持续荷载作用下外贴FRP片材加固混凝土梁承载力影响因素分析

为探讨结构不卸载或部分卸载情况下外贴纤维片材加固时,结构初始状态对承载力设计计算的影响,根据极限状态和容许应力法分析了受弯构件加固后承载力计算公式,考查了初始弯矩对承载力的不同影响。在改变配筋率、截面尺寸、初始弯矩、加固量等参数情况下,讨论了不同初始弯矩对加固梁分别按容许应力法和按极限状态计算的承载力变化影响情况。分析显示,初始弯矩对承载力的影响均比较小;而对大尺寸的等效桥梁模型试验结果表明,初始弯矩大小对承载力计算影响最大为6.2%,为保证安全和简化计算,加固设计时可在未考虑初始弯矩情况下承载力设计值基础上考虑一定的折减系数(0.9)进行设计,且偏于安全。     

FRP加固混凝土梁受弯剥离破坏的有限元分析

FRP加固钢筋混凝土梁受弯剥离破坏是一种非常常见的破坏形式。首先基于微观尺度有限元分析,对受弯剥离破坏的机理进行了研究,提出了一个受弯剥离的双重剥离破坏准则,以及相应的界面粘结滑移关系,使得受弯剥离可以由基于普通弥散裂缝模型的混凝土单元来加以模拟,并开发出了相应的FRP-混凝土界面单元模型。将该界面单元嵌入通用有限元程序MSC.MARC,对45根受弯剥离破坏的试验梁进行了有限元分析。分析结果表明,提出的计算模型与试验结果吻合良好,可以真实模拟受弯剥离破坏过程。     

钢筋混凝土染粘钢加固抗裂及截面刚度研究

通过对两种不同高跨比钢筋混凝土梁在其受拉区粘钢的受弯性能试验研究,得出了通过对钢筋混凝土梁在受拉区粘钢不仅可提高受弯构件的抗弯能力,同时也可提高其截面抗弯刚度和构件抗裂度。对钢筋混凝土梁在受拉区不卸载粘钢、卸载粘钢不同情况,提出钢筋混凝土粘钢加固受变构件变形计算方法。     

“板—桁模型”计算预应力混凝土复合受力构件抗扭强度

弯、剪、扭复合受力下混凝土构件的受力特性非常复杂,准确计算构件的抗扭强度对构件的设计及工作性能的分析都非常重要。在钢筋混凝土构件的板-桁模型理论基础上,以预应力混凝土构件在复合受力下的板—桁模型理论为基础,以箱形截面预应力混凝土构件为例,由力及力矩的平衡、变形关系、材料本构关系及构件的破坏准则,建立了在小扭弯比情况下复合受力构件抗扭承载力的计算方法。所建立的计算模型及公式理论基础强、概念清晰,经试验值与理论计算值对比,符合情况较好,证实了板—桁模型理论分析复合受力构件抗扭性能的正确性。     

钢筋活性粉末混凝土简支梁受弯力学性能试验与计算

活性粉末混凝土（RPC）与普通混凝土（OC）相比，具有超高的强度、高韧性和优异的耐久性，其构件承载力与刚度计算方法必然不同于普通混凝土构件。该文对4根钢筋活性粉末混凝土简支梁开展受弯性能足尺试验，获得了梁的开裂弯矩、极限弯矩及荷载-跨中位移曲线，揭示了RPC简支梁受弯变形特征与破坏模式，推导了钢筋RPC简支梁的开裂弯矩与正截面受弯承载力计算公式。结果表明:钢纤维RPC极限压应变为4394 με~5200 με，开裂应变为690 με~820 με，均远大于普通混凝土；由于添加了钢纤维，公式推导时必须考虑RPC拉区拉应力的影响，推导所得开裂弯矩、正截面受弯承载力及刚度公式计算值与试验值吻合较好，计算公式具有较高的精度，可用于钢筋RPC梁的设计计算。     

再生混凝土梁挠度计算方法研究

对再生混凝土梁进行了正截面弯曲性能试验.结果表明,再生混凝土梁挠度随着纵向钢筋配筋率和混凝土强度的增加而减小,再生混凝土梁实测挠度较普通混凝土结构设计规范计算值大10%左右.依据普通混凝土受弯构件挠度计算理论,通过试验回归出再生混凝土梁刚度公式中的3个系数,从而建立了再生混凝土梁挠度计算方法.国内外有关试验数据验算表明...     

功能梯度混凝土受弯构件形成机制和力学性能研究

基于理论分析提出了一种功能梯度混凝土受弯构件，通过梯级FRP筋+钢筋混合配筋，构建同地震弯矩分布梯度相适应的构件抗弯承载能力分布梯度，使多个功能梯度段进入塑性状态，以更充分利用构件的抗震能力。功能梯度可以有效控制构件塑性的分布和发展程度，确保发生延性的破坏模式，并实现较好的损伤自恢复性。功能梯度混凝土受弯构件的变形能力大幅提高，侧向承载力增大，极限刚度降低，抗震性能显著增强。模型试验证明了功能梯度混凝土受弯构件理念的可行性及其力学效果，也验证了所提功能梯度构建方案的有效性和工程实用性。     

RC框架弹塑性位移的解构规则与构件的目标侧移角

研究了RC框架在弹塑性状态下层间位移与构件变形之间的关系。引入塑性变形分布因子作为参数,建立了框架节的塑性变形分布因子与位移延性系数、强柱系数之间的关系,即chmcd--关系;在此基础上,提出了RC框架弹塑性位移的解构规则,可用于求解构件的目标侧移角,从而实现了基于位移设计的关键一步。     

钢管约束混凝土纯弯构件抗弯力学性能研究

采用有限元软件建模对钢管约束混凝土纯弯构件的荷载-变形关系进行了计算,计算结果分别得到了两个圆形及两个方形构件试验结果的验证。在此基础上,利用有限元方法对钢管约束混凝土纯弯构件受力过程中钢管及核心混凝土之间的相互作用以及构件的荷载-变形关系进行了分析,最后探讨了钢管约束混凝土纯弯构件抗弯承载力的实用计算方法。     

基于可靠度的FRP筋材料分项系数的确定

基于中国混凝土结构设计资料以及构件几何尺寸与材料力学性能的概率统计参数,建立了FRP筋混凝土梁受弯正截面承载力的计算公式,并采用验算点法计算了在六种荷载效应比情况下的受弯承载力可靠指标平均值,最后在考察FRP筋材料分项系数对可靠指标平均值影响的基础上,建议GFRP筋和CFRP筋的材料分项系数统一取为1.25。可靠度分析表明:过于保守的FRP筋材料分项系数只能使得截面发生混凝土破坏模式的概率增大,而这种破坏模式对应的可靠度水平基本与FRP筋材料分项系数无关,因此只能造成巨大的材料浪费。截面的设计破坏模式与实际破坏模式不完全一致,这种不一致现象存在的范围随着FRP筋材料分项系数的增大而扩大。     

Low-velocity flexural impact response of fiber-reinforced aerated concrete

Impact response of fiber-reinforced aerated concrete was investigated under a three-point bending configuration based on free-fall of an instrumented impact device. Two types of aerated concrete: plain autoclaved aerated concrete (AAC) and polymeric fiber-reinforced aerated concrete (FRAC) were tested. Comparisons were made in terms of stiffness, flexural strength, deformation capacity and energy absorption capacity. The effect of impact energy on the mechanical properties was investigated for various drop heights and different specimen sizes. It was observed that dynamic flexural strength under impact was more than 1.5 times higher than the static flexural strength. Both materials showed similar flexural load carrying capacity under impact, however, use of 0.5% volume fraction of polypropylene fibers resulted in more than three times higher flexural toughness. The performed instrumented impact test was found to be a good method for quantifying the impact resistance of cement-based materials such as aerated concrete masonry products.     

钢板-高延性混凝土组合低矮剪力墙抗震性能试验研究

为提高低矮剪力墙的抗震性能,提出外包钢板-高延性混凝土(HDC)组合低矮剪力墙。设计了1片HDC低矮剪力墙、2片内置钢板-HDC组合低矮剪力墙和2片外包钢板-HDC组合低矮剪力墙,通过拟静力试验,研究了轴压比、配钢形式对试件破坏形态、滞回性能、承载能力、变形能力、耗能能力和刚度退化的影响。试验结果表明:HDC低矮剪力墙发生剪切破坏,内置钢板-HDC组合低矮剪力墙发生弯剪破坏,外包钢板-HDC组合低矮剪力墙发生弯曲破坏;与HDC低矮剪力墙相比,钢板-HDC组合低矮剪力墙的变形能力和承载力明显提高;钢板-HDC组合低矮剪力墙的峰值荷载和刚度受轴压比的影响较小;轴压比从0.5变到0.7时,内置钢板-HDC组合低矮剪力墙的变形能力降低,外包钢板-HDC组合低矮剪力墙的变形能力没有降低;提出钢板-HDC组合低矮剪力墙受弯承载力计算公式,其计算值与试验值吻合较好。     

Nonlinear failure prediction of concrete composite columns by a mixed finite element formulation

This study focuses on developing a mixed frame finite element formulation of reinforced concrete and FRP composite columns in order to give more accuracy not only to predict the global behavior of the structural system but also to predict the local damage in the cross-section. A hypo-elastic constitutive law of concrete is presented under the basis of a three-dimensional stress state in order to model the compressive behavior of confined concrete wrapped with FRP jackets. To predict the nonlinear load path-dependent confinement model of FRP-confined concrete, the strength enhancement of concrete was determined by the failure surface of concrete in a tri-axial stress state, and its corresponding peak strain was computed by the strain-enhancement factor proposed in this study. The behavior of FRP jacket was modeled using the two-dimensional classical lamination theory. The flexural behavior of concrete and composite members was defined using a nonlinear fiber cross-sectional approach. The results obtained by developed mixed finite element formulation were verified with the experiments of concrete composite columns and also were compared with a displacement-based finite element formulation. It is shown that the proposed formulation gives e more accurate results in the global behavior of the column system as well as in the local damage in the column sections.     

Effect of mineral admixtures and steel fiber volume contents on the behavior of high performance fiber reinforced concrete

High Performance Fiber Reinforced Concrete (HPFRC) is a structural material with advanced mechanical properties. The structural design of HPFRC members is based on the post-cracking residual strength provided by the addition into the mix of the fibers. Moreover, the addition of different types of mineral admixtures influences the overall behavior of this material. In order to optimize the performance of HPFRC in structural members, it is necessary to evaluate the mechanical properties and the post-cracking behavior in a reliable way. As a result, an experimental study on six different sets of HPFRC specimens was carried out. The main parameters that varied were the fiber volume content and the types of mineral addition. The behavior in compression, in flexural tension and the shrinkage properties were evaluated and critically analyzed in order to give a guide for structural use.The results showed that by adding high fiber volume content and the Algerian blast furnace slag into the mix, the HPFRC material obtained has a very good performance and it is suitable for use in practice.     

Ductile strengthening using externally bonded and near surface mounted composite systems

Externally bonded fiber reinforced polymers (FRP) has been established as an effective technique for strengthening concrete members. Other techniques, like near surface mounted (NSM) FRP bars, and steel reinforced polymers (SRP) have emerged as viable alternatives. In this study, four composite-based strengthening systems were used to provide equivalent flexural performance, namely: externally bonded CFRP sheets, NSM prefabricated CFRP strips, externally bonded SRP sheets and NSM stainless steel bars. The strengthening design was based on achieving approximately 38% increase in flexural capacity over the unstrengthened control beams. The mode of failure by design was brittle failure controlled by concrete crushing at 0.003 strain. However, the experimental program was designed to demonstrate the effectiveness of transverse anchoring reinforcement to control premature debonding failure modes and fully utilize the high strength of the composite systems. A more ductile behavior was also observed as a result of transverse strengthening and concrete confinement effects. Accordingly, an increase of approximately 50% in flexural strength is accomplished.     

Textile reinforced mortar (TRM) versus FRP as strengthening material of URM walls: out-of-plane cyclic loading

The application of a new structural material, namely textile reinforced mortar (TRM), as a means of increasing the load carrying capacity and deformability of unreinforced masonry walls subjected to cyclic out-of-plane loading is experimentally investigated in this study. The effectiveness of TRM overlays is evaluated in comparison to the one provided by fiber reinforced polymers (FRP) in the form of overlays or near-surface mounted (NSM) reinforcement. TRM systems may be considered as alternative to FRPs, tangling with some of the drawbacks associated with the application of the latter without compromising performance. Medium-scale tests were carried out on 12 masonry walls subjected to out-of-plane bending. The parameters under investigation comprised mortar-based versus resin-based matrix materials, the number of layers, the orientation of the moment vector with respect to the bed joints and the performance of TRM or FRP jackets in comparison to NSM strips. It is concluded that TRM jacketing provides substantial increase in strength and deformability. Compared with their epoxy-resin counterparts (FRP), TRM may result in generally higher effectiveness in terms of strength and deformability. NSM strips offer lower strength but higher deformability, due to controlled debonding. From the results obtained in this study it is believed that TRMs comprise an extremely promising solution for the structural upgrading of masonry structures under out-of-plane loading.     

Analysis of hybrid beams composed of GFRP profiles and polymer concrete

In this paper, a numerical approach for the analysis of a new type of hybrid composite beams is presented. Such beams are composed of composite materials that resist shear and tensile stresses, and polymer concrete acommodating compression stresses.The beams offer an optimized configuration in terms of stiffness and strength, with much potential for mechanical and civil engineering structures.The current numerical model is based on a finite element formulation for layered shell-like structures. The model accounts for a polymer concrete material model, orthotropic material model, and considers the analysis of geometric and material nonlinearities. The first-order shear deformation theory is used in order to describe the shell deformation under a total Lagrangian formulation. The model is validated experimentally and a close agreement with experimental results makes this model an attractive solution method for this type of composite hybrid beams.     

The effect of time and temperature on flexural creep and fatigue strength of a silica particle filled epoxy resin

Composite materials that use an epoxy resin as a matrix resins have superior mechanical properties over standard structural materials, but these materials exhibit time and temperature behavior when used for long periods and under high temperatures. This time and temperature behavior has not been fully explained. The purpose of this paper is to further describe this time and temperature behavior, increasing the reliability of this class of composite materials. The time and temperature dependence of flexural strength was examined by creep and fatigue testing. Flexural creep tests were carried out at various temperatures below the glass transition temperature. Flexural fatigue tests were carried out at various stress ratios, temperatures below the glass transition temperature and 2 frequencies. The time-temperature superposition principle held for the flexural creep strength of this material. A method to predict flexural creep strength based on the static strength master curve and the cumulative damage law is proposed. When the fatigue frequency was decreased while temperature and stress ratio are held constant the flexural fatigue strength decreases. The time-temperature superposition principle was also found to hold for the flexural fatigue strength with respect to frequency.     

Effect of cryogenic temperature on the flexural and cracking behaviors of ultra-high-performance fiber-reinforced concrete

This study investigates the flexural and cracking behaviors of ultra-high-performance fiber-reinforced concrete (UHPFRC) before and after exposure to cryogenic temperatures for liquefied natural gas (LNG) storage tank applications. Normal concrete (NC), which has been used to make LNG storage tanks in Korea, was also considered for comparison. In order to evaluate the cracking resistance of NC and UHPFRC, several edge-type slabs were fabricated and tested by restraining their thermal deformation. Four-point bending tests were also performed to estimate the flexural performance before and after cryogenic cooling. Test results indicate that UHPFRC exhibited higher resistance to microcrack formation under these conditions. UHPFRC also showed substantially better flexural performance, both before and after exposure to cryogenic cooling, compared to NC. In addition, the microcracks in UHPFRC that were induced by the pre-cracking load were suddenly and effectively filled with calcium carbonate (CaCO₃), which was formed by a chemical reaction between melting water and calcium ions. This was verified by energy dispersive X-ray spectroscopy analysis. CaCO₃ formation resulted in enhanced flexural performance, including higher strength, deflection capacity, and energy absorption capacity, as compared to the virgin UHPFRC specimens without any cracks.