Behavior and performance of RC T-section deep beams externally strengthened in shear with CFRP sheets

A series of experimental tests were carried out to investigate the behavior and performance of reinforced concrete (RC) T-section deep beams strengthened in shear with CFRP sheets. Key variables evaluated in this study were strengthening length, fiber direction combination of CFRP sheets, and an anchorage using U-wrapped CFRP sheets. A total of 14 RC T-section deep beams were designed to be deficient in shear with a shear span-to-effective depth ratio (a/d) of 1.22. Crack patterns and behavior of the tested deep beams were observed during four-point loading tests. Except the CS-FL-HP specimen, almost all strengthened deep beams showed a shear–compression failure due to partial delamination of the CFRP sheets. From the load–displacement (p–u) curves, the effects of key variables on the shear performance of the strengthened deep beams were addressed. It was concluded from the test results that the key variables of strengthening length, fiber direction combination, and anchorage have significant influence on the shear performance of strengthened deep beams. In addition, a series of comparative studies between the present experimental data and theoretical results in accordance with the commonly applied design codes were made to evaluate the shear strength of a control beam and deep beams strengthened with CFRP sheets.     

Structural performance of corroded RC beams repaired with CFRP sheets

Corrosion of reinforcement is a serious problem and is the main cause of concrete structures deterioration costing millions of dollars even though the majority of such structures are at the early age of their expected service life. This paper presents the experimental results of damaged/repaired reinforced concrete beams. The experimental program consisted of reinforced concrete rectangular beam specimens exposed to accelerated corrosion. The corrosion rate was varied between 5% and 15% which represents loss in cross-sectional area of the steel reinforcement in the tension side. Corroded beams were repaired by bonding carbon fiber reinforced polymer (CFRP) sheets to the tension side to restore the strength loss due to corrosion. Different strengthening schemes were used to repair the damaged beams. Test results showed detrimental effect of corrosion on strength as well as the bond between steel reinforcement and the surrounding concrete. Corroded beams showed lower stiffness and strength than control (uncorroded) beams. However, strength of damaged beams due to corrosion was restored to the undamaged state when strengthened with CFRP sheets. On the other hand, the ultimate deflection of strengthened beams was less than ultimate deflection of un-strengthened beams.     

Experimental Behaviour of RC Beams Shear Strengthened with NSM CFRP Laminates

Abstract: The near‐surface mounted (NSM) is one of the most recent techniques applied for the increase of the shear resistance of reinforced concrete (RC) beams. This technique involves the installation of carbon fibre reinforcement polymers (CFRP) laminates into thin slits open on the concrete cover of the elements to strengthen. The effectiveness of this technique for the shear strengthening of T cross‐section RC beams was assessed by experimental research. For this purpose, three inclinations of laminates were tested (45º, 60º and 90º) and, for each inclination, three percentages of CFRP were applied in RC beams with a percentage of steel stirrups of 0.10% (ρ_sw). The highest percentage of laminates was designed to provide a maximum load similar to the reference RC beam, which was reinforced with a reinforcement ratio of steel stirrups of 0.28% (ρ_sw = 0.28%). For each percentage of laminates, a homologous RC beam strengthened with unidirectional U‐shaped CFRP wet lay‐up sheets (discrete strips) applied according to the externally bonded reinforcement technique was also tested, with the purpose of comparing the effectiveness of these two CFRP‐strengthening techniques. To evaluate the influence of the percentage of steel stirrups in the effectiveness of the NSM technique, some of the abovementioned CFRP configurations were also applied in beams with ρ_sw = 0.17%.     

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.     

模拟不中断交通状况下粘贴碳纤维布加固钢筋混凝土梁试验研究

为了调查动荷载作用下碳纤维布与钢筋混凝土梁的粘贴性能及加固效果,进行了5根模拟交通荷载(等幅正弦波形动荷载)作用下粘贴碳纤维布加固钢筋混凝土梁和1根保持荷载下粘贴碳纤维布加固钢筋混凝土梁以及2根对比梁的试验研究.试验中考虑了混凝土等级、配筋率、有无锚固条、粘贴长度、荷载幅值5个变化参数.试验结果表明,在模拟交通荷载的作用下,碳纤维布加固的钢筋混凝土梁粘贴性能满足要求,粘贴效果良好.在模拟交通荷载后的静载作用下,试验梁的抗弯承载力提高较多,加固效果明显,进一步验证了粘贴效果.     

Different CFRP strengthening techniques for prestressed hollow core concrete slabs: Experimental study and analytical investigation

This study presents an experimental–analytical investigation on the structural behavior of precast prestressed hollow core RC slabs strengthened in flexure by CFRP laminates. Externally bonded and near surface mounted (NSM) laminates were used. The CFRP area and using transverse anchorage were also investigated. Results demonstrated that NSM technique resulted in optimum strengthening efficiency. The increased bond strength also resulted in full activation of the NSM laminates at failure. However, the NSM flexural strengthening level should be carefully designed to avoid unfavorable shear-tension failure mode. Moderate efficiency was associated with the externally bonded technique due to the premature de-bonding. However, this efficiency was optimized by using transverse CFRP laminates as anchorage, which re-directed de-bonding further away from the laminates’ ends and delayed failure, but at much lower deformations than those of the control slab. A rational analytical study was conducted. Comparisons between the experimental and analytical results demonstrated satisfactory agreements.     

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。     

碳纤维束铆钉锚固下CFRP布加固梁的力学性能

粘贴碳纤维布(CFRP)加固梁时,通常需对其进行锚固,避免发生因CFRP布过早剥离而无法充分利用其抗拉强度。但当梁施工空间较小,梁侧面形状不规则时,CFRP布锚固较困难,采用新型碳纤维束铆钉锚固的方式能够很好地解决这一弊端。本文通过对四组不同锚固方式下的CFRP布加固梁进行四点受弯承载力试验,对比分析加载过程中各梁的力学性能和破坏规律,研究表明碳纤维束铆钉锚固下CFRP布加固梁强度和刚度都有显著增强。进一步对碳纤维束铆钉锚固方法优化,得到最佳碳纤维束铆钉锚固深度约为80mm,最佳锚固间距约为200mm。     

碳纤维布加固RC梁中粘结性能的非线性有限元分析

碳纤维布加固钢筋混凝土(RC)梁中,碳纤维布与梁底混凝土的剥离破坏使碳纤维布的强度不能得到充分发挥.分析碳纤维布与梁底混凝土的粘结应力,是研究碳纤维布加固剥离破坏承载力的基础问题.根据4根碳纤维布加固RC梁的试验研究结果,采用商业有限元程序MSC.Marc建立有限元模型,进行了非线性计算分析.通过分离总粘结应力中的局部粘结应力,得到粘结延伸长度范围内的锚固粘结应力分布,并结合试验数据对其分布规律进行了研究.根据分析和试验结果,引入了“有效锚固粘结长度”和“锚固粘结应力”的概念,给出了极限荷载下锚固粘结应力的计算建议.     

Seismic retrofit of reinforced concrete short columns by CFRP materials

Nowadays, reinforcing buildings or bridges against earthquake damage is a real technico-economic challenge. Composite materials applied by the wet lay-up method have been the main reinforcement technology for civil engineering structures since the 1990s. The research developed in this paper concerns seismic reinforcement. The main objectives are to evaluate CFRP’s contribution to mechanical and energetic performance and to the modification of the cracking pattern on short columns. During earthquakes, short columns undergo shear stress due to their low resistance to high imposed horizontal displacements.

Eight short columns were tested; their longitudinal reinforcement was higher than the Eurocode 8 upper limit whereas transverse reinforcement was insufficient, in order to ensure shear failure. Seven were continuously or discontinuously reinforced by CFRP or GFRP. They were tested under a constant compression load combined with a horizontal quasi-static cyclic load. It was therefore possible to evaluate the efficiency of such reinforcement by measuring the gain in terms of load and ductility.     

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.     

Performance of CFRP-strengthened RC beams subjected to repeated loads

In this paper, the effects of fatigue leading to crack formation and potential durability-bonding problems in reinforced concrete (RC) beams strengthened by carbon fiber reinforced polymer (CFRP) are studied. These effects are shown to cause CFRP de-bonding and loss of load carrying capabilities under static or low cyclic loading. Two series of RC beams with CFRP strengthening system are constructed and designed to fail in shear and flexural failures, respectively, under static loading. Repeated loading tests are conducted according to various loading ranges and loading cycles, and the experimentally determined fatigue properties are discussed. The test results show that it is possible to eliminate the debonding modes for longitudinally bonded CFRP using a U-wrap CFRP combination. The fatigue loads tested showed a significant effect on concrete rather than the CFRP system especially for the strengthened beams bearing a higher shear level. Moreover, the proposed equation to fit the testing S–N curve and the discussion of the stress in the component materials could be used for fatigue life predictions of beams with CFRP strengthening systems.     

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.     

Post-repair performance of eccentrically loaded RC columns wrapped with CFRP composites

This paper presents the results of a research program for examining the post-repair performance of eccentrically loaded corrosion-damaged reinforced concrete (RC) columns wrapped with carbon fiber reinforced polymer (CFRP) composites. The specimens, except a control undamaged group, were initially exposed to accelerated corrosion for 30 days using an impressed current technique. Following the initial corrosion, the damaged specimens were either repaired with full or partial CFRP wrapping systems or kept unrepaired. A group from the damaged specimens were further exposed to 60 days of corrosion exposure. All specimens were tested to failure under various eccentric loading with a nominal eccentricity-to-section height ratio (e/h) in the range of 0.3–0.86. Test results showed that full CFRP wrapping system effectively reduced the post-repair corrosion rate relative to that of the unwrapped specimens whereas partial CFRP wrapping had almost no effect on the steel mass loss. The strengths of the damaged specimens fully wrapped with CFRP were higher than those of the control specimens at the end of the post-repair corrosion phase. The strengths of the damaged partially wrapped specimens were higher than those of the control at nominal e/h values 0.43. At higher e/h values, the strengths of the partially wrapped specimens were lower than those of the control but still higher than those of the damaged unrepaired specimens. An analytical model that accounts for the confinement effect of the CFRP and the change in geometry under eccentric loading was employed to predict the columns’ strength. The model’s predictions were validated against test results.     

分段SFC预制壳壁抗震加固RC墩柱的效能研究

针对国内外强震作用下钢筋混凝土桥梁RC桥墩震损普遍存在的问题,在消化和吸收国内外已有RC墩柱抗震加固成果的基础上,研制开发一种了新型RC墩柱抗震加固措施-RC墩柱塑性铰区域外包分段钢纤维混凝土(Steel Fiber Concrete,SFC)预制壳壁,选取实际桥梁中的RC桥墩为原型,通过加固前后模型RC桥墩伪静力对比试验研究,校验这种新型RC墩柱抗震加固措施的有效性,试验结果表明新型RC墩柱抗震加固措施能在不改变RC桥墩塑性铰位置的前提下实现承载力和耗能能力的提高。此外,选取不同场地条件下的典型地震动,进行实验RC墩柱加固前后的非线性地震时程反应对比分析,进一步验证新型抗震加固措施的加固效能。     

Flexural response predictions of reinforced concrete beams strengthened with prestressed CFRP plates

Existing experimental studies showed that the reinforced concrete (RC) beams strengthened with prestressed carbon fiber-reinforced polymer (CFRP) plates had three possible flexural failure modes (including the compression failure, tension failure and debonding failure) according to the CFRP reinforcement ratio. Theoretical formulas based on the compatibility of strains and equilibrium of forces were presented to predict the nominal flexural strength of strengthened beams under the three failure modes, respectively, and a limitation on the tensile strain level developed in the prestressed CFRP plate was proposed as the debonding failure occurred. In addition, the calculation methods for cracking moment, crack width and deflection of strengthened beams were provided with taking into account the contribution of prestressed CFRP plates. Experimental studies on five RC beams strengthened with prestressed CFRP plates and a nonlinear finite element parametric analysis were carried out to verify the proposed theoretical formulas. The available test results conducted by other researchers were also compared with the predicted values.     

Seismic flexural behavior of concrete connections reinforced with CFRP bars and grids

The corrosion of steel reinforcement in concrete and the resulting deterioration of structures prompted research on fiber reinforced polymers (FRP) as potential reinforcement for concrete members, for use in new construction. FRPs have more favorable advantages for new concrete buildings subjected to seismic loads particularly in corrosive environment.     

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.     

Fatigue performance of RC beams strengthened with CFRP under coupling action of temperatures and vehicle random loads

Considering significant influence of servicing environments and vehicle random loads on fatigue performance of main load‐bearing members of bridges, in this paper, fatigue performance of reinforced concrete bridge structures strengthened with carbon fibre–reinforced polymer under coupling action of environmental temperatures and vehicle random loads was studied. A vehicle random loading spectrum for fatigue tests was simulated and compiled. A fatigue testing method with coupling action of random loads and temperatures was proposed, and 3‐point bending fatigue tests of the reinforced concrete beams strengthened with carbon fibre–reinforced polymer under coupling action of temperatures and vehicle random loads were performed. Effects of temperatures and loading form on the fatigue damage mechanism were preliminarily discussed. A modified Palmgren‐Miner rule and semiempirical fatigue equations were proposed and proved effective.