International Concrete Abstracts Portal

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.

Effects of alkalies in Class C fly ash on Alkali-aggregate reaction were studied by using two cements, a type I high-alkali cement and a type II low-Alkali cement, and three Class C fly ashes. Mortar bar expansion was measured according to ASTM C 441. Reaction products of alkali-aggregate reaction were examined n by XRD, SEM, and EDAX. were to study: The purposes of this research (1) the significance of the standard mortar bar test in determining the degree to which high and low-alkali cement could be replaced by Class C fly ashes, and (2) effect of fly ash alkali contents on alkali reactivity. Expansion of mortar bars prepared using high-alkali cement increased at low replacement levels but decreased at high replacement levels for curing periods up to 12 weeks at 38 C; whereas expansion of mortars prepared using low-alkali cement increased at all Levels of fly ash replacements up to 40% by volume. A critical equivalent Na20/Si02 mole ratio was identified and found to characterize alkali reactivity. No crys-talline reaction products could be identified by XRD. Results of SEM and EDAX showed that the reaction product was an alkali-silicate gel, composed mainly of silica, sodium, potassium, and calcium, with their relative amounts varying within the gel.

Reports on study of the chloride distribution profile in hardened cement paste cylinders of 5 cm diameter. The specimens were made from ordinary portland cement and blended cement with 10 percent fly ash. The condensed silica fume was used as cement replacement, with replacement levels of 5, 10, and 15 percent by weight of cement. Other experimental variables were water-to-(cement + silica fume) ratio of 0.5, 0.7, and 0.9. The specimens were immersed in stagnant seawater at 20 C. After 6 months of exposure, the specimens were cut, ground, dried, and the chloride ion content determined by a potentiometric titration procedure. By applying Fick's second law, the effective diffusion coefficient and the effective supply concentration of chloride were calculated by using an approximation method. Results show the effective diffusion coefficient is reduced highly when condensed silica fume is used as cement replacement.

The use of silica fume (microsilica) to improve the compressive strength at a given cement level or as a cement replacement is on the rise. Additional benefits of adding silica fume to improve the corrosion resistance of embedded steel and improve concrete durability in erosive or severe chemical exposure were investigated. Concretes with embedded steel were produced with silica fume levels varying from 0 to 15 percent by mass of cement. Additional variables were water-cement ratio and calcium nitrite content. All concretes were air-entrained and had high-range water-reducers. Plastic properties of the concretes are reported as well as compressive strength, freeze-thaw, and resistivity and rapid chloride data. Corrosion rates and chloride contents are reported and show substantial improvements with silica fume and/or calcium nitrite. An accelerated hydraulic erosion test was conducted, in which ball bearings impact the concrete surface, simulating abrasive action of waterborn particles. Mass loss was measured for concretes with 0 to 15 percent silica fume by mass of cement. Silica fume significantly improved erosion resistance. Chemical testing was performed in 5 percent acetic acid, 1 percent sulfuric acid, 5 percent formic acid, and mixed sulfates. A cyclic method involving drying, weighing, and wire brushing was used. Results show that silica fume concretes had superior chemical resistance that improved as silica fume levels increase.