the tensile bonding strength of the basalt fiber-reinforced polymer–concrete interface. The effects of high temperature, freezing–thawing cycles, type of resin, and concrete crack widths on the tensile bonding strength are also investigated. The pull-off experiment is conducted according to ASTM D7522/D7522M-15. A total of 205 core specimens of 50 mm diameter and 10 mm depth were taken from 41 concrete beams. The experimental results illustrate that both freezing–thawing and high-temperature condition have a substantial effect on the bonding strength of the basalt fiber-reinforced polymer–concrete interface. Bonding strength was decreased within the range of about 9%–30% when the number of freezing–thawing cycles increases from 100 to 300; likewise, it was decreased up to 30% when the exposure temperature rises to 200°C. Also, the specimens which were repaired to close their cracks by epoxy resin had no significant effect on the bonding strength of basalt fiber-reinforced polymer–concrete interface, when the specimens had crack width of less than 1.5 mm.

Evaluation of Pull-Out Strength of a Basalt Fiber-Reinforced Polymer-Concrete Interface via Temperature Variation and Surface Crack Repair

Evaluation of Pull-Out Strength of a Basalt Fiber-Reinforced Polymer-Concrete Interface via Temperature Variation and Surface Crack Repair

Effect of silica fume and calcium carbonate whisker on interfacial behavior of near-surface-mounted FRP with magnesium phosphate cement

Magnesium phosphate cement (MPC) is a potential substitute of epoxy as an adhesive material due to its advantages in setting time, early strength, and good fire and corrosion resistance. In this study, silica fume (SF) and calcium carbonate whisker (CCW) were employed to improve bond capacity of MPC which were used instead of epoxy in near-surface-mounted (NSM) fiber-reinforced polymer (FRP) systems. A direct pull-out test (DPT) was carried out to investigate bond performances of FRP–concrete interface after incorporating SF and CCW. According to the mix proportion of the MPC, a total of twenty-seven specimens in nine sets were divided into four groups: one group without additive, one group with SF alone, one group with CCW alone, and the last group with SF and CCW combination. Results showed that SF or CCW alone could improve MPC bond capacity, but their excess application would reduce it. SF and CCW combination, however, did not improve bond capacity as effectively as SF or CCW alone. Moreover, the addition of CCW would improve MPC’s bond ductility, with or without the adding of SF, but with the increase in CCW concentration, this improvement effect would decrease. Meanwhile, SF alone lowered its bond ductility.

Numerical investigation of the uplift performance of prestressed fiber-reinforced polymer floating piles

ABSTRACTFinite element analyses of prestressed fiber-reinforced polymer floating piles subjected to uplift force have been conducted in this paper. First, parameters of the modified BPE model (bond–slip model at the fiber-reinforced polymer–concrete interface) were calibrated using existing pullout testing data on fiber-reinforced polymer rebars embedded in concrete. Nonlinear spring elements were used in numerical modeling to characterize the bond–slip behavior at the fiber-reinforced polymer–concrete interface. A parametric study was performed to assess the influence of rebar diameter, fiber-reinforced polymer material, embedment length, and concrete strength on the mobilized bond stress. Upon the successful modeling of the pullout performance of fiber-reinforced polymer rebars in concrete, numerical models were developed to investigate the dependence of the uplift performance of floating piles on the prestress level, uplift force, fiber-reinforced polymer type, and compressive strength of concrete.

Thermomechanical Behavior of Near-Surface-Mounted Carbon Fiber-Reinforced Polymer Concrete Interface

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A two-scale damage model for high-cycle fatigue at the fiber-reinforced polymer–concrete interface

This paper presents a new two-scale damage model of the fiber-reinforced polymer (FRP)–concrete bond under high-cycle fatigue. The material behavior is modeled as elastic-plastic coupled with damage for the microscale analysis and as elastic for the mesoscale analysis. A new damage law for the interface joint is described. The two-scale damage model has been implemented as a material model for a three dimensional an eight-node interface element of zero thickness and used to simulate a double shear joint specimen under high cycle fatigue. The numerical calculations were performed with a full incremental cycle solution and a new cycle jump approach.

Strain rate effect on bond stress–slip relationship between basalt fiber-reinforced polymer sheet and concrete

Reinforced concrete structures that are strengthened with fiber-reinforced polymers frequently suffer dynamic loadings. The success of this strengthening method depends on the effectiveness of the bond between the fiber-reinforced polymer sheet and concrete. Determining the bond behavior between the fiber-reinforced polymer and concrete substrate is an important issue in technology because the typical failure mode of fiber-reinforced polymer-to-concrete joints is debonding of the composite from the concrete substrate. Although numerous experimental studies have investigated this static bond behavior, experimental data concerning dynamic tests on maximum bond stress, ultimate load, and the bond–slip relationship of the interface between basalt fiber-reinforced polymer sheets and concrete under different strain rates are lacking. This study presents an experimental investigation of the dynamic performance between the basalt fiber-reinforced polymer sheet and concrete under different strain rates. Double-lap shear specimens were used in the tests. The dynamic test results are reported and discussed in this paper to evaluate and compare the influence of the strain rate on the dynamic maximum bond stress, ultimate load, as well as the bond–slip relationship between the basalt fiber-reinforced polymer sheets and concrete. The data show that the maximum bond stress, ultimate load, and bond–slip relationship are sensitive to the strain rate. The test results indicate that (1) the dynamic maximum bond stress of the basalt fiber-reinforced polymer–concrete interface increases with the increase of the strain rate; (2) the dynamic ultimate load of the basalt fiber-reinforced polymer–concrete interface increases with the increasing strain rate as a logarithmic function; and (3) the strain rate has no effect on the slip at the maximum bond stress, and the dynamic bond–slip relationships of basalt fiber-reinforced polymer–concrete interface under different strain rates are similar. The calculation models of the dynamic maximum bond stress, ultimate load, and bond–slip relationship were established through a regression analysis of the dynamic test data while considering the influence of the strain rate.

A novel numerical model of debonding of FRP-plated concrete beam

Accurate modeling is required to estimate the debonding in a plated fiber-reinforced polymer (FRP) concrete beam. In the present investigation, a numerical method is developed to model a crack in the FRP–concrete interface. An initial notch is located at the mid-span of the concrete beam. A modified crack closure integral method is implemented to model Mode-I fracture in the concrete. In the present research, a special interface element is formulated to simulate and to predict the distribution of interfacial shear stresses by using drilling degrees of freedom in the nodes of interface elements. Cohesive forces in the nodes of interface elements are formulated by finite element methods. A crack propagation criterion is presented to evaluate when the crack grows in FRP–concrete interface. If the principal stress in the node at the tip of an interface element reaches the maximum shear stress along the FRP–concrete interface, debonding happens. The model is robust, accurate, independent of mesh size, and it is able to model the crack growth in the concrete and debonding of the FRP–concrete interface, simultaneously. The model presented in this study showed acceptable similarity to previous research data.

The effects of splices and bond on performance of bridge deck with FRP stay-in-place forms at various boundary conditions

An investigation is conducted on a novel system of Fiber Reinforced Polymer (FRP) stay in place (SIP) structural formwork for concrete bridge decks. The deck system is composed of FRP composite ribbed panels, spanning between girders and acting as both permanent formwork and bottom slab reinforcement. The testing, conducted at 1:2.75 scale, consisted of ten bridge deck sections and examined several critical parameters, namely: varying of the width (w) of the deck specimens relative to their spans (s), and varying interface bond condition, concrete strength and loading location on the deck. It was shown that as the width (w) is increased relative to span (s), the performance of the deck approaches the actual built performance. An aspect ratio (w/s) of greater than 1.6 was found to adequately represent performance and avoid overly conservative results produced at lower aspect ratios. Varying concrete strength from 17MPa to 42MPa in identical decks resulted in 20% increased capacity but did not influence stiffness. Applying adhesive bond at FRP–concrete interface to create a fully composite section increased the deck strength and initial stiffness by 30% and 73%, respectively. In decks with adhesive bond, loading directly above the FRP splice resulted in a 20% lower strength than loading half-way between splices. This is an opposite trend to that observed in decks without adhesive bond.

FBG Sensor Technology to Interfacial Strain Measurement in CFRP-Strengthened Concrete Beam

Fiber-reinforced polymer (FRP) composite materials are very attractive for use in strengthening of civil engineering structures because of their high strength to weight and stiffness to weight ratios, being corrosion free, light weight, and potentially high durability. The requirement for any embedded sensor for monitoring strain in FRP-strengthened concrete structures is that the sensor should not be detrimental to the operational requirement of the strengthened structure. Owing to the compatibility with FRP materials, fiber Bragg grating (FBG) sensor is a good choice for embedding at the interface of the FRP-strengthened concrete structures. As the FBG sensors are more flexible and small size (diameter = 250 μm), it may be easily instrumented at the FRP–concrete interface with sufficient surface preparation and suitable adhesive. This article deals with the issues/methods in application of FBG sensors for interfacial strain measurement of reinforced concrete beams strengthened with carbon fiber-reinforced polymer (CFRP). It also describes the experimental studies which were carried out to identify the initiation and propagation of debonding of CFRP from concrete surface using FBG array sensors.

Shear-Peeling Bond Strength between Continuous Fiber Sheet and Concrete

Fiber-reinforced polymer (FRP) sheets can be externally bonded to reinforced concrete structural members to increase strength. However, the bond between the FRP sheet and concrete may adversely affect the performance of flexurally strengthened members when there are diagonal tension cracks in the members. This study investigates the bond strength between the FRP sheet and concrete interface for both shear bond and peeling conditions. In the experiment, 27 rectangular specimens with FRP sheets bonded on two sides were tested in uniaxial tensile loading. The specimens were designed for different step angles at the middle to ensure that the interface acts for both shear and peeling conditions. Three types of woven composite sheets made of aramid and carbon were used to allow the study of various sheet stiffnesses and strengths. The results showed that the bond strength decreases considerably due to the peeling effect. Step angle and fiber stiffness greatly influence the bond strength between the FRP sheet and concrete interface for combined shear and peeling conditions. The highest bond strength was observed for the lowest axial stiffness of the laminate. Based on the test results, a modification to an existing bond strength model is proposed that improves the prediction of the bond strength between FRP laminate and concrete for shear-peeling conditions.

Limiting shear creep of epoxy adhesive at the FRP–concrete interface using multi-walled carbon nanotubes

Fiber reinforced polymer (FRP) composites are widely used in structural strengthening and retrofitting due to their high strength-to-weight ratio and non-corrosive properties. However, one of the recently recognized drawbacks of common FRP strengthening systems is the relatively high shear creep deformation of epoxy adhesives when FRP sheets are used to strengthen concrete structures against sustained loads. On the other hand, carbon nanotubes (CNTs) are reported to provide significant enhancement to various mechanical properties when used in epoxy adhesives. This enhancement is attributed to the extraordinary mechanical properties of the CNTs and their ability to bond to epoxy. In this article, we report the results of experimental and analytical investigations conducted to examine shear creep behavior of multi-walled carbon nanotubes (MWCNTs) reinforced epoxy nanocomposite used at the FRP–concrete interface. Double shear tests were performed on FRP sheets bonded to concrete blocks with MWCNTs reinforced epoxy nanocomposite. Various levels of pristine and functionalized MWCNTs by weight were examined including 0.1%, 0.5%, 1.0% and 1.5%. The viscoelastic behavior of MWCNTs reinforced epoxy nanocomposite was simulated with rheological models and the models' parameters were extracted and discussed. The results show the ability of MWCNTs to significantly reduce creep compliance of epoxy at the FRP–concrete interface making it a viable solution if FRP is used to strengthen concrete structures subjected to sustained stress.

Numerical Analysis on the Load-Carrying Capacity for the FRP Reinforced Four-Point Bending Concrete Beam

In this paper, different types of debonding failure modes are described. Study of concrete cracking behavior and interfacial debonding fracture in fiber reinforced polymer (FRP)-strengthened concrete beams are carried out. A finite element analysis is performed to investigate the different types of debonding propagation along FRP–concrete interface and crack distribution in concrete. The proposed FE, denoted as FRP–FB (force-based) beam, is used to predict the load-carrying capacity and the applied load-midspan deflection response of RC beams subjected to four-point bending loading. Numerical simulations and experimental measurements are compared based on numerous tests available in the literatures and published by different authors. The numerically simulated responses agree remarkably well with the corresponding experimental results. It demonstrates that the proposed two-dimensional frame finite element (FE) is able to accurately estimate the load-carrying capacity of reinforced concrete (RC) beams flexurally strengthened with externally bonded fibre reinforced polymer (FRP) strips and plates.

Analytical Studies on Mixed-Mode Debonding along the FRP–Concrete Interface in the Peeling Test

The bonding of fiber reinforced polymer (FRP) strips and plates to the concrete structures has been found to be an effective technique for flexural strengthening. The FRP is then under both pulling and peeling forces, resulting in a combination of shear sliding and opening displacement along the FRP/concrete interface. A novel experimental set-up is studied that a peeling load is applied on the FRP sheet by a circular rod placed into the central notch of the beam. Based on the linear-elastic fracture mechanics approach, a theoretical analysis is conducted on specimens representing the peeling behavior. From the numerical analysis, the load–displacement curves, load–stiffness of FRP sheet curves, and load–fracture energy curves affected by different variables are discussed. The peel load is related to the FRP sheet stiffness and to the interfacial fracture energy. Therefore, only two material parameters, the interfacial fracture energy of FRP–concrete interface and stiffness of FRP sheets, are necessary to represent the interfacial fracture behavior. The theoretical load–deflection curves of specimens agree well with the corresponding experimental results in the literatures.

Numerical Studies on Mode-II Debonding along the FRP–concrete Interface in the Pulling Test

The bonding of fiber reinforced polymer (FRP) plates or sheets to the concrete structures has been found to be an effective technique to improve the capacity. As a result, a large number of studies have addressed debonding failures in FRP-strengthened RC structures, with many of them being focussed on understanding the behaviour of simple FRP-to-concrete bonded joints in which an FRP plate/sheet is bonded to a concrete prism and is subject to a tensile force. With the help of features of FRP–concrete interface and experimental data in the literatures, a non-linear mode II interface law is presented. The proposed interface law includes non-linear contributions of adhesive and concrete cover at high shear stresses. The numerical results presented in this study show that FRP strains, shear stresses, slips and values of delamination force in the bonded region are in good agreement with experimental results. The agreement verifies the validity of the proposed interface law.