Fiber-reinforced Polymer Debonding Research Articles

Seismic flexural rehabilitation of RC coupling beams with FRP sheets: Evaluation of EBROG technique

Coupled shear walls (CSW) in reinforced concrete (RC) structures must be connected with stiff and strong coupling beams (CBs) to perform effectively during an earthquake. However, many RC buildings with CSW resisting systems have been designed and constructed with insufficient seismic requirements based on old codes and standards. During major earthquakes, these systems respond unsatisfactorily and may prematurely collapse. Strengthening of different RC members including CBs with fiber-reinforced polymer (FRP) sheets attached through externally-bonded reinforcement (EBR), though attractive, has always suffered from premature debonding of FRP from concrete substrate. The alternative externally-bonded reinforcement on grooves (EBROG) technique has been shown to overcome premature debonding when used instead of the EBR method. Therefore, this experimental study investigated the efficiency of rehabilitation with carbon FRP (CFRP) through the EBROG technique on the seismic behavior of flexural-deficient RC coupling beams. Furthermore, different patterns of flexural retrofit, as well as the effectiveness of FRP fans, have been studied to prevent undesirable debonding of CFRP strips. Four large-scale RC coupling beams have been identically constructed. The CB specimens were reinforced in such a way as to be flexural deficient, i.e., their loading capacities in shear were much more than that in bending. The CB specimens were then retrofitted with CFRP sheets in different patterns. The maximum peak loads of specimens CBF-1 L, CBF-1 L.F, and CBF-2 L.F increased by 37.0 %, 44.3 %, and 46.0 % over the control, respectively. Upon comparing the loading capacities of specimens CBF-1 L and CBF-1 L.F, it was found that FRP sheets installed through the EBROG method could delay premature debonding and utilize approximately 80 % of the CFRP's capacity. It was observed that using the EBROG technique and FRP fans at the termination point of FRP sheets, fully eliminated the debonding, and the full capacity of the CFRP sheets was utilized accompanied by their rupture, with improved ductility and increased load-carrying capacity of up to 46 %. It was also found that CFRP composites attached to concrete through the EBROG method significantly increased the strength, stiffness, ductility, and energy dissipation of CB specimens in terms of seismic rehabilitation.

Behavior of concrete voussoir flexible arch bridges reinforced with FRP composites

Flexible arch bridges, which can be pre-fabricated, transported and assembled on-site, have become increasing popular. This study presents an experimental investigation into the behavior of concrete voussoir flexible arch bridges reinforced with fiber-reinforced polymer (FRP) sheets by loading four arch specimens to failure at their left third-spans. The variables included the concrete voussoir strengths, bond quality between FRP and concrete voussoirs, and the FRP reinforcement locations. The results showed that two specimens with good bonding between FRP sheets and concrete voussoirs failed from sudden FRP rupture, while the other two specimens experienced FRP debonding and ultimately exhibited a four-hinge collapse mechanism. For the specimens with good FRP bonding, the typical load-deflection curve obtained at the loading point was an approximately monotonic increasing line, while load reductions and recoveries, as well as slope deteriorations were noted for the specimens experienced FRP debonding. It is also compared that bond quality could be more important than concrete voussoir strength, as the specimens with good bonding had higher ultimate load capacities. Applying one additional FRP sheet at intrados of arch could significantly enhance the initial ascending slope by 100%, but it cannot enhance the peak load as the FRP sheet at extrados of arch was more efficiently utilized in preventing more hinge formations at extrados joints.

Cracking and failure mode behavior of hybrid FRP strengthened RC column members under flexural loading

AbstractStrengthening of reinforced concrete (RC) members in bridges and buildings is often required to improve flexural performance. Strengthening using fiber‐reinforced polymer (FRP) composites has become popular as FRPs offer multiple advantages compared with conventional strengthening practices like steel jacketing and concrete enlargement. External bonding (EB) using carbon FRP (CFRP) laminates/fabric has become a common strengthening practice. The effectiveness of the EB strengthened system is reduced due to possible debonding of FRP from concrete substrate. The near‐surface mounting (NSM) of FRP laminates is efficient in reducing the debonding failure, but it is not effective under dominant compression loading. Hybrid FRP (HYB) strengthening combines the advantages of both the NSM and EB techniques. HYB strengthening can enhance the performance of RC members under all loading combinations. The main objective of this study is to understand the flexural crack opening behavior of control and hybrid FRP strengthened RC column members under pure bending. The digital image correlation (DIC) analysis shows that the crack width and its propagation are significantly reduced due to hybrid FRP strengthening. The crack widths measured using the DIC are compared with the analytical predictions based on Eurocode 2 and fib bulletin 90. HYB FRP strengthening increased the strength and post‐cracking stiffness of RC elements under flexure without compromising the ductility. In addition, the HYB strengthening distributed the damage under flexural loading, unlike localized damage observed in control RC elements.

Strengthening of two-way slabs with fiber-reinforced polymer composites: A new system using grooving technique and fans

The flexural capacity of a reinforced concrete (RC) two-way slab can be efficiently enhanced using fiber-reinforced polymer (FRP) composites if FRP debonding is postponed. On the other hand, with the increase in flexural capacity, the vulnerability of the slab to punching shear failure increases. In this context, the current study examines FRP strengthening of lightly reinforced slabs using the externally bonded reinforcement on grooves (EBROG) technique, which has already been proven successful in postponing FRP debonding. Four slab specimens are tested in two groups under concentric loading. The first group consists of two specimens reinforced with headed shear studs: one reference specimen and one specimen strengthened in flexure with FRP. The second group includes two specimens without internal punching shear reinforcement: one reference specimen and one specimen strengthened with a new simultaneous flexural and punching shear strengthening system, taking advantage of FRP fans. According to the results, flexural strengthening of the slab in Group 1 increased its ultimate load capacity by 77%, and the proposed strengthening system in Group 2 led to a 94% enhancement of the ultimate load capacity. A calculation method based on the yield-line theory is presented to predict the load capacity of the test specimens. According to this method, graphs simplifying the estimation of the flexural capacity of FRP-strengthened slabs are proposed.

A full-range fatigue life prediction model for RC beams strengthened with prestressed CFRP plates accounting for the impact of FRP debonding

To predict the fatigue life of a reinforced concrete (RC) flexural member strengthened with externally bonded prestressed Fiber Reinforced Polymer (FRP) laminate accurately, the interaction between components with different degrees of degradation must be carefully taken into account. Furthermore, as the debonding of FRP plates is a recurrent problem, this has to be incorporated into the prediction model. In this study, a nonlinear analysis method for the entire process of fatigue damage in RC beams strengthened with prestressed Carbon Fiber Reinforced Polymer (CFRP) plates was proposed. In this model, all the components of the strengthened beam had independent fatigue degradation properties and failure criteria, and debonding growth was also explicitly considered. The comparison between the analytical and experimental results were used for model validation and good agreement was found.

A method using information theory to select and rank existing FRP/concrete bond strength models

In order to achieve the desired resistance in reinforced concrete structures externally strengthened with fiber-reinforced polymer (FRP), an appropriate design for strengthening is necessary. The designer engineer must have a precise estimate of the bond strength between FRP and concrete before proposing a strengthening design. The bond strength is significantly lower than the FRP composite’s capacity due to the premature debonding of FRP from the concrete surface. Numerous studies have examined the bond strength of FRP/concrete and its influencing factors. As a result, a number of models have been proposed to evaluate the bond strength of FRP/concrete. The methodology, parameters, and data used in these models are distinct; consequently, the accuracy of evaluating these models differs. The selection of an accurate bond strength model presents a challenge for design engineers. Using information theory and the concept of relative entropy, this study compares the relative sufficiency of one model to another. For this comparison, 20 bond strength models and 410 experimental data are selected and compiled. None of these data is utilized in the development of the chosen models. According to the scores each model received from the information theory, the ranking results of the models have been established. The model with the highest score is ranked 1st and the model with the lowest score is ranked 20th. The superior model provides more information about the random parameter uncertainty. The results indicate that five of top seven models are provision models. The majority of the models that are among the 10 models with the highest score (rank 1 to 10) are based on fracture mechanics, whereas the 10 models with the lowest score (rank 11 to 20) are based on regression. Consideration of the effective bond length, along with the number and source of tests used to derive each model, has a significant impact on the ranking and accuracy of the models.

Experimental assessment and effective bond length for RC columns strengthened with aramid FRP sheets under cyclic loading

Fiber reinforced polymer (FRP) materials continue to demonstrate outstanding performance for strengthening and repairing reinforced concrete structures. However, one of the most significant problems that still has to be tackled is premature failure brought on by the FRP debonding. This research sought to determine how damaged RC columns could be strengthened with aramid (one type of FRP) composites and how that would affect their seismic responses. To further understand the individual effects of each strengthening material on seismic performance, hysteretic responses, such as strength, ductility, and energy dissipation, were examined and compared. Analytical modeling is validated based on the experimental results. Experimental results are validated with comparative study against other solid experimental results within the broader field. Effective bond length (EBL) is crucial for understanding the bonding between aramid fibers and concrete, however various models have different ways of calculating the value of EBL. Therefore, this research examined earlier experimental investigations of EBL and employing finite element (FE) methods established a novel relationship between the EBL and the aramid-to-concrete bond, based on experimental results.

Seismic performance of exterior RC beam-column junction strengthened with Stainless Steel Wire Mesh (SSWM)

Strengthening of concrete structures is necessary to enhance their performance in terms of serviceability and strength requirements as well as to increase the life span of the structure. In the past, different types of fiber-reinforced polymer (FRP) materials are widely employed for strengthening concrete structures. However, FRP-strengthened structural elements exhibit brittle failure as well as debonding of FRP strips or wraps. Due to debonding of FRP, its tensile strength is not fully utilized. To explore an alternative to FRP, Stainless Steel Wire Mesh (SSWM) is chosen in the present study. SSWM is a locally available material having lower tensile strength compared to FRP but having good bond characteristics with concrete and ductile behavior. The present study mainly focuses on experimental investigations to evaluate the behavior of beam-column junction strengthened using SSWM. In this experimental study, the performance of reinforced concrete beam-column junction is evaluated under cyclic loading. The reduced (one-third) scale test specimen representing an external beam-column junction of a 6-story regular moment resisting frame structure was tested under cyclic loading till its ultimate capacity. Subsequently, the damaged beam-column junction was strengthened using SSWM and its performance was again examined under cyclic loading. Minor cracks developed in the specimen were first filled with grout. Subsequently, epoxy-based adhesive SIKADUR 30 LP epoxy was used for bonding SSWM on the surface of the beam-column junction. The performance of test specimen was evaluated in terms of hysteretic response, displacement ductility, energy dissipation, degradation of stiffness, and failure modes of specimens. Results of experimental investigations show that SSWM effectively contributes to load resistance and enhances ultimate load carrying capacity and ductility of the beam-column junction without brittle or any significant debonding from concrete surface. Therefore, SSWM can be an economical alternative of FRP material for strengthening of concrete structures.

Shear Strengthening of RC Beams Using Fabric-Reinforced Cementitious Matrix, Carbon Plates, and 3D-Printed Strips

Existing reinforced concrete (RC) structures suffer from degradation in their structural capacity. These structures require strengthening and retrofitting to integrate sustainability and improve their serviceability and durability. RC members strengthened with fiber-reinforced polymer (FRP) composites usually suffer from FRP debonding; therefore, researchers proposed several types of sustainable materials to overcome the shortcomings of FRP composites. Limited experimental studies have been conducted for shear strengthening of RC beams using sustainable fabric-reinforced cementitious matrix (FRCM) composites; moreover, the application of 3D-printed strips in strengthening RC beams has never been established. The current research experimentally investigates the efficiency of FRCM composites, 3D-printed sheets (CD), and CFRP plates (CP) in strengthening RC beams that are weak in shear. Various strengthening configurations were adopted, including vertical, oblique, zigzag, and several-slanted layouts. Eight simply supported beams were prepared to find the most efficient shear-strengthening configuration and material for RC beams. Test results showed that FRCM and CP are both efficient for shear strengthening in terms of maximum load capacity, initial stiffness, and ductility. However, CD showed a limited effect on enhancing the performance of shear-strengthened beams. The best shear enhancement was found in the beam strengthened with vertical CP, with improvements in load-carrying capacity, stiffness, and ductility of 43%, 23%, and 23%, respectively. The vertical and oblique strengthening configurations were more efficient than the zigzag and several-slanted layouts. The ACI 440.2R-17 model yielded accurate predictions with an average (Vc, test/Vc, ACI 440) of 1.11.

Shear strengthening of RC beams with NSM FRP strips: Concept and behaviour of novel FRP anchors

The high efficiency of the near-surface mounted (NSM) fibre-reinforced polymer (FRP) technique for improving the shear capacities of reinforced concrete (RC) beams has been proved by numerous studies. However, premature debonding failures including interfacial debonding and concrete cover separation, which could largely limit the utilization of FRP strength, were frequently observed in RC beams strengthened in shear with NSM FRP. Proper anchoring measures of NSM FRPs can mitigate or even prevent such debonding failures and thus enhance the FRP strengthening efficiency, but relevant research on anchoring measures for NSM FRP shear-strengthened beams is still limited. Against the above background, a novel FRP anchor for NSM FRP shear-strengthened beams is proposed in this paper. A test program consisting of eight full-scale RC beams is carried out to investigate the effectiveness of the proposed FRP anchor system. It can be seen from the test results that the proposed FRP anchor is capable of mitigating or preventing the FRP debonding failures and thus significantly enhancing the load-carrying and deformation capacities of RC beams strengthened in shear with NSM FRP strips, and the configuration of the anchor can largely influence the performance of strengthened beams.

A nonlinear prediction model of the debonding process of an FRP-concrete interface under fatigue loading

Externally bonded Fiber Reinforced Polymer (FRP) strengthening has been proven to be an efficient and reliable method for structural strengthening of reinforced concrete (RC) members. However, the beneficial effects of this method can be diminished due to the debonding of the FRP laminates. The mechanism of FRP debonding still requires further research, especially for strengthened members under fatigue loading. To understand and predict the FRP fatigue debonding process better, eleven FRP-concrete joint specimens were tested under static or fatigue loading. Both the theoretical derivation and the experimental study indicated that the debonding growth rate of the FRP laminate depended not only on the mean level (S¯), but also the amplitude (ΔS) of the applied fatigue load. In addition, the debonded portion of the FRP laminate had a significant impact on the following debonding process due to the friction and mechanical interaction between the debonded FRP and the concrete surface. Therefore, a new nonlinear prediction model is proposed in this paper. The proposed model explicitly took into account the amplitude and the mean level of the fatigue loading, which enabled the effect of both to be modelled. Meanwhile, a correction term was also introduced into the model to account for the influence of the previously debonded FRP laminate. The predicted results of the debonding growth rate and the debonding length agreed well with the experimental results.

Influence of anchor design parameters on flexural performance of hybrid bonded-fiber reinforced polymer strengthened reinforced concrete beams

Strengthening concrete structures using adhesively bonded fiber-reinforced polymer (FRP) has been demonstrated as an effective method to improve the structural capacity. However, premature debonding of FRP material has limited the application of this technique. A number of anchorage methods have been proposed to enhance the bond performance, among which the hybrid bonded FRP (HB-FRP) has appeared to be more attractive as it can provide adhesive bond, frictional bond and dowel action. Previous studies have reported that the HB-FRP technique can considerably improve the bond strength and the flexural capacity of the strengthened beam. Yet, the influence of some key parameters on the flexural performance of the strengthened member is still not completely clear, and an accurate design method that can account for these parameters is not available for estimating the ultimate load capacity. In this paper, an experimental study was conducted on the HB-FRP strengthened beams to investigate the effects of anchoring pressure, anchor spacing, anchor distribution pattern, and FRP bonding mode. The test results indicate that the ultimate load-carrying capacity of the HB-FRP retrofitted member can be enhanced by increasing the anchorage pressure or decreasing the anchor spacing provided that the retrofitted beam fails by FRP debonding. The anchor distribution pattern has negligible influence on the flexural strength of the strengthened member, but uniform distribution of anchors along the bond length can yield higher flexural stiffness than the end anchoring approach. In addition, different FRP bonding mode can cause various strengthening mechanism in the HB-FRP system, leading to distinct flexural behavior of the retrofitted member. The dowel action of the anchor is suggested to be released by means of unbonding the FRP with the steel cap plate of the anchor since it may induce premature brittle anchorage failure. Based on the test results, a modified model is proposed for predicting the adhesive bond strength in HB-FRP system, in which a new parameter, referred to as spacing coefficient, is introduced to quantify the influence of the anchor spacing. Lastly, a theoretical model is developed in this study to design the ultimate moment capacity of the HB-FRP strengthened beam. A good correlation is achieved between the model predictions and the experimental results.

Vibration-based FRP debonding detection using a Q-learning evolutionary algorithm

The secured bonding between the externally bonded fiber reinforced polymer (FRP) and the host structure is critical to provide the composite action of the FRP strengthened structure. Conventional FRP debonding assessment is usually based on nondestructive testing methods, which have limited sensing coverage and thus cannot detect debonding far away from the sensors. In this study, the global vibration-based method is developed to identify the debonding condition of FRP strengthened structures for the first time. An FRP strengthened cantilever steel beam was tested in the laboratory. As debonding damage is non-invertible, a series of FRP debonding scenarios were specially designed by a stepwise bonding procedure in an inverse sequence. In each scenario, the first six natural frequencies and mode shapes were extracted from the modal testing and used for detecting the simulated debonding damage via the model updating technique. An l0.5 regularization is adopted to enforce sparse damage detection. A new Q-learning evolutionary algorithm is developed to solve the optimization problem by integrating the K-means clustering, Jaya, and the tree seeds algorithms. The experimental results show that the debonding condition of the FRP strengthened beam can be accurately located and quantified in all debonding scenarios. The present study provides a new FRP debonding detection approach.

An experimental study on fatigue debonding growth of RC beams strengthened with prestressed CFRP plates

Externally bonded Fiber Reinforced Polymer (FRP) laminates are increasingly used to strengthen Reinforced Concrete (RC) members. However, FRP debonding remains a major drawback of this strengthening method. To better understand the mechanisms of FRP debonding, six RC beams strengthened with prestressed or non-prestressed Carbon Fiber Reinforced Polymer (CFRP) plates were subjected to static and fatigue loading. CFRP plate debonding was observed in both cases. However, the mechanism of debonding differed: under cyclical fatigue loading, debonding was initiated under both loading points simultaneously and propagated synchronously towards the nearest support whereas in static tests debonding began under a single loading point and progressed suddenly towards its adjacent support. The results also showed that stress redistribution induced coupling between accumulated fatigue damage in the steel reinforcement and fatigue debonding of the CFRP plate, accelerating the fatigue failure of the specimens.

Effect of CFRP layers on the behavior of shear-strengthened RC-beams: U-wrapping versus full wrapping

The practice of externally bonded (EB) carbon fiber-reinforced polymer (CFRP) fabrics for structural strengthening increasingly shows evidence of effectiveness for the rehabilitation of existing structures. However, debonding of fiber-reinforced polymer (FRP), particularly in shear, is still a major premature failure mode when using FRP sheets to strengthen concrete structures. This paper presents the results of an experimental investigation of the performance of full-scale reinforced concrete (RC) beams strengthened in shear using EB-FRP U-wrapping (UW) and full wrapping (FW) with different fabric thicknesses. Full wrapping is used mostly on columns with all sides accessible and does not generally apply to beams. The main objectives of this study are to optimize FRP capacity and to evaluate the effect of confinement and the effectiveness of FRP using different strengthening configurations for RC beams, such as FW and UW. The results obtained in this study reveal that fully wrapped beams exhibit enhanced CFRP contribution to shear resistance compared to U-wrapped beams. In addition, the FW configuration eliminated the CFRP debonding failure mode and resulted in an increase in concrete strain.

Experimental behavior of fire-exposed reinforced concrete slabs without and with FRP retrofitting

In this study, experiments were conducted on nine reinforced concrete slabs which were divided into three groups, namely S0, S45, and S75. Group S0 included one slab which was not exposed to fire to use as the control specimen. Each of groups S45 and S75 included four slabs which were subjected to 45 min and 75 min of fire, respectively. After exposure to fire, two fire-exposed slabs of each group were retrofitted using fiber reinforced polymer (FRP). These slabs were monotonically or cyclically loaded to failure. The test results showed that the control and fire-exposed slabs exhibited flexural failure while the FRP-retrofitted fire-exposed slabs failed in the form of FRP debonding. Fire slightly decreased the ductility but it was still much larger than 4, classifying the specimens to be highly ductile. Fire marginally reduced the yield and ultimate strengths while it significantly reduced the stiffness by about 30%. Retrofits with 1/2 and 1 FRP layer for fire-exposed slabs increased the yield strength by 30% and 45% and increased the ultimate strength by 46% and 65%, respectively. The effect of cyclic loading caused negligible strength degradation, but it caused considerable stiffness degradation of 20% and 35% for the 1/2 and 1 FRP retrofitted fire-exposed slabs, respectively. Finally, a theoretical model was developed to compute the capacity of FRP retrofitted fire-exposed slabs. The model can be useful for engineers to estimate the capacity of FRP retrofitted fire-exposed slabs in practice.

Damage localization in reinforced concrete beams strengthened with FRP sheets using modal strain energy method

Civil structures are affected by many different factors from the environment, loads, aging of materials, … These factors are uncertain variables and affect the health of the structures. Therefore, structural health monitoring (SHM) is very essential to detect damages early for necessary maintenance. In this paper, damaged locations in reinforced concrete beams strengthened with FRP (Fiber Reinforced Polymer) sheets are identified by using the modal strain energy method. First, a reinforced concrete beam strengthened with FRP sheets is simulated by ANSYS APDL software in order to analyze the beam’s behavior and get vibration responses. The reliability of the simulation is verified by comparing the load - displacement relationship between numerical and experimental results. Next, the modal strain energy method is employed to determine the damaged locations (crack and debonding) in the beam. In which, a set of indicators to evaluate the accuracy of damage localization results is proposed. The feasibility of the method is demonstrated through two problems. For problem 1, five different damage scenarios including concrete damage and FRP debonding are examined to evaluate the modal strain energy method’s feasibility for damage detection in the target beam. For problem 2, damages occurring in the beams are analyzed and determined according to each load level corresponding to the real working of the target beam. The results show that the modal strain energy method has high accuracy in detecting and locating damages in reinforced concrete beams strengthened with FRP sheets.

Experimental investigation on Non-Destructive behaviour of repaired FRP concrete beams

Concrete is widely used for the construction of infrastructures namely bridges, buildings, power station structures, etc. Non-Destructive Testing (NDT) is widely adopted for testing the hardened concrete as a quality control measure. The reinforced concrete structure undergoes damages due to various reasons. Different repair techniques are adopted in restoring the structure to the original conditions. The fiber warping is commonly used technique for the repairing of concrete structure. For the evaluation of the efficiency of repair, advanced NDT techniques such as GPR (Ground Penetrating Radar) and Impact Echo (IE) are being adopted. This paper highlights the efficiency of the radar and Impact Echo techniques in the evaluation of de-bonding of FRP (Fiber Reinforced Polymer) in concrete specimen as well as determination of thickness from one side. In addition, the UPV (Ultrasonic Pulse Velocity) test was also adopted for comparison.

Evaluation on the effect of anchor embedment depth on the flexural capacity of concrete prisms

Using fiber-reinforced polymer (FRP) composites to strengthen and retrofit existing reinforced concrete (RC) structures has gained wide acceptance owing to the superior properties of FRPs such as high strength-to-weight ratios and corrosion resistance. Externally bonded FRP (EB-FRP) laminates can be applied to enhance the flexural and shear capacities of RC beams and improve the ductility of RC columns. However, the premature debonding of FRP laminates from the concrete substrate is a major concern and usually results in a brittle member failure. To tackle this issue, many studies proved that adequate anchorage systems could improve the FRP-to-concrete bond, and therefore results in delaying the FRP debonding and further developing the tensile strength of the FRP laminates. One of the most commonly used anchorage systems is FRP spike anchors. Compared to other anchorage systems (for e.g., metallic anchors), FRP anchors have high compatibility with the FRP laminates. In addition, FRP spike anchors are noncorrosive and have high tensile strengths. Although several studies addressed FRP spike anchors in flexural and shear strengthening of RC beams and slabs, the literature still lacks comprehensive information on the effect of all anchor parameters on the performance of the strengthening system. As a result, the aim of this study is to investigate the effect of anchor embedment depth on the behavior, strength, and effectiveness on the FRP spike anchorage system. Six concrete prisms were cast and strengthened with carbon FRP (CFRP) laminates and spike anchors then tested in flexure under four-point bending tests. Test results showed that installing anchored CFRP laminates increased the capacity of the plain concrete prism by 80-120%. Anchoring the laminates at different embedment depths of 50mm, 75mm, 100mm, and 125mm enhanced the capacity of the unanchored prisms by 11%, 13%, 15%, and 33% respectively. Thus, it was concluded that increasing the anchor embedment depths increased the ultimate load carrying capacity of the prisms.

Strengthening effect of natural fiber reinforced polymer composites (NFRP) on concrete

To date a large number of studies have been carried out on the use of different materials such as concrete, steel, fiber reinforced polymer (FRP) composites, to enhance the strength and ductility of the reinforced concrete (RC) members. Among these materials, concrete and steel are heavy weight, whereas FRP are expensive along with skin problems during handling and installations. In this paper, the efficiency of natural sisal fiber reinforced polymer composites is evaluated on strength and ductility of RC members. The salient features of natural sisal FRPs are low density, higher toughness, acceptable strength properties, reduced irritation to the skin and respiratory system, biodegradable and sustainable since they are natural products. The objectives of this research were twofold. The first was to evaluate the compressive and flexural behavior of small-sized concrete specimens (cylinders and beams) strengthened with natural sisal FRPs, considering different resin matrices, FRP thickness and concrete strengths. Whereas, the second objective was to investigate the strengthening effect of natural sisal FRP strengthening on the flexural response of RC beams. Test results showed that externally bonded natural sisal FRPs are significantly effective to increase the strength and ductility of the concrete members. In general, the experimental results demonstrated a higher increase in the load carrying capacity when the thickness of the sisal FRPs was increased. However, RC beams strengthened by natural sisal FRP, may fail by de-bonding of FRP from the concrete surface. To avoid this failure, a new anchoring system has been proposed and tested to evaluate their performance in preventing the de-bonding of FRP from concrete surface. Based on experimental results, proposed anchoring system was found to be effective to prevent the delamination of natural sisal FRPs from concrete surface.