钢纤维混凝土动态抗拉强度的实验研究

基于线弹性和一维应力波假定,采用Φ75mmSHPB对钢纤维体积率Vf分别为0、0.75%和1.50%的三种混凝土材料进行了一维杆层裂实验,考虑了应力波在混凝土材料内传播时的波形弥散效应和应力幅值衰减,通过计算应变片记录的应力信号确定了材料的动态抗拉强度。结果表明,钢纤维混凝土的动态抗拉强度受应变率和钢纤维体积率的影响,本文为测试脆性材料的动态抗拉强度提供了一种有效方法。基于微观扫描技术,对钢纤维增强机理进行了分析。     

Bond failure performances between near-surface mounted FRP bars and concrete for flexural strengthening concrete structures

Near-surface mounted (NSM) fiber reinforced polymer (FRP) has been established as an effective technique for strengthening concrete member. In preview literatures, bond failure was observed usually in the strengthened beam test for increasing flexural capacity. Bond behavior is of primary importance for the transfer of stress between the concrete and the FRP reinforcement to develop composite action. In this paper, a total of 22 tests were conducted to study the bond failure performance between NSM FRP bars and concrete besides only one test as a comparison. Failure modes, load–deflection curves, strain distribution of FRP bars, and local bond stresses at the FRP-epoxy adhesive interface from the tests were analyzed in detail. Some of the factors expected to affect bond performance were presented, namely: diameter of FRP bars, type to FRP material, concrete compressive strength and bonded length. The test results reported in this paper should be useful for further establishing local bond–slip constitute relationship and further verification of numerical simulation models, in addition to gaining a better understanding of bond failures for flexural strengthening concrete structures with NSM FRP bars.     

External reinforcement of high strength concrete columns

With the technology development on the compressive strength of concrete over the years, the use of high strength concrete has proved most popular in terms of economy, superior strength, stiffness and durability due to many advantages it could offer. However, strength and ductility are inversely proportional [J. Mater. Civil Eng. 11 (1999) 21]. High strength concrete is a brittle material causing failure to be quite sudden and ‘explosive' under loads. It is also known that structural concrete columns axially compressed rarely occur in practice. The stress concentrations caused by the eccentric loading further reduce the strength and ductility of high strength concrete. Therefore, studies for high strength concrete columns under eccentric loading are essential for the practical use.

This paper experimentally investigates a number of high strength concrete columns that are externally reinforced with galvanised steel straps and fibre-reinforced polymers subjected to concentric and eccentric loading. The experimental results show that external reinforcement can enhance the properties of high strength concrete columns.     

装配式混凝土框架节点基本性能研究进展

提出了一种新型的钢筋连接器——可调组合钢筋连接套筒,并基于此组装了一套耗能连接件。对耗能连接件开展了轴向的低周往复加载试验,验证了其荷载传递的可靠性以及优异的耗能性能。将耗能连接件内置于装配式混凝土框架梁端底部,提出了一种带可更换耗能钢棒的单侧屈服梁柱节点(REDB-SYBC)。对试件开展拟静力试验研究,分析节点的损伤分布、破坏形态、滞回特性、耗能能力等抗震性能。试件滞回曲线稳定饱满无捏缩,抗震性能良好。试验结果表明：新型节点单侧屈服的变形模式减少了楼板的变形与损伤,充分发挥了梁底连接件的耗能能力,其主要损伤及破坏均发生在梁底耗能钢棒上,实现了损伤集中的设计目标以及“强柱弱梁”的抗震设计原则;利用新型连接套筒的内部空间即可实现耗能钢棒的更换,经过更换和修复后的节点各项抗震性能与初始节点基本相当。     

Bond behaviour of high‐strength concrete flexural member under low cyclic fatigue loading

Use of high‐strength concrete can lead to more economical design reducing the material requirements, weight of structure and extended service life of structure. The effect of fatigue loading is more prominent on the structures using high‐strength concrete. Bond between concrete and reinforcing bars is a major factor affecting the performance and sustainability of reinforced concrete structures. Less research is available on the effect of low cyclic fatigue loading on the bond strength of high‐strength concrete. In this research, reinforced concrete beams (1165 × 150 × 225 mm) were tested under low cyclic loading with different stress range levels. It can be concluded that the bond strength of high‐strength concrete is more than for normal‐strength concrete. Low‐cyclic fatigue loading decreased the bond strength under monotonic loading by about 43–45%. Energy dissipation during cycling is found to be good. At higher cycles, energy dissipation decreased because of local damages in front of bar ribs. With the increase in number of cycles, change in slip behaviour was found.     

球罐应力腐蚀开裂研究与安全性分析

对球罐应力腐蚀开裂的原因和主要影响因素进行了分析.针对16MnR和SPV50Q球罐用钢,在分析湿硫化氢环境下应力腐蚀开裂形式的基础上,通过改进的WOL预裂纹试样的应力腐蚀开裂试验,对不同球罐用钢、不同硫化氢浓度、不同焊接状态条件下的应力腐蚀开裂进行了研究,并对设备的安全性能进行了分析,进而提出了防止应力腐蚀开裂的对策.     

局部火灾下钢筋混凝土平面框架结构的耐火性能研究

高温后混凝土与钢筋间的黏结性能影响钢筋混凝土结构的安全评估,实际工程中钢筋混凝土常承受侧向约束作用。为此,该文以高温温度和侧压力为参数,完成了48个钢筋混凝土试件的中心拉拔试验,分析侧压力对高温后试件破坏形态、峰值黏结应力、峰值滑移、残余黏结强度的影响规律,建立了峰值黏结应力随高温温度和侧压力变化的经验公式。将侧压下钢筋-混凝土黏结应力场简化为肋前混凝土挤压力和侧压力的线性叠加,并基于微观传力模型及高温后混凝土的多轴强度破坏准则推导计算了无侧压及单向侧压下的黏结强度理论。研究结果表明：侧压作用和高温温度直接影响试件破坏形态;随着侧压力增加,高温后混凝土与钢筋间的峰值黏结应力、残余黏结强度、峰值滑移逐渐增大,但温度达到500 ℃后,有侧压试件峰值滑移较无侧压试件小;黏结强度理论计算值与实测值较为吻合,能较好地预测高温后混凝土与钢筋间的黏结强度。     

连续钢箱梁桥面铺装层结构应力分析

正交异性钢桥面的构造复杂,用理论算法对铺装层进行力学计算,较难得到精确的计算结果。本文利用有限元通用软件对曲线钢箱梁桥浇筑式沥青铺装层进行受力分析,将正交异性钢桥面板、铺装层作为整体,建立有限元模型,研究铺装层受力变形特点。根据钢桥面铺装层常出现的病害,并结合超载及刹车产生的水平荷载对铺装层受力的影响,提出相应的桥面铺装层破坏的综合控制指标。     

高温后圆钢管高强混凝土界面黏结性能试验研究

为揭示历经高温作用后圆钢管与高强混凝土界面间的黏结滑移性能,设计17个试件进行推出试验,试验考虑了高强混凝土强度等级、历经最高温度和界面黏结长度3个变化参数,通过试验,观察了试件的破坏过程及形态,得到试件加载端及自由端的荷载-滑移曲线,基于试验实测数据深入分析各变化参数对黏结强度、界面耗能及损伤的影响规律,提出了高温后圆钢管高强混凝土界面黏结强度的计算式以及黏结滑移本构方程。结果表明：随历经温度的升高,圆钢管与高强混凝土界面黏结强度呈现先增大后减小的变化规律;随着历经温度的升高,黏结强度随混凝土强度等级的提高而增大的影响程度逐渐减弱;界面黏结损伤随历经温度的升高有所推迟;界面黏结耗能能力随历经温度与混凝土强度等级的提高而下降。     

复合加固方形木柱的抗震性能分析

基于性能抗震设计思想的600 MPa 级高强钢筋混凝土柱位移角限值研究,对于推广高强钢筋混凝土柱的应用至关重要。通过17根HTRB630高强钢筋混凝土柱试件和3根HRB400钢筋混凝土柱试件的拟静力试验研究了其抗震性能。与HRB400钢筋混凝土柱相比,HTRB630高强钢筋混凝土柱的滞回曲线的形状没有发生明显改变,试件具有较好的承载能力和延性。将基于 Kunnath 损伤模型计算的损伤指数与试验中测量的损伤指数范围进行了比较,结果表明,Kunnath 损伤模型可以准确计算HTRB630高强钢筋混凝土柱的损伤指标。根据HTRB630高强钢筋混凝土柱的破坏特点,以屈服点、峰值点和极限点作为高强钢筋混凝土柱的性能水平控制点。基于性能抗震设计的思想,将600 MPa 级高强钢筋混凝土柱的性能水平划分为5个性能水平,即正常使用、暂时使用、修复后使用、生命安全和接近倒塌性能水平。结合参考文献中600 MPa 级高强钢筋混凝土柱的试验数据,对65个600 MPa 级高强钢筋混凝土柱各特征点的位移角进行了相对频率统计分析,得到了600 MPa级高强钢筋混凝土柱在暂时使用、修复后使用和接近倒塌3个性能水平下具有超过90%安全保证率的位移角限值分别为1/150、1/80和1/60。

     

Effect of shrinkage reducing agent on pullout resistance of high-strength steel fibers embedded in ultra-high-performance concrete

The interfacial bond strength of long high-strength steel fibers embedded in ultra-high-performance concrete (UHPC) reinforced with short steel microfibers was investigated by conducting single-fiber pullout tests. In particular, the influence of the addition of a shrinkage-reducing to a UHPC matrix on the pullout resistance of high-strength steel fibers was investigated. The addition of a shrinkage-reducing agent produced a noticeable reduction in the fiber pullout resistance owing to the lower matrix shrinkage, although the reduction of pullout resistance differed according to the type of fiber. Long smooth and twisted steel fibers were highly sensitive to the addition of the shrinkage-reducing agent whereas hooked fibers were not. Among the various high-strength steel fibers tested, twisted steel macrofibers showed the highest interfacial bond resistance, although twisted fibers embedded in UHPC showed slip softening pullout behavior rather than the typical slip hardening behavior observed in mortar.     

冻融作用后超高性能混凝土中钢纤维的拔出行为研究

将超高性能混凝土冻融0～1500次后进行钢纤维的拔出实验和纳米压痕实验,研究了倾斜角度和埋置深度对钢纤维拔出行为的影响.结果表明,不同倾角和埋深的钢纤维其拔出荷载峰值都随着冻融次数的增加而逐渐降低,倾角为50°时拔出荷载峰值达到最大值;钢纤维-水泥浆体界面过渡区中的微孔洞逐渐增多和汇聚,过渡区的厚度由20 μm增加到6...     

Improvement of historic reinforced concrete/mortars by impregnation and electrochemical methods

Old reinforced concrete (RC) structures often exhibit unsatisfactory performances in terms of both residual serviceability and seismic behavior. The durability requirements were not considered at all at the age of design and construction, exposing such structures to severe decay in aggressive urban and/or industrial environment; in the meantime, the use of plain steel bars (with poor bond strength) was common at the age of construction and this jeopardizes the RC performance under cyclic load reversals like those induced by earthquakes.In this paper, the deep impregnation of cement-based mortars and concrete with a solution of ethyl silicate and with electrochemical treatments by means of a solution of sodium carbonate was investigated, with a twofold aim: (1) increasing the bond strength of the embedded plain steel bars in existing structures without any need for demolition and (2) increasing the durability and safety of the reinforced concrete elements. Both the treatments are already exploited in other fields: ethyl silicate solutions are widely used for stone consolidation (and only recently proposed for concrete surface protection), while migration of sodium carbonate under DC voltage is used for RC realkalization.In the present study these treatments are expected to penetrate inside mortar and concrete and to modify their microstructure, hence improving their properties. The effects of the treatments are investigated by comparing the characteristics of treated and untreated mortar and concrete samples reinforced with plain bars, in terms of penetration inside the samples, pull-out strength, water absorption, pore size distribution, carbonation resistance and corrosion resistance.     

Durability analysis of bamboo as concrete reinforcement

Durability is an actual challenge concerning all construction materials. If these materials are natural, the necessity to understand their long term behaviour is extremely important, because they are considered as having a low capacity to maintain their properties with time. Bamboo is a high strength material that can be used, in certain cases, as reinforcement in concrete. As concrete matrix has a high pH, many authors have discussed the decay of vegetal materials when used to reinforce cementitious matrix. This paper presents results of an experimental investigation made to evaluate bamboo durability to be used as concrete reinforcement. The durability was evaluated by changing the tensile strength and Young’s Modulus of bamboo. Five hundred specimens were extracted from a Dencrocalamus giganteus bamboo culms and part of them was set into concrete prisms. A set up was developed to expose the specimens to wetting and drying cycles. Each exposure to wetting and drying lasted 24 h. The specimens without concrete were submitted to a calcium hydroxide solution and the samples with concrete were immersed in tap water. Tensile strength and Young’s Modulus were measured after 7, 15, 30, 45 and 60 cycles. Results did not show any significant variation on these mechanical properties, attesting the durability of bamboo in these aggressive tests.      

Tensile tests and analyses of notched specimens fabricated from high strength steels using a generalized yield model

This article summarises the uniaxial tension tests for 20 notched bars fabricated from high strength steel Q345 specified in Chinese National Standards. The effects of the notch radius, r, and that of the notch depth ratio, d/D , on the ductility and fracture resistance of this high strength steel are examined. The experimental data are further analyzed using a generalized yield model together with an elliptical fracture stress envelope originally proposed by the first author. The experimental results demonstrate that cracks initiate at the notched section, with the fracture surface filled with many dimples and shearing marks. Specimens with a sharper notch radius (a smaller r) and a larger notch depth (a smaller d/D ratio) show poor ductility, but high fracture strength. The stress field computed from the numerical procedure including the generalized yield model indicates that the crack initiation occurs at the centre of the notched section which experiences the highest stress triaxiality ratio (σ_m/σ_seq) . As the stresses at the notched section reach the limiting values determined from the elliptical fracture criterion, macroscopic fracture failure in the notched bar occurs.     

Bond behavior of GFRP and steel bars in ultra-high-performance fiber-reinforced concrete

The bond behavior of glass fiber-reinforced polymer (GFRP) and steel bars embedded in ultra-high-performance fiber-reinforced concrete (UHPFRC) was investigated according to embedment length and bar diameter. Post-peak bond stress-slip softening curve of the GFRP bars was obtained, and a wedging effect was quantitatively evaluated. Test results indicated that a normalized bond strength of 5 was applicable for steel bars embedded in UHPFRC, and the development lengths of normal- and high-strength steel bars were determined to be 2 and 2.5 times the bar diameter, respectively. The GFRP bars exhibited approximately 70% lower bond strength than the steel bars, and the bond stress additionally applied by the wedging effect increased almost linearly with respect to the slip. Based on dimensionless bond stress and slip parameters, an appropriate theoretical model for the bond stress and slip relationship of steel bars in UHPFRC was suggested, and it was verified through comparison with the test data.     

高温后型钢混凝土粘结滑移性能研究

高温后型钢与混凝土之间的粘结滑移对型钢混凝土结构的高温后力学性能具有重要影响,为了研究高温后型钢混凝土的粘结强度与滑移特性,进行18个高温后型钢混凝土短柱推出试验及3个常温下的对比试验。试验结果表明：高温后型钢混凝土短柱试件的荷载-滑移曲线与常温下大体相似,但随着曾经经历最高温度的提高及最高温度持续时间的增大,型钢与混凝土之间的极限粘结强度和残余粘结强度降低,与极限粘结强度相对应的滑移减小,与残余粘结强度相对应的滑移增大。在试验结果基础上,构建了高温后型钢混凝土粘结滑移本构模型,提出了本构模型中极限粘结强度、残余粘结强度及相应滑移的计算方法,计算结果与试验结果总体相符,研究成果为高温后型钢混凝土构件考虑粘结滑移影响的数值模拟分析提供了条件。     

Influence of reinforcing bar type on autogenous shrinkage stress and bond behavior of ultra high performance fiber reinforced concrete

This study investigated the effects of reinforcing bar type and reinforcement ratio on the restrained shrinkage behaviors of ultra high performance fiber reinforced concrete (UHPFRC), including autogenous shrinkage stress, degree of restraint, and cracking potential. In addition, the influence of the type and embedment length of reinforcing bars on the bond behavior of UHPFRC was evaluated by performing pullout test. Three different reinforcing bars (deformed steel bar, round steel bar, and GFRP bar) were investigated in the restrained shrinkage and pullout tests. The GFRP bar exhibited the best performance in relation to the autogenous shrinkage stress, degree of restraint, and cracking potential because of its low stiffness. The highest bond strength was obtained for the deformed steel bar, and the bar yielding was observed when the bar embedment length of l_b = 2d_b was used. The round steel bar exhibited the poorest behaviors for both of the restrained shrinkage and pullout.     

Correlation of constitutive response of hybrid textile reinforced concrete from tensile and flexural tests

This papers addresses the disparities that exist in measuring the constitutive properties of thin section cement composites using a combination of tensile and flexural tests. It is shown that when the test results are analyzed using a simplified linear analysis, the variability between the results of tensile and flexural strength can be as high as 200–300%. Experimental results of tension and flexural tests of laminated Textile Reinforced Concrete (TRC) composites with alkali resistant (AR) glass, carbon, aramid, polypropylene textile fabrics, and a hybrid reinforcing system with aramid and polypropylene are presented. Correlation of material properties is studied analytically using a parametric model for simulation of flexural behavior using a closed form solution based on tensile stress–strain constitutive relation. The flexural load carrying capacity of TRC composites is computed using a back-calculation approach, and parameters for a strain hardening material model are obtained using the closed form equations. While the parametric model over predicts the simulated tensile response for carbon and polypropylene TRCs, predictions are however consistent with experimental trends for aramid and glass TRCs. Detailed discussion of the differences between backcalculated and experimental tensile properties is presented. Results can be implemented as average moment–curvature relationship in the structural design and analysis of cement composites.