Brian B. Hope

Corrosion rates of steel in concrete

Abstract This paper critically examines DC and AC electrochemical techniques as used to determine the rate of corrosion of steel in concrete. Experimental evidence is presented to show that film resistance and mass transport can be factors determining the rate of corrosion and rates calculated from DC polarization resistance values can be in error. The values of the Tafel parameters required to estimate the corrosion rate appear to vary widely and estimation of the appropriate values is difficult. The AC technique involving measurement of the impedance over a range of frequencies gives information concerning the corrosion process and the factors controlling the rate.

The influence of surface finish of reinforcing steel and ph of the test solution on the chloride threshold concentration for corrosion initiation in synthetic pore solutions

Electrochemical tests of reinforcing steel in the laboratory are often conducted on metallographically polished specimens in alkaline solutions to simulate the behaviour of steel in concrete. The present project illustrates two of the drawbacks associated with this procedure but suggests that the results may provide a guide to improving rebar corrosion resistance.

Corrosion and electrical impedance in concrete

Abstract The experimental results confirm the findings of other workers that electrical resistivity of concrete is influenced by water/cement ratio, moisture content and temperature. A nomograph is presented which indicates the conditions which result in a resistivity of 10,000 ohm cm, the value considered to be the upper limit for the likelihood of significant corrosion of reinforcing steel in concrete for the experimental conditions used in the investigation. The results show the very great dependence of the corrosion activity of the reinforcing steel on those concrete properties which influence resistance.

Corrosion inhibitors in concrete—part I: the principles

Chemical corrosion inhibitors present an alternate method for preventing and/or delaying corrosion of reinforcement in concrete. However, not only is it necessary to evaluate their effectiveness as a corrosion inhibitors, it is also essential that the mechanism of inhibition be understood to ensure proper use. This review of the various possible mechanisms of inhibition and passive concerns regarding the suitability of inhibitors in a concrete environment is presented as Part I of a series of papers describing the results of an investigation of commercial corrosion inhibitors.

Corrosion inhibitors in concrete. Part III. Effect on time to chloride-induced corrosion initiation and subsequent corrosion rates of steel in mortar

The effectiveness of four commercially available corrosion inhibitors for use in cement-based materials was assessed in mortar exposed to chloride solution. Although, in a parallel study of steel in synthetic pore solutions, the inhibitors were observed to be ineffective in increasing the chloride threshold value of reinforcing steel exposed to chlorides, in the present study all inhibitors were found to delay the onset of corrosion, but to differing degrees. Moreover, in agreement with the parallel study, the inhibitors were found to have little detectable effect on the corrosion rate of the embedded steel once active corrosion had been initiated. This study confirmed that calcium nitrite inhibitors delay the onset of corrosion but raises questions regarding the mechanism of inhibition.

CARBON FIBER-REINFORCED POLYMER WRAPS FOR CORROSION CONTROL AND REHABILITATION OF REINFORCED CONCRETE COLUMNS

This paper discusses the use of carbon fiber-reinforced polymer (CFRP) wraps as a rehabilitation technique for corroded reinforced concrete columns. The effect of applying the wraps at early ages of the columns and its effect on corrosion propagation is addressed. Interaction between corrosion phenomena and the CFRP wrapping is studied through simple combinations of both factors. The lab work included testing of 52 reinforced concrete cylinders. The testing procedures are described. It was concluded that applying CFRP wraps significantly decreases corrosion activity when applied over the entire specimen. The application of the wraps before corrosion propagation will prevent corrosion from taking place, while the application of the wraps after corrosion occurrence will drop the rate of corrosion sharply. This effect is likely due to the epoxy saturant used to apply the CFRP sheets and not from the fibers themselves.

Corrosion inhibitors in concrete Part II: Effect on chloride threshold values for corrosion of steel in synthetic pore solutions

The effectiveness of four commercially available corrosion inhibitors for use in cement-based materials was assessed in synthetic concrete pore solution containing chlorides. The effect of the surface topography of the sample and the composition of the pore solution was also assessed. Although in a parallel study the inhibitors were observed to delay the onset of corrosion, in these tests in pore solution they were found to be ineffective in increasing the chloride threshold value of reinforcing steel exposed to chlorides and had little influence on the progression of corrosion once initiated. This suggests that chemical reactions within the cement phase are responsible for the observed results. Metallographically polished samples proved the most resistant to corrosion regardless of electrolyte composition and samples with all surface finishes exhibited lower resistance in solutions containing only calcium hydroxide than in the higher pH synthetic concrete pore solutions.

Pull-out and bond degradation of steel rebars in ECE concrete

Abstract It is demonstrated in this study that application of electrochemical chloride extraction (ECE) to reinforced concrete alters the pull-out strength and bond between the embedded high strength steel (HSS) and surrounding concrete. Concrete cubes were cast using two chloride (Cl − ) ion concentrations of 1.7% and 3.0% by weight of cement respectively, and reinforced axially with a central ASTM #6 plain steel bar. Two cathodic (d.c.) current densities, 1.0 A/m 2 and 3.0 A/m 2 of concrete surface, were used for the ECE studies, with 0.1M sodium borate electrolyte solution and inert titanium anode mesh. Axial pull-out tests as per ASTM Standard Test Method C234-91a, were carried out within 24 hours of terminating treatment to investigate the differences in bond stress and bar slip behaviour. ECE treated specimens showed significant reductions in pull-out strength, with the degradation in bond being dependent on the applied cathodic current density and initial chloride contamination. At an impressed current density of 3.0 A/m 2 , the 1.7% Cl − ions specimens showed about 58% loss in bond strength compared with about 44% for specimens premixed with 3.0% Cl − ion. The bond strength reduction at an impressed current density of 1.0 A/m 2 was about half the reduction at current density of 3.0 A/m 2 . The loss in bond strength were based on (untreated) control specimens of each type. The alkali ion accumulations around the steel rebars were observed to follow similar trend as bond degradation. It is predicted the softening effect of the alkali (sodium and potassium) ions on the cement silicate hydrates around the steel-concrete interface will have severe effect on concrete structures reinforced with HSS irrespective of their stress state. Also, concrete structures containing steel rebars with substantial corrosion product coverage are likely to experience pronounced cathodic disbondment during ECE application and within a few days of terminating the treatment.

The effect of the electrochemical chloride extraction treatment on steel-reinforced mortar. Part I: Electrochemical measurements

A study has been made of the effectiveness of electrochemical chloride extraction in reducing chloride-induced corrosion of rebar embedded in steel with chlorides added with the mixing water, ingressed by ponding with a NaCl solution, or both. After exposure for 1 year, specimens with and without chlorides were subjected to an electrochemical chloride extraction treatment. Corrosion measurements taken before and after extraction showed that the treatment halted chloride-induced pitting in those specimens that were under attack. However, the extraction treatment increased the overall corrosion rate for all specimens studied due to reduction of the passive film and significant changes in pore solution and cement chemistry adjacent to the rebar.

Carbonation and electrochemical chloride extraction from concrete

Abstract Chloride ingress into steel reinforced concrete (r.c.), and the subsequent application of electrochemical chloride extraction (ECE) are shown to be considerably retarded by the presence of a carbonation front. Four concrete blocks each reinforced with two layers of steel mats in two different configurations were electrochemically treated. One block of each type was initially carbonated to a depth of about 30 mm and subsequently ponded with saturated chloride solution (as NaCl) for 18 months. Electrochemical treatment using an applied current density of 1 A/m2 of concrete surface was applied after sampling the concretes chloride ion profiles. Carbonated concrete specimens operated at about twice as much polarizing voltage at the same applied current density as the non-carbonated concrete specimens. Higher chloride extraction was obtained in non-carbonated concrete than in carbonated concrete blocks irrespective of the configuration of the reinforcement system. Accumulations of alkali ions were greater in the non-carbonated concrete specimens in addition to higher initial chloride ion concentrations and this was significantly greater in the specimens reinforced with different rebar mats at the top and lower sections. The changes in alkali ion accumulations followed a similar trend as chloride ion profiles. Hence, any softening effect due to these alkalies on the cement silicate hydrates is expected to be more pronounced in the non-carbonated than carbonated concrete. It is predicted that chloride contaminated concrete which has its cover considerably carbonated will likely show an inefficient ECE performance.