International Concrete Abstracts Portal

Freezing and thawing cycles in northern latitudes have resulted in the breakdown of some aggregates and concrete. Deicing salts have accelerated the problem. However, freezing of water cannot be the principal cause of deterioration, since in the fine-grained aggregates and cement paste the pores are too small to allow freezing. Yet it is these materials that deteriorate the most. Deicing salts likewise lower the freezing point and the number of freeze-thaw cycles, yet cause increased breakdown. The same materials susceptible to freeze-thaw breakdown also deteriorate significantly under repeated wetting-drying cycles. Laboratory experiments show these materials to expand on wetting and contract on drying. NaCl solution causes significantly greater expansion. Ice formation in the pores, therefore, is not the primary cause of breakdown. The answer may be found in the nature of the water in the small pores--water affected by the capillary and surface forces of the pore material. The pore water has lower vapor pressure, which prevents it from freezing, but which results in osmotic pressure differential, causing expansion. Deicing salt cations are preferentially adsorbed and concentrated on pore surfaces, further increasing the osmotic potential, expansion, and breakdown. Methods that determine the pore size distribution, surface sorption characteristics of the pore walls, and volume changes on wetting are suggested as more definitive measures of aggregate (and concrete) durability, compared to some of the currently accepted tests. This paper presents an overview of processes causing the aggregate breakdown, based on the theoretical and laboratory-derived evidence accumulated in the author's lab over the last 15 years.

With the adoption of the new test method ASTM C 1-12-84, "Standard Test Method for Length Change of Hydraulic-Cement Mortars Exposed to a Mixed Sodium and Magnesium Sulfate Solution," it is now possible to evaluate the performance of mortars made with portland cements, blended cements, and blends of portland cement with pozzolans or slags in producing a sulfate resisting cement mortar. This paper summarizes a laboratory research study parallel to the cooperative inter-laboratory testing program conducted by the ASTM Subcommittee on Sulfate Resistance (Col.29). The purpose of the study was to provide data needed to establish expansion limits for sulfate resistance. Twenty cements, including portland cements and blends of Type I cement with Class F fly ashes used for manufacture of Type IP cements, were used in the study. Data are presented showing the strength development of the companion mortar cubes and the expansions of the mortar bars measured at ages up to 1 year. From these data, acceptance limits on expansion were proposed for moderate and high sulfate resistance of hydraulic cements.

A concrete fish-breeding tank built near the north shore of Lake Superior failed during the first winter. The 250 mm thick concrete wall failed by cracking and delamination in the center of the concrete. The outside surfaces were generally unaffected. The concrete leaked water at such a rate that the tank became unusable. Spring-fed water inside the tank had been maintained at a constant temperature of 4.5 C. At this site, the mean daily temperature in December is -10 C; in January and February it is about -15 C. Minimum temperatures are commonly less than -30 C and may drop to -40 C. The concrete had been delivered by ready-mix trucks following a two-hour haul. Quality control on the site had been poor. The concrete in the failed areas was non-air-entrained and had a high water-cement ratio. Failure was attributed to formation of an ice lens within the permeable concrete.

Results are presented on recent studies undertaken in England, the state of Georgia, and Australia, on the durability of a variety of concrete structures containing fly ash. Five of these structures have been in service for over 15 years, and some provided comparisons between fly ash concrete and plain concrete. Details of the fly ashes used in the structures are supported by background data on fly ash availability and use in the areas concerned. Design, construction, and service details for the structures are presented along with findings of on-site inspections. Assessments were made of: fly ash hydration characteristics; depth of carbonation; functional efficiency; and visible deterioration. Links are ascribed between the determined values.

Presents three experimental programs employing high-range water-reducing admixtures to modify durability performance in portland cement-fume concrete. Resistivity and chloride permeability are shown to be significantly improved where fume-replaced concrete uses superplasticizers. Superplasticizing has some demonstrated drawbacks where fume is introduced to reduce alkali aggregate reactions.