International Concrete Abstracts Portal

Factors affecting the chloride threshold level and the active corrosion rate are reviewed, with a special emphasis on the effect of macro cracks in high performance concrete. It was found that while the initiation time in uncracked high performance concrete may be more than a hundred years, reinforced high performance concrete is likely to crack, reducing the initiation time substantially as compared with uncracked concrete. Measurements of long term corrosion rates in cracked high performance concrete indicate however, that initially high corrosion rates decrease rapidly to very low corrosion rates, in the same range as for the passive corrosion rate in uncracked concrete. Therefore the initiation time for cracked high performance concrete may be very short, but the propagation time can be very long before any critical damage will occur. As a consequence, it seems more appropriate to define the service life for cracked high performance concrete as the time to reach a certain degree of corrosion damage, instead of using the (unpredictable) initiation time.

A modified version of the Dundee rapid chloride test (1) was used to characterise the physical resistance of a range of portland cement and condensed silica fume (CSF) concrete mixtures to chloride ingress. Cubes from these mixtures were placed in submerged, tidal and spray marine exposure zones. The solution to Fick's Second Law was applied to the measured on-site chloride profiles (after 2 years) to obtain apparent diffusion coefficients, Da and surface chloride levels, Cs. The depth of the threshold chloride level, x0.4, was calculated from Da, and Cs. The rapid test results (chloride (C1-) index values) were then correlated with the on-site chloride ingress parameters, Da and x0.4. The best correlation was found between x0.4 and the C1- index values. This can be attributed to the fact that x0.4 is calculated from both Da and Cs, and therefore eliminates a large portion of the scatter arising from the process of chloride profile determinations. The differences in correlations for the different cement types were ascribed to the lower chloride binding capacities of CSF concretes. The need for a suitable chloride binding test, which could be used in conjunction with a rapid chloride test to characterize the potential durability of concrete in ternis of chloride ingress, was identified.

This paper describes deterioration of reinforced concrete structures at a liquefied gas/sulfur plant that occurred shortly after completion of the construction. Investigations have shown that the principal cause of the deterioration was corrosion of reinforcing steel due to the presence of chlorides, with marine salts being the main source. The primary contributing factor was the increased level of industrial pollutants, which in combination with marine salts rendered the plant environment highly aggressive to the reinforcement. Other contributing factors included the use of sulfate resisting portland cement, the presence of hair-line cracks in the concrete, the elevated W/C, and the insufficient concrete cover. The problem of corrosion was also aggravated by simultaneous action of multiple enhancing each other environmental factors, such as the high salinity of Gulf water with a consequent high rate fallout of marine salts, high relative humidities and air temperatures over much of the year, unfavorable wind speed and direction, intense solar radiation, and corrosive ground conditions. Considering all data available, it appears that the corrosion process of reinforcing steel at the liquefied gas/sulfur plant is a result of complex synergistic effects of chlorides and acid solutions enhanced by highly unfavorable environmental conditions prevailing in the region.

After damage due to reinforcement corrosion in carbonated concrete, repair principle W - limiting the water content of concrete - as specified in the Draft Recommandation for Repair Strategies for Concrete Structures Damaged by Reinforcement Corrosion (l), can be applied in the repair of concrete compo-nents. The repair principle involves sealing the concrete surface with a suitable surface protection system after exchanging and reprofiling the damaged concrete areas. As a result the water content in the concrete shall be reduced to such an extent as to bring the corrosion process of the reinforcement to a standstill. The effectiveness of surface coatings as corrosion protection systems has been investigated on carbonated concrete specimens using the simultaneous measurement of corrosion rates and electrolytic concrete resistivities.

Throughout the world, sizable portions of national budgets must be appropriated for the repair and rehabilitation of concrete structures which have suffered damage due to corrosion of reinforcing steel. This paper presents a critical review of the current state-of-the-art on the mechanisms responsible for deterioration of concrete and reinforcing steel. The protective effect of the passive film and the depassivation of reinforcing steel by carbonation and chloride ingress are discussed. In addition to the passive film, some researchers believe that a mineral scale may contribute to the protection of embedded steel against corrosion. The following ambiguities in the current understanding of the deterioration of concrete due to corrosion of the reinforcing steel are discussed: mechanisms of passivation of steel in the concrete environment; stoichiometry of the passive film; applicability of Fick’s second law for the prediction of chloride diffusion in concrete; mechanisms of depassivation of steel by chloride ions; threshold Cl-/OH ratio; the composition and mechanism of protection provided by mineral scales; mechanisms of expansion of the steel corrosion product in concrete; and the detrimental effects of aggressive ions on concrete properties.