Carolyn M. Hansson

Pore solution expression as a method to determine the influence of mineral additives on chloride binding

Abstract A collaborative project on cement paste pore solution analysis has been carried out by Institutes in Sweden, Denmark and Austria to determine: (i) the reproducibility of the pore solution expression method; (ii) the effect of added chlorides on the OH − concentration of OPC pastes from the three countries and (iii) the influence of flyash, slag and silica fume additions on the degree of chloride binding by the cement.

Comments on electrochemical measurements of the rate of corrosion of steel in concrete

Abstract The electrochemical processes involved in the corrosion of steel are briefly described in terms of their thermodynamics and kinetics. The purpose is to explain and discuss the currently used laboratory techniques of corrosion rate measurements of reinforcement steel in concrete.

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.

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.

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.

The effect of the electrochemical chloride extraction treatment on steel-reinforced mortar. Part II: Microstructural characterization

A study has been made of the changes in cement composition and microstructures resulting from electrochemical chloride extraction applied to mortar samples in which the chlorides were added with the mixing water, ingressed by ponding with an NaCl solution, or both. After exposure for 1 year, specimens with and without chlorides were subjected to an electrochemical chloride extraction treatment. Microstructural analyses of fracture surfaces through the steel/mortar interface revealed a significant alteration of the cementitious phases. In untreated samples, calcium-silicon-rich phases consistent with Types I and II calcium silicate hydrate were observed. After the extraction treatment, these phases were not detectable and instead, sodium-rich, calcium-rich, iron-rich, and calcium-aluminum-rich phases were observed.

The influence of silica fume on the corrosion resistance of steel in high performance concrete exposed to simulated sea water

This investigation examined the influence of silica fume on the corrosion behaviour of steel in high performance concrete (HPC) by comparing the behaviour of HPC concretes with and without a 10% by mass of cement addition of silica fume. Reinforced concrete prisms (500 × 100 × 100 mm) with embedded corrosion probes were loaded in three-point bending to achieve a 0.3 mm crack and exposed to simulated sea water for up to four years. Corresponding prisms without induced cracks were also studied as controls. For the HPC with silica fume, pore size distribution measurements showed that after exposure to the simulated sea water, the hydration and pozzolanic reactions near the induced crack blocked almost all continuous pores in the 0.01 to 10 μm range. This affected the type and distribution of corrosion products that formed by restricting the access of chlorides and oxygen to the surface of the steel. Thus, only magnetite (Fe3O4) formed and was confined the space provided by the induced crack, effectively plugging the crack. In HPC without silica fume, oxygenated corrosion products such as goethite (α-FeOOH) and akaganeite (β-FeOOH) formed in the induced crack region. The implications of these observations on the service life of high performance concrete structures are discussed.

Ion-conduction in cement-based materials

Abstract Those factors which control electrical conduction in cement based materials, namely, the concentration and mobility of ions in the pore solution and the porosity and pore size distribution, are discussed in terms of the time dependency of the electrical conductivity. An experimental study has been made of the influence of water/cement ratio and age of the cement on the various parameters which can be determined from conductivity measurements. While the measurements have been made on dense silica-cement, the conclusions are applicable to other Portland cement based materials.

Nuclear magnetic resonance monitoring of capillary imbibition and diffusion of water into hardened white cement paste

Nuclear magnetic resonance (NMR) experiments monitoring the imbibition (sorption) and diffusion of water into white cement paste are reported. The sample was a 1.3 cm long cylinder (6 mm o.d.) of hardened ordinary white cement paste, with a water/cement ratio of 0.42 containing 0.5% Ca(NO2)2 and 2% NaCl. Water proton magnetization and T2 values were obtained as functions of time. Imbibition of H2O and diffusion of H2O and D2O were monitored with 1H NMR at 26 and 30 MHz. The countercurrent water imbibition experiments revealed a two-stage process. A rapid uptake of water, involving about 85% of the total, took place in about 45 min. Maximum saturation was reached in about 2 days. Both stages of the process were well described by a nonlinear diffusion-like equation. Diffusion of both H2O and D2O was characterized by a single diffusion coefficient. The diffusion coefficient for H2O and D2O, derived by fitting the data to the diffusion equation, is well predicted by D0/(Fφ).