International Concrete Abstracts Portal

The results of studies on the contributions of alkalies in fly ash, slag, and silica fume to the expansion of concrete due to alkali-silica reaction are presented and discussed. A wide range of concrete mixtures was made. Each mixture contained a different amount of cement and different proportions of one type of fly ash, one type of slag, or one type of silica fume. All mixtures were made with amorphous-fused silica as a synthetic reactive aggregate. The alkalilevel of some mixtures was increased by sodium hydroxide to study the effects of pozzolans or slag mixtures at higher concentrations of alkalies. Concrete prisms were made and stored in water at 38 C. The test results indicate that the effectiveness of these supplementary cementing materials in reducing or increasing expansion due to alkali-silica reaction varies widely. The results also indicate that the supplementary cementing materials can contribute significant quantities of alkalies to the reaction, under particular replacement and test conditions employed.

Describes recently completed construction of a 175,000 mý (1.85 million ftý) 72-story office building. A unique feature of the structural system is the high-strength reinforced concrete-structural steel composite columns. Conventional aggregates and portland cement, with about 29 percent fly ash replacement, produced average 56 day compressive strengths of over 83 Mpa (12,000 psi), easily surpassing the design specified strength of 69 Mpa (10,000 psi). Most of the concrete was placed at a slump of 200 mm (8 in.), using a high-range water-reducing admixture. The structural design was based on 16 exterior columns, an arrangement that allowed excellent utilization of tenant space. Preconstruction research and development of the high-strength fly ash concrete mix designs proceeded simultaneously with the architectural and structural design. Results of these tests produced important data on modulus of elasticity, shrinkage, and creep that influenced the composite column design. Much of the concrete was placed under hot weather conditions typical of Texas, where summer ambient temperatures sometimes exceed 40 C (104 F). While fly ash was employed in all of the concrete mixes, this paper focuses on the high-strength aspects in which the use of fly ash was an essential ingredient.

Gives results of an investigation on the chloride ion permeability of concretes incorporating supplementary cementing materials, using the Rapid Determination of Chloride Permeability Test (AASHTO T277-83). A total of 18 concrete mixtures were made. These included mixtures incorporating silica fume (8 percent replacement or addition to the cement by mass) or ground granulated blast-furnace slags (50 percent replacement by mass), or fly ash (25 percent replacement by mass). The w/c of the mixtures investigated ranged from 0.21 to 0.71. From each mixture, a number of 152 x 305 mm cylinders for compressive strength testing and 102 x 203 mm cylinders for determining the chloride permeability were made. Porosity measurements were also performed on some of the concrete specimens. The test results showed that the use of supplementary cementing materials significantly reduced the chloride ion permeability of concrete. Silica fume and blast furnace slags investigated seem to be particularly efficient for producing concrete almost impermeable to chloride ions.

An extensive research program was performed on the rate of carbonation of concrete produced with ordinary portland cement, portland blast furnace slag cement, and portland fly ash cement, both with replacement of cement by fly ash and without. After various periods of wet curing, concrete samples were exposed to various exposure conditions. The wet curing ranged from 1 to 90 days, the exposure concerns outdoor sheltered from rain and at 20 C, 65 percent relative humidity in the laboratory. For the various exposure conditions, a relation has been found with respect to the carbonation rate as a function of the compressive strength at 7 days or at 28 days per type of cement and for all types of cement when the lime content is involved. Results of measurements over a period up to 2 years are presented.

Chemical characteristics of the cement paste-aggregate interfacial zone have been considered to influence the durability and mechanical properties of concrete. Particularly, effects of mineral admixtures such as fly ash and slag on the microstructure of the interfacial zone deserve attention. An x-ray diffraction technique was used to evaluate the amounts of Ca(OH)2, ettringite, and the orientation of Ca(OH)2 crystals in the interfacial zone. Composite specimens with several types of rocks were broken to produce a fracture surface on the cement paste prism to which the x-ray diffraction analysis was applied. The analyses showed that the addition of fly ash or slag considerably affected the peak height and orientation of Ca(OH)2 crystals in the interfacial zone, which normally extends up to 50 to 100 æm from the interface. The formation of ettringite in the vicinity of the aggregate surface was restricted by the addition of the admixtures. These results also suggest that the addition of the mineral admixtures favorably affects the resistance of the interfacial zone against aggressive agents from the surroundings. The x-ray fluorescence analysis was conducted to quantify calcium and silicon in the zone. The results obtained complemented the conclusions described previously. 137-389