Limitations of the linear polarization method to determine stainless steel corrosion rate in concrete environment

The linear polarization is a non-destructive way of estimating the instantaneous corrosion rate of a metal. In case of reinforced concrete, the method is restricted mainly because of a concrete resistance, unknown area of the polarized reinforcement and non-uniform corrosion. In spite of that, thanks to the polarization resistance it is possible to estimate whether the reinforcement corrodes in activity or passivity. The results imply that sometimes even the knowledge of R_p is insufficient for stating activity or passivity of the material. It has been proved, however, that the shape of the curve determining R_p by linear polarization may frequently indicate the corrosion behaviour. It is possible to evaluate the deviation of the point of zero current from the free corrosion potential which was measured prior to the linear polarization and also to evaluate the deviation of the polarization curve from the regression line, which is defined by the coefficient of determination.     

Prediction of ductility factor of corroded reinforced concrete beams exposed to long term aging in chloride environment

An experimental study using a three-point bending test on RC beams with dimensions of 150 × 280 × 3000 mm, naturally corroded over many years was conducted to evaluate the influence of steel corrosion on structural performance and, in particular, to better understand the change in ultimate deflection in bending and then in ductility. Some previous works by different authors are also discussed. The results show that the conventional ductility factor hardly applies to the assessment of ductile behaviour of corroded beams. A new ductility factor, based on the ratio between ultimate deflection of corroded and non-corroded beams, is proposed. In addition, the relation between ductility factor of corroded beams and cross-section loss in the corroded reinforcing steels was studied on the RC beams tested. The service life of corroded structures appears to be limited by the reduction of ductility in bending behaviour, which is more pronounced on the reduction of load-bearing capacity. This was linked to the change in mechanical properties of corroded steel bars in comparison with non-corroded steel bars.     

Residual strength of reinforced concrete columns exposed to fire

Abstract

Several methods were employed to evaluate the residual strength of reinforced concrete columns exposed to different durations of fire. These methods included the analytical method, ultrasonic tests, hammer tests and load tests. Fifty columns were involved in the tests. Calculated temperatures and residual strengths of the test columns were compared with those measured. Comparisons were also made between results from load tests and those from nondestructive tests. The results showed that using analytical procedures is acceptable while the nondestructive test methods are accurate only for shorter durations of fire.     

On-site detection of corrosion in reinforced concrete structures

The availability of on-site methods of diagnosis of the deterioration rate in reinforced concrete structures is a pressing technical need all over the world. Two new methods are proposed in this paper which are derived from the polarization resistance method, but which, differing from this method, are applicable to full-size structures, in spite of the non-uniform distribution of the electric signals in them. The two proposed procedures are of a simplicity and reliability comparable with those offered by direct measurement of polarization resistance.     

Modeling of chloride-induced corrosion in reinforced concrete structures

A numerical procedure is presented in this paper for the prediction of chloride induced steel corrosion in reinforced concrete structures. Finite element analysis is introduced for the mechanical analysis of crack initiation and propagation due to the accumulation of corrosion products around the reinforcement, while the alternating direction implicit method is used to solve the transport equations of temperature, humidity, chloride ions and oxygen in concrete. Based on the assumption of a uniform distribution of corrosion products, a self-adaptation process for the variation of boundary conditions is proposed through a series of diffusion analyses together with crack propagation in concrete. Therefore, the interaction between the corrosion rate and the propagation of cracks in concrete is taken into account. Furthermore, a numerical program is developed and a case study involving bridge deck exposed to a marine environment in Hong Kong is investigated. The results show that interactive behavior has a significant effect on the corrosion rate of the reinforcement, and the non-cracking model significantly overestimates the service life of structures.     

Short-term corrosion rate measurement of OPC and HPC reinforced concrete specimens by electrochemical techniques

This paper presents the results of a laboratory investigation in which the applicability of Tafel plot and linear polarization techniques in short-term corrosion rate measurement of reinforcing bar in concrete has been evaluated. One hundred and four OPC and HPC concrete cylinders, fifty-two of each kind and each cylinder with a single embedded reinforcing bar, have been subjected to various controlled conditions, and the corrosion rate of each specimen has been monitored. Results indicate that sodium chloride concentration plays an effective role on the propagation of corrosion,i.e., the higher the NaCl concentration, the higher the corrosion rate.     

锈蚀对钢筋混凝土结构耐久性的影响研究

钢筋混凝土结构在工程建设中得到广泛的应用,然而结构中的钢筋锈蚀是影响钢筋混凝土结构耐久性的主要因素。文章阐述了混凝土结构的耐久性理论,锈蚀影响混凝土耐久性的因素以及预防锈蚀的措施等。     

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。     

恶劣环境下纤维增强聚合物片材拉伸性能

通过拉伸试验,研究了恶劣环境作用后纤维增强聚合物(FRP)片材的拉伸性能。试验参数包括恶劣环境类别和作用方式、FRP片材种类和层数。试验结果表明,常温环境下、冻融和干湿循环作用后,碳纤维增强聚合物(CFRP)片材和玻璃纤维增强聚合物(GFRP)片材的拉伸应力-应变关系近似为直线;常温环境下,CFRP片材和GFRP片材的拉伸强度和延伸率几乎不受片材层数的影响;冻融循环对GFRP片材的影响大于CFRP片材,冻融循环75次时,CFRP片材和GFRP片材的拉伸强度分别是未冻融的0.978倍和0.898倍,并且随着循环次数的增加,CFRP片材和GFRP片材拉伸强度逐渐下降;干湿循环作用对GFRP片材拉伸性能没有明显的影响。基于对有关文献及本文试验结果的分析,提出了恶劣环境下FRP片材拉伸强度的计算方法。     

Use of the time constant to detect corrosion speed in reinforced concrete structures

The relatively simple to measure ‘time constant’ is presented as an index for characterizing the level of corrosion experienced by steel in concrete. This communication, largely in the form of a technical note, explicitly illustrates that the time constant determined from electrochemical testing has significant merit for monitoring corrosion of steel in concrete, and appears to be insensitive to the area of electrode being probed. In this work, the time constant, κ, is determined following a galvanostatic excitation signal, revealing a good correlation between the value of κ and corrosion status across specimens of widely varying geometry. Although this notion has been suggested in the past, this work presents both a survey and consolidated review to indicate the utility of the parameter as an index to corrosion rate on-site.     

Crack growth resistance in fibre reinforced alkali-activated fly ash concrete exposed to extreme temperatures

This paper examines the crack growth resistance of alkali-activated fly ash concrete under extreme temperatures. Plain and hybrid fibre reinforced alkali activated concrete prepared with fly ash were subjected to a range of temperatures from ? 30 to 300 °C, sustained for 2 h. The alkali activation was effected with a blend of sodium hydroxide and sodium silicate. A fibre blend of steel to polypropylene in the volume ratio of 4:1 and a total as high as 1% by volume fraction reinforced the mixtures. The resulting systems were examined for residual strength under compression and splitting tension. Further, notched prisms were loaded under 4-point flexure to evaluate the residual fracture toughness. Based on the results, four different stages for fracture behaviour were identified with superior fibre efficiency seen at sub-zero temperatures. Across the breadth of temperatures examined, adding fibres led to higher residual fracture toughness for those systems that displayed a narrow range of thermal conductivity.     

Lattice modelling of corrosion induced cracking and bond in reinforced concrete

A lattice approach is used to describe the mechanical interaction of a corroding reinforcement bar, the surrounding concrete and the interface between steel reinforcement and concrete. The cross-section of the ribbed reinforcement bar is taken to be circular, assuming that the interaction of the ribs and the surrounding concrete can be captured by a cap-plasticity interface model. The expansive corrosion process is represented by an Eigenstrain in the lattice elements forming the interface between concrete and reinforcement. Several pull-out tests with varying degree of corrosion are analysed. The numerical results are compared with experiments reported in the literature. The influence of the properties of concrete are studied. The proposed lattice approach offers insight into corrosion induced cracking and its influence on bond strength.     

The design of an instrumented rebar for assessment of corrosion in cracked reinforced concrete

An instrumented rebar is presented which was designed to have a realistic mechanical performance and to provide location dependent measurements to assess the environment with regards to reinforcement corrosion. The instrumented rebar was constructed from a hollowed 10 mm nominal diameter standard rebar with 17 electronically isolated corrosion sensors. Instrumented and standard rebars were cast into concrete beams and bending cracks were induced and held open using steel frames. Epoxy impregnation was used to assess and compare cracks in the concrete around the instrumented and standard rebar. As bending-induced cracks reached the reinforcement, slip and separation occurred along the concrete–reinforcement interface. Cracks in the concrete surrounding standard and instrumented rebars are largely similar in appearance; however, sensors protruding from the instrumented rebar reduced the separation between the steel and concrete. Cracked beams with cast-in instrumented and standard rebars were ponded with a 10% chloride solution and the open circuit corrosion potential (OCP) of the 17 sensors was measured for up to 62 days. Measurements from the individual sensors indicate when and where active corrosion may be thermodynamically favored based upon the local environmental conditions. Results indicated the length along the instrumented rebar where active corrosion was thermodynamically favored increased with exposure time due to the increased aggressivity of the local environmental conditions.     

聚丙烯-钢纤维增强高强混凝土高温性能

通过对聚丙烯-钢纤维增强高强混凝土(混杂纤维/高强混凝土)试块的高温试验, 研究不同目标温度后混凝土表观特征、高温爆裂、质量损失及力学性能。结果表明: 高强混凝土在600 ℃时发生爆裂, 混杂纤维/高强混凝土直至800 ℃未出现爆裂, 混杂纤维有效抑制了高强混凝土的高温爆裂。混杂纤维/高强混凝土的质量损失随所受温度的升高而增大, 其抗压强度、抗折强度随温度的升高而降低, 并且400 ℃以后显著降低。相同温度下, 混杂纤维的加入提高了高强混凝土高温后强度。通过对试验结果的统计分析, 分别建立了混杂纤维混凝土质量损失、抗压强度和抗折强度随温度变化的关系式。     

Effect of corrosion layer of steel bar in concrete on time-variant corrosion rate

Corrosion current density of steel bar in concrete was measured in active corrosion process under a designed artificially controlled climate environment. The active corrosion process shows the characteristics of the time-variant corrosion rate, and the three phases of the corrosion process are presented. The corrosion rate decreases at first; this is followed by a steady state phase; finally after concrete cover cracking caused by corrosion, an ascending phase of the corrosion rate is observed. The mechanism of the time-variation characteristics is discussed based on the microstructure of the interfacial transition zone (ITZ) between steel bars and concrete at different corrosion levels. The microstructure shows that the porous interfacial transition zone gradually transforms into a dense corrosion layer composed of concrete and corrosion products due to expansion of the corrosion products. The layer is called as corrosion layer for short in this paper. The main reason for the descent of the corrosion rate is that transportation of oxygen and moisture is retarded due to the dense corrosion layer. When the equilibrium between rates of consumption and transportation of oxygen is reached, the corrosion rate tends to be steady. The concrete cover cracking offers new access for transporting oxygen and the corrosion rate speeds up.     

Numerical modeling for predicting service life of reinforced concrete structures exposed to chloride environments

Degradation of reinforced concrete (RC) structures due to chloride penetration followed by reinforcement corrosion has been a serious problem in civil engineering for many years. In the present paper, a systematic and robust model for predicting service life of RC structures is developed which takes environmental humidity and temperature fluctuations, chloride binding, diffusion and convection, as well as the decay of structural performance into account. The interactions between the decay of structural performance, heat and moisture transfer are considered in a coupled thermal-hygro-mechanical model. The governing equations of heat, moisture and chloride transport into nonsaturated concrete are described particularly and solved numerically by finite element analysis in space and time domains. Comparisons of numerical results with analytical solutions and experimental observations are conducted to establish the validity of the proposed numerical model. Applications of the numerical model are demonstrated by predicting service life of a RC slab exposed to a chloride environment.     

Natural marine exposure results for reinforced concrete slabs with corrosion inhibitors

Reinforced concrete slabs were cast with a concrete cover of 20 mm. The water-to-cement ratio was 0.40, and the concrete slabs were either uncracked or precracked. A simulated crack 0.2 or 0.4 mm wide was formed transverse to the axis of the reinforcing bar during the casting. Two types of commercial corrosion inhibitors were added to concrete mixtures for corrosion protection. Slabs were placed about 1 m below high tide at the Treat Island, Maine, USA, natural marine exposure site. The specimens were visually inspected and the corrosion rates were measured annually using the linear polarization technique. Some of the concrete slabs were broken open after 12 months of exposure and corrosion damage was evaluated. Water-soluble chloride content analysis was performed at this time. After three years of exposure, it was found, that both corrosion inhibitors were effective in reducing the corrosion rate for uncracked concrete slabs, but relatively ineffective in preventing localised corrosion of reinforcing steel in the crack area for precracked concrete slabs.