International Concrete Abstracts Portal

Choosing the optimum cementing material system is a key question when designing a concrete structure to last for a long time in a chloride environment. There is a lack of reliable comparative information on the short and long term performance of cementing material blends, incorporating materials such as silica fume, fly ash and slag. In this contribution, various cementing material blends are examined for their potential ability to resist chloride ion penetration. A range of mixtures was tested to assist designers in material selection for concrete in a chloride environment. Silica fume, blast furnace slag, Class C and F fly ash were examined at 0.30 and 0.40 water to cementing materials ratios (w/cm) for mortars and concrete. Ternary blends of normal portland cement/slag/silica fume and normal portland cement/slag/fly ash were also tested. The diffusion coefficients were determined by chloride migration using a D.C. potential gradient, and by chloride pending/profile grinding. Silica fume was found to be essential to obtain low diffusion coefficients particularly at an early age. Some additional reduction in diffusion coefficient is obtained with ternary blends of silica fume plus slag or silica fume plus Class F fly ash.

The resistance to chloride penetration is one of the prime parameters in specifying concrete in marine applications and in a quality assurance scheme during construction. The aim of this research was to compare alternative accelerated laboratory test procedures for the assessment of chloride penetration into concrete containing supplementary cementitious materials. Three binder systems, a normal portland cement (PC), one with 30% fly ash and a third with 50% slag, were investigated under three curing conditions, 7 days water curing, air curing, and 12 hours 65 C water curing. Chlorides penetration was measured by static ponding (5% NaCl solution) and cyclic ponding (2%, 5% and 15% NaCl solutions with 12 hour wetting and drying cycles). The effect of the age at start of testing and the test duration were also investigated. The water absorption was tested by measuring weight gain, and the results were compared with that of the chloride penetration tests. It was found that the cyclic ponding test with 15% NaCl solution resulted in accelerated chloride penetration and a clearer chloride penetration front compared to the use of 2% and 5% NaCl solutions. Good correlation was found between the results from cyclic chloride ponding and static ponding. While the chloride penetration depth was found not vary significantly with test age from 7 to 56 days for all the three mixes, the increase of chloride penetration with the test duration from 3 to 14 days was much pronounced for the PC mix than the fly ash and slag mixes. The chloride penetration was highest in the air cured specimens and the lowest in the water cured specimens for all the three mixes, and the slag concrete had the lowest chloride penetration within the three mixes under each of the three curing conditions. It was also found that water absorption correlated poorly with chloride penetration.

Electrochemical tests on mortar electrodes in alkaline chloride solutions were carried out to determine the critical chloride content depending on the mortar composition (type of cement, cement content, water-cement ratio and concrete additions). Sodium chloride, with varying chloride concentrations (between c(NaC1) = 14 and 508 mmol*L-1), was used to initiate corrosion. Depassivation times of the mortar electrodes as well as corresponding total and free chloride contents were determined. By means of macrocell corrosion tests without polarisation and at a constant concentration of the test-solution of c(NaC1) = 282 mmol*L-1, the diffusion process of the chloride and the chloride binding (dependent on the age of the test amples) were investigated in order to obtain reference values for the depassivation time of the steel. Potentiostatic tests with constant polarisation at USHE = +500 mV in alkaline chloride solutions of different concentrations were carried out for 11 different mortar mixtures to determine the critical corrosion-initiating chloride content. A linear relation between corrosion-initiating free and total chloride content was found. A minimum critical chloride content of 0.25 wt.-% relative to cement was determined.

A total of 56( 16 indoor and 40 outdoor) simulated bridge deck slabs containing 5 electrically isolated steel reinforcing bars were cast. Study variables were 6 corrosion rates, 2 concrete cover depths, 2 bar sizes and 2 bar spacings. Monthly measured corrosion parameters for the 4 year study period reported here were corrosion current density, concrete temperature at the bar depth and ohmic resistance of the cover concrete. An unguarded (3LP) and guarded (Geocor) device were used to take the monthly measurements. Results demonstrate that the corrosion current density decreases with time at constant chloride contamination levels. Also, the corrosion current density increases with increasing chloride content, decreases with increasing concrete resistance and increases with increasing temperature. Cover depth had little to no influence on the measured corrosion current density. A non-linear model was developed for the 3LP device which demonstrates that the corrosion current density is a function of the water soluble chloride content, temperature at the bar depth, resistance of the cover concrete and the corrosion time after initiation. No similar model could be developed for the Geocor device because of the lack of precision (highly variable instantaneous corrosion current densities) for this device. The predicted corrosion rates are in good agreement with the measured rates. Variability of the 3LP measurements are greater at low corrosion rates than at high corrosion rates.

The use of epoxy coated steel reinforcement in concrete serves to contribute to service life extension in aggressive environments especially where proper procedures for handling and placing of the steel are followed. In this study, it is shown the durability of these steel bars exposed in saturated calcium hydroxide (Ca(OH)2) solution and/or concrete is affected by the presence of chlorides. Test parameters included measurement of electrochemical potential and electrical resistance changes with exposure time, and corrosion monitoring using DC linear polarization (LPR) and electrochemical impedance spectroscopy (EIS). Corrosion damage was particularly critical at all as received flaws and holidays, and also deterioration was detected around the bends and patched coating areas.