Physical Properties of Glass Fiber Reinforced Polymer Rebars in Compression

Forty-five glass fiber reinforced polymer (GFRP) rebars were tested in compression to determine their ultimate strength and Young’s modulus. The rebars (or C-bars), produced by Marshall Industries Composites, Inc., had an outside diameter of 15 mm (#15 rebar), and unbraced lengths varying from 50 to 380 mm. A compression test method was developed to conduct the experiments. Three failure modes, that are directly related to the unbraced length of the rebar, are identified as crushing, buckling, and combined buckling and crushing. The crushing region represents the failure mode a GFRP rebar would experience when confined in concrete under compression. The experimental results showed that the ultimate compressive strength of the #15 GFRP rebar failing by crushing is approximately 50% of the ultimate tensile strength. Based on a very limited number of tests, in which strain readings were acceptable, Young’s modulus in compression was found to be approximately the same as in tension.     

Thermal Compatibility of Concrete and Composite Reinforcements

Experiments on commercially-produced composite reinforcements have shown that the transverse coefficients of thermal expansion (CTE) of these rebar are 3–5 times higher than the CTE of concrete. In this paper, thermoelastic solutions of plain and spirally wrapped composite rebar, embedded in concrete and subject to a uniform temperature increase, are presented. Comparisons are made between bars with no spiral wrapping and those produced with a spiral wrap of unidirectional rovings. Results show that the spiral wrapping, included primarily to improve the bond between the concrete and the composite rebar, also aids in the reduction of overall thermal expansion of the rebar. Thermoelastic solutions of unidirectional composite rods and concrete predict that the tensile stresses in the concrete surrounding the rebar will exceed the tensile strength of the concrete for relatively small temperature increases. The extent of this cracking cannot be determined from the analytical predictions. To predict the extent of the cracking, nonlinear thermoelastic simulations, completed using the COSMOS-M finite-element code, are used to determine the extent of concrete cracking. Results show that cracking decreases with a decreasing rebar diameter and increasing confining pressure of the concrete.     

600MPa级钒氮微合金化热轧高性能钢筋的研制

介绍了济钢钒氮微合金化HRB600钢筋的生产实践.通过转炉冶炼、炉外精炼和全连续轧机进行了工业试制,应用光谱分析仪、显微镜、拉伸试验机、多功能焊机等设备对试制钢筋的成分、组织、性能,特别是钢筋的时效性、焊接工艺性能、疲劳性能进行了分析.结果证明,采用钒氮微合金化工艺试制的产品的屈服强度达到670 MPa、抗拉强度达到800 MPa,断后伸长率达到19％以上,钢筋具有在500万次高周疲劳应力下未发生断裂的疲劳性能,时效试验表明产品性能稳定,并且适用于多种焊接和机械连接方式,满足行业标准的要求,其生产工艺具有一定的经济优势.     

Analyses of powder segregation in MIM

The effects of main process parameters on final components made by metal injection moulding have been studied through experiments correlated with accurate modelling and numerical simulations. A mould specially designed for experiments was made in our laboratory for components with different processing conditions, then the components were debound and sintered. Mechanical properties were measured by tensile and bending tests and correlated with processing conditions. The prediction of segregation during injection moulding was based on a bi-phasic model with a newly developed algorithm. The results from simulation were compared with experimental results and the agreement demonstrates that proposed methods are well adapted for mould design and determination of injection moulding parameters.     

热轧带肋钢筋强度特性沿径向的分布

从钢筋的机械连接出发,研究了轧后弱冷和强冷钢筋强度性能沿径向的分布,发现剥肋后承载力的变化和力学性能沿径向的分布都与轧后冷却方式密切相关,强冷者显著降低而弱冷者基本不变。由此说明,强冷和弱冷钢筋分别适用于不剥肋和剥肋的滚轧直螺纹连接技术。从冷却方式、组织分布特征上对其现象做了解释。     

热轧带肋钢筋强度特性沿径向的分布

从钢筋的机械连接出发,研究了轧后弱冷和强冷钢筋强度性能沿径向的分布,发现剥肋后承载力的变化和力学性能沿径向的分布都与轧后冷却方式密切相关,强冷者显著降低而弱冷者基本不变。由此说明,强冷和弱冷钢筋分别适用于不剥肋和剥肋的滚轧直螺纹连接技术。从冷却方式、组织分布特征上对其现象做了解释。     

Prediction of the Failure Time of Glass Fiber Reinforced Plastic Reinforced Concrete Beams under Fire Conditions

A model is proposed to predict the time to failure of reinforced concrete beams in a fire. The model is developed specifically to predict the lifetime of beams reinforced with glass fiber reinforced plastic rebar, but is applicable to beams with any form of reinforcement. The model is based on the calculations for flexural capacity and shear capacity of beams embedded within ACI design codes where time and temperature dependent values for rebar modulus and strength and concrete strength replace the static design values. The base equations are modified to remove safety factors and where necessary the temperature induced reductions in strength for concrete and steel are derived using the equations presented by EUROCODE 2. In order to validate the model it was used to predict the failure times of steel rebar reinforced beams that had been documented in the literature. There was excellent agreement between the model and the reported lifetimes for these conventional beams. The model was applied to predict the lifetimes of two beams that had been manufactured and tested for destruction in a fire by the research group. The model predicted that the failure mode of the beams would be because of rebar rupture as opposed to the design condition of concrete crushing and this was confirmed by the experimental test results. The model provided reasonable agreement with experimental results with a lifetime of 108?min predicted based on flexural failure and 94 and 128?min observed in the experiments.     

Ti-6Ta合金等离子焊焊接接头的组织与性能

采用等离子弧焊对3mm及12mm厚Ti-6Ta合金板材进行了焊接实验,通过光学显微镜和腐蚀试验研究了焊接接头的组织形貌和耐腐蚀性能,同时测试了焊接接头的力学性能和弯曲性能。结果表明：采用等离子弧进行焊接的Ti-6Ta合金,其焊接热影响区为粗大的等轴晶粒,焊接熔区晶粒组织则呈粗大的不规则形貌。并且,Ti-6Ta合金等离子焊焊接接头的强度和弯曲性能与母材相当,塑性稍有下降。此外,焊接热应力对接头的强度和弯曲性能影响较小,焊接接头在沸腾硝酸中具有良好的耐腐蚀性能,且焊接接头的耐蚀性对焊接热应力不敏感。     

不同相区轧制对螺纹钢组织和性能影响

为了降低螺纹钢生产线坯料的生产成本,通过安装在16号轧机之后的预水冷装置对进入17号轧机的螺纹钢进行不同温度的控制,再经过17号和18号轧机对不同温度的螺纹钢进行轧制。探究了钢坯在不同相区进行轧制时对其组织性能的影响,结果表明,当钢种为HRB400-1NbS(Nb质量分数为0.025%)的螺纹钢在奥氏体未再结晶区轧制时(进入17号轧机的温度为(880±20) ℃),其屈服强度为437 MPa,抗拉强度为595 MPa;当钢种为HRB400-0NbS(Nb质量分数为0.015%)的螺纹钢在两相区轧制时(进入17号轧机的温度为(780±20) ℃),其屈服强度为435 MPa,抗拉强度为605 MPa;两者力学性能相差不大,这是因为HRB400-0NbS钢种在两相区轧制时,其晶粒度/级为10.5,相比HRB400-1NbS钢种在奥氏体未再结晶区轧制时晶粒度/级为9.5更加细小,通过细晶强化弥补了Nb所产出的第二相强化作用,为螺纹钢生产线坯料节约了每吨40~50元的成本。     

Large-Bar Connection for Precast Bridge Bents in Seismic Regions

A new beam-to-column connection has been developed for assembling precast concrete bridge bents in regions of high seismicity. The connection is made with a small number of large column bars, which are grouted into large corrugated-metal ducts embedded in the cap beam. Bents built with these connections can be erected quickly and permit generous construction tolerances. To evaluate the seismic performance of the proposed connection, lateral-load tests were performed on three manifestations of the connection, as well as on a comparable cast-in-place connection. The tests demonstrated that the force-displacement response and damage progression in the precast connection are similar to those of typical cast-in-place concrete connections. Deliberate partial debonding of the longitudinal reinforcement only slightly affected the force-displacement response and observed damage.     

纵筋锈蚀无腹筋混凝土梁抗剪性能细观数值研究

以钢筋混凝土梁为研究对象,考虑钢筋非均匀锈蚀膨胀效应,建立三维钢筋混凝土梁剪切破坏分析的数值分析模型。通过多阶段分析方法(钢筋锈蚀阶段,构件性能退化阶段)探索锈蚀对结构力学行为的影响。钢筋的非均匀锈蚀膨胀以施加非均匀径向位移的方式模拟,获得保护层的破坏状态,并以此“最终状态”作为之后混凝土梁静载试验的“初始条件”输入,进而模拟构件的力学行为。在验证了多阶段数值模型合理性的基础上,分析了纵筋锈蚀、剪跨比对无腹筋混凝土梁抗剪性能的影响规律。结果表明,纵筋锈蚀使混凝土梁产生明显的纵向裂缝。纵筋锈蚀率增大,保护层开裂区域增加,梁的抗剪承载力下降严重。另外,剪跨比对梁的抗剪承载力产生影响,剪跨比对未锈蚀梁的影响明显大于对锈蚀梁的影响程度。最后,基于模拟结果对相关设计规范中的抗剪承载力计算公式进行了讨论,发展建立了考虑锈蚀影响的无腹筋混凝土梁抗剪承载力计算方法。      

Closed-Form Solutions for Flexural Response of Fiber-Reinforced Concrete Beams

A constitutive law for fiber-reinforced concrete materials consisting of an elastic perfectly plastic model for compression and an elastic-constant postpeak response for tension is presented. The material parameters are described by using Young’s modulus and first cracking strain in addition to four nondimensional parameters to define postpeak tensile strength, compressive strength, and ultimate strain levels in tension and compression. The closed-form solutions for moment-curvature response are derived and normalized with respect to their values at the cracking moment. Further simplification of the moment-curvature response to a bilinear model, and the use of the moment-area method results in another set of closed-form solutions to calculate midspan deflection of a beam under three- and four-point bending tests. Model simulations are correlated with a variety of test results available in literature. The simulation of a three- and four-point bending test reveals that the direct use of uniaxial tensile response underpredicts the flexural response.     

Model for Reinforced Concrete Members under Torsion, Bending, and Shear. I: Theory

A model has been developed that can predict the load-deformation response of a reinforced concrete (RC) member subjected to torsion combined with bending and shear to spalling or ultimate capacity. The model can also be used to create interaction surfaces to predict the failure of a member subjected to different ratios of applied torsion, bending, and shear. The model idealizes the sides of an reinforced concrete member as shear “wall panels.” The applied loads are distributed to the wall panels as uniform normal stresses and uniform shear stresses. The shear stress due to an applied torsional moment and shear force are summed over the thickness of the shear flow zone. Stress-strain relationships are adopted for tension stiffening and softened concrete in compression. The crack alignment rotates to remain normal to the principal tensile stress and the contribution of concrete in shear is neglected. The model has been validated by comparing the predicted and experimental behavior of members loaded under torsion combined with different ratios of bending and shear. The torque-twist behavior, reinforcement stress, and concrete surface strain predicted by the model were in agreement with experimental results.     

Interface Shear Connection Analysis of Ultrahigh-Performance Fiber-Reinforced Concrete Composite Girders

The purpose is to analyze the interface shear connection behavior for ultrahigh-performance fiber-reinforced concrete (UHPFRC) and normal concrete (NC) composite girders. The shape and dimension of the shear stud in the conducted tests are referenced from the traditional interface connection design and engineering experiences. The interface shear connection parameters, i.e., initial stiffness and slippage capacity of a single shear stud, are measured from three groups of lateral direct push test specimens with different numbers of studs. Based on the UHPFRC tensile failure characteristics and cracked section rotational mechanisms of the UHPFRC-NC composite structures with flexural, or flexural and shear failure, the limit state is defined as a full pullout from the bottom fiber of the UHPFRC girders. Pseudostrain hardening behavior of the UHPFRC is simplified as an equivalent rectangular stress block. From this mechanism, the interface equilibrium equations are constituted and the interface shear connection degree of the UHPFRC-NC composite girders is derived. It is recommended that the interface shear connection degree may be used as minimum design standard for UHPFRC-NC composite interface shear connection design.     

Ultimate Condition of Fiber Reinforced Polymer-Confined Concrete

One important application of fiber reinforced polymer (FRP) composites is as a confining material for concrete in the retrofit of existing concrete columns by the provision of FRP jackets. Such jackets are commonly formed in a wet layup process, with the fibers being only or predominantly in the hoop direction. It has been well established in recent studies that the rupture strains/strengths of FRP measured in tests on such FRP-confined concrete cylinders fall substantially below those from flat coupon tensile tests, but the causes are unclear. This paper presents the results of a study that is aimed at clarifying these causes. To this end, the paper reports and compares the ultimate tensile strains of two types of FRP (carbon FRP and glass FRP) obtained from three types of tests—flat coupon tensile tests, ring splitting tests, and FRP-confined concrete cylinder tests. Based on comparisons of these test results, it can be concluded that the FRP hoop rupture strains in FRP-confined concrete cylinders are reduced below the ultimate tensile strains from flat coupon tests by at least three factors—(1) the curvature of the FRP jacket; (2) the deformation localization of the cracked concrete; and (3) the existence of an overlapping zone. While the first factor that reduces the in situ strain capacity of FRP on confined concrete is material dependent, the last two factors that result in a nonuniform strain distribution in the jacket are independent of the FRP material properties. The third effect reduces the average hoop rupture but does not affect the distribution of the confining pressure, as the FRP jacket is thicker in the overlapping zone.     

HRB400热轧带肋钢筋的性能分析

为了扩大经营品种，2002年3月韶钢用大电炉生产工艺试制与开发了HRB400Ⅲ级钢筋，本文重点对成品的性能和V微合金化的强化机理进行了分析，分析结果表明，试制的HRB400钢筋性能符合GB1499－1998标准要求，并同时满足二级抗震钢筋的性能要求，生产HRB400钢筋比生产HRB335钢筋可增加销售利润约90元／t。     

Bond between Carbon Fiber Reinforced Polymer Bars and Concrete. I: Experimental Study

The bond characteristics of four different types of carbon fiber reinforced polymer (CFRP) rebars (or tendons) with different surface deformations embedded in lightweight concrete were analyzed experimentally. In a first series of tests, local bond stress-slip data, as well as bond stress-radial deformation data, needed for interface modeling of the bond mechanics, were obtained for varying levels of confining pressure. In addition to bond stress and slip, radial stress and radial deformation were considered fundamental variables needed to provide for configuration-independent relationships. Each test specimen consisted of a CFRP rebar embedded in a 76-mm-(3 in.)-diam, 102-mm-(4 in.)-long, precracked lightweight concrete cylinder subjected to a constant level of pressure on the outer surface. Only 76 mm (3 in.) of contact were allowed between the rebar and the concrete. For each rebar type, bond stress-slip and bond stress-radial deformation relationships were obtained for four levels of confining axisymmetric radial pressure. It was found that small surface indentations were sufficient to yield bond strengths comparable to that of steel bars. It was also shown that radial pressure is an important parameter that can increase the bond strength almost threefold for the range studied. In a second series of tests, the rebars were pulled out from 152-mm-(6 in.)-diam, 610-mm-(24 in.)-long lightweight concrete specimens. These tests were conduced to provide preliminary data for development length assessment and model validation (Part II).     

LNG用镍基合金焊条熔敷金属的组织与性能

目前国产LNG用镍基低温焊条存在的主要问题是无法同时满足强韧性要求。采用金相显微镜和扫描电镜对自主生产的3种ENiCrMo-6焊条熔敷金属的显微组织进行观察,应用硬度、冲击和拉伸试验对接头力学性能进行测试分析。结果表明:焊缝金属主要由奥氏体以及析出物构成,晶界间存在的低熔点共晶物,易形成沿晶裂纹,降低熔敷金属的拉伸、冲击和弯曲性能。随着合金含量的升高,强度和硬度总体呈上升趋势,对塑性和韧性有所损害,Nb的碳化物析出对裂纹扩展具有阻碍作用,有利于提高韧性。新研制的镍基焊条满足LNG设备对焊缝金属的性能要求。     

Closed-Form Model and Parametric Study on Connection of Concrete-Filled FRP Tubes to Concrete Footings by Direct Embedment

Concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) have been introduced as a new system for piles, columns, and poles. A simple moment connection based on direct embedment of the CFFT into concrete footings or pile caps, without using dowel-bar reinforcement, has been proposed by the authors. Robust analytical models to predict the critical embedment length (Xcr) were also developed and experimentally validated. In this paper, a comprehensive parametric study is carried out using the models developed earlier along with a newly developed closed-form model for the general case of axial loading, bending, and shear applied to the CFFT member. The parameters studied are the diameter (D), thickness (t), length outside the footing (L), and laminate structure of the FRP tube, as well as the tube-concrete interface bond strength (τmax?), concrete compressive strength in the CFFT (fct′) and footing (fc′), and the magnitude and eccentricity of axial compressive or tensile loads. It was shown that increasing D, L/D, τmax?, and fc′ of the footing, or the axial compression load, reduces (X/D)cr, whereas increasing t and fct′ of the CFFT, the fraction of longitudinal fibers in the tube, or the axial tension load, increases Xcr. As the axial load eccentricity increases, Xcr reduces for tension loads and increases for compression loads until both cases converge asymptotically to the same Xcr value, essentially that of pure bending.     

Bond-Slip Analytical Formulation toward Optimal Embedment of Concrete-Filled Circular FRP Tubes into Concrete Footings

This paper presents a robust analytical model for a moment connection of concrete-filled fiber reinforced-polymer (FRP) tubes (CFFTs) to concrete footings. The CFFT connection is based on a simple approach of direct embedment into the footing, thereby eliminating the need for connection rebar or mechanical devices. The CFFT is externally subjected to lateral and axial loads, resembling practical applications such as piles affixed to pile caps, bridge columns, or utility poles. The model adopts the concepts of equilibrium, deformations compatibility, and nonlinear concrete stress-strain behavior. It also employs a “bond stress-slip” relation that can be obtained from simple push-through tests on some of the commercially used tubes. The model can predict the critical embedment length Xcr, which is the minimum length required to achieve material failure of the CFFT outside the footing, and bond failure inside the footing, simultaneously. If the actual embedment length is less than Xcr, bond failure occurs prematurely at a lower strength that can also be predicted by the model. The model was verified using experimental data and showed that Xcr was only 0.7 of the diameter for that case. A sensitivity parametric analysis was carried out that led to some approximations. Based on which, a simple closed-form expression was established for Xcr in the case of lateral loading only.