Bond behaviour of flat stainless steel rebars in concrete

In recent years there has been an increasing interest in applying stainless steel (SS) reinforcement in concrete structures to avoid corrosion derived durability problems. Concrete-steel bond behaviour and applicability of existing standards have not yet been extensively studied with respect to SS. Moreover, an interest in applying flat rebars as reinforcement elements has emerged. The idea is based on the optimization of the rebar surface in contact with the concrete as well as on the reinforcement optimization for shallow slabs. To advance in the evaluation of the bond between concrete and flat SS strips and to compare with the behaviour of standard carbon steel (CS) round reinforcement 72 bond tests have been conducted. The test program consists of pull-out tests to centrally reinforced concrete specimens. Traditional concrete and self compacting concrete are used for embedding. For a better analysis and understanding of the failure aspect of the specimens, fluorescent epoxy injection has been conducted. Results are compared to the CEB-FIP 1990 bond model. The authors conclude that, the use of SS instead of CS is not considerably influencing the bond capacity of the reinforcement when ribbed samples are used while parameters as reinforcement shape or rib pattern are more deterministic. An adaptation of the CEB-FIP 1990 bond model is proposed for flat reinforcements tested in this work.     

Numerical simulation of rebar/concrete interface debonding of FRP strengthened RC beams under fatigue load

The aim of this paper is to simulate the rebar/concrete interface debonding of FRP strengthened RC beams under fatigue load and also, to ascertain the influence of design parameters such as the elastic modulus, thickness and length of the FRP plate on the debonding performance. In order to simplify the simulation, some basic equilibrium equations are formulated and then the stresses of the rebar and FRP plate are numerically solved, and stress intensity factor is avoided in the simulation by fundamentals of fracture mechanics because of its complexity around the crack tip of bi-material interface. With the combination of finite element method and difference approximation, authors program the degradation model of coefficient of friction, debond criterion, propagation law and loop of load process into a commercial finite element code to investigate the fatigue debonding. The relationships between the debond length as well as other fatigue parameters and number of cyclic load are obtained and discussed.     

Numerical modeling on concrete structures and steel–concrete composite frame structures

A steel–concrete composite fiber beam-column model is developed in this study. The composite fiber beam-column model consists of a preprocessor program that is used to divide a composite section into fibers and a group of uniaxial hysteretic material constitutive models coded in the user defined subprogram UMAT in ABAQUS. The steel–concrete composite fiber beam-column model is suitable for global elasto-plastic analysis on composite frames with rigid connections subjected to the combined action of gravity and cyclic lateral loads. The model is verified by a large number of experiments and the results show that the developed composite fiber model possesses better accuracy and broader applicability compared with a traditional finite element model. Although the fiber beam-column model neglects the slip between the steel beam and concrete slab, there are essentially no effects on the global calculation results of steel–concrete composite frames. The proposed model has a simple modeling procedure, high calculation efficiency and great advantage when it is used to analyze composite frames subjected to cyclic loading due to earthquake.     

FE modeling of expansive oxide induced fracture of rebar reinforced concrete

This paper develops a finite element based simulation strategy which can model the time dependent expansive oxide induced fracturing of corroding rebar reinforced concrete. The oxide growth history is modeled by empirical relations which can be correlated to represent the expansive process for a given in situ condition. Due to the generality of the procedure developed, problems with multiple long progressively growing cracks can be handled in an entirely automated manner. To ascertain crack tip physics, moving templates are attached to each crack. These individually move with the cracks as they progress under a given expansive oxide growth history. To quantify the procedure several benchmark example problems are presented.     

Bond characteristics of carbon and stainless steel flat rebars with an alternate rib pattern

The good performance of reinforced concrete depends on the appropriate transfer of forces between reinforcing rebars and concrete, which relies on the bond interaction between the two materials. At an uncracked section, both materials work together by means of the bond forces; however, if the tensile strength of the concrete is reached at a certain part of the structure, a crack will appear and the steel will be the only active element at the cracked section. At increasing loads, the crack will continue opening and large crack openings may lead to a failure of the rebar and to the collapse of the whole structure. A new idea to positively influence the cracking behaviour has emerged, which is based on the combination of smooth and rib zones within the same reinforcing rebar. Furthermore, use of stainless steel flat reinforcement has been considered as an option to optimize the reinforcing of shallow slabs. This paper presents bond tests performed on carbon and stainless steel flat reinforcements embedded in concrete and with different alternate rib configurations. Test results are presented in terms of bond strength and force transfer stiffness, as well as in terms of bond stress–slip relationship. Results show no differences between the bond capacity of carbon and stainless steel rebars if other parameters are kept constant. The use of an alternate surface configuration combining smooth and ribbed zones within the bond length, does affect the bond capacity of the rebar, and the position of the smooth zone within the bond length plays an important role.     

A three-dimensional steel/concrete interface model including corrosion effects

This paper presents a new constitutive law for modelling the steel/concrete interface by including main corrosion effects. A three-dimensional formulation has been proposed based on continuum damage mechanics. The theoretical framework of the thermodynamics of irreversible processes has been applied in order to guarantee the respect of both conservation and evolution principles. Results are presented in order to point out the main features of the proposed model. The identification of material parameters, one of the major points when invoking this model, is detailed. In order to show its efficiency, pull-out tests with and without the presence of corrosion have been simulated. The effects of corrosion on failure loads are clearly in accordance with experimental results. A qualitative comparison between the damage pattern and the experimental crack path due to corrosion is also included in the paper.     

钢筋与结构型合成纤维对混凝土抗冲击性能混杂效应的分析

为了研究钢筋与结构型合成纤维混杂后对混凝土抗冲击性能的影响,采用改进的自由落球冲击试验装置,对素混凝土、钢筋混凝土、结构型合成纤维增强混凝土以及钢筋-结构型合成纤维混杂增强的混凝土试件的抗冲击性能进行了试验研究,分析了钢筋、结构型合成纤维以及钢筋与结构型合成纤维混杂后对混凝土抗冲击性能的影响及其增强机理。同时,利用Weibull分布理论分析了试件初裂冲击次数和破坏冲击次数的分布规律。研究表明:结构型合成纤维可以提高混凝土抗冲击性能;对于提高混凝土的抗冲击性能,钢筋与结构型合成纤维表现出显著的正混杂效应;两参数Weibull分布能较好的描述钢筋-结构型合成纤维混凝土抗冲击次数的分布特征。     

Electrochemical chloride removal for reinforced concrete with steel rebar cage using auxiliary electrodes

In this paper, a new approach of electrochemical chloride removal (ECR) using the auxiliary electrodes was proposed and the performances of this method were investigated by experiments. Two kinds of electrode setups were investigated: the radial type and the layer type. The results showed no matter which setup was used the chloride removal percentage all exceeded 70% after 8-week treatment. The chloride enclosed by the steel rebar cage could be successfully pushed out from concrete using auxiliary electrodes. The half cell potentials of rebars after 8-week treatment were all higher than −270 mV(CSE) and the corrosion rates were lower than 0.05 mpy, it indicated that after ECR treatment the corrosion risk of rebars was low. In addition, after ECR the surface hardness was enhanced via the results of rebound strength test. The pH value of concrete was also enhanced during the ECR process.     

Bond strength prediction for deformed steel rebar embedded in recycled coarse aggregate concrete

This paper summarizes the results of an experimental investigation into the bond behavior between recycled aggregate concrete (RAC) and deformed steel rebars, with the main variables being the recycled coarse aggregate replacement ratio (RCAr) and water-to-cement ratio of the concrete mixture. The investigation into splitting cracking strength indicates that the degradation of the bond splitting tensile stress of the cover concrete was affected by not only the roundness of the coarse aggregate particles but also the weak interfacial transition zone (ITZ) between the cement paste and the RCA that has a more porous structure in the ITZ than normal concrete. In this study, a linear relationship between the bond strength and the density of the RCA was found, but the high compressive strength reduced the effects of the parameters. To predict the bond strength of RAC using the main parameters, a multivariable model was developed using nonlinear regression analysis. It can be inferred from this study that the degradation characteristic of the bond strength of RAC can be predicted well, whereas other empirical equations and code provisions are very conservative.     

Bond mechanism of FRP rebars to concrete

The bond mechanism of Fiber Reinforced Polymer (FRP) rebar to concrete was studied. Five different types of 12.7 and 12.0-mm rebar subjected to different surface treatments were tested, and the bond mechanism was compared with that of untreated FRP rods and ordinary deformed steel. High bond values were obtained for rods exhibiting a stiff deformed surface, on which large deformations were molded by resin, and for rods with a rough surface whereby the roughness has resulted either from winding a helical fiber together with embedded sand particles or from using excess polymer. The bond values recorded were equivalent to or larger than those of ordinary deformed steel. Low bond strength was obtained both for rods with a thick polymeric layer of low mechanical for rods with a thick polymeric layer of low mechanical properties and for rods with smooth surfaces. Different pre-peak and post-peak behavior was observed for the various rods when the entire set of P-s (Pullout load vs. slip) curves were compared. Brittle behavior was apparent wherever the external layer of the rod exhibited large deformations formed in a stiff matrix. Where the surface was rough, more ductile behavior was detected. The wedging of particles into the surface can alter the load-slip behavior, from one of slip-weakening to one of slip-hardening.     

Efficiency of coatings applied on rebars in concrete

The performance of three reinforcing steel bars (rebars) coatings is analyzed by means of electrochemical methods. The coatings are representative of those commonly used during the repair of concrete structures affected by corrosion in the coastal regions of Argentina: an epoxy rust conversion coating, a zinc-rich epoxy, and a sprayed zinc coating. Two exposure conditions were investigated: immersed in a saline solution (wet) and exposed to an indoors atmosphere (dry). The rebar corrosion potential (E_corr), the corrosion rate (CR) and the electrical resistance (R) between bars were measured during approximately 800 days. In the dry condition the three coatings presented a satisfactory performance characterized by passive E_corr values and CR values lower than 1 m/year. On the other hand, the performance of the coatings in the wet condition depended on their formulations. The rust conversion coating showed active E_corr values and CR values higher than those measured on the uncoated bars. The zinc-rich epoxy and the sprayed zinc coatings presented E_corr values typical of active zinc. This indicates a certain degree of cathodic protection provided to the reinforcing steel. Besides, R values showed that in this condition, coatings do not provide a barrier type of protection.     

钢纤维增韧微膨胀钢管混凝土界面粘结性能研究

进行了30根钢纤维微膨胀钢管混凝土试件推出试验,系统研究了核心混凝土与管壁的界面粘结性能,阐述了推出试验过程中试件界面粘结破坏特征,并对界面粘结性能的影响因素进行了分析。结果表明,钢纤维微膨胀钢管混凝土界面粘结强度较普通钢管混凝土明显提高。径厚比和含钢率是钢纤维微膨胀钢管混凝土界面粘结强度的主要影响因素。钢纤维的掺加导致微膨胀钢管混凝土界面粘结性能降低,其体积掺量建议不宜超过0.75%。含钢率达到22%时,钢纤维掺量与界面长度对界面粘结性能影响不明显。     

Durability issues of FRP rebars in reinforced concrete members

The use of fibre reinforced polymers (FRPs) as rebars in reinforced concrete (RC) elements is a viable means to prevent corrosion effects that reduce the service life of members employing steel reinforcement. However, durability of FRP rebars is not straightforward as it is related to material properties as well as bar–concrete interaction. A state of the art of durability of FRP rebars is presented herein in order to highlight issues related to the material properties and interaction mechanisms which influence the service life of RC elements. The design approach implemented in international codes is discussed and the reduction factors taking into account the durability performances are summarized.     

In situ measurement of Cl- concentrations and pH at the reinforcing steel/concrete interface by combination sensors

This paper presents an in situ, nondestructive method of monitoring Cl- concentrations and pH values at the steel/concrete interface. The Ag/AgCl electrodes prepared by the electrochemical anodization and the Ir/IrO2 electrodes prepared by thermal oxidation in carbonate served as Cl- concentration and pH sensors, respectively. The potentiometric response of the Ag/AgCl electrode to the logarithm of Cl- concentrations ranging from 1 x 10(-4) to 2 M in saturated Ca(OH)2 solution simulating the inner electrolytic medium of concrete shows good linearity. The Ir/IrO2 electrode also exhibits an ideal Nernstian response in the range of pH 1-14. The Ag/AgCl and Ir/IrO2 electrodes were combined into a multiplex Cl-/pH sensor, and the sensor was embedded in concrete close to the steel/concrete interface to realize an in situ and long-term measurement of Cl- concentrations and pH values. The results indicate that the combined sensor is robust and sensitive enough to in situ measure Cl- concentrations and pH quantitatively at the steel/concrete interface, which is of indispensable importance to the study of corrosion and protection of the steel in concrete.     

Effect of zinc phosphate chemical conversion coating on corrosion behaviour of mild steel in alkaline medium: protection of rebars in reinforced concrete

We outline the ability of zinc phosphate coatings, obtained by chemical conversion, to protect mild steel rebars against localized corrosion, generated by chloride ions in alkaline media. The corrosion resistance of coated steel, in comparison with uncoated rebars and coated and uncoated steel rebars embedded in mortar, were evaluated by open-circuit potential, potentiodynamic polarization, cronoamperometry and electrochemical impedance spectroscopy. The coated surfaces were characterized by x-ray diffraction and scanning electron microscopy. First, coated mild steel rebars were studied in an alkaline solution with and without chloride simulating a concrete pore solution. The results showed that the slow dissolution of the coating generates hydroxyapatite Ca₁₀(PO₄)₆(OH)₂. After a long immersion, the coating became dense and provided an effective corrosion resistance compared with the mild steel rebar. Secondly, the coated and uncoated steel rebars embedded in mortar and immersed in chloride solution showed no corrosion or deterioration of the coated steel. Corrosion rate is considerably lowered by this phosphate coating.     

Comprehension of demoulding mechanisms at the formwork/oil/concrete interface

The implementation of concrete and its association with a release agent influence the aesthetics of the concrete facings. The mineral oils tend to be replaced by vegetable formulations, to reduce the impact of the substances spilled in the environment. From a technical point of view, it is important to characterize the action of these new formulations at the interface concrete/oil/formwork. Two performing techniques have been used to study the physico-chemical processes, the tribometry and electrochemical impedance spectroscopy. The correlation of the results obtained allowed to improve the understanding of the mechanisms involved at the interface mould/oil, in connection with the use of an acidifier in the formulation.      

Effect of zinc phosphate chemical conversion coating on corrosion behaviour of mild steel in alkaline medium: protection of rebars in reinforced concrete

Abstract

We outline the ability of zinc phosphate coatings, obtained by chemical conversion, to protect mild steel rebars against localized corrosion, generated by chloride ions in alkaline media. The corrosion resistance of coated steel, in comparison with uncoated rebars and coated and uncoated steel rebars embedded in mortar, were evaluated by open-circuit potential, potentiodynamic polarization, cronoamperometry and electrochemical impedance spectroscopy. The coated surfaces were characterized by x-ray diffraction and scanning electron microscopy. First, coated mild steel rebars were studied in an alkaline solution with and without chloride simulating a concrete pore solution. The results showed that the slow dissolution of the coating generates hydroxyapatite Ca₁₀(PO₄)₆(OH)₂. After a long immersion, the coating became dense and provided an effective corrosion resistance compared with the mild steel rebar. Secondly, the coated and uncoated steel rebars embedded in mortar and immersed in chloride solution showed no corrosion or deterioration of the coated steel. Corrosion rate is considerably lowered by this phosphate coating.