复杂应力状态对混凝土梁外贴FRP条带抗剪贡献的影响

FRP剥离是外贴FRP抗剪加固混凝土梁主要的破坏模式之一。以往研究中往往简单的将面内剪切试验得到的FRP-混凝土界面粘结滑移关系应用于外贴FRP抗剪加固梁的剥离承载力计算。外贴FRP抗剪加固梁中FRP下的混凝土的应力状态与面内剪切试验情况有较大差别,这对FRP-混凝土界面的力学性能具有较大的影响。因此,以往的方法高估了FRP条带的抗剪贡献。该文研究了混凝土多轴应力状态对FRP-混凝土界面性能的影响,并根据试验研究结果,提出了U形FRP加固混凝土梁中FRP剥离应变的折减系数。与试验结果的对比计算分析表明:使用该折减系数修正后的设计公式更加合理。     

表层嵌贴预应力CFRP-strip加固钢筋混凝土梁的受力性能研究

体外粘贴预应力FRP技术和表层嵌贴FRP加固技术均是FRP应用于混凝土结构加固的重要技术,但存在各自的局限和不足。表层嵌贴预应力FRP技术有望结合上述两种技术的优点,进一步提高FRP加固混凝土结构的优势和效能。该文在研制成功FRP板条夹具的基础上,加固并进行了5根钢筋混凝土受弯构件的试验研究,深入考察了各试件尤其是表层嵌贴加固试件的力学行为与破坏形态,对比了体外与表层嵌贴加固试件的承载与变形性能差异,分析了预应力的存在对表层嵌贴加固构件力学性能与破坏模式的影响。试验结果表明:表层嵌贴预应力FRP板条加固显著提高了受弯构件的承载性能,其改善效果与体外粘贴预应力FRP板加固非常接近;嵌贴方式可使粘结树脂代替机械锚具锚固预应力FRP板条,但该文试验中表层嵌贴加固试件的破坏形态为粘贴端部的树脂-混凝土界面剥离和保护层混凝土撕裂,因此有需要就提高界面粘结能力和抑制保护层混凝土撕裂的构造措施开展进一步研究。     

内爆作用下不同类型填充墙对RC框架结构的影响

为了研究内爆作用下不同类型填充墙对RC框架结构的影响,减轻RC框架结构在爆炸荷载作用下的破坏程度,采用ANSYS/LS-DYNA软件对已有的RC框架结构、碳纤维布加固砌块填充墙爆炸试验进行数值模拟。通过模拟结果和试验结果的对比分析,验证了采用的数值模拟方法及参数设置是合理和适用的。在此基础上,通过数值模拟研究2层L型RC框架结构在相同内爆条件下,分别设置普通混凝土砌块填充墙、碳纤维布加固砌块填充墙、加固泄爆组合填充墙时构件的损坏程度。研究结果表明:在相同内爆作用下,设置不同类型填充墙的RC框架结构的破坏程度和破坏形态有明显差异,填充墙体对内爆作用下RC框架结构的影响不容忽视;全面采用碳纤维布加固砌块填充墙虽可减少爆炸引起的墙体碎片飞溅,但会使内爆作用下的RC框架结构产生较为严重的破坏,顾此失彼;与其他两种填充墙相比,采用加固泄爆组合填充墙可以有效减轻内爆作用下RC框架结构的破坏程度,减少爆炸引起的墙体碎片飞溅。     

FRP布加固混凝土框架子结构抗连续倒塌的精细有限元分析EI北大核心CSCD

外贴FRP布加固是一种有效提高既有建筑抗连续倒塌性能的手段,但现有FRP布加固方式存在降低结构抗震性能、加固施工不便等缺点。该文采用数值模拟方法分析了FRP布加固方式对现浇和装配式混凝土框架子结构抗连续倒塌与抗震性能的影响,并开展了优化方案研究。基于通用有限元软件LS-DYNA建立了FRP布加固混凝土框架子结构的连续倒塌精细数值模型,其中混凝土、钢筋与FRP布分别采用实体、梁与壳单元进行模拟,考虑了FRP布和钢筋的滑移、新旧混凝土界面的粘结失效和机械套筒处的钢筋截面损失。试验验证表明该方法可准确模拟试验试件的破坏模式和承载力发展。分析试验试件的不同粘贴方案结果发现:对现浇混凝土子结构,梁底与梁侧中性轴粘贴纵向FRP布并在梁端塑性铰区粘贴U形横向FRP布后,小变形下的结构倒塌抗力提升有限(最大仅2.6%)、基本不影响结构抗震性能,而对大变形下的结构倒塌抗力提升幅度可达49.5%;对于装配式混凝土子结构,在梁底、梁顶与梁侧底部外贴纵向布并在梁端塑性铰区粘贴U形横向FRP布可将小变形和大变形下的结构抗力最大提升24.2%和48.1%,使得装配式子结构在小变形下受力等同现浇结构,提升了原装配式子结构的抗震性能。对上述最优方案进一步的分析表明:保持FRP布用量不变而将塑性铰区内U形横向FRP布的分布范围和条数增加可提高大变形下的结构倒塌抗力,而不影响小变形下的加固效果。     

内爆作用下不同类型填充墙对RC框架结构的影响

为了研究内爆作用下不同类型填充墙对RC框架结构的影响,减轻RC框架结构在爆炸荷载作用下的破坏程度,采用ANSYS/LS-DYNA软件对已有的RC框架结构、碳纤维布加固砌块填充墙爆炸试验进行数值模拟。通过模拟结果和试验结果的对比分析,验证了采用的数值模拟方法及参数设置是合理和适用的。在此基础上,通过数值模拟研究2层L型RC框架结构在相同内爆条件下,分别设置普通混凝土砌块填充墙、碳纤维布加固砌块填充墙、加固泄爆组合填充墙时构件的损坏程度。研究结果表明:在相同内爆作用下,设置不同类型填充墙的RC框架结构的破坏程度和破坏形态有明显差异,填充墙体对内爆作用下RC框架结构的影响不容忽视;全面采用碳纤维布加固砌块填充墙虽可减少爆炸引起的墙体碎片飞溅,但会使内爆作用下的RC框架结构产生较为严重的破坏,顾此失彼;与其他两种填充墙相比,采用加固泄爆组合填充墙可以有效减轻内爆作用下RC框架结构的破坏程度,减少爆炸引起的墙体碎片飞溅。     

水下接触爆炸对舷侧空舱结构破坏载荷测试技术研究

为解水下接触爆炸对舷侧空舱结构破坏载荷特性,该文对其测试技术进行研究。对于冲击波的破坏载荷,采用DPS(多普勒光纤探针)获取外板在冲击波作用下的速度时程,分析得到不同接触爆炸条件下冲击波破坏载荷的时空分布特性;对于爆轰产物的破坏载荷,在外板上预制破口,在内板位置放置质量块作为效应物,采用激光位移传感器测得质量块在爆轰产物作用下的位移时程,分析得到爆轰产物破坏载荷随爆距的变化规律。研究结果表明,该文提出的水下接触爆炸对舷侧空舱结构破坏载荷的测试技术是可行的,可为相关理论研究和试验测试提供技术基础。     

爆炸冲击波作用下管道穿墙板防护密闭性破坏研究

爆炸冲击波作用下人防工程各种管道穿墙板构造部位防护密闭性是否受到破坏,是一个值得深入研究的问题.对爆炸冲击波作用下穿墙板管道进行了受力分析与计算,并对管道穿墙板的构造部位进行了爆炸冲击波模拟实验.试验结果表明人防工程在各抗力等级爆炸冲击波作用下,管道穿墙板做法可采用刚性防水套管做法替代刚性密闭套管做法,且穿墙板管道直径可以达到100 mm以上,同时,试验结果与提出的模型计算结论是相一致的,表明该模型的建立是合理的.     

爆炸冲击波与高速破片对夹层结构的联合毁伤效应试验研究

为探讨爆炸冲击波和高速破片联合作用对夹层结构的毁伤机理,进行了钢-玻璃钢-钢夹层结构空中近爆模型试验,分析了结构变形破坏模式及冲击波与高速破片的联合毁伤机制。结果表明,爆炸冲击波和高速破片联合作用下,结构的毁伤程度远大于冲击波的单独作用。爆炸冲击波单独作用时,夹层结构前、后面板均产生褶皱变形,玻璃钢夹芯板则以大面积分层破坏和纤维层的脱落为主。而在爆炸冲击波和高速破片联合作用下,前面板以反向大变形和中部穿甲大破口为主,后面板以大变形和花瓣开裂为主,玻璃钢夹芯板则产生了较大的穿甲破口和分层破坏。爆炸冲击波能量主要通过前、后面板的塑性大变形和玻璃钢夹芯板的分层破坏吸收。高速破片的动能则主要通过前、后面板的剪切破坏、玻璃钢板的分层破坏和纤维层的拉伸断裂吸收。     

高强钢丝绳网片-聚合物砂浆加固RC板抗爆性能试验研究

为了研究高强钢丝绳网片-聚合物砂浆对钢筋混凝土(RC)板的抗爆加固效果,对5块加固RC板和1块未加固RC板进行了野外现场爆炸试验,研究了砂浆强度、钢丝绳间距、钢丝绳预应力和界面增设销钉等因素对试件的破坏形态、裂缝分布及发展、跨中位移、钢筋应变等影响规律,并对爆炸试验后的试件进行了剩余承载力试验和爆炸损伤评估。研究表明:高强钢丝绳网片-聚合物砂浆加固能显著提高RC板的抗爆性能,相比于未加固板,加固板的裂缝宽度,板底跨中的峰值位移、残余位移和钢筋应变均大幅减小;加固后,构件剩余承载力大幅增加,其损伤程度大为降低。     

钢支撑加固RC框架抗连续倒塌试验与数值模拟研究EI北大核心CSCD

为研究钢支撑对RC框架抗连续倒塌性能的加固效应,在实验室制作了2榀RC平面子框架试件,并对其中1榀框架通过钢支撑进行加固,采用pushdown加载方式研究钢支撑对RC框架破坏模式和抗倒塌承载能力的影响。试验结果表明:钢支撑可以提高峰值荷载超过122%,其承载能力随拉杆的断裂而突然下降;由于压杆在加载初期就发生严重的平面外屈曲破坏,故仅对试件的初始刚度有影响,对峰值荷载作用很小。为进一步量化拉杆与压杆的加固贡献,通过商用有限元软件LS-DYNA进行精细有限元分析。有限元结果进一步验证了试验结果:采用角钢加固RC框架,压杆在加载初期容易发生平面外屈曲,对结构抗力影响不大,抗力提升主要由拉杆提供;通过拓展分析发现:采用防屈曲支撑加固可以显著提升压杆对结构抗力贡献,如压杆使用方钢管混凝土截面比L形截面峰值荷载提升超过42.8%。     

Strengthening slabs using externally-bonded strip composites: Analysis of concrete covers on the strengthening

This study pertains to the experimental and theoretical behaviour of slabs strengthened by fibre reinforced polymer (FRP). The experimental results show that FRP significantly increases punching failure stress, resulting in a reduction of slab rotation around the loading column. The theoretical investigation presents a finite element model for the bending of strengthened slabs. The developed model considers the concrete as a 3D multi-layered non-linear material and explicitly takes into account the steel reinforcement and the FRP strips. The proposed model is then used to analyse the effects of a concrete cover on the reinforcement and repairs. In the analysed cases, the results show that an average reduction in the concrete shear modulus, between steel rod and FRP, of more than 30% leads to significant reductions of stress and slab stiffness.     

RC two-way slabs strengthened with CFRP strips: experimental study and a limit analysis approach

This paper deals with strengthening of reinforced concrete (RC) two-way slabs with carbon fibre reinforced plastic (CFRP) strips bonded to the tensile face. The first part deals with an experimental study. The fibre reinforced plastics (FRP) strengthened slab test presents a failure mode with debonding of the external FRP strips from the slab. The second part deals with a limit analysis modelling. The strengthened slab is designed as a three-layered plate. A simplified laminated plate model is used to describe the behaviour of three-layered plate supported in four sides, which is subjected to a load in the centre. The upper bound theorem of limit analysis is used to approximate the ultimate load capacity and identify the different collapse mechanisms. Experimental results are compared with theoretical predictions.     

Nonlinear transient analysis of reinforced concrete slabs subjected to blast loading and retrofitted with CFRP composites

Computational models using the finite element method for nonlinear transient analysis of reinforced concrete (RC) two-way slabs subjected to blast loading are presented. Both as-built and retrofitted slabs with carbon fiber reinforced polymer (CFRP) composite strips are analyzed. The models are used to investigate different parameters including (a) loading duration, and (b) effect of CFRP retrofit on damage accumulation. In this study, damage is globally quantified by the amount of reduction of the first two vibrational frequencies of the slabs. Local representation of damage in terms of reinforcing steel strains is also discussed. The computational models for both the as-built and the retrofitted slabs are verified using experimental results. In these experiments, a slowly increasing uniform pressure is applied to the bottom surface of large-scale RC slab specimens using high-pressure water bag. Experimental results showed that an increase up to 200% in the load carrying capacity is achieved when using the CFRP composite retrofit system. Transient nonlinear analysis results proved the efficiency of the CFRP composite retrofit in improving the slab behavior under blast loading for different loading durations, i.e. for small, medium, and large charge weights at the same applied maximum pressure. In particular, less than 50% reduction of the fundamental frequency due to concrete damage is obtained for the retrofitted slab compared to more than 85% reduction for the as-built slab. Moreover, the maximum displacement is reduced by 40–70% with the CFRP retrofit compared to the as-built slab. As for reinforcing steel strains, the application of CFRP retrofit significantly limited the spread of yielding in time and space. The improved slab behavior with CFRP is best when retrofitting is applied to both sides of the slab.     

Design limits for RC slabs strengthened with hybrid FRP–HPC retrofit system

A polymeric hybrid composite system made of high-performance concrete (HPC) and an innovative carbon/epoxy reinforced polymer (CFRP) unidirectional laminates was proposed as a retrofit system to enhance flexural strength and ductility of reinforced concrete (RC) slabs. The effectiveness of the proposed system was confirmed through experimental evaluation of three full-scale one-way slabs having two continuous spans. In this study, the results of the loading tests for the hybrid high-performance retrofit system are presented and discussed. Design limits to derive a flexural failure of a continuous RC slab strengthened with the hybrid retrofit system are extracted. Using the proposed design limits, the procedure of a flexural failure design for a continuous RC slab strengthened with the hybrid retrofit system is demonstrated with numerical examples for two types of the retrofit systems with respect to overlay strength. The flexural failure design limits can be extended for flexural and shear strengthening design with externally bonded FRP to ensure flexure failure for a continuous flexural members.     

Impact resistant behaviour of RC slab strengthened with FRP sheet

To investigate the impact resistance of RC slabs strengthened with Fibre Reinforced Plastic (FRP) sheet to the back of the slab, falling-weight impact tests were conducted. Two loading types were applied: iterative loading and single loading. The impact load was applied to the centre of the RC slab with a free falling 300 kg steel striker with a diameter of 60 mm. A total of 12 RC slabs that were 1,650 (L) × 1,650 (W) × 150 (h) mm were used for these experiments. In this study, the strengthening method and material properties of the FRP sheet and the number of FRP sheet layers were varied. The results obtained from this study are as follows: (1) the impact resistance of the RC slabs can be improved by attaching a strengthened FRP sheet to the back surfaces; (2) the load bearing mechanism of RC slabs depends on the loading type and strengthening volume of the FRP sheet; and (3) the dynamic amplification factor is about two, which is independent of the load bearing mechanism of the RC slab and the strengthening volume and tensile rigidity of the FRP sheet.     

Use of infrared thermography for quantitative non-destructive evaluation in FRP strengthened bridge systems

The increasing use of fiber reinforced polymer (FRP) composites as externally bonded reinforcement for strengthening of concrete structures has created a need for the development and implementation of rapid methods of field level non-destructive evaluation (NDE) both for quality assurance during installation of the material and also for long-term monitoring of in-field performance. These methods must provide inspectors with the ability not only to inspect FRP strengthened structures for defects, but also should provide the means to quantify the shape, size and severity of the defect for continued monitoring and comparison. Infrared (IR) thermography, which uses data from temperature differentials to detect and characterize defects and anomalies, presents a potentially efficient technique for non-contact, real-time inspection and quantitative data interpretation. In this research a three-girder two bay reinforced concrete bridge deck segment was loaded under field representative loading conditions and the girders and slabs were sequentially strengthened with FRP composites to study the effect of FRP strengthening at systems level. The two slabs were strengthened with two different composite systems, prefabricated strips and field impregnated fabric laminates. The appearance and progression of damage in the FRP systems and at the FRP–concrete interface with an increase in the level of loading was quantitatively monitored using IR thermography. Based on the thermal intensity, the damage could be classified into types such as interlaminar debonding inside the composite or composite–concrete interface debonding. Also based on the magnitude of the thermal intensity, the severity of the damage could be quantitatively monitored. The results were also correlated to the visual observations of crack patterns and failure mechanisms as well as to strain and displacement data measured during the test.     

Failure diagrams of FRP strengthened RC beams

Amongst various methods developed for strengthening and rehabilitation of reinforced concrete (RC) beams, external bonding of fibre reinforced plastic (FRP) strips to the beam has been widely accepted as an effective and convenient method. The experimental research on FRP strengthened RC beams has shown five most common modes, including (i) rupture of FRP strips; (ii) compression failure after yielding of steel; (iii) compression failure before yielding of steel; (iv) delamination of FRP strips due to crack; and (v) concrete cover separation. In this paper, a failure diagram is established to show the relationship and the transfer tendency among different failure modes for RC beams strengthened with FRP strips, and how failure modes change with FRP thickness and the distance from the end of FRP strips to the support. The idea behind the failure diagram is that the failure mode associated with the lowest strain in FRP or concrete by comparison is mostly likely to occur. The predictions based on the present failure diagram are compared to 33 experimental data from the literature and good agreement on failure mode and ultimate load has been obtained. Some discussion and recommendation for practical design are given.     

Effective strain of RC beams strengthened in shear with FRP

The failure modes of Reinforced Concrete (RC) beams strengthened in shear with Fiber Reinforced Polymer (FRP) sheets or strips are not well understood as much as those of RC beams reinforced with steel stirrups. When the beams are strengthened in shear with FRP composites, beams may fail due to crushing of the concrete before the FRP reaches its rupture strain. Therefore, the effective strain of the FRP plays an important role in predicting the shear strength of such beams. This paper presents the results of an analytical and experimental study on the performance of reinforced concrete beams strengthened in shear with FRP composites and internally reinforced with conventional steel stirrups. Ten RC beams strengthened with varying FRP reinforcement ratio, the type of fiber material (carbon or glass) and configuration (continuous sheets or strips) were tested. Comparisons between the observed and calculated effective strains of the FRP in the tested beams failing in shear showed reasonable agreement.