Repair and structural performance of initially cracked reinforced concrete slabs

Crack is one of the most common defects observed in reinforced concrete slabs and beams. Major cracks in concrete structures may occur due to overloading, corrosion of reinforcement or differential settlement of support. To restore the structural capacity of the distressed elements, retrofitting and/or strengthening are needed. There are different techniques available for retrofitting and strengthening of different reinforced concrete structural elements reported in the literature. This paper investigates the structural behaviour of cracked reinforced concrete one-way slab, which is repaired using different techniques.Five different techniques are used for the purpose of repair in the cracked concrete slab namely; cement grout, epoxy injection, ferrocement layer, carbon fibre strip and section enlargement. The slabs were loaded to failure stage and the structural response of each slab specimens have been predicted in terms of deflection, variation of strain in concrete and steel, collapse loads and the failure modes.The efficiency of different repair and strengthening techniques and their effects on the structural behaviour of cracked one-way reinforced concrete slab had been analyzed. It was observed that the type of repair technique used will affect the load carrying capacity of the slab and will lead to a redistribution of the strains and hence stresses in both concrete and steel reinforcement. All repair techniques are found to be able to restore or enhance the structural capacity of cracked concrete slabs.     

GFRP筋地下连续墙混凝土板受弯性能试验研究及有限元分析

为了解GFRP筋地下连续墙的受弯性能,通过GFRP筋混凝土板和钢筋混凝土板的对比受弯试验,分析了两者的受力-变形过程和破坏形态,对比了两者的挠度、开裂荷载、极限荷载以及混凝土应变。结果表明：GFRP筋混凝土板的受力-变形曲线大致可划分为开裂前和开裂后两个阶段,其破坏表现为脆性;混凝土开裂前两种板的截面应变变化规律均基本符合平截面假定,但开裂后GFRP筋混凝土板的挠度增长速率远大于钢筋混凝土板,且该速率基本不变;两种板的开裂荷载较为接近,而GFRP筋混凝土板的极限荷载为钢筋混凝土板的1.2倍。在试验基础上,建立了GFRP筋混凝土板的有限元模型,通过参数分析表明,GFRP筋混凝土板的抗弯刚度在开裂后随配筋率的增大而增大。图13表6参8     

基于改性磷酸盐水泥的CFRP网格加固钢筋混凝土板的受弯试验

采用碳纤维(Carbon Fiber Reinforced Polymer,CFRP)网格增强无机改性磷酸盐水泥基材料(Magnesium Phosphate Cementitious,简称MPC)加固钢筋混凝土板,MPC是对磷酸盐水泥在配合比和矿物掺合料两个方面进行改性优化,该加固方法充分利用了MPC具有早期强度高、低收缩、与旧混凝土黏结力强、良好的耐久性、防火防老化等优点。主要开展了CFRP网格增强MPC水泥基体加固混凝土板的抗弯试验,试验结果表明,破坏模式为混凝土板与加固层界面的剥离破坏;加固后板的开裂荷载提高了133%,极限荷载提高了108%。CFRP网格增强无机磷酸盐水泥基体复合材料加固技术不仅能够有效地提高混凝土板的抗弯承载力,还可以提高构件的刚度和抗裂性能。     

纤维增强复合材料加固钢筋混凝土单向板受弯性能研究

通过纤维增强复合材料（FRP）布加固钢筋混凝土单向板的受弯性能试验,研究了FRP布种类、层数、宽度、粘贴方式和锚固措施等因素对加固效果的影响。结果表明：在板底粘贴CFRP布和高强GFRP布均可明显提高钢筋混凝土单向板的受弯承载力和纵筋屈服后刚度。当采用横向粘贴1层U形CFRP布条作为端部锚固措施时,所有加固单向板试件的破坏模式均为跨中弯曲裂缝引起的剥离破坏。当FRP布宽度和层数相同时,采用CFRP布加固效果优于高强GFRP布加固。与对比试件相比,FRP布加固钢筋混凝土单向板试件的位移延性略有降低。高强GFRP布加固钢筋混凝土单向板的位移延性优于CFRP布加固钢筋混凝土单向板。FRP布的粘贴方式对FRP加固钢筋混凝土单向板的位移延性也有影响,单层FRP布加固单向板试件的延性较好。通过对试验结果的分析,提出了FRP布加固钢筋混凝土单向板构件受弯破坏模式的判别方法,验证了已有文献和GB 50608-2010《纤维增强复合材料建设工程应用技术规范》中给出的跨中弯曲裂缝引起的FRP剥离应变计算公式对FRP布加固混凝土单向板的适用性,建立了FRP布加固钢筋混凝土单向板受弯承载力计算方法。     

GFRP筋加固混凝土板的试验分析

本文介绍了GFRP筋混凝土板和钢筋混凝土板的试验室抗弯试验,描述了试件的受力～变形过程和破坏形态,对两种混凝土板的开裂荷载和极限荷载、挠度以及混凝土的应变进行了对比和分析。同时,对不同配筋率GFRP筋混凝土板进行了有限元模拟试验,探讨了配筋率对GFRP筋混凝土板刚度的影响,给出了GFRP筋混凝土板开裂后抗弯刚度的计算公式。     

钢筋再生混凝土楼板的承载性能试验研究

为了研究钢筋再生混凝土楼板的力学性能,共设计了6个试件,其中3个单向板和3个双向板,主要以再生粗骨料取代率为变化参数,取代率分别为0%,50%,100%。通过对两类楼板进行抗弯试验,观察试件的破坏形态、获取了试件受力的全过程曲线、跨中挠度发展情况及构件的极限承载力等重要数据,并分析再生粗骨料取代率对单向板和双向板的影响,利用相关规范的计算方法对再生混凝土楼板承载能力进行计算,并与试验实测结果进行对比。试验结果表明:不同骨料取代率的楼板破坏形态相似;在混凝土出现裂缝前,随着荷载的增加,楼板的挠度基本呈线性增大;当混凝土开裂后,钢筋再生混凝土楼板的挠度增大有突变;钢筋再生混凝土单向板的刚度比钢筋普通混凝土单向板好,双向板则相反;钢筋再生混凝土楼板与钢筋普通混凝土楼板相比,两者开裂荷载较为接近,但钢筋再生混凝土楼板有极限承载力高的优点。研究结果可为钢筋再生混凝土板构件的进一步研究和推广应用提供参考。     

页岩陶粒轻质混凝土双向叠合楼板

根据3块足尺页岩陶粒轻质混凝土双向叠合楼板试件和1块足尺现浇双向楼板对比试件的静力堆载试验结果,分析了双向叠合楼板预制底板的普通混凝土和后浇叠合层的页岩陶粒轻质混凝土采用相同强度等级时的挠度变形特点,并根据试验结果和弹性力学薄板理论推导了页岩陶粒轻质混凝土双向叠合楼板的挠度计算公式。推导了四边固支页岩陶粒混凝土和普通混凝土叠合板在竖向均布荷载作用下截面中和轴高度、抗弯刚度和挠度的计算表达式,并分别对挠度和初始刚度计算公式进行了修正,提出了考虑四边支承转动的页岩陶粒轻质混凝土双向叠合楼板挠度计算公式。结果表明:页岩陶粒轻质混凝土双向叠合楼板跨中位移呈现线弹性变化特征,页岩陶粒轻质混凝土材料的弹性模量、叠合楼板中设置的钢筋桁架以及双向叠合板的板端约束条件是影响叠合楼板刚度和跨中挠度的主要因素;采用所推导公式计算的页岩陶粒混凝土叠合楼板的挠度值与试验值吻合良好。     

Steel-Concrete-Steel sandwich slabs with lightweight core — Static performance

This paper investigates the static performance of Steel-Concrete-Steel (SCS) sandwich slabs which consist of an ultra-lightweight concrete core sandwiched between two steel plates. Special J-hook connectors have been developed to develop composite action between the concrete core and the two steel plates. The core is made of lightweight concrete of density less than 1450 kg/m³. Laboratory tests were carried out on eight SCS sandwich slabs under centrally applied patch load. Test results showed that the mode of failure and crack pattern of SCS sandwich slabs were very similar to those of reinforced concrete slabs especially when the concrete core and steel plates act in a fully composite manner. Flexural and punching are the primary modes of failure. After flexural yielding, membrane action developed in the slab due to the effectiveness of J-hook connectors in maintaining composite action which further increases its load carrying capacity after flexural yielding. Theoretical models are proposed to predict the flexural and punching resistance and a good correlation with test results is obtained. A large deflection analysis considering plate membrane action is also proposed to predict the force deflection relation of SCS sandwich slabs.     

预应力碳纤维布加固钢筋混凝土梁力学性能试验

基于预应力碳纤维布加固钢筋混凝土梁能够提高其承载能力的理论,进行了6根外贴预应力碳纤维布(CFRP)加固钢筋混凝土梁的力学性能试验。研究了在不同预应力和不同端部锚固方式下加固后的钢筋混凝土梁的预应力损失、开裂荷载、跨中挠度、极限荷载及抗弯抗剪性能。结果表明:碳纤维布预应力在一定范围内能够决定钢筋混凝土梁的加固效果,同时在梁端部采取锚固方式能有效阻止预应力碳纤维布在放张过程中的预应力损失;预应力碳纤维布加固后梁的开裂荷载随着预应力的增大而增大,最大增大幅度达到81.8%;极限荷载在一定范围内随预应力的增大先增大后减小,极限荷载最大增大幅度为41.07%。     

碳纤维布加固预应力空心板承载力与延性试验分析

本文通过7块足尺寸预应力混凝土空心板粘贴碳纤维布后,在均布荷载作用下的试验,进行了抗弯承载力和位移延性的研究。提出了碳纤维布加固预应力混凝土空心板延性的概念和计算方法,分析了碳纤维布不同粘贴方式、不同粘贴量对承载力和延性的影响,可以得出如下结论:碳纤维布加固预应力混凝土空心板可有效提高承载力,提高程度达48%~109%;粘贴碳纤维布后预应力混凝土空心板在预应力钢筋屈服后,其跨中挠度增长显著,显示出较好的延性。     

某高层住宅楼大跨度楼板加固处理

通过加固工程实例介绍了大跨度楼板板面有墙荷载,楼板承载力不足时,采用板面浇筑混凝土叠合层,板底粘贴碳纤维布的加固方法。对该大跨度楼板进行加固处理后,后期使用荷载作用下的挠度进行了计算,找出加固时需要进行顶升处理的楼板,针对现场情况提出了一种简单、易行的顶升处理方法。     

Repair of heat-damaged reinforced concrete slabs using fibrous composite materials

Sixteen under-reinforced high strength concrete one-way slabs were cast, heated at 600 °C for 2 h, repaired, and then tested under four-point loading to investigate the coupling effect of water recuring and repairing with advance composite materials on increasing the flexural capacity of heat-damaged slabs. The composites used included high strength fiber reinforced concrete layers; and carbon and glass fiber reinforced polymer (CFRP and GFRP) sheets. Upon heating then cooling, the reinforced concrete (RC) slabs experienced extensive map cracking, and upward cambering without spalling. Recuring the heat-damaged slabs for 28 days allowed recovering the original stiffness without achieving the original load carrying capacity. Other slabs, recured then repaired with steel fiber reinforced concrete (SFRC) layers, regained from 79% to 84% of the original load capacity with a corresponding increase in stiffness from 382% to 503%, whereas those recured then repaired with CFRP and GFRP sheets, regained up to 158% and 125% of the original load capacity with a corresponding increase in stiffness of up to 319% and 197%, respectively. Control, heat-damaged, and water recured slabs showed a typical flexural failure mode with very fine and well distributed hairline cracks, propagated from the repair layers to concrete compression zone. RC slabs repaired with SFRC layers failed in flexural through a single crack, propagated throughout the compression zone, whereas those repaired with CFRP and GFRP experience yielding failure of steel prior to the composites failure.     

CFRP片材加固混凝土梁收缩徐变效应研究

按换算截面法推导了FRP片材加固混凝土梁的短期与长期挠曲变形计算公式,在此基础上编制了相应的分析计算程序,该分析方法能较准确预测开裂混凝土梁的长期挠曲变形。制作了10根钢筋混凝土梁,其中6根为普通钢筋混凝土梁,另外4根为CFRP片材加固梁,并对试件的短期与长期受力性能进行了试验观测,持续荷载作用下对试件进行了近700d的收缩徐变试验。运用本文计算方法预测了试件的长期挠曲变形,理论计算值与本文试验结果吻合较好。研究表明:碳纤维片材对混凝土梁的长期挠曲变形影响很小,但碳纤维片材能有效地控制持续荷载作用下裂缝的进一步开展。研究结果可为CFRP加固混凝土结构工程设计与理论分析提供参考。     

预应力高强钢绞线网加固钢筋混凝土板的试验研究

采用新型张拉锚固系统对钢绞线网施加预应力,研究预应力水平值对板加固效果的影响规律。对3块不同预应力水平加固板、1块非预应力加固板和1块对比板进行抗弯试验研究。试验结果表明:较对比板,预应力加固板的承载力、截面刚度大幅度提高,裂缝宽度明显减小;较非预应力加固板,其开裂荷载显著提高,屈服荷载、极限荷载、截面刚度均有提高,裂缝宽度减小;随预应力水平的提高,加固效果、钢绞线强度的利用率明显提高。对钢绞线的预应力损失进行分析,提出预应力加固板承载力的计算公式,计算结果与试验值吻合良好,可作为预应力钢绞线抗弯加固混凝土板理论分析和设计的参考。预应力高强钢绞线加固技术解决了非预应力加固方法钢绞线应变滞后、发生剥离破坏、高强度利用率低等缺点,是一种主动高效的加固技术,具有较高的推广应用价值。     

Zeitabhängiges Verhalten von Makrokunststofffaserbeton und dessen Einfluss auf die Bemessung von Industriefußböden

Time Dependent Behaviour of Macro Synthetic Fibre. Reinforced Concrete and its Influence on the Design of Industrial Slabs on Ground Material tests and tests on structural elements with synthetic macro fibre reinforced concrete are presented. The experimental program included the investigation of the load carrying capacity and the deformation behaviour under short‐term as well as under long‐term loading conditions. Long‐term load tests were carried out on pre‐cracked fibrereinforced concrete beams to measure the load level above which creep failure can occur. At service load level creep coefficients for a time period of up to three years were determined. Centre point load tests were carried out on concrete slabson‐ground. Two slabs were reinforced with steel mesh and two with synthetic macro fibres only. The tests were performed under short‐term and long‐term loading conditions over a period of 18 month. The current main application of synthetic macro fibre reinforced concrete is slab‐on‐ground. Slabs‐on‐ground can be designed under the assumption of elastic subgrade reaction. The concrete slab itself can be calculated either by elastic theory for uncracked concrete or by yield line theory taking fibre reinforcement into account. Based on the test results of the experimental programme, design recommendations for permanently loaded slabs‐on‐ground are presented.     

爆炸荷载作用下外贴FRP加固钢筋混凝土双向板试验研究

通过集团装药隔土爆炸荷载作用下4块外贴FRP条带加固钢筋混凝土双向板和1块普通板的对比试验,考察了裂缝的产生、开展过程及分布形状,分析了FRP加固板的荷载、位移、加速度、钢筋和混凝土以及FRP应变动力响应时程,研究了FRP加固板的抗爆破坏特征。研究结果表明：外贴FRP条带加固能有效延缓混凝土的开裂,限制裂缝的开展,改善钢筋混凝土板的抗爆性能;外贴FRP条带加固后,RC双向板的跨中位移响应、混凝土和钢筋应变响应明显降低,结构的抗爆炸冲击波能力得到明显提高;外贴FRP条带加固双向板在爆炸冲击荷载作用下的破坏形态有受弯破坏和弯曲屈服后的剪切破坏,外贴FRP条带在极限状态时发生了剥离及断裂破坏。图12表6参10     

集中荷载作用下后浇UHTCC简支双向混凝土板的试验研究

在既有钢筋混凝土板的受弯面上浇筑一定厚度超高韧性水泥基复合材料(UHTCC)或混凝土材料制成复合板,通过集中荷载作用下简支双向板试验,考察复合板试件的弯曲破坏形态和底面裂缝的走向特征等。试验结果表明,后浇材料有效提高整体构件的承载力和前期刚度。后浇混凝土复合板与对比混凝土板相比较,韧性改善较小,开裂时中心挠度基本没有变化;而UHTCC后浇层的加入,改善混凝土板的整体韧性,开裂时中心挠度与原构件相比最高可提高至235%。并且UHTCC层有效限制受弯底面裂缝的开裂和扩展,延迟有害裂缝的出现,加载过程中没有出现类似于后浇混凝土复合板的脆性破坏。运用非线性三维有限元软件对试验进行数值计算,通过合理的建模和定义材料属性,对试验结果进行分析预测,包括荷载-挠度曲线、受弯底面的挠度分布和应变分布等。分析结果与试验结果基本一致,表明此方法适用于文中复合板件的计算,为采用UHTCC作为保护层或加固层的复合双向板的设计、分析和优化提供分析依据。     

反拱法加固混凝土受弯构件理论及试验研究

对采用反向起拱粘贴碳纤维布加固混凝土梁构件的基本原理和设计方法进行了初步研究,提出了基于预应力原理的反拱法碳纤维布材加固梁构件的施工工艺。在此基础上,进行了3根试件的试验。通过试验结果对比了不同加固工艺的受弯构件的开裂荷载、极限荷载、抗弯刚度等工作性能,分析了反拱加固的预应力效应。试验发现,反拱加固试件的开裂荷载较普通粘贴加固试件的开裂荷载提高了36.2%,屈服条件下的变形减小了24.2%。试验结果表明,采用反拱加固可方便有效地卸除加固构件的残余应力,并产生预应力效应,可充分发挥碳纤维布材料高强度的特性,提高构件刚度和承载能力。     

Assessing the effectiveness of embedding CFRP laminates in the near surface for structural strengthening

Near surface mounted (NSM) is a recent strengthening technique based on bonding carbon fiber reinforced polymer (CFRP) bars (rods or laminate strips) into pre-cut grooves on the concrete cover of the elements to strength. To assess the effectiveness of the NSM technique, an experimental program is carried out involving reinforced concrete (RC) columns, RC beams and masonry panels. In columns failing in bending the present work shows that the failure strain of the (CFRP) laminates can be attained using the NSM technique. Beams failing in bending are also strengthened with CFRP laminates in order to double their load carrying capacity. This goal was attained and maximum strain levels of about 90% of the CFRP failure strain were recorded in this composite material, revealing that the NSM technique is also very effective to increase the flexural resistance of RC beams.The effectiveness of externally bonded reinforcing (EBR) and NSM techniques to increase the flexural resistance of masonry panels is also assessed. In the EBR technique the CFRP laminates are externally bonded to the concrete joints of the panel, while in the NSM technique the CFRP laminates are fixed into precut slits on the panel concrete joints. The NSM technique provided a higher increase on the panel load carrying capacity as well as a larger deflection at the failure of the panel.The performance of EBR and NSM techniques for the strengthening of RC beams failing in shear is also analyzed. The NSM technique was much more effective in terms of increasing the beam load carrying capacity as well as the beam deformability at its failure. The NSM technique was easier and faster to apply than the EBR technique.     

预应力碳纤维布材加固混凝土受弯构件的抗弯性能研究

本文对预应力碳纤维布材加固混凝土梁构件的性能做了初步的研究,并提出了应用预应力碳纤维布材加固梁构件的施工工艺。在此基础上,作者进行了9根试件的试验。通过试验结果对比了预应力碳纤维布加固的受弯构件与非预应力碳纤维布加固的受弯构件的开裂荷载、极限荷载、抗弯刚度等工作性能,分析了预应力对构件弯曲性能的影响,讨论了预应力水平变化引起的构件使用荷载以及变形能力的变化。试验发现预应力碳纤维布材加固的试件的开裂荷载较对比试件提高了43%至214%,在对比试件屈服荷载下的变形为对比试件的29.8%至56.3%。试验结果与分析表明预应力碳纤维布材极大地提高了梁构件的工作性能。另外,本文提出了预应力碳纤维布材加固的受弯构件的界限补强率、极限承载力、抗弯刚度以及挠曲变形的计算公式。