Shear strengthening of old concrete beams

In our March‐April issue last year, the author described work at the Czech Technical University in Prague on the use of epoxy resins to apply new flat reinforcement to old concrete beams in order to enhance or prolong their service life. Here, some further test results and general conclusions are presented on the application of this technique to shear strengthening. The author is Associate Professor of Civil Engineering at the University.     

Shear strengthening of old concrete beams

In our March-April issue last year, the author described work at the Czech Technical University in Prague on the use of epoxy resins to apply new flat reinforcement to old concrete beams in order to enhance or prolong their service life. Here, some further test results and general conclusions are presented on the application of this technique to shear strengthening. The author is Associate Professor of Civil Engineering at the University.     

Shear strengthening of reinforced concrete beams by means of vertical prestressed reinforcement

Strengthening reinforced concrete (RC) elements critical to shear with prestressed transversal reinforcement can be an efficient method to increase the shear resistance of structures, allowing the development of the full flexural capacity. However, research on the performance of this technique is very limited, and methods for designing the optimum amount of prestressed transversal reinforcement and assessing the retrofitted structure have not been produced yet. Nonlinear finite element models are an important tool regarding predicting the efficiency of these interventions. In this paper, a shear-sensitive fibre beam formulation is extended in order to account for the effects of unbonded vertical external prestressed reinforcement in the structural response of RC beams. The model is validated with experimental tests available in literature, succeeding in capturing the gain of shear strength brought by different strengthening solutions. A parametric study is performed to find the optimal quantity of transversal reinforcement that ensures flexural failure mechanism in a beam with insufficient internal shear reinforcement. The relative simplicity of the numerical model makes it suitable for engineering practice.     

Shear failure of reinforced concrete beams

It has been suggested in a previous work that the causes of shear failure exhibited by reinforced concrete (RC) beams are associated with the stress conditions in the region of the path along which the compressive force is transmitted from support to support. The work described in this paper presents experimental evidence supporting the above concept. The work is based on a comparative study of the behaviour of concrete beams reinforced in compliance with this concept and that of beams reinforced in compliance with current design procedures.     

Shear strength model of the reinforced concrete beams with embedded through-section strengthening bars

In this study, finite element (FE) analysis is utilized to investigate the shear capacity of reinforced concrete (RC) beams strengthened with embedded through-section (ETS) bars. Effects of critical variables on the beam shear strength, including the compressive strength of concrete, stiffness ratio between ETS bars and steel stirrups, and use of ETS strengthening system alone, are parametrically investigated. A promising method based on the bond mechanism between ETS strengthening and concrete is then proposed for predicting the shear resistance forces of the strengthened beams. An expression for the maximum bond stress of the ETS bars to concrete is developed. This new expression eliminates the difficulty in the search and selection of appropriate bond parameters from adhesion tests. The results obtained from the FE models and analytical models are validated by comparison with those measured from the experiments. Consequently, the model proposed in this study demonstrates better performance and more accuracy for prediction of the beam shear-carrying capacity than those of existing models. The results obtained from this study can also serve researchers and engineers in selection of the proper shear strength models for design of ETS-strengthened RC beams.     

Shear strengthening of RC beams with textile reinforced concrete (TRC) plate

Carbon–epoxy composite materials are of considerable interest for reinforcement, but they need to be improved and have constraints, particularly in terms of cost and criteria of sustainable development. Alternative materials like textile reinforced concrete (TRC) should be seriously considered as substitutes for traditional composite materials.This experimental study focuses on the mechanical feasibility of this type of solutions by comparing them with traditional solutions such as CFRP. The first comparative results are promising in terms of strength and stiffness. It can be established that, quantitatively, the materials have very similar behaviours, especially in the final stage. Finally, the Ritter–Mörsch truss model can be used with sufficient accuracy for the evaluation of the tensile strength, by considering a non-uniform stress distribution along the shear cracks that leads to failure.     

碳纤维加固钢筋混凝土施工技术

阐述了采用碳纤维布加固的优点、材料特性及其构造做法,详细介绍了碳纤维布加固钢筋混凝土的施工工艺,指出该技术是一种经济、适用、新型的结构方法,且具有轻质高强、施工便捷、耐腐蚀性和耐久性强等优点,值得推广使用。     

Shear strengthening of continuous reinforced concrete T-beams using wire rope units

A simple unbonded-type shear strengthening technique for reinforced concrete beams using wire rope units is presented. Ten two-span reinforced concrete T-beams externally strengthened with wire rope units and an unstrengthened control beam were tested to failure, to explore the significance and shortcomings of the developed unbonded-type shear strengthening technique. The main parameters investigated were the type, amount and prestressing force of wire rope units. All beams tested failed, owing to significant diagonal cracks within the interior shear span. However, beams strengthened with closed type wire rope units exhibited more ductile failure than the unstrengthened, control beam or those strengthened with U-type wire rope units. The diagonal cracking load and ultimate shear capacity of beams with closed-type were linearly increased with the increase of vertical confinement stresses in concrete owing to the prestressing force in wire rope units, while those of beams with U-type were minimally influenced. It was also observed that average stresses in closed-type wire ropes crossing diagonal cracks at ultimate strength of beams tested were much higher than those in U-type wire ropes, showing better utilization in the former case. The shear capacity of beams with closed-type wire rope units is conservatively predicted using the equations of ACI 318-05, modified to account for the external wire rope units. A mechanism analysis based on the upper bound approach of the plasticity theory is also developed to assess the load capacity of beams tested. The predictions by the mechanism analysis for beams with closed-type wire rope units are in good agreement with test results and showed a coefficient of variation slightly less than the modified ACI 318-05 equations. However, the modified ACI 318-05 equations are more conservative and simpler to use for design purposes.     

Flexure strengthening of lightweight reinforced concrete beams using carbon fibre-reinforced polymers

A series of 40 lightweight reinforced concrete (LWRC) beams of 1400 mm length and a rectangular cross section of 150 × 200 mm were cast, strengthened and then tested under four-point bending test to study the effectiveness of using externally applied carbon fibre-reinforced polymer (CFRP) composites as a method of increasing the flexural strength of under-reinforced LWRC beams. Parameters investigated include reinforcement ratio, ρ; ρ = 0.55ρ_b and ρ = 0.27ρ_b, CFRP sheet length; 600, 800 and 1000 mm, CFRP sheet width; beam width and half-beam width. Three types of strengthening schemes were used: jacketing covers the beam from bottom and two sides with total width of 500 mm, sheets at the tension side with width equal to beam width and sheets with width equal to half-beam width. Test results showed a limited increase in ultimate load-carrying capacity accompanied with some reduction in mid-span deflection for the strengthened beams. Among the strengthening schemes investigated, jacketing was the most effective for strength enhancement (about 41%) with respect to control beam; however, it reduced ductility significantly. An analytical model was proposed for predicting the ultimate load-carrying capacity of LWRC beams strengthened with CFRP composites.     

Shear strength of reinforced concrete deep beams

An hypothesis is made for the shear mechanism of concrete deep beams based on the failure mode observed during testing. Using the hypothesis an equation is developed for predicting the ultimate strength of the beam. The equation is found to give reasonable values for the ultimate loads for beams with depth to span ratio as low as .     

火灾后钢筋混凝土梁抗剪承载力

研究火灾后钢筋混凝土梁抗剪承载力,建立了钢筋混凝土梁的非线性有限元模型,对其在火灾后的抗剪性能进行了数值模拟。有限元模型考虑了火灾作用后混凝土和钢筋的材料损伤,分析了火灾时间、剪跨比、混凝土强度、箍筋直径和箍筋间距对钢筋混凝土梁抗剪承载力和刚度的影响程度。结果表明,随着火灾时间的增加、剪跨比的增大或混凝土强度的下降,火灾后钢筋混凝土梁的抗剪承载力和刚度均明显下降。随着箍筋直径的下降或箍筋间距的增大,火灾后钢筋混凝土梁的抗剪承载力下降,而刚度变化较小。     

锈蚀钢筋混凝土梁的抗剪承载力分析模型

锈蚀钢筋混凝土梁的传统抗剪承载力模型大多属于经验模型,考虑的影响因素不全面且缺乏严密的理论推导,导致计算精度有限。基于修正压力场理论,建立了可以综合考虑钢筋锈蚀对临界斜裂缝倾角、梁有效抗剪截面积、配筋率、配箍率等关键因素影响的锈蚀钢筋混凝土梁的抗剪承载力模型,通过与85组试验数据和现有模型的对比分析,验证了模型的适用性和计算精度。结果表明,该模型具有严密的理论基础,考虑的影响因素全面,计算精度较高、离散性较小。     

Shear behaviour of reinforced palm kernel shell concrete beams

The shear behaviour of palm kernel shell concrete (PKSC) beams prepared using palm kernel shell (PKS) as lightweight aggregate (LWA) is reported here. The shear strength of grade 30 PKSC with a density of 1850 kg/m³ was found 24% higher than the corresponding normal weight concrete (NWC). Good aggregate interlock in PKSC was evident as it produced shorter jagged cracks compared to longer plain cracks of NWC. Further, PKSC was able to produce twice as many flexural and shear cracks compared to NWC. Tension stiffening between the tensile cracks of PKSC enhanced flexural rigidity and dowel action. The non-linear numerical analysis predicted the shear strength within an average 20% of the experimental results.     

Shear strength prediction for steel reinforced concrete deep beams

This study proposes an analytical method for determining the shear strengths of steel reinforced concrete deep beams under the failure mode of concrete crushing originally based on the softened strut-and-tie model. The proposed method is a good physical model that can correlate well with the observed failure phenomenon of steel reinforced concrete deep beams. By comparing the predictions of the proposed method with the available test results from the literature, it was found that the proposed method is capable of predicting the shear strengths for steel reinforced concrete deep beams with sufficient accuracy. The shear-carrying behavior of steel reinforced concrete deep beams is highly influenced by the ratios of flange width to gross width, the shear span-to-depth ratios, and the concrete strengths. When the ratio of flange width to gross width is low, the shear-carrying capacities of steel reinforced concrete deep beams increase with the increasing ratio. However, if the ratio of the flange width to gross width is higher than a critical value, then the failure mode of steel reinforced concrete deep beams will be converted from diagonal compression failure into bearing failure.     

Fracture mechanics approach for flexural strengthening of reinforced concrete beams

This paper reports on the experimental testing of 9 notched reinforced concrete specimens under four point bending. The beams comprise three beam sizes and three tension reinforcing steel ratios. All beams have constant span/depth ratio of 4, initial notch/depth ratio of 0.3. Two strengthening fiber laminates were used: Glass fiber for the two lower tension reinforcing steel ratios and Carbon fiber for the higher tension reinforcing steel ratio. The strengthening laminates were designed to enhance beam moment capacity by 15% to 150% depending on the beam size and reinforcement ratio. To simulate real life strengthening situations, beams were first loaded until the notch propagated to 0.5 the beam depth. The strengthening fiber laminate was then introduced to the tension side of the beam while the load was kept applied to the other side of the beam. The fracture moment for a given crack depth was calculated through an analytical algorithm which employs Linear Elastic Fracture Mechanics. The approach takes into consideration the previous loading history of the beam prior to introducing the strengthening laminate. Test measurements of crack extension and applied load were used to compare the fracture moment recorded experimentally to that one calculated analytically. The application of the solution algorithm to different specimen sizes–cross-section dimensions, reinforcement ratio, and strengthening fiber laminate–showed that the solution algorithm is able to effectively predict the behavior of larger beam size and/or reinforcement better than that of smaller beam size and/or reinforcement. A sensitivity analysis was conducted to explore this point.     

碳纤维复合材料加固钢筋混凝土梁的应用研究

结合具体工程实例,详细介绍了碳纤维加固钢筋混凝土梁的设计方法和相关参数计算方法,并具体阐述了该技术的施工工艺及质量要求,指出采用该技术施工效果良好,并对其未来发展方向和应用空间进行了展望,以期促进该技术的推广及应用。     

Retrofitting of reinforced concrete beams using composite laminates

This paper presents the results of an experimental study to investigate the behaviour of structurally damaged full-scale reinforced concrete beams retrofitted with CFRP laminates in shear or in flexure. The main variables considered were the internal reinforcement ratio, position of retrofitting and the length of CFRP. The experimental results, generally, indicate that beams retrofitted in shear and flexure by using CFRP laminates are structurally efficient and are restored to stiffness and strength values nearly equal to or greater than those of the control beams. It was found that the efficiency of the strengthening technique by CFRP in flexure varied depending on the length. The main failure mode in the experimental work was plate debonding in retrofitted beams.     

钢筋混凝土梁受弯加固对比试验研究

通过钢筋混凝土矩形截面梁的抗弯测试,对用碳纤维布补强和钢板粘接加固的钢筋混凝土梁进行了抗弯性能对比试验研究,分析了碳纤维和粘钢加固钢筋混凝土梁抗弯性能的主要影响因素。计算结果与试验数值吻合较好,得到了两种加固方法的适用性设计指标。     

三角形分布荷载下钢筋混凝土无腹筋梁受剪性能研究

在试验基础上,分析了三角形分布荷载下钢筋混凝土无腹筋梁抗剪机理,指出斜截面受剪承载力由斜裂缝末端剪力决定,并提出了建议计算方法。     

Shear capacity evaluation of steel reinforced recycled concrete (RRC) beams

The applicability of some major concrete design standards and other pertinent methods to calculate the concrete contribution to the shear resistance of reinforced recycled concrete (RRC) beams without stirrups is investigated. Results of a relatively comprehensive experimental program are used to compare the actual shear strength of the tested beams with their corresponding predicted values. The concrete mixes for the RRC beams were proportioned by the so-called Equivalent Mortar Volume (EMV) method. The method is predicated on the fact that recycled concrete aggregate (RCA) is a composite material, comprising mortar and natural aggregate, and the volumetric content and properties of each phase must be quantitatively accounted for when proportioning concrete mixes containing RCA. The test variables included in the test program are shear-span/depth ratio, beam size, RCA source, and coarse aggregate type. The results show that the shear capacity of a RRC beam is comparable, or sometimes superior, to that of a companion beam made of conventional concrete. The analyses performed in the current investigation show, contrary to previous findings, that existing shear design methods, such as the ACI and CSA codes methods, are applicable to RRC beams, provided the EMV method of mix design is used.