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Presents results of investigations carried out on high fly ash content concrete (HFCC) cores removed from several structures constructed in the U.K. since 1979. Structures investigated included a road pavement, a major road viaduct, water-retaining and industrial structures, and a slipway subjected to marine exposure. Concrete properties measured after 10 years of service include compressive strength, depth of carbonation, permeability, and chloride and sulfate penetration profiles. In addition, petrographic analysis of thin sections was also undertaken. The HFCCs studied were designed considering the fly ash to be just a further ingredient in the concrete rather than as a cement replacement. This led to higher fly ash contents and lower cement contents than is generally normal practice. The structures examined were in excellent condition after 10 years. Results show a durable concrete exhibiting increases in compressive strength beyond 28 days, little evidence of carbonation, low to average permeability, and resistance to chloride penetration. In this respect, it is significant that at the marine exposure sites, the chloride concentrations decreased significantly with depth. No evidence of alkali-silica reaction was detected in spite of reactive aggregates being present in some of the concretes.

Cubes and slab specimens prepared from three concrete mixtures containing three types of fly ash (FA), Classes F, C, and F/C, and a control portland cement concrete (PC) were cast and moist cured for either 7 or 28 days. Following 14 days of air drying, the cubes were immersed for 21 days in, and the slabs ponded for 12 months with, 15 percent NaCL solution. Chloride-ion distribution profiles were determined at four depth intervals. For all four concretes, the chloride ion permeability decreased when the length of moist curing was increased from 7 to 28 days. However, after 12 months of testing, the two mixes containing FA Class F and F/C exhibited such low chloride-ion contents at depths below 25 mm that the length of moist curing had little influence on chloride-ion reduction in contrast to that found for PC concrete, and to a lesser degree, concrete containing FA Class F. Use of all thre FA materials resulted in remarkable improvement in concrete impermeability when compared with PC concrete.

It is well known that reduction of free lime in reinforced concrete by air carbon dioxide neutralization and calcium carbonate formation makes steel reinforcement more susceptible to corrosion, since the reduction of pH eliminates the passivity conditions and can promote iron corrosion. On the other hand, the addition of pozzolan also causes a reduction of free lime by producing calcium silicate and calcium aluminate hydrates. It may therefore seem that fly ash addition, which improves many aspects of concrete durability, can specifically reduce the iron protection from corrosion promoted by reduction of pH. Concretes containing different amounts of ordinary portland cement and fly ash were produced. Fly ash (an additional 20 percent by weight of cement), was used either to replace cement or as an additional ingredient without any cement reduction. Plain and reinforced specimens were manufactured. All the specimens were kept in a carbon dioxide enriched (30 percent by volume) room to accelerate the carbonation process. The carbonation depth on plain concrete specimens and electrochemical measurements on reinforced specimens were carried out. The results indicated that fly ash addition reduces the carbonation rate when used without cement reduction, whereas it accelerates the process when used to replace cement. The electrochemical measurements of electric potential and polarization resistance are not significantly affected by the presence of fly ash, especially when added without cement reduction. The visual observation of iron reinforcement indicated that fly ash addition does not substantially modify the corrosive aspects, even when it replaces cement.

Sulfate damage in concrete is a result of the reaction between C 3A in portland cement, which produces the hydration of calcium silicates and water soluble sulfates. The damage is reduced or overcome by replacing some of the cement with low-lime fly ash. High-lime fly ashes usually contain some free lime, anhydrite, iron oxide, and C 3A contents of PC-fa mortars and their sulfate expansions determined according to ASTM C 452 showed that, beside the sulfate resistance factor R, free CaO/Fe 2O 3 and C 3A + free alumina/free CaO ratios of the PC-fa mixes containing high fly ashes can be used to determine their sulfate susceptibility.

The resistance of fly ash concrete to sulfate attack is dependent on both its chemical and physical characteristics. The chemistry and mineralogy of both fly ash and cement influence the sulfate resistance of hydration products, while the permeability of concrete influences the rate of ingress of sulfate ions. Researchers are studying the interdependence of the chemical and physical characteristics of fly ash concrete, as they affect sulfate resistance. Both ASTM Type I and II cements have been partially replaced by both ASTM Class F and C fly ashes. Cement replacement levels of 25 and 35 volume percent have been studied. Fly ashes have been added as mineral admixtures at the concrete mixing stage and interground with the cement constituents, as in blended cement production. Concrete specimens have been monitored for linear expansions and mass changes while immersed in 10-percent sodium sulfate solution. This paper reports on the sulfate resistance of fly ash/Type II cement concrete mixtures after 450 days of sulfate exposure. Data from 57 concrete mixtures are included, 28 of which utilized fly ash/ cement intergrinding procedures.