Effect of temperature on the passivation behavior of steel rebar

Steel rebar normally forms an oxide or rusty skin before it is embedded into concrete and the passivation properties of this skin will be heavily influenced by temperature. To study the effect of temperature on the passivation properties of steel rebar under different surface conditions, we conducted scanning electron microscopy （SEM） observations and electrochemical measurements, such as measurements of the free corrosion potential and polarization curves of HPB235 steel rebar. These measurements identified three kinds of surfaces： polished, oxide skin, and rusty skin. Our results show that the passivation properties of all the surface types decrease with the increase of temperature. Temperature has the greatest effect on the rusty-skin rebar and least effect on the polished steel rebar, because of cracks and crevices on the mill scale on the steel rebar＇s surface. The rusty-skin rebar exhibits the highest corrosion rate because crevice corrosion can accelerate the corrosion of the steel rehar, particularly at high temperature. The results also indicate that the threshold temperatures of passivation for the oxide-skin rebar and the rusty-skin rebar are 37℃and 20℃, respectively.

Deformation Uniformity of Cold-Rolled Q235 Steel Rebar by FEM in Bending and Rolling Processes

An analytic model based on ANSYS/LS-DYNA has been developed on the cold rolling process for Q235 steel rebar with 12 mm in diameter. The elastic-plastic finite element method (FEM) and the cold deformation resistance model of Q235 steel were adopted in this model. Deformation uniformity of the final product has been analyzed using this model. The results indicate that the uniformity of the final product is obtained only as the centerline of the bending rolls is vertical to the centerline of the driven roll and parallel to the centerline of the drive roll in the whole rolling process. Besides, the number of the bending rolls must even realize the continuous bending and reverse bending process. Also, the number of the bending rolls must match the deformation degree of the workpiece in the cold rolling process. The validity of this finite element model was verified by the size and distribution of grains from the billet to the rebar in a practical cold rolling process.

Corrosion behavior of steel rebar in coal gangue-based mortars

Corrosion of steel rebar is the most important durability problem of reinforced concrete. The aim of this research was to investigate the corrosion behavior of steel rebar in simulated pore solutions and gangue-blended cement mortar. The simulated pore solutions were based on the pore solution composition of gangue-blended cement. The pH and Cl-concentration of simulated pore solutions had significant effects on corrosion potential. However,an increase in pH reduced the influence of Cl-concentra-tion on corrosion potential. The corrosion behavior of steel rebar in gangue-blended cement is different from that in simulated solutions. The gangue cementitious mortar surrounding steel rebar provides stable passivity environments for steel,leading to a decrease in ion diffusion coefficients. Alternating current impedance (ACI) analysis results indicated that the indicator Rc for concrete resistivity is higher for gangue mortar than for ordinary Portland cement (OPC),which improves its corrosion potential. The results from energy dispersive X-ray analysis (EDX) showed more aluminates and silicates at the rebar interface for gangue-blended cement. These aluminates improve the chloride binding capacity of hydrates in mortar,and increase the corrosion protection of steel rebar.

Analysis of inhibition of concrete steel-rebar corrosion by Na_2Cr_2O_7 concentrations:Implications for conflicting reports on inhibitor effectiveness

Corrosion test data were measured using non-destructive electrochemical techniques and analysed for studying inhibition effectiveness by different concentrations of Na2Cr2O7 on the corrosion of concrete steel-rebar in NaCl and in H2SO4 media. For these, specifications of ASTM G16-95 R04 were combined with the normal and the Gumbel probability density functions as model analytical methods for addressing issues of conflicting reports of inhibitor effectiveness that had generated concerns. Results show that reinforced concrete samples admixed with concentrations having 4 g(0.0127 mol), 8 g(0.025 4 mol) and 6 g(0.019 1 mol) Na2Cr2O7 exhibited, in that order, high inhibition effectiveness, with respective efficiency, η, of(90.46 1.30)%,(88.41 2.24)% and(84.87 4.74)%, in the NaCl medium. These exhibit good agreements within replicates and statistical methods for the samples. Also, optimal inhibition effectiveness model in the H2SO4 medium was exhibited by 8 g(0.025 4 mol) Na2Cr2O7 concentration having η=(78.44 1.10)%. These bear implications for addressing conflicting test data in the study of effective inhibitors for mitigating steel-rebar corrosion in aggressive environments.

Effect of Toluene and Dioctylphthalate on the Rebar Corrosion of Medium Carbon Steel in Seawater and Cassava Fluid

Chemical mitigation is regularly used as one of the principal prevention and control techniques in reinforcement corrosion. Hence this study presents the effect of toluene and dioctylphthalate on the rebar corrosion of medium carbon steel in seawater and cassava fluid with a view to determining inhibitive potentials of the different inhibitors in the two media. Gravimetric and voltametric techniques were employed in this study and a total of forty-five corrosion coupons of different dimensions were produced. Forty coupons were used for gravimetry and the remaining five for corrosion potentials measurements. Eight of the samples were used as control;while other eight samples were admixed with dioctylphthalate and toluene in concrete cubes. It was later immersed in seawater and cassava fluid for a total duration of 32 days and the measurements were taken at interval of 4 days in order to determine the corrosion rates in mils per year (mmpy). Two controls and admixed samples were later immersed in seawater and cassava fluid, respectively, for durations of thirty-two days to determine the corrosion potentials using a voltmeter and a Copper-Copper Sulphate Electrode (Cu/CuSO4). The pH of each medium was also measured throughout the period of exposure. The results obtained showed that all the samples except the control samples, displayed some degree of inhibition. The inhibition levels for the admixed samples in seawater were on the higher side compared with those in cassava fluid. The inhibition efficiencies for different inhibitors followed different trends in different environment. The inhibition efficiencies for toluene in cassava fluid and seawater were 21.64% and 45.78% respectively. The study concluded that organic inhibitors were effective in inhibiting corrosion in cyanide and chloride contaminated concrete cubes.

Effect of Calcium Nitrate and Sodium Nitrite on the Rebar Corrosion of Medium Carbon Steel in Seawater and Cassava Fluid

Inhibitors are regularly used as one of the principal prevention and control techniques in reinforcement corrosion. Hence this study investigates the effect of calcium nitrate and sodium nitrite inhibitors on the rebar corrosion of medium carbon steel in seawater and cassava fluid with a view to determining inhibitive potentials of the different inhibitors in the two media. Gravimetric and voltametric techniques were employed in this study and a total of forty-five corrosion coupons of different dimensions were produced. Forty coupons were used for gravimetric analysis and the remaining five for corrosion potentials measurements. Eight of the samples were used as control;while other eight samples each were admixed with calcium nitrate and sodium nitrite in concrete cubes. It was later immersed in seawater and cassava fluid for a total duration of 32 days and the measurements were taken at the interval of 4 days in order to determine the corrosion rates in mils per year (mmpy). Two controls and admixed samples each were later immersed in seawater and cassava fluid, respectively, for durations of 32 days to determine the corrosion potentials using a voltmeter and a Copper-Copper Sulphate Electrode (Cu/CuSO4). The pH of each medium was measured throughout the period of exposure. The results obtained expressed that all the samples except the control samples, displayed some degree of inhibition. The inhibition levels for the admixed samples in seawater were higher compared with those in cassava fluid. Inhibition efficiencies for various inhibitors followed different trends in different environment. The inhibition efficiencies for calcium nitrate in cassava fluid and seawater were 26.81% and 64.85% respectively. The study concluded that inorganic inhibitors were effective in inhibiting corrosion in cyanide and chloride contaminated concrete cubes.

基于黏聚力模型的螺纹钢锚杆拉拔数值模拟

为研究螺纹钢锚杆锚固结构变形破坏机理,采用ABAQUS软件建立锚固结构轴对称数值模型,基于黏聚力模型,开展数值试验,获得锚固结构拉拔过程中锚杆变形、荷载以及锚固体位移场分布演化规律。试验结果表明:在相同的拉拔位移下,小肋间距锚杆也能够承受更高的拉拔荷载,表明小肋间距锚杆对巷道围岩变形的承载能力更强;在锚杆拉拔过程中,随着拉拔位移的增加,岩体破坏和离层区域呈倒碗形向岩体深部扩散,当拉拔位移增加至9.06 mm后,岩体锚固段末端处位移增速加大,岩体的最大位移为4.79 mm,出现在锚固段端口处,最终导致锚固段基体破坏;锚杆轴力与锚杆肋间距呈负相关,锚杆的轴力随着肋间距的减小而增加。

高强螺栓连接装配式混凝土剪力墙抗震性能试验研究

提出一种用于装配式剪力墙结构的螺栓连接的干式连接方法,即预制墙板上预埋钢件,上、下层墙体通过高强螺栓和预埋钢件进行连接。为研究高强螺栓连接装配式混凝土剪力墙的抗震性能,对1个现浇试件和2个高强螺栓连接试件进行拟静力试验,对试件的破坏特征、滞回性能、变形能力、承载能力、耗能能力及延性等进行分析。结果表明:高强螺栓连接装配式剪力墙有效实现了剪力墙构件的工作机制,滞回曲线饱满,耗能能力较强,抗震性能良好。现浇试件RCSW、钢筋焊接试件DCSW1、钢筋环绕型试件DCSW2的极限位移角分别达到了1/60、1/60和1/41;钢筋焊接试件DCSW1的裂缝开展形态、受力性能及破坏形态与现浇试件RCSW相似,耗能曲线基本一致,耗能能力接近;钢筋环绕型试件DCSW2的屈服荷载、峰值荷载和极限荷载均小于RCSW和DCSW1,墙体塑性变形偏小,耗能能力偏低。

界面焊接环向钢筋的钢管混凝土叠合柱抗剪性能研究

为研究界面焊接环向钢筋的钢管混凝土叠合柱的抗剪性能,以环向钢筋的布置间距和直径为研究参数,对6根叠合柱试件进行了横向剪切静力试验和数值模拟,分析了各参数对试件的破坏过程、破坏形态、荷载-跨中挠度曲线以及抗剪承载力的影响规律。研究结果表明:界面环向钢筋布置方案不同的叠合柱试件破坏形态基本相同,均发生斜压破坏;试件的受力过程可划分为弹性阶段、弹塑性阶段、强化阶段和破坏阶段四个阶段;随着界面环向钢筋布置间距的减小,试件的抗剪承载力提高显著,最大增幅达22%;增大界面环向钢筋的直径可提高试件的抗剪承载力,但作用效果有限,最大增幅仅为6.1%。通过试验数据对有限元模型的有效性进行验证,并进一步分析了不同阶段有限元模型的应力状态,明确了此类叠合柱的受剪机理。

配置桁架钢筋的山区钢-混组合人行桥关键技术研究

为进一步推广绿道人行桥梁在山区地带的发展,本文总结了传统组合梁桥桥面板施工方式在山区地形下的局限性,同时依托云南省怒江美丽公路绿道项目,以一座30m跨的钢-混组合梁桥为例,提出了一种利用桁架钢筋模板施工组合梁桥新形式,详细介绍了桁架钢筋模板的构造特点与独特优势;建立全桥的有限元模型,进行结构的静力分析,并采用Abaqus有限元软件对桁架钢筋模板在施工工程中承载力进行了数值分析。结果表明:该类组合梁受力模式好,刚度大,同时桁架钢筋模板在施工工程中安全储备高,配置桁架钢筋的钢-混组合梁桥值得在山区桥梁建设中推广应用。

无筋钢纤维混凝土管片正截面结构设计方法

为完善无筋钢纤维混凝土管片的设计方法,加速其在城市轨道交通中的应用,提出一种新的无筋钢纤维混凝土管片正截面结构设计方法。根据已有对钢纤维混凝土本构关系的研究成果,依据地铁管片的受力特点,分析了影响地铁管片正截面承载力的主要因素。依据无筋钢纤维混凝土管片极限拉应变与钢筋混凝土的极限拉应变不在一个数量级上的差异,结合现行混凝土规范,采用平面假设,把应力和应变作为无筋钢纤维混凝土管片设计的特征值;用钢纤维混凝土应变的取值范围对应不同的设计工况,将不同工况下的无筋钢纤维混凝土管片的设计验算结果归结到同一张应力应变图中。论述了采用轴力-弯矩包络图进行正截面设计验算的主要流程,并通过一个工程实例展示不同工况下无筋钢纤维混凝土轴力-弯矩包络图的相互关系。计算结果表明:在正常使用极限状态下,强度是影响承载能力的主要因素;而承载能力极限状态下,特别是偶然荷载作用下,无筋钢纤维混凝土的韧性是影响承载能力的主要因素。

双钢板混凝土组合结构起波钢筋连接件的拉伸力学性能研究

提出将起波钢筋作为双钢板混凝土组合结构的连接件,并设计开展12个组合板单元体试件的拉伸试验,研究了直径和起波角度对起波钢筋连接件拉伸力学性能的影响。研究结果表明:连接件的破坏模式分为拉直后断裂和未拉直即断裂两种,当起波钢筋连接件在拉直后发生破坏时,变形能力和耗能能力得到大幅提高;随着连接件直径和起波角度的增大,破坏位移、拉断力和耗能能力也增大,但起波角度对拉断力影响较小;提出起波钢筋连接件强度的计算方法,计算结果与试验结果吻合较好。结合试验结果,对双钢板混凝土组合结构起波钢筋连接件的设计提出了合理化建议。

Strengthening of High Strength Rebars

The influence of alloying elements and cooling conditions after rolling on the microstructures and mechanical properties of a series of medium carbon Si-Mn-Mo-V high-strength precision-formed rebars were investigated.It is found that steel with combined addition of Mo and V appears to be the most promising one than those containing V and Nb or containing V alone under the same pro- cessing conditions.From CCT curve of the steel and the cooling curve of the bar determined by sim- ulating the cooling rate in mill practice,the reason why 40Si2MnMoV bar has excellent combination of strength and ductility is clarified.

A Non-Linear 3D FEM to Simulate Un-Bonded Steel Reinforcement Bars under Axial and Bending Loads

This paper presents development of 3D non-linear finite element model to simulate the response and predict the behavior of un-bonded mild steel bars under axial and bending loading. The models were successfully analyzed with the finite element software ANSYS, taking into account the nonlinear material properties of the reinforced mild steel bars. A bending strain relationship is derived based on a parametric study involving multiple nonlinear finite element models. A mild steel fracture criterion based on low-cycle fatigue models is proposed to control the total (elastic and plastic) strains in the mild steel bar below a maximum permissible limit. In addition, FE predictions of bar elongation due to strain penetration reasonably agreed with a proposed empirical equation by Raynor and Lehman. It was concluded that the equation proposed by Raynor and Lehman is considered valid for estimating the additional unbounded length and can be used in both analysis and design.

Protection of steel hooks embedded in glass-fiber-reinforced concrete against macrocell corrosion

This study evaluated the corrosion behavior of steel hooks embedded in GFRC,which were protected by a zinc-rich(96%Zn)galvanizing coating.The coating provided the hooks with active cathodic protection and a passive physical shield.Macrocell corrosion may form when the anode is smaller than the total steel surface.Thus,the steel hooks at the embedment juncture were additionally sealed against water ingress and air exchange using a construction sealant.The study was conducted in three phases in a salt-spray chamber.First,the electrogalvanized steel hooks embedded in GFRC were allowed to freely corrode for 7 days.In the second phase,the electrogalvanized steel hooks were painted with the zinc-rich coating and observed over 7 days.In the third phase,the steel hooks were protected by the zinc-rich coating together with a primer and construction sealant,and observed over 7 days.To evaluate the electrogalvanized hooks and the corrosion products formed,the thickness of the material was measured.Corrosion on the metal surface was inferred by studying the surface morphology of the hooks at various points of contact and after different periods of time.

螺纹钢表面缺陷和组织性能不稳定的原因分析与控制实践

针对国内某钢厂电炉流程生产的HRB400E螺纹钢,生产过程中出现屈服强度波动大、产品表面存在“黑线”缺陷等问题,选取问题严重的Φ18mm规格螺纹钢筋试样,开展了金相显微组织和电镜扫描分析,发现存在外来的硅铝酸盐类复合夹杂物以及采用二切分轧制,是造成螺纹钢表面缺陷和组织性能不稳定的主要原因,再结合生产工艺提出了改进措施,有效降低了螺纹钢中夹杂物含量水平,从而改善了螺纹钢力学性能波动,且未发现“黑线”缺陷。

新型钢筋与混凝土黏结本构关系的试验研究

NPR钢筋是一种抗冲击、吸能强以及延伸率高的新型钢筋。为了探究这种新型钢筋应用于实际工程可能性,进行了NPR新型钢筋的拉伸试验、黏结试件的电化学加速锈蚀试验和中心拔出试验,提出了NPR新型钢筋的本构关系模型;描述了NPR钢筋黏结试件的破坏形态;对锈蚀前后的NPR新型钢筋与混凝土黏结退化机理进行了分析;阐述了不同种类钢筋、锈蚀率、黏结锚固长度等因素对钢筋混凝土的黏结性能影响;分别拟合了NPR新型钢筋锈蚀率与混凝土极限黏结强度降低系数、黏结锚固长度与相对黏结强度关系表达式,以及锈蚀前后NPR新型钢筋、普通钢筋与混凝土的极限黏结强度表达式。

多向式强加电流的钢筋加速腐蚀方法优化研究

强加阳极电流的方法被广泛应用于钢筋的人工加速腐蚀,但串联或并联形式加速腐蚀电路往往由于电流分布的差异导致钢筋腐蚀程度不均匀。为了优化强加阳极电流的钢筋加速腐蚀方法,提出了一种多向式的混合连接电路形式并开展钢筋加速腐蚀试验,对比分析了混合连接、串联、并联3种加速腐蚀模式下,各组钢筋试件的电流情况、腐蚀电位与腐蚀电流密度的变化、腐蚀率以及腐蚀产物情况。结果显示,串联和并联电路形式中钢筋的电流分布存在差异导致钢筋腐蚀程度不一致;混合连接形式中,电流分布均匀且钢筋表面腐蚀程度相近,各钢筋试件锈蚀率接近理论值。通过多向式的混合连接电路可以实现均匀加速腐蚀钢筋或钢筋混凝土构件,有效优化了钢筋加速腐蚀的效果,为加速腐蚀钢筋或钢筋混凝土构件提供了参考。

控冷工艺对Mo-Cr-Nb耐火钢筋组织性能的影响及耐火机理研究

采用Gleeble、TEM及SEM等研究了模拟控冷工艺下Mo-Cr-Nb系耐火试验钢的连续冷却相转变过程并系统分析其耐火机理。结果表明:终冷温度从700℃增至1000℃,试验钢软相铁素体比例从14.5%增至36.1%,强度降低而塑性提升;冷速从0.5℃/s增至20℃/s,硬相贝氏体及马氏体增多并占据主体,强度增加而塑性降低。控制终冷温度800℃和冷速1℃/s,试验钢铁素体占28.2%,尺寸为12.3μm,强塑性匹配最佳。同时试验钢经600℃加热保温1 h后,大块上贝氏体细化为高稳定性合金碳化物团,细小的析出粒子聚集长大形成含Mo复合析出相,两者共同作用保证耐火性能。