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A new bond-slip model for NSM FRP systems using cement-based adhesives through artificial neural networks (ANN)
Abstract
This paper introduced a novel Artificial Neural Networks (ANN)-based bond–slip model for the Near-surface mounted (NSM) FRP system using cement-based adhesives, as an alternative to epoxy adhesives due to their high-temperature resistance and moisture-durability problems, employing experimental data. Therefore, closed-form formulas were presented for key components of the bond-slip law, including maximum bond stress, corresponding slip, fracture energy, and post-peak branch, while taking important factors into account. Compared to available bond-slip laws, this innovative model demonstrates promising potential in predicting the bond behaviour, thereby enabling more efficient and reliable designs for the NSM FRP strengthening applications using cement-based adhesives.
Concrete structures with shallow grooving depths require minimal adjustments to accommodate horizontal near-surface-mounted (NSM) carbon fibre-reinforced polymer (CFRP) strips. This method proves advantageous in minimizing damage to reinforced concrete structures, establishing itself as an effective reinforcement technique. To delve into the interfacial performance of the horizontal CFRP strip reinforcement, single shear pull-out testing is utilized to analyze and compare the interfacial bonding performances of concrete specimens reinforced with external bonding (EB), NSM, and horizontal NSM CFRP strips. The study also explores the influence of groove depth-to-width ratio on the interfacial bonding performance of horizontal NSM CFRP strip reinforcement. An interfacial bond-slip principal structure model of horizontal NSM CFRP strips and concrete is established, factoring in the groove depth-to-width ratio. Subsequently, the proposed interfacial bond-slip model is validated through finite element numerical simulation. The results show that the horizontal NSM CFRP strips reinforcement method exhibits commendable bonding capacity and interfacial bond stiffness. Within a specific range of groove widths, increasing the groove depth-to-width ratio diminishes the distance between the peak bond shear stress location and the loading end of the horizontal NSM CFRP strip-reinforced specimens. This improves the interfacial bonding performance between the horizontal NSM CFRP strips and concrete. The theoretical curve derived from the interfacial bond-slip principal structure model aligns well with test data across all reinforced specimens. FEM numerical simulations based on this interface bond-slip principal structure model demonstrate minor relative errors compared to experimental values. Furthermore, the theoretical distribution curve of CFRP strip strain closely follows the measured distribution curve. This model concerning the bond-slip principal structure of the interface between horizontal NSM CFRP strips and concrete, accounting for the groove depth-to-width ratio, exhibits applicability and stands as a valuable reference for practical engineering applications.
Use of organic resins such as epoxy and vinyl esters as bonding materials in fibre reinforced polymer (FRP) strengthening of concrete members is widely accepted. However, the performance of organic resins is compromised when exposed to high temperature and extreme weather conditions leading to reduced durability of the strengthened systems. The present study attempts to evaluate the effectiveness of inorganic (cement mortar and geopolymer mortar) bonding materials for shear strengthening of prestressed concrete (PSC) beams using the near-surface mounting (NSM) technique. Different types of bonding materials are used in this study for NSM shear strengthening including: (i) epoxy resin, (ii) high strength cement grout (HSCG) and (iii) geopolymer mortar. Bond tests were first conducted to evaluate the pull-out/bond strength of different bonding materials. Bond tests revealed that epoxy resin had the highest bond strength followed by geopolymer mortar and HSCG. Sixteen full-scale PSC beams were cast with and without stirrups. The beams were strengthened using NSM CFRP laminates oriented at 45-degree configuration and then tested under a three-point bending configuration. Experimental results revealed that the performance of high strength cement grout and geopolymer mortar was similar but with a lesser efficiency compared to the epoxy resin.
The repair efficiency of fiber‐reinforced polymer (FRP) is crucially linked to bond strength between FRP and concrete. Artificial neural networks (ANNs) technique is employed for the prediction of FRP–concrete bond strength based on more than 440 data points collected from literature work for training and testing of the proposed ANNs model. Such a model facilitates investigating the effect of various key parameters in controlling the bond. These are concrete compressive strength, maximum aggregate size, FRP thickness and modulus of elasticity, FRP‐to‐concrete length and width ratios, and adhesive tensile strength. The proposed ANNs model shows high fitting and prediction capability of training and testing data, respectively, with low mean square errors. Its accuracy of prediction far exceeds that of literature empirical models. Furthermore, the present comparative and sensitivity study of the predicted bond strength promotes the understanding of the impact of the above key parameters.
One of the main concerns of using structural composites as an effective technique for strengthening and rapid restoration of concrete structures is the behaviour of these systems in fire condition. Epoxy resins are currently used to bond structural composites to concrete substrate, but the vulnerability of their properties to high temperatures can compromise the strengthening effectiveness of these systems. Hence, finding an alternative adhesive is of a great importance. Recent studies presented promising results with the use of cement based materials as adhesives due to their good ability for transferring stresses and compatibility to the substrate. This study explores the adoption of a pre-treatment procedure for carbon fibre laminates for increasing the bond strength according to the near surface mounted (NSM) strengthening technique. Pull-out tests results confirmed the effectiveness of the proposed approach for enhancing the bond strength.
This paper presents design procedures for fibre reinforced polymer (FRP) systems inserted in the cover of concrete elements according to the near-surface mounted (NSM) technique. Such strengthening system depends greatly on their bond strength. Two existing design formulations to estimate the bond strength of NSM FRP systems in concrete are studied. A reliability analysis is conducted with the purpose of making the design formulations consistent with the partial safety factors philosophy, including the Eurocodes. Hence, the necessary probabilistic distributions are calibrated based on a large database of bond tests. The results presented herein show that the existing guidelines can be extended and adopted under the framework of the Eurocodes. However, mainly due to their limitations in addressing individually all the possible failure modes, the variability of the probabilistic distributions found are quite high, leading to high partial coefficients of safety. Thus, in the future, new and improved formulations should be developed.
Near-surface mounted (NSM) fiber-reinforced polymer (FRP) bars are being increasingly recognized as a valid alternative to externally bonded FRP laminates for enhancing flexural and shear strength of deficient concrete, masonry, and timber members. The ultimate capacity and service performance of strengthened members are deeply influenced by the bond characteristics of the strengthening system on which, in the case of NSM bars, limited data is available to date. This paper follows up to previous investigations on the mechanics of bond of NSM bars to concrete. Experimental results completing a previous test series are reported and discussed, and a global evaluation of results of three different test series is attempted. A three-dimensional finite element model for bond of NSM reinforcement is proposed and calibrated on the basis of some experimental results.
This paper presents a review of current knowledge on the bond behavior of fiber reinforced polymer (FRP) systems inserted in the cover of concrete elements, commonly known as the near-surface mounted technique (NSM). In the first part, by studying the physics of the phenomenon, the typical failure modes, the most common bond tests and two of the most important design guidelines for FRP NSM systems are introduced. In the second part, a database of bond tests composed by 431 records is presented and the accuracy of existing design guidelines is assessed with this data. Lastly, the formulations proposed by these design guidelines are recalibrated based on the experimental results in the database.
This study proposes the use of artificial neural networks (ANNs) to calculate the compressive strength and strain of fiber reinforced polymer (FRP)confined square/rectangular columns. Modeling results have shown that the two proposed ANN models fit the testing data very well. Specifically, the average absolute errors of the two proposed models are less than 5%. The ANNs were trained, validated, and tested on two databases. The first database contains the experimental compressive strength results of 104 FRP confined rectangular concrete columns. The second database consists of the experimental compressive strain of 69 FRP confined square concrete columns. Furthermore, this study proposes a new potential approach to generate a user-friendly equation from a trained ANN model. The proposed equations estimate the compressive strength/strain with small error. As such, the equations could be easily used in engineering design instead of the invisible processes inside the ANN.
Near surface mounted fiber reinforced polymer reinforcement has become very popular method in strengthening of reinforced concrete structures. The major problem that accompanied the near surface mounted technique is the bond between fiber-reinforced polymers and concrete that corresponds to the stress transfer from concrete into fiber-reinforced polymers. This article presents the test results of pullout tests of near surface mounted glass fiber reinforced polymer bars to investigate the effect of different parameters on the bond performance of this strengthening technique. The test parameters include: adhesive type, groove size, bonded length, environmental condition, and concrete strength. A total of 40 near surface mounted–fiber-reinforced polymer bars, installed in C-shaped concrete blocks, were constructed and tested to failure. Five types of cement and epoxy-based adhesives were used. Two groove sizes and three bonded lengths were also investigated. Normal and high strength concrete were also used. In addition, the effect of two harsh environmental conditions on the performance of the strengthening system was also investigated. The results are presented in terms of pullout loads, free end slip, and mode of failure.
The current research demonstrates the application of artificial neural network (ANN) in predicting the fracture under mixed-mode I/II loadings. To this end, based on the analysis of the relative importance of different input factors, the crack parameters of modes I and II, stress intensity factors (KI and KII), T-stress, mode I fracture toughness (KIc), and ultimate tensile strength (σu) are selected and used as input data to the ANN model. Subsequently, a large number of empirical data are used, different ANN models are trained and built with the help of Levenberg-Marquardt (LM), Bayesian regularization (BR), and Broyden-Fletcher-Goldfarb-Shanno (BFGS) optimization algorithms. Finally, the trial-and-error procedure is used to determine the optimal number of hidden layers and neurons. The amounts of mean absolute percent error (MAPE) for the optimized models with LM, BR, and BFGS algorithms are equal to 8.4%, 5.1%, and 6.3%, respectively. All three models (i.e., ANN-LM, ANN-BR, and ANN-BFGS), estimate the new testing data successfully with approximately 91%, 95%, and 93% accuracy, respectively. This paper shows the effectiveness and the potential wide range application of the data-driven based fracture predictions in comparison with the traditional physics-based criteria.
This paper presents the experimental and analytical results of a large study on the bond performance of near-surface mounted (NSM) fiber-reinforced polymer (FRP) bars. The investigated parameters included the bar material (basalt-FRP (BFRP), glass-FRP (GFRP), carbon-FRP (CFRP), and stainless steel (SS) bars), the bar surface configuration (deformed and sand-coated bars), the filling adhesive (NSM-Gel, Sikadur-30, and Sika Grout-214), and the bonded length of the bar (6, 12, and 24 times its diameter). Sixty-six C-shape concrete specimens were tested under direct pullout loading configuration. The bond strength, the free-end and loaded-end slip, the strains in the NSM bar, and the modes of failure of the tested specimens are reported and discussed. The NSM-Gel adhesive outperformed other adhesives in developing the bond strength of the tested specimens regardless of their bar material. Both the deformed and sand-coated NSM-BFRP and GFRP bars showed almost similar bond strengths while the NSM-CFRP bars showed the highest strengths. The images obtained from the scanning electron microscope confirmed the obtained results in terms of the modes of failure and the bond failure mechanism. Analytically, the BPE model was calibrated using the experimental results to describe the bond stress-slip relationships of the FRP bars.
Organic adhesives (e.g. epoxy) have been widely utilized in near-surface mounted (NSM) carbon fibre reinforced polymer (CFRP) strengthening technique for reinforced concrete (RC) structural members. Their inevitable shortcomings, such as poor fire resistance, emission of toxic fumes, and incompatibility with concrete substrate, necessitate the advancement of eco-friendly inorganic adhesives. In this study, two types of inorganic adhesives, including ordinary Portland cement mortar (OPCM) and alkali-activated slag mortar (AASM), were adopted in the NSM CFRP bar strengthening method for RC beams. The inorganic adhesive type, bonding length, and additional anchorage device on CFRP bar were considered as parameters in the strengthening scheme. One control beam and four strengthened beams were fabricated and tested, followed by comparison of their failure modes, loading behaviour, and reinforcement strains. The results show that all the strengthened beams exhibit higher ultimate loads than the control beam. Comparing to the OPCM, the AASM is more effective in mitigating debonding of CFRP bar and enhancing the ultimate load of beams. Furthermore, it is essential to provide a sufficient bonding length for AASM bonded NSM CFRP bars to guarantee their strengthening effectiveness. The addition of anchorage device for CFRP bars can further enhance the synergistic action between CFRP bar and adhesive, leading to a further enhancement in flexural strength and CFRP bar strain. In addition, the flexural strengths of OPCM or AASM bonded NSM CFRP bar-strengthened beams can be accurately predicted by the equations provisioned in ACI 440.2R-2017.
The aim of this research is to investigate the efficiency of modified cementitious bonding material to sustain the loads on retrofitted concrete beams under high temperature exposure. The evaluation was based on the performance of CFRP reinforcement to sustain a considerable load over time and to maintain the corresponding service load at high temperature, in comparison with reinforced concrete beams. The experimental program reported in this research investigated the high temperature endurance for RC beams retrofitted using CFRP composites. The beams were subjected to their constant service load and then subjected to elevated temperature up to failure. The failure temperature was compared to that of the reinforced concrete beam. The same temperature at failure demonstrates the efficiency of the adhesive under high temperature exposure. The retrofitted beams with the cementitious adhesive at high temperature exhibited significant performance compared to that of epoxy adhesive.
Interest in FRP composite bars as reinforcement to concrete has increased over the years as it showed solutions to the drawbacks of steel such as its corrosion issues and vulnerability when employed in adverse environmental conditions. However, it is still not widely incorporated as a replacement to conventional steel primarily due to the complexity of its bond strength mechanism. This, therefore, imposes the need to establish a comprehensive relationship for the bond property of the FRP reinforced concrete. This paper developed a novel Artificial Neural Network (ANN) bond strength prediction model for FRP reinforced concrete using 184 hinged beam database from various existing experiments. From series of simulations performed, the model N 7-10-1 with ten nodes in the hidden layer appeared to be the best fit with the experimental results yielded the most favorable performance among other existing models. From the parametric analysis conducted, the compressive strength of the FRP reinforced concrete has proved to be the most dominant parameter in evaluating its bond behavior as determined by relative importance of 17.82%. Overall, the proposed ANN model has demonstrated the best prediction for FRP bond strength in comparison to previous studies and code equations.
Near surface mounted (NSM) strengthening technique, using fiber reinforced polymer (FRP) composite materials, is one of the most promising solutions for dealing with the deterioration problems of current reinforced concrete (RC) structures. However, intense research is still ongoing to keep improving this technique in the cases where its application shows limitations, e.g. in cases of fire hazard. The bonding in NSM systems is usually guaranteed by polymeric matrices like epoxy adhesives. However certain drawbacks result from the use of these adhesives, such as low resistance to elevated temperatures. This characteristic leads to premature failure under these circumstances, preventing the mobilization of the exceptional load carrying capacity of carbon fibers. Thus, the development of solutions involving alternative adhesives seems technically and economically relevant. Cement-based materials, which are incombustible and show low thermal diffusivity, show great potential as a valid alternative. Recent investigations suggest the possibility of transferring stresses between the NSM system and concrete substrate using cementitious matrices. However, the performance of these materials in different loading conditions, including high temperature exposure, need to be better explored.
The superb performance of carbon fiber reinforced polymer (CFRP) composites as near surface mounted (NSM) reinforcement in strengthening solutions for structures is already well recognized. Due to their deficiencies in fire conditions, cement-based adhesives as an alternative to polymeric matrices are recently suggested as a solution in these systems. However, the interface between the CFRP laminate and cement-based adhesives should have good stress transferring capacity. Thus, it is of great importance the research on improving this interface to increase the bonding capacity of CFRP/cement-based adhesive system. For that purpose, pull-out tests were conducted to examine the interfacial debonding process of two types of CFRP laminates: conventional smooth surface laminates and sand surface treated laminates. Digital image correlation (DIC) technique was used to verify the potentiality of the proposed sand treating approach. Therefore, the interlocking mechanism of sand treated laminates with the developed cement-based adhesive is assessed and the results are compared to those with non-treated smooth surface laminates. Furthermore, the bond-slip behaviour from pull-out tests is compared to obtained data through the DIC technique. The results verified the effectiveness of sand treatment approach applied to NSM CFRP reinforcements. Moreover, the DIC technique has revealed capable of providing qualitative and quantitative information in this regard.
Organic adhesives (e.g. epoxy resin) have been widely used in the near-surface-mounted (NSM) fibre reinforced polymer (FRP) strengthening system for reinforced concrete (RC) structures. However, the drawbacks of the organic adhesives in terms of high-temperature resistance and compatibility with concrete urge the development of inorganic adhesives (e.g. cement-based and alkali-activated materials). Moreover, a premature debonding of NSM FRPs from concrete substrate adversely affects the performance of strengthened RC structures, as well as the efficiency of NSM FRPs. Various anchorage strategies have been consequently proposed to improve the bond between the NSM FRPs and the concrete. This paper presents an overview on the bond behaviour between inorganic adhesive based NSM FRPs and concrete. Flexural strengthening of RC beams with inorganic adhesive based NSM FRPs, and the anchorage strategies for NSM FRPs are also summarized. Different from epoxy-based adhesives, the bond strength of inorganic adhesive based NSM FRPs decreases as the groove dimension increases. The RC beams strengthened by inorganic adhesive based NSM FRPs tend to fail due to an intermediate crack debonding of FRPs, which is probably caused by the low deformation capacity of inorganic adhesives. Anchorage strategies, such as adding external wrapping FRP fabrics or internal additional ribs, could significantly enhance the strengthening efficiency and the constructability of NSM FRPs. In the end, recommendations for future studies on the use of inorganic adhesive based NSM FRPs for structural strengthening are outlined.
Fiber reinforced polymer (FRP) composite has been used effectively for the rehabilitation of concrete and steel structures in the last decades due to its excellent properties compared with conventional reinforcing materials such as steel. Despite the major role of epoxy resins as a bonding material between fibre and substrate, the rapid deterioration of the mechanical properties at elevated temperature is the critical issue for the epoxy matrix. Therefore, substitution of epoxy adhesive with cementitious bonding agent will be beneficial in order to provide more resistant systems at elevated temperatures. Near-surface mounted (NSM) strengthening technique with cement adhesives has only been investigated for torsion. Flexural performance is a significant factor in the strengthening of different structures. This paper reports the experimental study on the behavior of small scale retrofitted beams using Near-Surface Mounted strengthening system with carbon fiber reinforced polymer (CFRP) textile and laminate and cement-base adhesives for flexure. The beams retrofitted with modified cement-based adhesive achieved 98% to 100% ultimate loads of that beams strengthened with epoxy adhesives. Numerical analyses is utilized to evaluate the experimental results, and comparable results were observed between the experimental results and finite element analysis.
This paper presents two artificial intelligence techniques (e.g., gene expression programming (GEP) and random forest (RF)) for predicting the bond strength of near-surface-mounted (NSM) fiber-reinforced polymer (FRP) strips or rods bonded to concrete. Experimental data from 145 direct pullout tests collected from the literature and five parameters, namely, the bond length, FRP axial rigidity, groove depth-to-width ratio, epoxy tensile strength, and concrete compressive strength, were used to develop the GEP and RF models. A comparison was conducted between the proposed GEP and RF models and two existing empirical models, namely, Seracino’s model and Zhang’s model, and six statistical indices were used to evaluate the performance of these four models. The results show that the proposed GEP and RF models had higher coefficient of determination (R²) values and lower root mean squared error (RMSE), mean absolute error (MAE), root relative squared error (RRSE), mean absolute percentage error (MAPE), and integral absolute error (IAE) values than the two existing empirical models. Finally, a detailed parametric study was conducted to investigate the influence of each input variable on the bond strength. The results showed that the bond strength increased with increasing bond length, FRP axial rigidity, groove depth-to-width ratio, and concrete compressive strength, while the epoxy tensile strength had little effect on the bond strength.
There is currently a high demand in the construction industry for high-strength and durable cement-based adhesives (CBAs) for use in a variety of applications such as ground anchors, ground improvement, concrete repair or as a substitute for epoxy resins to bond carbon fiber reinforced polymers (CFRPs) to concrete structures where increased fire performance is required. Limited research has been conducted on the use of nano-materials such as graphene oxide (GO), graphene nanoplatelets (GNPs) and nano silica (NS) to improve the mechanical properties and durability of CBA. This paper presents a new mix design comprising ordinary portland cement, GNP, GO and NS to improve the strength and durability of CBA. Furthermore, the paper presents bond-slip measurements between carbon fibre reinforced polymer (CFRP) and CBA embedded in concrete prisms. Single lap shear prisms are tested using different CBA mixes and structural epoxy to assess the bond for near surface-mounted (NSM) strengthened structures. The results show improvement in the compression and flexural strength of the CBA by up to 46% and 57% respectively by using NS and GO at low concentrations. The durability tests indicate that the modified CBA has 61% lower chloride penetration than standard CBA due to reduced porosity. The pull-out tests were conducted of FRP-to-concrete joints using the CBA materials and results of GNP-reinforced CBA show an improvement of 27% and 73% for CFRP laminate and textile, respectively.
Concrete is a strain-rate sensitive material and shows relatively low ductility and energy dissipation capacity under high strain rate loads (HSRL) such as blast and impact, representative of terrorist attacks and accidents. Experimental research in the literature has evidenced that introducing steel fibers, into the concrete mixtures can significantly improve the concrete behavior under HSRL. Besides the experimental research, development of design models is an important aspect to provide more confidence for engineers to use SFRC in structural elements when subjected to HSRL. The existing design codes (e.g. CEB-FIP Model Code 1990 and fib Model Code 2010) propose models for the prediction of the strengths of concrete under different HSRL, but they are only function of strain rate. In this regard, the current paper deals with the improvement of design models in the fib Model Code 2010 for the prediction of the compressive behavior of SFRC by considering the effects of the important parameters such as volume fraction, aspect ratio and tensile strength of steel fibers, and concrete compressive strength, besides the strain rate effect. The developed artificial neural network mathematical model is calibrated and its predictive performance is assessed using a database collected from the existing compressive impact tests results on SFRC specimens.
Numerous studies have been conducted on flexural strengthening using epoxy adhesive for both externally-bonded and Near-Surface Mounted carbon fibre reinforced polymer (NSM CFRP) composites. Substitution of epoxy with cementitious bonding material for flexural retrofitting of concrete structures has great potential to overcome the drawbacks and disadvantages associated with the use of epoxy adhesive. However, limited studies have been conducted for the use of cementitious adhesives for near-surface mounted strengthening system due to the special physical properties required for this technique. A modified cement-based adhesive has been developed for this purpose and comparable bond properties have been achieved. In this paper, an experimental program has been conducted to investigate the flexural behaviour of reinforced concrete beams retrofitted with NSM CFRP textile and laminate using modified cement-based adhesive. Numerical analysis using finite element has been used to evaluate the experimental results and parametric study for future design requirements. The findings showed significant and excellent increase in flexural strength, achieving almost same that results gained from epoxy adhesives and comparable finite element results.
Studies have shown that the near-surface mounted (NSM) FRP technique provides better bond performance than the externally bonded (EB) FRP method in strengthening applications. However, undesirable intermediate crack-induced debonding failures and end debonding failures are still frequently observed in NSM FRP reinforcements. In this study, the additional ribs (ARs) anchorage system and the wire mesh mortar protection layer (WML) anchorage system were proposed in an attempt to eliminate such premature debonding failures in NSM FRP strengthening system. The testing results of 20 direct pull-out specimens were used to verify the feasibility of the two proposed anchorage systems in improving the bond behaviour of NSM FRP bars. The individual use of the ARs anchorage system and WML anchorage system enhanced the bond strength of the NSM FRP bars up to 40.7% and 69.7%, respectively. The combined use of the two developed anchorage systems was found to be the most effective in increasing the bond performance of the NSM FRP bars, providing a maximum bond enhancement of 114.3%.
The influence of the adhesive type on bond behaviour between concrete and Carbon Fibre Reinforced Polymers (CFRP) laminate strips, in the context of Near Surface Mounted (NSM) strengthening technique, is considered as crucial for an efficient design. Thus, direct pullout tests were carried out to assess the influence of i) type of adhesive ii) CFRP cross-section and (iii) bond length on behaviour of NSM-CFRP system. Two types of stiff adhesives and one type of flexible adhesive were studied. For similar bond lengths, significantly higher maximum pullout force and bond stiffness were observed in the case of stiff adhesives, while noticeably higher slip at maximum force was achieved with the flexible adhesive. Analytical and numerical investigations were carried out in order to determine the local bond stress-slip relationships for both stiff and flexible adhesives. After demonstrating its good predictive performance, the analytical approach was used to design curves of the required anchorage lengths of NSM-CFRP system at ultimate limit state conditions. For the analytically calibrated mechanical parameters of NSM bond-slip law a two-dimensional numerical model of the direct pullout tests was worked out.
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.
The use of fibre reinforced polymer (FRP) in civil construction applications with near-surface mounted (NSM) method has gained considerable popularity worldwide and can produce confident strengthening and repairing systems for existing concrete structures. The response of concrete structural members rehabilitated using FRP with externally-bonded (EB) method under monotonic and fatigue loading has been well reported. On the other hand, the monotonic and fatigue response of NSM FRP-rehabilitated concrete structures is less known. This paper reviews current research on concrete members retrofitted by NSM FRP system and exposed to monotonic and fatigue loading. It provides an outline of FRP composites and the fatigue behaviour of concrete, steel and FRP materials. In addition, the review focuses on the bond characteristics between NSM FRP, adhesive and concrete substrate, and on the flexure behaviour of NSM FRP-strengthened and repaired reinforced concrete (RC) beams. Furthermore, the paper reviews the failure modes, factors affecting NSM FRP systems, guidelines and codes for the use of FRP in concrete structures, damage accumulation and residual strength, stress limits in steel reinforcement, and the limitations of using epoxy and polymer-based cementitious adhesives for the strengthening of concrete members.
Among the side effects caused by the blast, ground vibration (GV) is the most important one and can make serious damages to the surrounding structures. According to many scholars, the peak particle velocity (PPV) is one of the main indicators for determining the extent of blast‑induced GVs. Recently, following the rapid growth of soft computing approaches, researchers have tried to use these new techniques. This paper aims to explore three methods of soft computing including genetic programming (GP), response surface methodology (RSM), and multivariate adaptive regression splines (MARS) to predict the PPV values. For this purpose, a dataset of 200 published data including PPV, distance from the blasting face (D), and charge weight per delay (W) was used. The data have been recorded using blast seismograph, during the blast-induced earthquake triggered at 10 quarry sites in Ibadan and Abeokuta areas, Nigeria (https://doi.org/10.1016/j.dib.2018.04.103). The coefficient of determination for the MARS model as a most accurate model built in this research based on overall data results (R² = 0.81), compared with the most accurate empirical equations presented in the research literature, namely general predictor model (R² = 0.78), had a variation equal to 0.02. This variation for the root mean of squared error (RMSE), mean of absolute deviation (MAD), and mean of absolute percent error (MAPE) values were equal to 0.85, 0.25, and 0.38, respectively. In addition, the sensitivity analysis using cosine amplitude method (CAM) showed that the influence of each D and W parameters on PPV values based on developed models by this paper was more similar with the influence of these parameters based on the actual values, compared to empirical models. Finally, the parametric studies to investigate the behavior of various developed models were done to survey the changes to the values of the two variables D and W.
Bond failure due to a lack of anchorage capacity is often observed when using FRP reinforcement as a strengthening method in RC structures, thus restricting the efficiency in upgrading their structural performance. To address this issue, our research group has recently developed a simple and innovative additional ribs anchorage system for FRP bars, which has been verified to have the high efficiency of improving the anchorage performance of FRP bars reinforced in concrete. As a promising strengthening method in RC structures, the near-surface mounted (NSM) fiber-reinforced polymer (FRP) technique has become popular. However, bond failure also exists in the NSM FRP strengthening system. It is also important to initiate a study on the bond performance of NSM FRP bars which are mounted in concrete using the proposed additional ribs. In this study, a total of 36 direct pull-out specimens and 12 beam pull-out specimens were employed. In particular, the influences of additional ribs on the bond behavior of FRP bars mounted in concrete in terms of the failure modes, local bond stress distribution, and load-slip curves were discussed. Several important parameters expected to affect the bond performance were investigated, namely, the type and the bonded length of the FRP bars, groove size and section configuration, and adhesive species. The test results and corresponding analysis in this paper demonstrated the feasibility and significant efficiency of using the proposed additional ribs anchorage system to enhance the bond performance of NSM FRP bars in concrete.
The near surface mounted (NSM) fiber reinforced polymer (FRP) strengthening technique is a demonstrated, attractive and efficient alternative to externally bonded reinforcement (EBR) strengthening systems. NSM strengthening can be used to enhance the stiffness and the strength of deficient reinforced concrete members, with high utilisation of the FRP's mechanical properties (at ambient temperature) when epoxy is used as bonding agent. However, owing to epoxy adhesives’ sensitivity to elevated temperature exposure, recent research has focused on the use of cementitious adhesives, which are less sensitive to elevated temperature, in NSM FRP applications. This paper presents results from 22 bond pull-out tests at ambient temperature on concrete prisms with an embedded carbon FRP bar NSM strengthening system. Different bonding agents (i.e. epoxy resin or cementitious grout), positions of the bar in the groove (i.e. in the centre or at the top of the groove), bar surface treatments (smooth and ribbed) and bond length (300 and 400 mm) are investigated.
Concrete structures are normally strengthened using fibre reinforced polymer (FRP) with epoxy adhesives and polymer cementitious mortars. Epoxy adhesives have significant issues, such as the release of toxic fumes throughout curing, loss of strength and stiffness when exposed to hot temperatures, and low permeability and weakness to UV radiation. In the case of polymer cementitious adhesives, their properties are adversely affected by hydrothermal conditions. An innovative high-strength self-compacting non-polymer cementitious adhesive (IHSSC-CA) has recently been developed by the authors which uses graphene oxide and cementitious materials. This paper presents the bond response of carbon FRP strips bonded to concrete substrate using near-surface mounted (NSM) technique with IHSSC-CA, epoxy and polymer cement-based adhesives using direct pull-out tests. The behaviour of each adhesive is presented and compared and the local bond-slip relationship is calculated. Finally, an analytical model is proposed to predict the ultimate pull-out force (bond strength). The results of this study confirm the effectiveness of using IHSSC-CA to improve the bond strength, stiffness, CFRP strip utilisation, ductility and residual strength of NSM CFRP system. Moreover, the proposed analytical model can simulate experimental conditions reasonably well.
The bond between carbon fiber reinforced polymer (CFRP) composites and concrete surface is the key factor in strengthening systems of concrete structures. Near-Surface mounted carbon fiber reinforced polymer (NSM CFRP) has been used efficiently to increase the load-carrying capacity of concrete structures using epoxy adhesives. However, the toxic fumes, the flammability, and the degradation of mechanical properties at elevated temperatures are some of disadvantages associated with the use of epoxy adhesives. To overcome these disadvantages, modified cement-based adhesive has been used as a good alternative bonding agent for epoxy resin with NSM strengthening system. In this paper, full bond characteristics of near-surface mounted (NSM) CFRP textile and concrete using modified cement-based adhesive has been investigated and assessed using finite element modeling. Comparable bond properties of modified cement-based adhesive with NSM techniques have been achieved.
The repair and strengthening of existing concrete structures with composite material has become more common during the last decade. The near-surface-mounted (NSM) technique was used in this research, where fiber-reinforced polymer (FRP) laminates were bonded using an epoxy or cement-based adhesive in grooves cut into the concrete surface to study the behavior of the bond between the NSM carbon fiber-reinforced polymer (CFRP) strips and concrete after exposure to heating. Twenty-four heat-damaged concrete prisms were tested using a single-lap shear test. The results were compared with 24 reference specimens without heat damage. The results demonstrate that the residual bond strength after the repair of heat-damaged concrete with CFRP using epoxy adhesive is 94, 79, and 49% for temperature exposure of 1 h at 200, 400, and 600°C, respectively. The corresponding values for 2 h of exposure are 86, 75, and 41%, respectively. However, the residual bond strength after the repair of heat-damaged concrete with CFRP using cement-based adhesive is 91, 79, and 70% for temperature exposure of 1 h at 200, 400, and 600°C, respectively. The corresponding values for 2 h of exposure are 85, 69, and 66%, respectively.
Strengthening of concrete structures using fibre-reinforced polymer (FRP) composites has been utilized successfully for retrofitting a large number of structures around the world. This is required in case of increasing the applied loads, design and construction errors, updating of existing codes, or reduction of strength due to the deterioration of structure over time. The strengthening technique with FRP composites has been applied successfully using epoxy adhesives for concrete structures. The research showed the rapid degradation of properties of epoxy matrix with increasing temperature and this temperature is limited only below its glass transition temperature of the epoxy (Tg). Therefore, replacing the epoxy adhesive with cementitious bonding agent can enhance the performance of the strengthened structure at high temperatures. This test program was conducted to assess the efficiency of bonding strength between near-surface mounted (NSM) carbon fibre reinforced polymer (CFRP) textile and concrete using modified cement-based adhesive at elevated temperature. The conclusion showed a comparable efficiency of the modified-cement adhesive as a promising bonding material in FRP strengthening at high temperature climates.
Adhesive materials play a key role in any strengthening system for concrete structures. Traditional organic adhesives have a major limitation of poor fire resistance; hence, developing an alternative inorganic material is needed. This paper aims to present an innovative cement-based adhesive, which has been fabricated and verified using a number of tests. A nanomaterial represented by graphene oxide and a series of cementitious materials was used in this work. Fresh properties such as setting time and flowability were tested to ensure suitability for practical applications. Mechanical properties were tested in terms of compressive, tensile, and pull-off strengths. The experimental results proved the adequate performance of the innovative cement-based adhesive in terms of fresh properties and mechanical strength. The results show that the adhesive mixture has a pot life up to 120 min with a flow of 7.5%, and remarkably high 13.8 MPa tensile, 101 MPa compressive, and 1.2 MPa pull-off strengths.
This paper presents the results of an experimental study of near-surface mounted (NSM) carbon fibre reinforced polymer (CFRP) -repaired and strengthened reinforced concrete (RC) beams with innovative high-strength self-compacting cementitious adhesive (IHSSC-CA), with the particular objective of improving the ductility. Graphene oxide and cementitious materials were used to synthesise the IHSSC-CA. An analytical model is developed to predict the maximum tensile strain in NSM CFRP strips based on the experimental results. ACI 440.2R-08 guide was used to calculate the ultimate flexural capacity of the tested specimens based on the predicted tensile strain in NSM CFRP strips. The test results confirm the effectiveness of using IHSSC-CA to improve the flexural strength, CFRP strip utilisation, stiffness, residual strength and ductility of RC beams repaired and strengthened with NSM CFRP strips. Moreover, the statistical model proposed to predict the maximum tensile strain in CFRP strips has good agreement with the experimental results and can be used in the design of NSM CFRP- repaired and strengthened RC members with cement-based (IHSSC-CA) and epoxy adhesives. Furthermore, the ultimate flexural capacity of the repaired and strengthened RC beams using NSM CFRP strips with cement-based (IHSSC-CA) and epoxy adhesives can be predicted adequately using the ACI 440.2R-08.
Strengthening of reinforced concrete (RC) structures using the near-surface mounted (NSM) fibre reinforced polymer (FRP) method is becoming an attractive technique for upgrading the existing structural elements. The exposure of the NSM FRP strengthening system to high temperatures greatly affects the bond between FRP, adhesive material and the concrete substrate. Organic adhesives are widely used in the NSM strengthening technique. However, their low fire resistance is a major drawback. This study investigates the performance of NSM carbon FRP strengthening technique using single-lap shear tests (pull-out tests) with innovative high-strength self-compacting cementitious adhesive (IHSSC-CA) under high temperatures. Graphene oxide and cementitious materials were used to synthesise the IHSSC-CA. Single-lab shear tests were conducted on NSM strengthened specimens which were exposed to high temperatures of 800 °C. The results obtained from the pull-out tests signifies that the samples made with IHSSC-CA were less affected by high temperature, and maintain considerable residual bond strength. Theoretical equations are proposed to simulate the effect of high temperature on bond strength.
Near-surface mounted (NSM) techniques have been successfully utilized for the strengthening of concrete structures with CFRP composites. Although the adhesive most commonly used for strengthening of structures is epoxy, this adhesive is suitable only for low temperature environments due to the deterioration of its mechanical properties at elevated temperatures, in addition to its other disadvantages such as flammability and emission of toxic fumes. Substituting the epoxy with cementitious adhesive is therefore of great interest. However, few investigations on the strengthening of concrete structures using cementitious bonding materials have been reported, particularly for NSM CFRP strengthening systems. Since the bond between NSM FRP reinforcement and the concrete substrate plays a major role in the efficiency of NSM strengthening systems, in this paper, the bond properties of NSM CFRP laminate using modified cement-based adhesive are investigated. 15 concrete specimens were studied, and parameters including bond length, strain distribution, and bond-slip curve were considered. Finite element simulation was conducted to verify the experimental results using ATENA Software from Cervenka. The results showed excellent bond properties with reasonable correlation between experimental and finite element analysis results.
This paper presents the effectiveness of soft computing algorithms in analyzing the bond behavior of fiber reinforced polymer (FRP) systems inserted in the cover of concrete elements, commonly known as the near-surface mounted (NSM) technique. It focuses on the use of Data Mining (DM) algorithms as an alternative to the existing guidelines’ models to predict the bond strength of NSM FRP systems. To ease and spread the use of DM algorithms, a web-based tool is presented. This tool was developed to allow an easy use of the DM prediction models presented in this work, where the user simply provides the values of the input variables, the same as those used by the guidelines, in order to get the predictions. The results presented herein show that the DM based models are robust and more accurate than the guidelines’ models and can be considered as a relevant alternative to those analytical methods.
The strengthening system with near-surface mounted (NSM) has emerged in last decades as a promising technology to improve the flexural and shear strength of concrete structures. Large number of concrete structures have been strengthened worldwide by this technique using epoxy adhesives. However, the rapid deterioration of the mechanical properties of epoxy-based polymer matrix at elevated temperature, and the hazardous effects of toxic fumes during the application made the need of replacing this polymer with a new cementitious bonding agent to enhance the performance of concrete structures in high-temperature environments and reduce the environmental and health hazards. The application of cement-based adhesive has been studied for externally-bonded techniques using CFRP textile. Although, the good bond properties has been achieved, debonding was the critical failure between fibre and concrete substrate. To exploit the advantages of NSM technique, a new modification of cement-based adhesive has been achieved to be compatible with requirements of NSM application system. To assess the efficiency of this adhesive, an experimental investigation of pull-out testing using a single-lap shear test set-up was conducted to study the bond characteristics between CFRP textile and concrete. Four different mixes were used in this investigation. The test results show the efficiency of the NSM technique using modified cement adhesive and its superior bond properties compared to externally-bonded CFRP.
The efficiency of strengthening RC beams using the near-surface mounted (NSM) carbon fibre-reinforced polymer (CFRP) method depends on the bond between the NSM CFRP and the concrete substrate. This study presents the results of a study of the bond efficiency of NSM CFRP strips with two different CFRP strip surface types (smooth, rough) and dimensions (1.4 × 10 mm, 1.4 × 20 mm) with cement-based adhesive using single-lap shear tests. The study also contains development of a statistical model to predict the bond strength (pull-out force). It was found that the rough surface CFRP showed superior bond strength (pull-out force) when compared to the smooth surface CFRP. Moreover, there is a very good agreement between the experimental and predicted results.
A relatively recent method for the strengthening of concrete structures involves the embedding of fiber reinforced polymer (FRP) bars/strips into pre-cut, adhesive-filled grooves in the cover concrete. This paper presents a finite element (FE) study into the bond behavior of NSM CFRP strip-to-concrete bonded joints using a three-dimensional (3-D) meso-scale FE model developed by the authors. The effects of various parameters on the bond behavior of NSM CFRP strips are first clarified with the assistance of FE results, leading to the identification of the groove height-to-width ratio and the concrete strength as the two important parameters for the bond behavior. Based on the results of a FE parametric study on the effects of these two parameters, an analytical bond–slip model is proposed.
Research on the strengthening technique using near-surface mounted (NSM), fiber-reinforced polymer (FRP) composites has substantially advanced the knowledge on the bond-related issues such as bonded length, failure mechanisms, groove and bar dimensions, and local bond-slip behavior. In this study, several important design-related parameters that deserve more attention are carefully examined, including average bond strength, groove detailing, and bond-dependent coefficient. The accuracy of the existing analytical and semi-empirical models for average bond strength is assessed using the experimental data available in the literature. Optimum groove dimensions and a new empirical model for the bond-dependent coefficient are also proposed to complement the existing design guide for the NSM FRP technique used in concrete strengthening.
A relatively recent method for the strengthening of concrete structures involves the embedding of fiber reinforced polymer (FRP) bars/strips into pre-cut, adhesive-filled grooves in the cover concrete. This paper presents a finite element (FE) study into the bond behavior of NSM CFRP strip-to-concrete bonded joints using a three-dimensional (3-0) meso-scale FE model developed by the authors. The effects of various parameters on the bond behavior of NSM CFRP strips are first clarified with the assistance of FE results, leading to the identification of the groove height-to-width ratio and the concrete strength as the two important parameters for the bond behavior. Based on the results of a FE parametric study on the effects of these two parameters, an analytical bond-slip model is proposed.
This paper reports the results of an experimental program to investigate the bonding behavior of two different types of fiber-reinforced polymer (FRP) systems for strengthening RC members: externally bonded carbon (EBR) plates and bars or strips externally applied with the near-surface-mounted (NSM) technique. The overall experimental program consisted of 18 bond tests on concrete specimens strengthened with EBR carbon plates and 24 bond tests on concrete specimens strengthened with NSM systems (carbon, basalt, and glass bars, and carbon strips). Single shear tests (SST) were carried out on concrete prisms with low compressive strengths to investigate the bonding behavior of existing RC structures strengthened with different types of FRP systems. The performance of each reinforcement system is presented, discussed, and compared in terms of failure mode, debonding load, load-slip relationship, and strain distribution. The findings indicate that the NSM technique could represent a sound alternative to EBR systems because it allows debonding to be delayed, and hence FRP tensile strength to be better exploited. DOI: 10.1061/(ASCE)CC.1943-5614.0000204. (C) 2011 American Society of Civil Engineers.
The bond strength between GFRP bars and concrete is one of the most important aspects in reinforced concrete structures and is generally affected by several factors. In this study, experimental data of 159 notched, hinged, splice and inverted hinged beam specimens from an existing database in the literature were used to develop artificial neural network (ANN) and genetic programming (GP). The data used in modeling are arranged in a format of seven input parameters that cover the bar position, bar surface, bar diameter (d b), concrete compressive strength (f c), minimum cover to bar diameter ratio (C/d b), bar development length to bar diameter ratio (l/d b) and the ratio of the area of transverse reinforcement to the product of transverse reinforcement spacing, the number of developed bar and bar diameter (A tr/snd b). The MAE of testing data was found to be less than 1.06 and 0.76 MPa for the proposed ANN and GP models, respectively. Moreover, the study concluded that the proposed ANN and GP models predict the bond strength of GFRP bars in concrete better than the multi-linear regression model and existing building code equations. A parametric analysis was also conducted using the developed ANN and GP models to establish the trend of the main influencing variables on the bond capacity. Many of the assumptions made by the bond design methods are predicted by the developed models; however, few are inconsistent with the developed models’ predictions.