Fatigue behavior of RC beams strengthened with prestressed NSM CFRP rods

The fatigue performance of Reinforced Concrete (RC) beams strengthened using NSM CFRP rods was examined in this study. The testing matrix consisted of one un-strengthened beam and four beams strengthened using NSM CFRP rods prestressed to effective strain values of 0, 3260, 6899, and 9177 μ representing 0%, 20.4%, 43.1%, and 57.4% of the CFRP rod ultimate tensile strain, 1.6%. All beams were tested in four-point bending under fatigue conditions representing in-service loading for 3 million cycles at a frequency of 2 Hz. Upper and lower load limits were chosen to induce a stress range of 125 MPa in the tension steel during the first cycle. The fatigue results were compared with experimental test results of identical beams strengthened using prestressed NSM CFRP strips tested under identical fatigue conditions found elsewhere in literature. Test results showed that all strengthened beams experienced deflection increase lower than that of the un-strengthened beam which indicates the efficiency of the strengthening process in reducing the damage accumulation. Also, the percentage deflection increase as well as the stiffness degradation after 3 million cycles are almost the same for all the strengthened beams which indicates that damage accumulation is independent from the prestress level. The groove dimensionality, rather than the CFRP geometry, has a detrimental effect on the bond behavior.     

碳纤维布加固混凝土梁弯曲性能的试验研究

通过对4根矩形截面钢筋混凝土梁的静力加载试验,研究了碳纤维布对抗弯加固梁的破坏形态、刚度变化、极限承载力、变形能力和裂缝开展情况的影响,分析了碳纤维布粘贴层数和加固梁是否欲裂对碳纤维加固效果的影响.结果表明:碳纤维布可明显增强加固梁的极限承载力,改变加固梁的破坏形态,改善延性,延缓裂缝的发展,提高加固梁的整体刚度;同时碳纤维布粘贴层数越多,加固梁的极限承载力越大,但其提高程度并不与粘贴层数成正比;加固预裂梁后其极限承载力明显提高,但其延性降低.最后,针对现有的规范及理论研究结果,提出了抗弯加固梁的极限承载力计算公式,理论计算结果与试验值符合较好,满足工程实际要求.     

Hysteretic behavior of RC shear walls strengthened with CFRP strips

The purpose of this study was to investigate the hysteretic behavior of shear deficient reinforced concrete (RC) walls that were strengthened with carbon fiber reinforced polymer (CFRP) strips. Totally, ½ scale five specimens with 1.5 aspect ratio walls were constructed. One of them was tested without any retrofitting as a reference specimen and four of them were retrofitted specimens with CFRP strips. All of the specimens were tested under cyclic lateral loading. CFRP strips with different configurations were tested like X-shaped, horizontal and parallel strips or combinations of them. All of the CFRP configurations were symmetrically bonded to both sides of the shear wall and were anchoraged to the wall. The research focuses on the effect of using CFRP strips for enhancing strength and increasing ductility of the non-seismic detailed shear walls. Test results shows that all of the CFRP strip configurations significantly improves the lateral strength, energy dissipation and deformation capacity of the shear deficient RC walls. The specimen that was strengthened with X-shaped CFRP strips was failed with premature shear failure. The specimen that was strengthened with horizontal strips was showed flexural hysteretic behavior and plastic hinge was developed at the wall base. CFRP strips were controlled shear crack propagation and resulted in improvement of displacement capacity.     

Experimental study on seismic behavior of RC frames strengthened with CFRP sheets

The purpose of this study was to investigate experimentally the seismic performance and hysteretic energy capacity of strengthened reinforced concrete (RC) frames using carbon fiber reinforced polymer (CFRP) sheets under low-cyclic lateral loading. Two test specimens were constructed and tested under low-cyclic lateral loading. Two 1/3 scaled one-bay and one-storey RC frames specimens were constructed to simulate a two-storey industrial workshop. One specimen was reinforced by CFRP at the ends of beams, columns and at the joints; the other specimen was not reinforced and was used for comparison. This experimental study mainly investigated the effects of CFRP sheets on specimen seismic behavior. The information about the crack development, the damage characteristics, the hysteretic curves of the steel bar and CFRP sheets and the skeleton curves of frame were presented. In addition, the maximum crack width and the ultimate bearing capacity were measured. Test results indicate that the CFRP sheets reinforced frame shows a good hysteretic energy capacity and a higher ductility, which indicates that the CFRP sheets reinforced frame has a better seismic behavior. The results provide an important insight of the role of CFRP sheets in improving the earthquake resistance of frame buildings.     

Experiments and analytical comparison of RC beams strengthened with CFRP composites

This paper deals with strengthening, upgrading, and rehabilitation of existing reinforced concrete structures using externally bonded composite materials. Five strengthened, retrofitted, or rehabilitated reinforced concrete beams are experimentally and analytically investigated. Emphasis in placed on the stress concentration that arises near the edge of the fiber reinforced plastic strip, the failure modes triggered by these edge effects, and the means for the prevention of such modes of failure. Three beams are tested with various edge configurations that include wrapping the edge region with vertical composite straps and special forms of the adhesive layer at its edge. The last two beams are preloaded up to failure before strengthening and the ability to rehabilitate members that endured progressive or even total damage is examined. The results reveal a significant improvement in the serviceability and strength of the tested beams and demonstrate that the method is suitable for the rehabilitation of severely damaged structural members. They also reveal the efficiency of the various edge designs and their ability to control the characteristic brittle failure modes. The analytical results are obtained through the Closed-Form High-Order model and are in good agreement with the experiment ones. The analytical and experimental results are also used for a preliminary quantitative evaluation of a fracture mechanics based failure criterion for the strengthened beam.     

Assessment of CFRP strengthened RC beams through dynamic tests

The use of Fiber Reinforced Polymers (FRPs) for strengthening damaged RC beams has become common practice over recent years. Two methods adopted for repairing or strengthening such beams are FRP plates/sheets glued onto their concrete surface and FRP rods or strips inserted into grooves.This paper investigates experimental vibration monitoring of strengthening according to the two aforementioned methods through dynamic tests on six RC beam models strengthened using carbon FRP. Three beams were strengthened applying CFRP sheets on the tensile cracked surface after loading and three beams were strengthened by near surface mounted (NSM) CFRP rods. The experimental results include both the static tests to create damage and the dynamic tests of strengthened, measuring natural vibration modes and frequency values for free end beams.Comparison between experimental dynamic response and static behavior established that vibration monitoring is a convenient, non-destructive method for assessing strengthened beams under service loads. Further studies and tests must be developed in order to solve the issues that emerged following the analysis of the experimental data obtained.     

RC beams strengthened with NSM CFRP rods and modeling of peeling-off failure

The aim of the experimental programme developed in this work was to investigate the possibility of using Carbon Fibre-Reinforced Polymer (CFRP) rods to strengthen concrete structural members with the Near Surface Mounted reinforcement (NSM) technique. The global behaviour of reinforced cantilever concrete beams strengthened by the NSM technique and subjected to flexure is investigated. The specific problem of cantilever beams (strengthening outward pressure) was studied. The global behaviour of the cantilever concrete beams was compared with that of beams subjected to flexure with four points load test. A carbon–epoxy pultruded FRP (CFRP) rod of 6 mm in diameter was used. The study was carried out up to the failure load, and focused on the modifications in mechanical behaviour, cracking and failure mode of the beams. An analytical and Finite Element models to predict the peeling-off failure mode were compared.     

Experimental study of moment redistribution in RC frames strengthened with CFRP sheets

The present paper describes an experimental investigation of moment redistribution of continuous beams in 6 two-span RC frames strengthened with CFRP laminates. Design variables were the number of CFRP layers, and the configuration of the laminates. To prevent debonding of the CFRP laminates at the end region and at the beam-column connection, U-shaped CFRP anchorages were provided for all of the frame specimens. Furthermore, mechanical anchorages of steel plate strips and bolts were added to the laminates in one frame of these specimens. Test results showed that a maximum moment redistribution of 56% occurred in the strengthened frames. Furthermore, the load carrying capacities of the strengthened frames increased from 20% to 38%, while the flexural capacities had an increase of 9% to 20% and 35% to 55% at the negative and positive moment regions, respectively.     

Failure of RC beams strengthened in bending with unconventionally arranged CFRP laminates

This paper describes the experimental tests made on RC beams retrofitted by unconventionally arranged CFRP strips and on a reference, not retrofitted one. Diagonal CFRP strips were applied on the lateral faces of the specimens and connected to the longitudinal ones in order to improve the anchorage length of the latters. The experimental outcomes prove that this CFRP strips distribution can improve the load carrying capacity of the retrofitted beams, provided that the diagonal strips are long enough and that the longitudinal reinforcement is arranged along the whole beam. Comparison with the predictions based on CNR-DT 200 and ACI 440.2R-02 guidelines is finally displayed.     

Improved model for plate-end shear of CFRP strengthened RC beams

In case of RC members strengthened by means of externally bonded reinforcement, a premature failure can be detected in addition to the conventional modes of failure observed in RC unstrengthened beams. The premature failure occurs mainly due to both shear and normal stresses induced in either the external reinforcement–concrete interface or at the level of steel reinforcement. This research is part of a complete programme aiming to set up design formulae to predict the strength of CFRP strengthened beams, particularly when premature failure through laminates-end shear or concrete cover delamination occurs. Series of RC beams were strengthened with carbon-fiber-reinforced plastic (CFRP) laminates and tested to estimate the extent of the applicability of the formulae proposed by the authors, as well as to study the influence of the layout of the external reinforcement in terms of unsheeted length (the distance between CFRP laminates-end and the nearer support) and cross-sectional area, on the behaviour of strengthened beams. The predictions using the proposed formulae are compared with the obtained experimental results, as well as with the calculated design limit states. The interfacial shear stress and the maximum deflection corresponding to the predicted values at maximum and service loads are also studied.     

Preloaded RC columns strengthened with high-strength concrete jackets under uniaxial compression

Although use of high-strength reinforced concrete (RC) jackets has become common practice worldwide, there are still two unresolved issues regarding the contribution of the original concrete and the effects of existing loads. Twelve RC-jacketed columns were tested with and without preloading under uniaxial compression. Tests showed the entire core to contribute to the capacity of the jacketed column, as long as adequate confinement is provided. Also, preloading does not adversely affect the capacity of the jacketed column, while it may increase its deformability, especially in square sections. Transverse reinforcement in the jacket directly improves ductility of the strengthened column, especially in circular sections.     

Degradation of bond between FRP and RC beams

Beams and slabs externally reinforced with FRP are often in contact with moisture and temperature cycles that reduce the expected durability of the system. Bond degradation is a frequent cause of premature failure of structural elements and environmental conditions are known to relate to such failures. The study shows the effects of cycles of salt fog, temperature and moisture as well as immersion in salt water on the bending response of beams externally reinforced with GFRP or CFRP, especially on bond between FRP reinforcement and concrete. Temperature cycles (−10 °C; 10 °C) and moisture cycles were associated with failure in the concrete substrate, while salt fog cycles originated failure at the interface concrete–adhesive. Immersion in salt water and salt fog caused considerable degradation of bond between the GFRP strips and concrete. However, immersion did not lower the load carrying capacity of beams, unlike temperature cycles (−10 °C; 10 °C) that caused considerable loss. No significant differences were detected on the behavior of the systems strengthened with GFRP and CFRP, perhaps because the design of the tests impeded failure of the fibres.     

Fatigue behavior of RC T-beams strengthened in shear with EB CFRP L-shaped laminates

The use of externally bonded carbon fiber-reinforced polymer (EB-CFRP) to strengthen deficient reinforced concrete (RC) beams has gained in popularity and has become a viable and cost-effective method. Fatigue behavior of RC beams strengthened with FRP is a complex issue due to the multiple variables that affect it (applied load range, frequency, number of cycles). Very few research studies have been conducted in shear under cyclic loading. The use of prefabricated CFRP L-shaped laminates (plates) for strengthening RC beams under static loading has proven to be technically feasible and very efficient. This study aimed to examine the fatigue performance of RC T-beams strengthened in shear for increased service load using prefabricated CFRP L-shaped laminates. The investigation involved six laboratory tests performed on full-size 4520 mm-long T-beams. The specimens were subjected to fatigue loading up to six million load cycles at a rate of 3 Hz. Two categories of specimens (unstrengthened and strengthened) and three different transverse-steel reinforcement ratios (Series S0, S1, and S3) were considered. Test results were compared with the upper fatigue limits specified by codes and standards. The specimens that did not fail in fatigue were then subjected to static loading up to failure. The test results confirmed the feasibility of using CFRP L-shaped laminates to extend the service life of RC T-beams subjected to fatigue loading. The overall response was characterized by an accelerated rate of damage accumulation during the early cycles, followed by a stable phase in which the rate slowed significantly. In addition, the strains in the stirrups decreased after the specimens were strengthened with CFRP, despite the higher applied fatigue loading. Moreover, the addition of L-shaped laminates enhanced the shear capacity of the specimens and changed the failure mode from brittle to ductile under static loading. Finally, the presence of transverse steel in strengthened beams resulted in a substantially reduced gain in shear resistance due to CFRP, confirming the existence of an interaction between the transverse steel and the CFRP.     

Analytical approach for the flexural analysis of RC beams strengthened with prestressed CFRP

The objective of this paper is to propose a simplified analytical approach to predict the flexural behavior of simply supported reinforced-concrete (RC) beams flexurally strengthened with prestressed carbon fiber reinforced polymer (CFRP) reinforcements using either externally bonded reinforcing (EBR) or near surface mounted (NSM) techniques. This design methodology also considers the ultimate flexural capacity of NSM CFRP strengthened beams when concrete cover delamination is the governing failure mode. A moment–curvature (M–χ) relationship formed by three linear branches corresponding to the precracking, postcracking, and postyielding stages is established by considering the four critical M–χ points that characterize the flexural behavior of CFRP strengthened beams. Two additional M–χ points, namely, concrete decompression and steel decompression, are also defined to assess the initial effects of the prestress force applied by the FRP reinforcement. The mid-span deflection of the beams is predicted based on the curvature approach, assuming a linear curvature variation between the critical points along the beam length. The good predictive performance of the analytical model is appraised by simulating the force–deflection response registered in experimental programs composed of RC beams strengthened with prestressed NSM CFRP reinforcements.     

CFRP加固钢筋混凝土柱的多维拟静力试验研究

为了研究多维地震作用下钢筋混凝土柱的 CFRP(碳纤维增强塑料 )加固效果 ,对六个尺寸相同而加固方式不同的矩形截面钢筋混凝土柱进行了拟静力试验研究。荷载采用轴压、双向弯曲、扭转的分别组合或共同作用不同荷载模式 ,以模拟多维地震动。研究结果表明 ,尽管构件的力学响应与加固方式、荷载作用方式有密切的关系 ,但CFRP加固能有效提高试件的延性和承载能力 ,对于多维地震动下的结构是一种理想的加固方法。     

Mechanical behaviour of corroded RC beams strengthened by NSM CFRP rods

Corrosion of steel in reinforced concrete leads to several major defects. Firstly, a reduction in the cross-sectional area of the reinforcement and in its ductility results in premature bar failure. Secondly, the expansion of the corrosion products causes concrete cracking and steel–concrete bond deterioration and also affects the bending stiffness of the reinforced concrete members, causing a reduction in the overall load-bearing capacity of the reinforced concrete beams. This paper investigates the validity of a repair technique using Near Surface Mounted (NSM) carbon-fibre-reinforced polymer (CFRP) rods to restore the mechanical performance of corrosion-damaged RC beams. In the NSM technique, the CFRP rods are placed inside pre-cut grooves and are bonded to the concrete with epoxy adhesive.Experimental results were obtained on two beams: a corroded beam that had been exposed to natural corrosion for 25 years and a control beam, (both are 3 m long) repaired in bending only. Each beam was repaired with one 6-mm-diameter NSM CFRP rod. The beams were tested in a three-point bending test up to failure. Overall stiffness and crack maps were studied before and after the repair. Ultimate capacity, ductility and failure mode were also reviewed. Finally some comparisons were made between repaired and non-repaired beams in order to assess the effectiveness of the NSM technique. The experimental results showed that the NSM technique improved the overall characteristics (ultimate load capacity and stiffness) of the control and corroded beams and allowed sufficient ductility to be restored to the repaired corroded elements, thus restoring the safety margin, despite the non-classical mode of failure that occurred in the corroded beam, with the separation of the concrete cover due to corrosion products.     

预应力CFRP布加固损伤RC梁的动力特性研究

从理论上推导了预张力与损伤RC梁固有频率之间的定量关系。同时进行了预应力CFRP布加固RC梁的动力特性试验,测定不同预张力条件下,完好梁与损伤梁的一阶频率值。而后利用ANSYS软件建立钢筋混凝土完好梁的有限元模型,根据损伤梁的动力测试结果,运用优化分析的方法得到损伤梁混凝土刚度折减系数;利用一阶频率的试验值对公式进行线性拟合,得到频率影响因素及损伤梁频率计算公式。最后将加固损伤梁一阶频率的理论值与试验值进行比较,发现在低预应力作用下,理论计算结果基本能反映出试验值随预张力变化的趋势,试验值与理论值吻合较好。     

预应力CFRP布加固RC梁动力特性的试验研究

进行了预应力CFRP布加固完好RC梁的动力特性试验,结果表明:预应力加固梁的固有频率随着预张力的增加而增加。同时,考虑了相关的影响因素,从理论上推导了预张力与加固梁固有频率之间的定量关系,再利用一阶频率的试验值对公式进行拟合,得到频率影响系数。而后将二阶频率的试验值与理论值进行比较,发现理论值基本能反映频率随预张力的变化趋势,两者最大误差为0.173(在预张力为0kN时),试验值与理论值吻合较好。     

Flexural behaviour of RC slabs strengthened with prestressed CFRP strips using different anchorage systems

The Externally Bonded Reinforcement (EBR) technique using Carbon Fiber-Reinforced Polymers (CFRP) has been commonly used to strengthen concrete structures in flexure. The use of prestressed CFRP material offers several advantages well-reported in the literature. Regardless of such as benefits, several studies on different topics are missing. The present work intends to contribute to the knowledge of two commercially available systems that differ on the type of anchorage: (i) the Mechanical Anchorage (MA), and (ii) the Gradient Anchorage (GA). For that purpose, an experimental program was carried out with twelve slabs monotonically tested under displacement control up to failure by using a four-point bending test configuration. The effect of type of anchorage system (MA and GA), prestrain level (0 and 0.4%), width (50 mm and 80 mm) and thickness (1.2 mm and 1.4 mm) of the CFRP laminate, and the surface preparation (grinded and sandblasted) on the flexural response were the main studied parameters. Better performance was observed for the slabs: (i) with prestressed laminates, (ii) for the MA system, and (iii) with sandblasted surface preparation.     

Fatigue crack propagation behavior of RC beams strengthened with CFRP under cyclic bending loads

A fatigue crack propagation equation of reinforced concrete (RC) beams strengthened with a new type carbon fiber reinforced polymer was proposed in this paper on the basis of experimental and numerical methods. Fatigue crack propagation tests were performed to obtain the crack propagation rate of the strengthened RC beams. Digital image correlation method was used to capture the fatigue crack pattern. Finite element model of RC beam strengthened with carbon fiber reinforced polymer was established to determinate J‐integral of a main crack considering material nonlinearities and degradation of material properties under cyclic loading. Paris law with a parameter of J‐integral was developed on the basis of the fatigue tests and finite element analysis. This law was preliminarily verified, which can be applied for prediction of fatigue lives of the strengthened RC beams.