Seismic performance of steel reinforced ultra high-strength concrete composite frame joints

To investigate the seismic performance of a composite frame comprised of steel reinforced ultra high-strength concrete （SRUHSC） columns and steel reinforced concrete （SRC） beams, six interior frame joint specimens were designed and tested under low cyclically lateral load. The effects of the axial load ratio and volumetric stirrup ratio were studied on the characteristics of the frame joint performance including crack pattern, failure mode, ductility, energy dissipation capacity, strength degradation and rigidity degradation. It was found that all joint specimens behaved in a ductile manner with flexural-shear failure in the joint core region while plastic hinges appeared at the beam ends. The ductility and energy absorption capacity of joints increased as the axial load ratio decreased and the volumetric stirIup ratio increased. The displacement ductility coefficient and equivalent damping coefficient of the joints fell between the corresponding coefficients of the steel reinforced concrete （SRC） frame joint and RC frame joint. The axial load ratio and volumetric stirrup ratio have less influence on the strength degradation and more influence on the stiffness degradation. The stiffness of the joint degrades more significantly for a low volumetric stirrup ratio and high axial load ratio. The characteristics obtained from the SRUHSC composite frame joint specimens with better seismic performance may be a useful reference in future engineering applications.

Charactersitics of Stress-strain Curve of High Strength Steel Fiber Reinforced Concrete under Uniaxial Tension

A whole of 110 specimens divided into 22 groups were tested with varying the volume fraction of steel fibers and the matrix strength of these specimens. The stress-strain behaviors of four types of steel fiber reinforced concrete （SFRC） under uniaxial tension were studied experimentally. When the matrix strength and the fiber content increase, the tensile stress and tensile strain vary differently according to the fiber type. The mechanisms of reinforcing effect for different types of fiber were analyzed and the stress-strain curves of the specimens were plotted. Some experimental factors for stress or strain of SFRC were given. A tensile toughness modulus Re0.5 was introduced to evaluate the toughness characters of SFRC under uniaxial tension. Moreover, the formula of the tensile stress-strain curve of SFRC was regressed. The theoretical curve and the experimental ones fit well, which can be used for references in construction.

Structural Behavior of Continuous Prestressed Steel Fiber Reinforced High Strength Concrete Beam

The flexural behaviors of continuous fully and partially prestressed steel fiber reinforced high strength concrete beams are studied by experiment and nonlinear finite element analysis. Three levels of partial prestress ratio （PPR） are considered, and three pairs of two-span continuous beams with box sections varying in size are designed. The major parameters involved in the study include the PPR and the fiber location. It is concluded that the prestressed high strength concrete beam exhibits satisfactory ductility; the influences of steel fiber on the crack behaviors for partially prestressed beams are not as obvious as those for fully prestressed ones; steel fibers can improve the structural stiffness after cracking for fully prestressed high strength concrete beams; the moment redistribution from mid-span to intermediate support in the first stage should be mainly considered in practical design.

Seismic Performance of Steel Reinforced Ultra High-strength Concrete Columns

The seismic performance of steel reinforced ultra-high-strength concrete columns(SRSHC) with various shear-span ratios(λ) were studied through a series of experiments.The concrete compressive cube strength value of experimental specimens ranged from 92.9 MPa to 108.1 MPa.The main experimental variables affecting seismic performance of specimens were axial load ratio and stirrup reinforcement ratio.The columns(λ=2.75) subjected to low cyclic reversed lateral loads failed mainly in the flexural-shear mode failure and columns(λ≤2.0) subjected to low cyclic reversed lateral loads failed mainly in the shear mode failure.Shear force-displacement hysteretic curves and skeleton curves were drawn.Coefficient of the specimen displacement ductility was calculated.Experimental results indicate that ductility decreases with axial pressure ratio increasing,and increases with stirrup reinforcement ratio increasing.Limit values of axial pressure ratio and minimum stirrup reinforcement ratio of columns are proposed to satisfy definite ductility requirement.The suggested values provide a reference for engineering application and for the amendment of the current Chinese design code of steel reinforced concrete composite structures.

Properties of High Strength Steel Fiber Reinforced Concrete under Compression

This paper mainly discusses the properties of high strength steel fiber reinforced concrete under compression. Steel fibers with volume content of 1% do not display significant effect on the strain at peak stress and the area of the ascending portion of

Seismic Behavior of Steel Reinforced Ultra High Strength Concrete Column and Reinforced Concrete Beam Connection

To investigate the seismic behavior of connections composed of steel reinforced ultra high strength concrete (SRUHSC) column and reinforced concrete (RC) beam, six interior strong-column-weak-beam connection specimens were tested subjected to reversal cyclic load. Effects of applied axial load ratio and volumetric stirrup ratio on ductility, energy dissipation capacity, strength degradation and rigidity degradation were discussed. It was found that all connection specimens failed in bending in a ductile manner with a beam plastic hinge. The ductility and energy dissipation capacity increased with the decrease of applied axial load ratio or increase of volumetric stirrup ratio. The displacement ductility coefficient and equivalent damping coefficient lay between those of steel reinforced ordinary concrete connection and those of reinforced concrete connection. The applied axial load ratio and volumetric stirrup ratio had less influence on the strength degradation and more influence on the stiffness degradation. The stiffness degraded sharply with the decrease of volumetric stirrup ratio or increase of applied axial load ratio. The experimental results indicate that SRUHSC column and RC beam connection exhibited better seismic performance and can provide reference for engineering application.

Experimental Shear Study on Reinforced High Strength Concrete Beams Made Using Blended Cement

With the increased application of High Strength Concrete(HSC)in construction and lack of proper guidelines for structural design in India,behavioral study of high strength concrete is an important aspect of research.Research on the behavior of HSC reinforced beams with concrete strength more than 60 MPa has been carried out in the past and is still continuing to understand the structural behavior of HSC beams.Along with the many benefits of the high strength concrete,the more brittle behavior is of concern which leads to sudden failure.This paper presents the behavior of reinforced HSC beams in shear with considering the effects of various factors like shear reinforcement ratio,longitudinal reinforcement ratio,l/d ratio(length to depth ratio),etc.Ten numbers Reinforced Concrete Beams of various sizes using concrete mix with three different w/c ratios(0.46,0.26 and 0.21)were cast for shear strength assessment.The beams were tested in simply supported condition over two fixed steel pedestals with load rate of 0.2 mm/minute in displacement control.Mid-point deflection was measured using LVDT.A comparative analysis of theoretical approaches of Euro code,extension of current IS code up to M90 and the experimental data was done to understand the behavior of beams.Shear capacities of beams without any factors of safety were used to assess the actual capacities and then was compared with the experimental capacity obtained.Results of this study can be used in the design of high strength concrete and will be more reliable in Indian continent as the regional materials and exposure conditions were considered.

A Study on the Bond Strength between High Performance Concrete and Reinforcing Bar

As a preliminary study for the erection of floating structures using high performance concrete, this paper examines the bond characteristics between concrete and the reinforcing bar. Since the floating structure is constructed in aquatic environment, corrosion of the reinforcing steel is likely to develop more prematurely than in onshore structure in case of concrete cracking. A solution to this corrosion problem could use FRP rebar instead of steel reinforcement. To that goal, an experimental study is conducted on the concrete-FRP bond strength to verify if such FRP rebar develops performance comparable to the conventional steel rebar. A series of tests are performed considering the bond length of ordinary steel rebar and G-FRP rebar as test variable with respect to the strength of concrete, and the results are presented.

Spalling and Mechanical Properties of Fiber Reinforced High-performance Concrete Subjected to Fire

Spalling and mechanical properties of FRHPC subjected to fire were tested on notched beams. The results confirm that the internal vapor pressure is the leading reason for spalling of high-performance concrete （HPC）. At the same time, the temperature-increasing velocity and constrained conditions of concrete element also play significant roles in spalling. Steel fibers cannot reduce the risk of spalling, although they have obvious beneficial effects on the mechanical properties of concrete before and after exposure to fire. Polypropylene （PP） fibers are very useful in preventing HPC from spalling, however, they have negative effects on the strengths. By using hybrid fibers （steel fibers＋PP fibers）, both good anti-spalling performance and improved mechanical properties come true, which may provide necessary safe guarantee for the rescue work and structure repair after fire disaster.

纤维织物增强高延性混凝土加固RC短柱抗剪性能试验研究

为研究纤维织物增强高延性混凝土(TR-HDC)加固钢筋混凝土短柱的抗剪性能,设计了6根钢筋混凝土柱,包括2个对比柱和4个TR-HDC加固柱.通过低周反复荷载试验,对比分析剪跨比、纤维织物层数对试件破坏形态、变形、承载力和耗能能力的影响.结果表明:采用TR-HDC加固钢筋混凝土短柱,可显著提高其抗剪承载力;TR-HDC与原混凝土柱协同工作性能良好,加固后的混凝土柱的变形、承载力和耗能能力明显提高;增加纤维织物的层数对钢筋混凝土短柱的抗剪承载力提高幅度较小,但可大幅增强柱的耗能和变形能力;剪跨比较大时,更有利于发挥TR-HDC加固材料的力学性能.基于桁架-拱模型,提出TR-HDC加固钢筋混凝土短柱的抗剪承载力计算方法,计算结果较准确.

UHPC-RC节段组合梁的抗弯性能分析

为研究低预应力度条件下,不同节段接缝的超高性能混凝土-钢筋混凝土(ultra-high performance concrete-reinforced concrete,UHPC-RC)组合梁的抗弯性能,文章基于ABAQUS有限元软件,建立UHPC-RC节段组合梁的基准有限元模型,并利用已有的试验数据验证该模型的合理性。节段接缝分别为平面干接缝、平面胶接缝、齿键干接缝和齿键胶接缝,在基准有限元模型的基础上,进一步研究节段接缝类型对组合梁抗弯性能的影响。研究发现,在四点受弯加载方式下,接缝的几何特征(平面或齿键)对组合梁的极限荷载和开裂荷载影响较小,环氧树脂胶的使用对组合梁的抗弯性能影响较大。

Mechanism of improving ductility of high strength concrete T-section beam confined by CFRP sheet subjected to flexural loading

For the purpose of inventing a new seismic retrofitting method for the reinforced high strength concrete （HSC） T-section beam using carbon fiber reinforced polymer （CFRP） sheet, three series, a total of twelve T-section beams with nine specimens confined by CFRP sheet in the plastic zone and three control beams were conducted up to failure under four-point bending test. The effectiveness of confining CFRP sheet on improving the flexural ductility of tmstrengthened T-section beams was studied. The parameters such as the width and the thickness of CFRP sheet and the type of T-section were analyzed. The experimental results show that ductility and rotation capacity of plastic hinge can be improved by the confinement of CFRP sheet, and the ductility indices increase with the increment of width and thickness of CFRP sheet. A plastic rotation model considering the width of CFRP sheet and the effect of flange of T-section beam is proposed on the basis of the model of BAKER, and the test results show a good agreement with the perdicted results. The relevant construction suggestions for seismic retrofitting design of beam-slabs system in cast-in-place framework structure are presented.

RCS混合节点抗剪承载力计算方法对比研究

为研究适用性强的钢筋混凝土柱-钢梁(RCS)混合节点抗剪承载力计算方法,对近年来的RCS节点剪切破坏试验数据进行了统计,将试验结果与中国规程方法、Nishiyama方法、Parra方法和ASCE指南方法计算结果进行了对比,并对各方法的参数适用性进行了讨论。对比结果表明:4种方法均具有工程实用价值,其中Parra方法结果离散性最小,中国规程方法计算最为简便。参数分析表明:4种方法对不同配箍率和不同位置节点均有较好适用性,但对小轴压比节点(轴压比0~0.2)与柱贯通节点预测均偏于保守;中国规程方法对于混凝土强度高于60 MPa的节点预测结果偏于危险,同时对有直交梁节点承载力预测偏于保守。建议在公式中引入混凝土强度系数和直交梁约束系数,用以考虑两者对承载力的影响。

Seismic Performance of High-Strength Short Concrete Column with High-Strength Stirrups Constraints

The seismic performance of four short concrete columns was investigated under low cycle and repeated loads, including the failure characteristics, hysteretic behavior, rigidity degeneracy and steel-bar stress. The influences of reinforcement strength, stirrup ratio and shear span ratio were also compared. Test results reveal that the restriction effect of stirrups can improve the peak stress, so the bearing capacity of specimen can be improved; for the high-strength short concrete column with high-strength stirrups, it was more reasonable to use ultimate displacement angle to reflect the ductility of the specimen, and the yield strength of high-strength stirrups should be devalued when calculating the stirrup characteristic value; the seismic performance of short column would be improved with the increase of volume–stirrup ratio and shear span ratio; the high-strength stirrups and high-strength longitudinal reinforcements did not yield when the load acting on the specimen reached the peak value, which brought adequate safety stock to these short columns. © 2017, Tianjin University and Springer-Verlag Berlin Heidelberg.

Achievements of Truss Models for Reinforced Concrete Structures

Achievements are presented for truss models of RC structures developed in previous years: 1. Two constitutive models, biaxial and triaxial, are based on regular trusses, with bars obeying nonlinear uniaxial σ-ε laws of material under simulation;both models have been compared with test results and show a dependence of Poisson ratio on curvature of σ-ε law. 2. A truss finite element has been used in the nonlinear static and dynamic analysis of plane RC frames;it has been compared with test results and describes, in a simple way, the formation of plastic hinges. 3. Thanks to the very simple geometry of a truss, the equilibrium equations can be easily written and the stiffness matrix can be easily updated, both with respect to the deformed truss, within each step of a static incremental loading or within each time step of a dynamic analysis, so that to take into account geometric nonlinearities. So the confinement of a RC column is interpreted as a structural stability effect of concrete. And a significant role of the transverse reinforcement is revealed, that of preventing, by its close spacing and sufficient amount, the buckling of inner longitudinal concrete struts, which would lead to a global instability of the RC column. 4. The proposed truss model is statically indeterminate, so it exhibits some features, which are not met by the “strut-and-tie” model.

Mechanical Properties of Polypropylene Fiber Reinforced Concrete under Elevated Temperature

Apart from many advantages,High Strength Concrete(HSC)has disadvantages in terms of brittleness and poor resistance to fire.Various studies suggest that when polypropylene(PP)fibers are uniformly distributed within concrete,they play an active role in improving spalling resistance of concrete when exposed to elevated temperature while having no adverse effect on its mechanical properties.Therefore,there is a necessity to quantify the effect of the addition of polypropylene fibers in terms of the fiber dosage,the strength of the concrete,and the residual mechanical properties of fiber-reinforced concrete under exposure to high temperature from fire.The study was carried out on three water/cement(w/c)ratios(0.47,0.36&0.20)using granite aggregate for determining short term mechanical properties of Polypropylene fiber reinforced concrete in comparison to control mix.The experimental program includes 100×200 mm&150 x 300 mm cylinders with fiber volume of 0.5%,that were subjected to temperatures exposures of 400°C and 600°C for durations of 1 hour.From the results,it was observed that no significant enhancement in mechanical properties such as modulus of elasticity,Poisson’s ratio,split tensile strength,flexural strength,and compressive strength was observed at room temperature and at elevated temperatures.

高强钢绞线网/ECC加固RC柱小偏心受压性能研究

为研究高强钢绞线网/ECC(HSME)加固钢筋混凝土(RC)小偏心受压柱的正截面受力性能,对HSME加固RC柱、ECC加固RC柱和未加固RC柱进行了小偏心受压性能试验.结果表明:采用HSME加固后RC柱的整体性能得到显著提升,当采用相同的相对初始偏心距时,与未加固RC柱相比,HSME加固RC柱的开裂荷载、峰值荷载和延性分别提高了100.0%~113.3%、99.8%~108.0%、75.9%~77.8%;相同荷载下,HSME加固RC柱的ECC应变和纵筋压应变均小于ECC加固RC柱,HSME的加固效果更优;HSME加固层为核心混凝土提供了有效约束,改善了RC柱的损伤分布和破坏模式,显著提高了RC柱的承载力和延性;整个受压过程中,HSME加固层与RC柱混凝土协调变形,共同工作性能良好.

Effects of fibers on mechanical properties of high-performance concrete subjected to elevated temperatures

The compressive strength and ilexural toughness as well as fracture energy of fiber reinforced highperformance concrete （FRHPC） subjected to different high temperatures were studied. The results showed that after exposure at 300,600 and 900℃, the concrete mixes retained 88.1% , 41.3% and 10.2% of the original compressive strength on average, respectively. Steel fiber and polypropylene （PP） fiber were both effective in minimizing the damage effect of high temperatures on the compressive strength. The HPC reinforced with steel fibers showed higher flexural toughness and fracture energy before and after the high-temperature exposures. In comparison, PP fibers had minor beneficial effects on the flexural toughness and fracture energy. The mechanical properties of HPC reinforced with hybrid fibers （steel fiber ＋ PP fiber） were equivalent to or better than those of HPC reinforced with steel fibers alone. In addition, the failure pattern of FRHPC beams changed from pull-out of steel fibers at lower temperatures （20, 300 and 600℃） to tensile failure of steel fibers at higher temperature （900 ℃）.

基于黏滞阻尼器的RC框架抗震性能提升设计

基于性能化设计理念,对既有RC框架结构提出了一种基于黏滞流体阻尼器的抗震性能提升设计方法.以大震作用下的结构弹塑性位移作为性能控制目标,利用等价高阻尼弹性位移反应谱,通过迭代进行原结构和减震结构的弹塑性位移响应计算.利用算例展示了不同黏滞阻尼器速度指数减震方案的设计结果,并与工程实践中常用的基于小震工况设计方法结果进行对比.结果表明,所提方法可以实现性能设计目标且避免直接对结构多次进行弹塑性时程分析,设计过程高效快捷.该方法更适合采用低速度指数的黏滞阻尼器,在阻尼器成本和结构大震位移响应相同的情况下,相比速度指数为0.6的减震方案,采用速度指数为0.2的阻尼器在中、小震作用下的结构位移响应可以分别减小16%和30%.

Strengthening of RC T-beams with Shear Deficiencies Using GFRP Strips

The shear performance, modes of failure, and strain analysis of simply supported reinforced concrete （RC） T-beams, externally strengthened in shear using epoxy bonded glass fiber reinforced polymer （GFRP） strips are focused in the present paper. Six RC T-beams of 2.5 m span without shear reinforcement are cast. Three beams are used as control specimens and rest three beams are strengthened in shear with GFRP strips in U-shape, side bonded at 45° and 90° to the longitudinal axis of the beam. All the beams are tested in a Universal Testing Machine. The test results demonstrate the feasibility of using an externally applied, epoxy-bonded GFRP strips to restore or increase the shear strength of RC T-beams. It is also observed that the RC T-beams strengthened by diagonal side strips outperformed those strengthened with vertical side strips.