International Concrete Abstracts Portal

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.

The results of a great number of trial mixes for mix design of roller compacted concrete (RCC) are presented. This particular RCC used a local fly ash, rich in lime and sulfates, which does not meet any existing specification. This fly ash's performance in concrete has been studied for some time at the Laboratory of Reinforced Concrete of Aristotle University of Thessaloniki. Recently, this fly ash was used in the construction of a large RCC dam in northern Greece. Measurements of the strength development and the elasticity of RCC mixes showed that the 80 percent (by weight) of the cementitious material could be substituted for this fly ash. Therefore, it was proven that in RCC mixes this fly ash is more effective than in conventional concrete.

Effect of residual carbon content in fly ash on the hardened cement paste properties was investigated. Mortars and concretes were prepared with three different aluminosilicate-type fly ashes of the same mineralogical composition but containing, respectively, 5, 7, and 12 percent of residual carbon. Mixtures containing 15 and 30 percent of fly ash, as replacement of cement, were studied. The microstructure was studied by means of SEM observations and EDS analysis. Pore-size distributions were determined by mercury intrusion porosimetry. Compressive and flexural strengths were measured after 2, 7, 28, and 90 days. Results indicate that the residual carbon content in fly ash does not have any detrimental influence on the microstructure and on the mechanical strength development even at the highest carbon content.

Presents the results of a study to evaluate the potential of modified fly ash for use as a construction material. Areas of utilization investigated were: structural fill for embankments and dykes; lining material for canals, waste disposal sites, etc.; replacement of cement in concrete; and cement-stabilized base and subbase course. This modified fly ash was evaluated mainly on the basis of its compressive strength at various ages. Availability of only a limited quantity of fly ash necessitated use of miniature specimens for strength test. Results indicate that modified fly ash with and without cement developed adequate compressive strength to be used as fill material in embankments and dykes. Compacted modified ash also exhibited very low permeability characteristics. Replacement of 35 percent cement by modified fly ash produced 20 percent more strength than portland cement mix at 28 days.

Paper is concerned with the research of the effective utilization of fly ash produced from power plants. Three classes of "classified fly ash" produced by classifying conventional fly ash by air separation with the maximum particle diameters of about 20, 10, and 5 æm have been investigated. Special attention has been given to concrete strength enhancement effect due to classified fly ash. Experimental studies have reported on the basic properties of fresh concrete and hardened concrete having low water-binder ratio and high strength, produced by mixing the classified fly ash having the maximum particle diameter of about 10 æm, alone or in combination with ground granulated blast furnace slag. It is shown that the classified fly ash is an effective material that contributes to the reduction of superplasticizer requirements that are generally used in high-strength concrete, improvement of workability by reduced viscosity, and improvement of strength development, whether the classified fly ash is used alone or in combination with ground granulated blast furnace slag.