The corrosion pattern of reinforcement and its influence on serviceability of reinforced concrete members in chloride environment

This paper deals with two corroded reinforcement concrete beams, which have been stored under sustained load in a chloride environment for 14 and 23 years respectively. The evolution of corrosion pattern of reinforcement and its influence on serviceability are studied. In chloride-induced corrosion process, corrosion cracking affects significantly the corrosion pattern. During the corrosion cracking initiation period, only local pitting corrosion occurs. At early stage of cracking propagation, localized pitting corrosion is still predominant as cracks widths are very small and cracks are not interconnected, but a general corrosion slowly develops as the cracks widen. At late cracking stage, interconnected cracking with wide width develops along large parts of the beam leading to a general corrosion pattern. Macrocells and microcells concepts are used for the interpretation of the results.Mechanical experiments and corrosion simulation tests are performed to clarify the influence of this corrosion pattern evolution on the serviceability of the beams (deflection increase). Experimental results show that, when the corrosion is localized (early cracking stage), the steel–concrete bond loss is the main factor affecting the beams serviceability. The local cross-section loss resulting from pitting attack does not significantly influence the deflection of the beam. When corrosion is generalized (late cracking stage), as the steel–concrete bond is already lost, the generalized steel cross-section reduction becomes the main factor affecting the beams serviceability. But, at this stage, the deflection increase is slower due to the low general corrosion rate.     

Concrete cover cracking with reinforcement corrosion of RC beam during chloride-induced corrosion process

This paper deals with the evolution of the corrosion pattern based on two beams corroded by 14 years (beam B1CL1) and 23 years (beam B2CL1) of conservation in a chloride environment. The experimental results indicate that, at the cracking initiation stage and the first stage of cracking propagation, localized corrosion due to chloride ingress is the predominant corrosion pattern and pitting corrosion is the main factor that influences the cracking process. As corrosion cracking increases, general corrosion develops rapidly and gradually becomes predominant in the second stage of cracking propagation. A comparison between existing models and experimental results illustrates that, although Vidal et al.'s model can better predict the reinforcement corrosion of beam B1CL1 under localized corrosion, it cannot predict the corrosion of beam B2CL1 under general corrosion. Also, Rodriguez's model, derived from the general corrosion due to electrically accelerated corrosion experiments, cannot match natural chloride corrosion irrespective of whether corrosion is localized or general. Thus, for natural general corrosion in the second stage of cracking propagation, a new model based on the parameter of average steel cross-section loss is put forward to predict steel corrosion from corrosion cracking.     

Pitting and stress corrosion cracking behavior in welded austenitic stainless steel

The effect of microstructural changes in 304 austenitic stainless steel induced by the processes of gas tungsten arc welding (GTAW) and laser-beam welding (LBW) on the pitting and stress corrosion cracking (SCC) behaviors was investigated. According to the in situ observations with scanning reference electrode technique (SRET) and the breakdown potentials of the test material with various microstructures, the GTAW process made the weld metal (WM) and heat-affected zone (HAZ) more sensitive to pitting corrosion than base metal (BM), but the LBW process improved the pitting resistance of the WM. In the initiation stage of SCC, the cracks in the BM and HAZ propagated in a transgranular mode. Then, the crack growth mechanism changed gradually into a mixed transgranular + intergranular mode. The cracks in the WM were likely to propagate along the dendritic boundaries. The crack initiation rate, crack initiation lifetime and crack propagation rate indicated that the high-to-low order of SCC resistance is almost the same as that for pitting resistance. High heat-input (and low cooling rate) was likely to induce the segregation of alloying elements and formation of Cr-depleted zones, resulting in the degradation in the corrosion resistance.     

Prediction of reinforcement corrosion using corrosion induced cracks width in corroded reinforced concrete beams

This paper studies the evolution of reinforcement corrosion in comparison to corrosion crack width in a highly corroded reinforced concrete beam. Cracking and corrosion maps of the beam were drawn and steel reinforcement was recovered from the beam to observe the corrosion pattern and to measure the loss of mass of steel reinforcement. Maximum steel cross-section loss of the main reinforcement and average steel cross-section loss between stirrups were plotted against the crack width. The experimental results were compared with existing models proposed by Rodriguez et al., Vidal et al. and Zhang et al. Time prediction models for a given opening threshold are also compared to experimental results. Steel cross-section loss for stirrups was also measured and was plotted against the crack width. It was observed that steel cross-section loss in the stirrups had no relationship with the crack width of longitudinal corrosion cracks.     

Effect of moisture conditions and transfers on alkali silica reaction damaged structures

The aim of this paper is to point out the water driving effect on the alkali silica reaction (ASR) expansion and particularly when modifications of moisture conditions occur. After being submitted to a unidirectional moisture gradient during 14 months, the upper faces of ASR reactive beams were covered by water for 9 months. This late water supply on the upper face rapidly produced large expansions, which mainly occurred along the transverse and the vertical directions resulting in large longitudinal cracks. Companion nonreactive specimens were kept in the same environmental conditions in order to quantify the basic characteristics of moisture-dependent expansive behaviour of the material. The paper focuses on the effects of late water supply or late drying on already ASR-damaged concrete, which is a significant concern for real-life structures. Both structural effects of late water supply on ASR progress in already damaged structures and interpretation of such phenomena are described.     

Characterisation of bending cracks in R/FRC using image analysis

In this study, experiments were conducted to induce bending cracks of specific surface crack width to reinforced concrete beams made of plain concrete (RC) and reinforced concrete beams made of fibre reinforced concrete (R/FRC). After injecting and impregnating the cracks with dyed epoxy resin, image processing and analysis were employed to investigate the internal crack morphology. Several crack features including crack width (accumulated, effective and maximum), branching and tortuosity were defined and quantified. The results showed that in addition to arrested crack development, the presence of fibres yielded a distinctive change in the internal crack pattern, including increased branching and tortuosity, both of which have positive implications regarding concrete permeation. Likewise, specimens with fibres exhibited reduced maximum individual crack widths near the rebar, potentially increasing the ability of autogenous crack healing and reducing the risk of corrosion initiation.     

Probabilistic evaluation of initiation time in RC bridge beams with load-induced cracks exposed to de-icing salts

In this study, a reliability-based method for predicting the initiation time of reinforced concrete bridge beams with load-induced cracks exposed to de-icing salts is presented. A practical model for predicting the diffusion coefficient of chloride ingress into load-induced cracked concrete is proposed. Probabilistic information about uncertainties related to the surface chloride content and the threshold chloride concentration has been estimated from a wide review of previous experimental or statistical studies. Probabilistic analysis to estimate the time to corrosion initiation with/without considering the effect of the load-induced cracks on the chloride ingress into concrete has been carried out. Results of the analysis demonstrate the importance of considering the effect of the load-induced cracks for correct prediction of corrosion initiation in RC bridge beams exposed to chlorides.     

An efficient catalyst Co(salophen) for synthesis of diethyl carbonate by oxidative carbonylation of ethanol

The oxidative carbonylation of ethanol to diethyl carbonate (DEC) was investigated by an efficient catalyst system comprising of Co-Schiff base complexes. Effects of Schiff base ligands, reaction time, catalyst concentration, temperature and pressure on the catalytic activity were studied. Co(salophen) [N,N′-bis(salicylidene) o-phenylenediamine cobalt] catalyst exhibited better catalytic activity compared with other Co complexes. When the oxidative carbonylation was carried out at the reaction conditions: 0.12 mol/L Co(salophen), P(CO)/P(O₂) = 2:1, 3.0 MPa, 140 °C, 2.5 h, the conversion of ethanol is 15.8%, the selectivity to DEC is 99.5% and the turnover number (TON) is 22.2. The corrosion behavior of Co(salophen) catalyst to the stainless steel reactor was also examined. The corrosion rate to the stainless steel by Co(salophen) catalyst is below 0.005 mm/a. SEM images demonstrated that the pitting corrosion was not observed on the surface of the stainless steel.     

Comparison between surface and bulk hydrophobic treatment against corrosion of galvanized reinforcing steel in concrete

The effectiveness of bulk hydrophobic treatment against corrosion of galvanized steel reinforcement in concrete specimens with w/c = 0.45 and w/c = 0.75 was compared with that of surface treatment, even in the presence of cracks 0.5 and 1 mm wide in the concrete cover. In this case surface hydrophobic treatments were applied both before and after cracking as a preventive and a restorative method against reinforced concrete deterioration, respectively. The obtained results in terms of water absorption, electrochemical measurements, chlorides penetration, and visual observations carried out on reinforced concrete specimens during the exposure to wet–dry cycles in 10% NaCl solution showed that bulk hydrophobization is the most effective treatment in improving the corrosion resistance of galvanized steel reinforcements in concrete also in the presence of cracks. Surface hydrophobization is very effective just in the first few exposure cycles to the aggressive environment and when used as a restorative method which is able to cancel the deleterious effect of cracks only 0.5 mm wide.     

水介质中4340钢超高压换热器疲劳裂纹扩展速率

针对超高压换热管常用的ＡＩＳＩ４３４０钢，对表面裂纹的试板采用浸水腐蚀的方法，研究了４３４０钢在水介质中表面裂纹的疲劳扩展速率，并与空气中的扩展速率进行了比较。试验结果表明，腐蚀介质的存在加速厂裂纹扩展，并随裂纹的扩展，其加速作用越来越明显。水中存在的氧是导致腐蚀和加速裂纹扩展的主要原因。     

Electrochemical studies on the alternating current corrosion of mild steel under cathodic protection condition in marine environments

Alternating current (AC) corrosion of mild steel in marine environments under cathodic protection (CP) condition was studied. Electrochemical studies at the two protection potentials namely −780 and −1100 mV versus SCE were examined by different techniques. DC polarization study was carried out for mild steel in natural seawater and 18.5 g/L NaCl solution to evolve corrosion current density. The corrosion rate determination, pH of the end experimental solution and surface morphology of the mild steel specimens under the influence of different AC current densities were studied. The amount of leaching of iron into the solution was estimated using inductively coupled plasma spectrometry. All these techniques revealed that AC influences the corrosion of mild steel in the presence of marine environments even though CP was given. Surface micrographs revealed that spreading of red rust products noticed on the mild steel surface. At −780 mV CP, red rusts are visually seen when the AC source was above 10 A/m² in both the media but red rusts are appeared after 20 A/m² in the case of −1100 mV CP. Weight loss measurements coupled with surface examination and solution analysis is a effective tool to characterize and quantify the AC corrosion of mild steel in marine environments.     

Eco friendly inhibitor for corrosion inhibition of mild steel in phosphoric acid medium

The inhibition effect of Zenthoxylum alatum plant extract on the corrosion of mild steel in 20, 50 and 88% aqueous orthophosphoric acid has been investigated by weight loss and electrochemical impedance spectroscopy (EIS). Plant extract is able to reduce the corrosion of steel more effectively in 88% phosphoric acid than in 20% phosphoric acid. The effect of temperature on the corrosion behaviour of mild steel in 20, 50 and 88% phosphoric acid with addition of plant extract was studied in the temperature range 50-80 °C. Results on corrosion rate and inhibition efficiency have indicated that this extract is effective up to 70 °C in 88% phosphoric acid medium. Surface analysis (XPS and FT-IR) was also carried out to establish the mechanism of corrosion inhibition of mild steel in phosphoric acid medium.     

Influence of steel fibers on the development of alkali-aggregate reaction

This work presents the results of an experimental research concerning the use of fibers in mortar specimens subjected to alkali-aggregate reaction (AAR). Two types of steel fibers (0.16 mm diameter and 6.0 mm length, and 0.20 mm diameter and 13.0 mm length) were used with fiber volume contents of 1% and 2%. Besides the expansion accelerated tests, compressive tests and flexural tests have also been carried out to display the main mechanical characteristics of the fiber-reinforced mortars after being subjected to AAR. Moreover, the microstructure of the specimens was analyzed by scanning electron microscopy and energy dispersive X-ray. The results shown that the addition of steel fibers reduced the expansion due to AAR for the experimental conditions studied in this paper. The most expressive benefit corresponded to the addition of 13.0 mm fibers in the mixture containing 2% fiber content. This fiber volume content also corresponded to the maximum increment in the mechanical properties compared to the reference mortar, mainly for the post-cracking strength and for the toughness in bending. It was observed that the fibers have a beneficial effect on the material, without compromising its main mechanical properties.     

Chloride-induced corrosion products of steel in cracked-concrete subjected to different loading conditions

This project focused on examining the composition and distribution of chloride-induced corrosion products at the rebar-concrete interfaces and on crack surfaces in reinforced ordinary Portland cement concrete (OPCC) and high performance concrete (HPC) subjected to different loading conditions. The results indicated that, regardless of the type of loading, there was a larger distribution of corrosion products along the rebar surface in the HPC than in the OPCC. Also, dynamic loading caused a greater detachment of the aggregate-paste bond in OPCC than static loading. The opening and closing of the cracks in salt solution under dynamic load forced corrosion products to flow from the rebar-concrete interface into the cracks in both OPCC and HPC. As a result, corrosion products diffused from the crack into the cement paste in the dynamically loaded OPCC but remained in the cracks in the dynamically loaded HPC, where they induced branched cracks. The mill-scale on the rebar was also evaluated before and after being embedded in concrete and was observed to be porous allowing ingress of species from the cement paste.     

Characterization of Ballistic Impact Damage in Wired Glass

The ballistic impact behavior of wired glass has been studied. Three types of commercially available samples, with thin and thick wire mesh, were impacted with steel, glass, and tungsten carbide spheres of various sizes using a gas gun. Impact energies varied from 0.5 to 40 J. A system of cracks was found to develop that comprised primarily a major cone crack. At higher energies, additional radial and lateral cracking developed along with increased damage to the front face of the sample. The extent of damage was assessed in terms of basal cone crack area, number of cracks, mass loss, and fracture load of impacted samples. It was found that damage, at a given impact energy, increased with increasing projectile diameter. Moreover, 10 mm diameter glass spheres caused more damage than smaller (3–4.74 mm diameter) steel spheres. No significant differences were observed with regard to the ballistic behavior of the thin- and thick-wired glass types.     

Biogenic sulfuric acid attack on different types of commercially produced concrete sewer pipes

Laboratory experiments were conducted to compare the degradation of low and high quality concrete under conditions simulating sewer pipes with and without bacteria. Small concrete samples were exposed to hydrogen sulfide, multiple species of bacteria found in corroding sewer pipes and artificial wastewater. Experiments without bacteria were used as controls. The corrosion rates of the concrete samples exposed to bacteria over 227 days were 0.08 mm/yr (millimeters per year) for the concrete from a domestic manufacturer with moderate strength and a lower water-cement ratio (Low-w/c) versus 0.208 mm/yr for the concrete samples from a foreign country with low strength and a higher water-cement ratio (High-w/c). The (Low-w/c) concrete was more resistant to the biodegradation even though a lower pH was attained for its bioactive systems. Experiments showed the influence of biogenic sulfuric acid production on short term corrosion rates.     

Predicting carbonation in early-aged cracked concrete

Carbonation in cracked concrete is considered as one of major deteriorations accelerating steel corrosion in reinforced concrete structures. For durable concrete structures, it is necessary to control crack in concrete through crack resistance evaluation for early-aged concrete structures, but often unavoidable cracks in early-aged concrete may occur. These cracks become a main path for CO₂ penetration inside concrete so that the carbonation is accelerated in cracked concrete.In this study, an analytical technique for carbonation prediction in early-aged cracked concrete was developed for considering both CO₂ diffusion of pore water in sound concrete and in cracked concrete. Then, characteristics of diffusivity on the carbonation in early-aged concrete are studied through finite element analysis implemented with the so-called multi-component hydration heat model and micro-pore structure formation model. The carbonation behaviour in sound concrete and cracked concrete are also simulated by using the derived diffusivity with consideration of reaction with dissolved CO₂. Finally, numerical results obtained for cracked concrete made with 3 different W / C ratios (45%, 55%, and 65%) with different crack widths were compared with experimental results.     

Corrosion protection of iron in water by activated carbon fiber (ACF)

Commercial activated carbon fiber (ACF) was modified and employed to prevent iron corrosion in industrial water supply and circulation system. Static and dynamic experiments were carried out under varying conditions, including different pHs, different temperatures, different adsorbent quantities and different adsorbents. The primary objective was to experimentally demonstrate the suitability of ACF in effectively preventing iron corrosion in water under varying operating conditions, and compare its performance vis-à-vis to that of the other commercially available adsorbents, such as granular activated carbon (GAC) and powdered activated carbon (PAC). Iron sheet static corrosion simulation test as well as dynamic corrosion simulation test was performed to verify the idea. It was found out that ACF could significantly decrease the zeta potential (from 329 mV to 203 mV when 100 mg ACF was added to 200 ml water) and dissolved oxygen concentration (from 9.60 mg/l to around 9.18 mg/l when 200 mg ACF was added to 200 ml water) of the solution, thereby slowing down iron corrosion rate.     

The bond and flexural strengthening of reinforced concrete elements strengthened with near surface mounted prestressing steel (PS) bars

Abstract

The main objective of this work is to study the performance of prestressing steel (PS) bars as reinforcements in the reinforced concrete (RC) elements strengthened by the near-surface mounted method (NSM). The work includes two parts. In the first part, direct pull-out tests are performed in order to study the bond performance between PS reinforcement and concrete. The influences of groove sizes and PS surface conditions (smooth and sand coated) are evaluated. The results show that the sand coated PS (PS-Sc) reinforcement has the best adhesion behavior compared with the smooth bar, and its pull-out force is increased by 48%. For this reason, the PS-Sc bars are used in the second part of this work as NSM reinforcement to strengthen RC beams subjected to bending forces. Then, four-point bending tests are carried out to understand the flexural behavior of strengthened RC beams with PS-Sc reinforcements of different lengths and ratios. The obtained results demonstrate that the use of NSM-PS-Sc bars strengthening technique leads to important enhancement in the load carrying capacity of the RC beams. The first crack load and ultimate load of the strengthened RC beams attain 71.41 and 65.67%, respectively, which are higher than those of the control beam. Furthermore, the experimental values show a good agreement with the analytical values in both the ultimate deflection and the ultimate load. This proves that the NSM-PS-Sc bars studied in this work are promising reinforcement of the RC beams.     

The effect of Thyme leaves extract on corrosion of mild steel in HCl

The aqueous extract of Thyme leaves was investigated for its efficacy as corrosion inhibitor for mild steel in 2 M HCl using weight loss measurements and various electrochemical techniques. Experimental results show that corrosion inhibition efficiency increases with concentration of the Thyme leaves extract. A corrosion inhibition efficiency of 84% was achieved in 2 M HCl. Polarization studies show that Thyme leaves extract acts as a mixed inhibitor. The adsorption of Thyme leaves extract on the steel surface is found to follow Langmuir adsorption isotherm.