Modeling of debonding failure for RC beams strengthened in shear with NSM FRP reinforcement

The shear capacity of reinforced concrete members can be successfully increased using near-surface mounted (NSM) fiber-reinforced polymer (FRP) reinforcement. Tests conducted thus far have shown that failure is often controlled by diagonal tension associated to debonding between the NSM reinforcement and the concrete substrate. In absence of steel stirrups and/or when the spacing of the NSM reinforcement is large, debonding involves separately each of the bars crossed by the critical shear crack. In order for shear strengthening of beams with NSM reinforcement to be safely designed, an analytical model able to encompass the failure mode mentioned above must be developed. This paper presents two possible approaches, a simplified and a more sophisticated one, to predict the FRP contribution to the shear capacity. In the first approach, suitable for immediate design use, an ideally plastic bond–slip behavior of the NSM reinforcement is assumed, which implies a complete redistribution of the bond stresses along the reinforcement at ultimate. The second approach, implemented numerically, accounts for detailed bond–slip modeling of the NSM reinforcement, considering different types of local bond–slip laws calibrated during previous experimental investigations. It also takes advantage of an approach developed by previous researchers to evaluate the interaction between the contributions of steel stirrups and FRP reinforcement to the shear capacity. The paper illustrates the two models and compares their predictions, with the ultimate goal to evaluate whether the first simple model can be used expecting the same safety in predictions of the second model.     

Behavior and capacity of RC beams strengthened in shear with NSM FRP reinforcement

A recent and promising method for shear strengthening of reinforced concrete (RC) members is the use of near-surface mounted (NSM) fiber-reinforced polymer (FRP) reinforcement. In the NSM method, the reinforcement is embedded in grooves cut onto the surface of the member to be strengthened and filled with an appropriate binding agent such as epoxy paste or cement grout. Only a few studies have been conducted to date on the use of NSM FRP reinforcement for shear strengthening of RC beams. These studies identified some critical failure modes related to debonding between the NSM reinforcement and the concrete substrate. However, more tests need to be conducted to identify all possible failure modes of strengthened beams. Moreover, virtually no test results are available on the behavior of shear-strengthened beams containing steel shear reinforcement, and on the effect of variables such as the type of epoxy used as groove filler. This paper illustrates a research program on shear strengthening of RC beams with NSM reinforcement, aimed at gaining more test results to fill the gaps in knowledge mentioned above. A number of beams were tested to analyze the influence on the structural behavior and failure mode of selected test parameters, i.e. type of NSM reinforcement (round bars and strips), spacing and inclination of the NSM reinforcement, and mechanical properties of the groove-filling epoxy. One beam strengthened in shear with externally bonded FRP laminates was also tested for comparison purposes. All beams had a limited amount of internal steel shear reinforcement to simulate a real strengthening situation. Test results are presented and discussed in the paper.     

IC debonding resistance of groups of FRP NSM strips in reinforced concrete beams

The use of fibre reinforced polymer (FRP) externally bonded (EB) plates in the form of pultruded and wet lay-up plates is now generally accepted as an efficient and unobtrusive technique for retrofitting reinforced concrete structures and is applied worldwide. However, EB plates, and in particular EB pultruded plates, tend to debond at strains much lower than their fracture strains. An alternative technique of adhesively bonding pultruded plates or strips in narrow grooves sawn into the concrete cover, that is near surface mounted (NSM) plates or strips, is now gradually gaining acceptance as tests have shown that the debonding strains can be much higher than that for EB plates. However, tests have also shown that NSM plates can interact with adjacent parallel NSM plates to cause intermediate crack (IC) debonding of groups of NSM strips at reduced strengths. This paper develops a mathematical model for the IC debonding resistance of groups of NSM plates for use in the flexural and shear strengthening of reinforced concrete beams.     

模拟嵌入式FRP条增强钢筋混凝土梁抗剪的三维力学模型

嵌入式FRP增强钢筋混凝土梁抗剪是进行结构修复的新技术,由于嵌入式FRP所具有的一些优势,使其成为较好的外贴加固技术,越来越受到重视。嵌入式FRP主要优势在于材料开发、对结构完整性的保护以及相对较快的适用性。由于FRP条复杂的几何尺寸,粘结后的非线性力学性能,混凝土受拉性能的离散性,使得嵌入式FRP条加固的性能十分复杂,一般通过试验来获得其性能。为更好地掌握表面嵌贴FRP带的性能,提出三维力学模型,用于模拟在相似加载过程中,嵌入式FRP条体系对钢筋混凝土梁抗剪强度的贡献。模型很好地解释了试验结果,并在模型中考虑了粘结滑移力和混凝土破坏之间的相互作用以及相邻FRP条之间的相互作用等复杂现象。     

The effects of NSM CFRP reinforcements for improving the shear capacity of RC beams

One of the most efficient technique for improving the shear strength of deteriorated RC members is bonding external carbon fiber-reinforced polymer (CFRP) composites. However, delimitation and debonding of the strengthening material frustrates to achieve the expected requirements. Near surface mounting (NSM) is a recent strengthening technique that was developed with the anticipation of obstructing the drawbacks of external CFRP usage. To demonstrate the efficiency, an experimental program was conducted to validate the effect of CFRP reinforcements on behavior and ultimate strength of shear deficient (without stirrups) reinforced concrete (RC) beams under cyclic loading. Accordingly seven of eight beams except the flexural reference were fabricated and strengthened with CFRP reinforcements with distinct CFRP reinforcement arrangements. Spacing of CFRP reinforcements, variation of CFRP reinforcement diameter and application of CFRP reinforcements were the selected variables of the experimental program. Tests results confirmed that all in all an increase in strength was seen in every specimen to which CFRP reinforcements applied with no occurrence of delamination, debonding or fracture of CFRP reinforcements. To verify the reliability, experimental results were compared with ACI-440 guideline and the proposals of De Lorenzis and Nanni.     

Unified analytical approach for determining shear capacity of RC beams strengthened with FRP

This paper presents a rational model to predict the ultimate load capacity of reinforced concrete (RC) beams strengthened by a combination of longitudinal and transverse fiber reinforced polymer (FRP) composite plates/sheets (flexure and shear strengthening system). The model is based on the truss analogy and the theory of plasticity and is opportunely refined in order to incorporate some critical aspects, such as variable angle crack, non-uniform FRP stress distribution over the shear crack, shear span/depth ratio. It is a general and unified model that allows consideration of all the main possible failure mechanisms of strengthened RC beams, related to flexural-shear interaction, shear web-crushing and pure flexural mechanisms. The model is validated against a large number of beam tests reported in the literature, involving a wide range of geometrical and mechanical characteristics. The numerical investigation shows a very satisfactory correlation between predicted and experimental data.     

内嵌FRP加固钢筋混凝土梁的受弯承载力分析

对内嵌FRP加固矩形截面梁的三种弯曲破坏形式(钢筋屈服前混凝土压碎、钢筋屈服后混凝土压碎和钢筋屈服后FRP拉断)和两种界限破坏情形(钢筋屈服时混凝土压碎和FRP拉断时混凝土压碎)进行了极限状态分析,提出了各种弯曲破坏极限状态相应的承载力计算公式和界限条件,其计算结果与作者及国外已有试验实测值吻合较好。     

A generic design approach for EB and NSM longitudinally plated RC beams

There has been much good research on adhesive bonding plates to existing reinforced concrete structures which is now readily available in the form of published books or guidelines. Most of the fundamental and unique failure modes of plated structures have now been identified and quantified and research has reached the stage where it can be applied as design in practice and with confidence. This paper brings together this research and shows that the fundamental behaviour of plated structures can be described in generic terms for all forms of plating, which will eventually enable generic design rules to be developed for all types of adhesive plating. A generic design procedure is presented for adhesive bonding longitudinal plates to reinforced concrete (RC) beams and slabs: that applies to both externally bonded (EB) plates and near surface mounted (NSM) plates; that can be applied to prestressed and unprestressed beams; that covers all plate materials, plate configurations and plate adhesion to any surface; covers all four major debonding mechanisms at serviceability or ultimate limit states; and quantifies the flexural and shear strengths as well as the ductility associated with moment redistribution.     

RC beams shear-strengthened with FRP: Stress distributions in the FRP reinforcement

Reinforced concrete (RC) beams may be strengthened for shear with externally bonded fibre reinforced polymer (FRP) composites through complete wrapping, U-jacketing or bonding on their sides only. The two main shear failure modes of such strengthened beams are FRP rupture and debonding. In both modes of failure, the contribution of the bonded FRP reinforcement to the shear capacity of the beam depends strongly on the stress (or strain) distribution in the FRP at the ultimate limit state. This paper presents a numerical study of the FRP stress distribution at debonding failure in U-jacketed or side-bonded beams using a rigorous FRP-to-concrete bond–slip model and assuming several different crack width distributions. Numerical results indicate that Chen and Teng’s early simple assumption [Chen JF, Teng JG. Shear capacity of FRP-strengthened RC beams: FRP debonding. Constr Build Mater 2003;17:27–41] for the stress distribution in the FRP results in satisfactory predictions for the effective FRP stress in most cases for both U-jacketed and side-bonded beams. However, it may become unconservative for side-bonded beams that have only light flexural steel reinforcement.     

纤维材料抗剪加固钢筋混凝土梁的影响因素与试验结果分折

本文讨论了多种因素对纤维布加固钢筋混凝土梁效果的影响,分析了不同因素的影响程度,并分析了纤维布对梁刚度、开裂荷载的影响,最后对纤维布的应变变化情况进行了研究。     

纤维材料抗剪加固钢筋混凝土梁的影响因素与试验结果分析

本文讨论了多种因素对纤维布加固钢筋混凝土梁效果的影响，分析了不同因素的影响程度，并分析了纤维布对梁刚度、开裂荷载的影响。最后对纤维布的应变变化情况进行了研究。     

CFRP受剪加固钢筋混凝土梁的理论研究

对CFRP受剪加固钢筋混凝土梁的方法及破坏模式进行了分析,对受剪承载力的计算方法进行了研究,提出了采用CFRP加固混凝土构件的施工要点,以供参考。     

CFRP嵌入式加固混凝土梁抗弯性能试验研究

通过对3根对比梁和8根加固粱进行了试验研究,结果表明：CFRP布嵌入式加固能有效提高混凝土梁的抗弯承载力,另外,不同初始弯矩作用下加固梁的极限承载力几乎相同。     

Modeling error analysis of shear predicting models for RC beams

Analyses of the modeling error associated with several commonly referred models for predicting shear capacity of reinforced concrete beams were carried out using a database containing results of test beams that was established based on test specimens found in the literature. The database contains a total of 1146 test beams, each has sufficient information for calculating the shear capacity using the shear predicting model in the design code suggested by the American Concrete Institute (ACI), the simplified method recommended by the Canadian Standard Association (CSA), the modified compression field theory, the shear friction method and Zsutty’s method.Statistics of the modeling error suggest that the performance of the shear predicting model in the ACI code and the CSA simplified method is similar but significantly biased and associated with large coefficients of variation of the modeling error. Overall, Zsutty’s method may be considered to be the best predicting model among the models included in this study. However, for beams with stirrups, use of the modified compression field theory provides less biased prediction with less scatter. This is followed by Zsutty’s method and by one of the shear friction models.     

集中荷载下无腹筋梁抗剪承载力新的设计方法

采用数理统计的方法,通过对收集到的国内外集中荷载作用的钢筋混凝土无腹筋简支梁的大量试验数据进行回归分析,对无腹筋简支梁受剪承载力的主要影响因素综合加以考虑,通过引入纵筋率及截面高度综合影响参数,提出无腹筋简支梁抗剪承载力实用计算公式,并把利用该式得到的相应计算值与试验数据进行比较。研究表明,本文所建议的计算公式与试验结果有较好的吻合度且偏于安全,基本可以应用于实际工程。     

Shear-flexural strength mechanical model for the design and assessment of reinforced concrete beams

A conceptual model for the prediction of the shear-flexural strength of slender reinforced concrete beams with and without transverse reinforcement is presented. The model incorporates the shear transferred by the un-cracked concrete chord, along the crack's length, by the stirrups, if they are, and, in that case by the longitudinal reinforcement. After the development of the first branch of the critical shear crack, failure is considered to occur when the stresses at any point of the concrete compression chord reach the assumed biaxial stress failure envelope. A physical explanation is provided for the evolution of the shear transfer mechanisms, and the contribution of each one at ultimate limit state is formulated accordingly. Simple equations are derived for shear strength verification and for designing transverse reinforcement. The method is validated by comparing its predictions with the results of 1131 shear tests, obtaining very good results in terms of mean value and coefficient of variation. Because of its accuracy, simplicity and theoretical consistency, the proposed method is considered to be very useful for the practical design and assessment of concrete structures subjected to combined shear and bending.     

钢筋混凝土框架短柱抗剪承载力分析

总结了国内外几个主要的钢筋混凝土框架短柱抗剪承载力计算公式,结合太平洋地震研究中心(PEER)的短柱抗剪拟静力试验数据库,对各公式进行了对比分析,得出各公式计算的短柱抗剪承载力与试验结果基本成线性关系的结论.     

局部粘结退化钢筋混凝土梁变形计算模型

钢筋锈蚀会影响荷载作用下钢筋混凝土构件的变形。目前,因钢筋锈蚀导致的钢筋与混凝土之间的粘结退化(特别是局部粘结退化)对构件变形的影响问题尚未得到有效解决。本文以简支梁为例,揭示了局部粘结退化对钢筋混凝土梁变形的影响机制,建立了受拉纵筋局部粘结退化后钢筋混凝土梁的变形计算模型,并对比已有实验数据,验证了模型的合理性和准确性。     

Reduction in flexural strength in rectangular RC beams due to slenderness

This paper presents the results of 15 experimental tests on rectangular slender reinforced concrete (RC) beams. The results reveal limitations in existing theoretical formulations to estimate the failure moment capacity and mode of failure. An improved theoretical formulation is proposed here to predict the critical buckling moment including effects related to nonlinearity and cracking of concrete. Also, following the trends in steel design, an improved measure of slenderness ratio is proposed. Based on a study of 72 test results, it is shown that there is an interaction between flexural tension and instability modes of failure in moderately slender beams. To avoid lateral instability failure, it is suggested that the slenderness ratio be limited to unity. A ‘moment reduction factor’ is also proposed to account for slenderness effects in RC beams.     

基于桁-拱叠加模型的体外预应力混凝土梁抗剪承载力计算方法

通过剪力传递途径分析和结构拓扑优化,揭示了桁架-拱模型可以反映体外预应力混凝土梁主要受剪承载机理。通过桁架模型和拱模型的叠加,建立了体外预应力混凝土梁的抗剪承载力简化计算方法。公式形式简单,能够反映结构的受力机理,包含了梁的几何尺寸、剪跨比、配箍率、轴向预应力、混凝土强度、荷载形式等影响因素。公式可以统一考虑无腹筋梁和有腹筋梁,不同的剪跨比,以及体内体外混合配束的情况。应用本文方法与国内外已有试验进行对比,表明计算值与试验结果吻合良好且偏于安全。最后,通过参数分析得出了主要因素对抗剪承载力的影响规律。