International Concrete Abstracts Portal

Chloride-induced corrosion is a problem common to steel reinforced concrete exposed to chloride ions. A severe case is the use of reinforced concrete in seawater. The high-chloride concentration in salt water, the geometry of concrete piles, and the moisture differential between concrete above and below the water line are all factors that complicate the problem. The corrosion resistance of steel reinforced concrete is a function of the concrete cover of the steel, concrete permeability, surface chloride concentration, and ambient temperature. In this paper, the authors present diffusion curves for chloride ingress into concrete piles. The diffusion coefficients are based on extensive laboratory and field studies. They also discuss the usefulness of this model, based on Fick's law of diffusion. By estimating the chloride ion concentration at the steel reinforcement after a given amount of time, the lifetime of the structure can be predicted. In addition to concrete quality, concrete admixtures affect the corrosion of steel in concrete. Two concrete admixtures are discussed--calcium nitrite and microsilica. As demonstrated in other publications, both of these additives delay the onset of corrosion. It has also been shown that calcium nitrite affects the rate of corrosion upon initiation. The appropriate dosage of each admixture can be determined using the chloride diffusion curves. Examples are described in the paper.

When anaerobic conditions occur in a sewer pipe in the presence of sulfate, sulfur-reducing bacteria will produce hydrogen sulfide. As hydrogen sulfide is released, various populations of sulfur-oxidizing bacteria (thiobacilli), will proliferate. The proliferation of these organisms results in a decrease in pH due to the production of sulfuric acid. Different thiobacilli will be present depending on the pH of the environment. Samples from regions of deteriorated and nondeteriorated concrete pipe were taken to determine the presence of microorganisms that could cause microbially induced concrete deterioration. The results presented show that the degree of concrete deterioration can be correlated with the number and type of thiobacilli present. Extensive deterioration was observed at the crown of reinforced and asbestos concrete pipe, where the most acidophilic group of thiobacilli were present in elevated numbers. Areas of lesser deterioration were somewhat acidic, with a combination of different sulfur-oxidizing thiobacilli present. Areas that did not appear to be deteriorated were populated with the least acidophilic group of sulfur-oxidizing thiobacilli. The presence of microbially induced deterioration of concrete and the stage of deterioration can be determined by utilizing selective media to culture the various groups of sulfur-oxidizing bacteria associated with concrete decay.

Two concrete natural draft cooling towers exhibited honeycombing and freeze-thaw damage. This paper presents results of inspections and laboratory and field tests used to develop cooling tower rehabilitation repair strategies. Different repair materials were evaluated and tested in the laboratory. The repair strategy selected involved measures to dry out the marginally air-entrained saturated tower shell concrete to minimize future freeze-thaw damage and then replace concrete exhibiting honeycombing and condensate leakage with dry-mix shotcrete (gunite). The interior concrete shell was then coated with an impermeable membrane. Six materials for coating the interior shell concrete and two types of shotcrete processes were evaluated.

Twenty roller-compacted concrete loads were cast at St. Constant near Montreal during the fall of 1987. Three types of cement (Canadian Types 10, 30, and 10SF), four different aggregate gradings, and three water-cement ratios (0:27, 0:33, and 0:35) were used to prepare the various mixes. Most of these mixes contained an air-entraining admixture. Approximately one-third of each concrete surface was moist-cured for 7 days, another third was covered with a white curing compound, and the remaining portion was not cured at all. Samples representative of all mixes and all curing conditions were taken from the pavement after 28 days and then tested for freeze-thaw durability (ASTM C 666) and deicer salt scaling resistance (ASTM C 672). The characteristics of the air-void system of all concretes were determined in accordance with ASTM C 457. With no exception, all samples withstood, without any significant deterioration, 300 cycles of freezing and thawing in water. However, the loss of mass after 50 cycles in the presence of a deicer salt solution ranged between 2 and 18 kg/mý (i.e., higher than the usual 1 kg/mý limit in all cases), even if most of the spacing factor values were below 250 æm. The best results (a weight loss of approximately 2 kg/mý after 50 cycles) were obtained for a mix containing Type 10 cement and no air-entraining admixture. In addition, this mix was not cured at all. Overwoking of the concrete surface during compaction is considered to be one of the possible explanations for the discrepancy between the results of the C 666 and the C 672 tests. It is also possible that the relationship between spacing factor and freeze-thaw durability does not apply to such concretes with a high permeability, numerous irregularly shaped compaction air voids, and large porous zones in the paste. This series of tests is the first phase of a 3-year research project on roller-compacted concrete pavements at Laval University, in collaboration with Canada Cement Lafarge. In the second and third years of this project, various ways to improve the scaling resistance (mostly by micro structural changes) will be studied.

Paper reports the results of part of a program to determine the extent and severity of carbonation in buildings in Canada. About 350 core samples drilled from 28 buildings in Toronto were tested by two procedures to determine the depth of carbonation. Tests were made on cast-in-place balconies and vertical components and on precast cladding. A proportion of the total sample was found to be susceptible to carbonation damage within a reasonable service life.