International Concrete Abstracts Portal

Paper is concerned with the compressive strength of flowable mortars containing high-volume coal ash applicable to backfill or base construction. In addition to Type I portland cement, both Class F fly ash and bottom ash were used. The test specimens with flowability ranging from 13 sec to 5 min measured by a flow cone were fabricated by hand-rodding in the paper molds of dimensions 5 x 10 cm. The relationship between 28-day compressive strength and flowability as affected by fly ash content is studied. Compressive strength as a function of cement content is discussed. The effect of tasting condition and of curing condition on compressive strength is also evaluated. A comparison relating to strength gain is made between specimens utilizing tap water and seawater, respectively, as mixing water. Moreover, the influences of other factors such as mix proportion and curing temperature on compressive strength are reported. In this paper, 28-day compressive strength of about 1 MPa can be achieved for the specimens with 6 percent cement, by weight, at the excellent flowability of around 20 sec. For a given flowability, the replacement of fly ash by bottom ash generally can improve compressive strength. Compared to tap water, seawater as mixing water or as curing moisture definitely has more beneficial effect on compressive strength. The test results obtained from this study indicate that flowable mortar containing high-volume coal ash has a great potential as backfill or base construction material, particularly in hot weather regions.

The changes in physical and chemical properties of rice husk ash (RHA) fired at several temperatures from 400 to 800 C at 50 C increments were studied. The noncrystalline of RHA fired below 600 C could not be determined by x-ray diffraction (XRD), but it could be expressed by Luxan's method, which determines the variation in electric conductivity in a saturated solution of calcium hydroxide containing RHA. The effect of RHA on the properties of mortar, such as strength, drying shrinkage, resistance to acid attack, freeze-thaw resistance, and carbonation, was also determined. It was found that the compressive strength of RHA blended mortar with respect to that of plain mortar corresponded to the variation in conductivity. The RHA in mortar improved resistance to acid attack and developed the same degree of resistance to freeze-thaw action as that with silica fume, while it increased the drying shrinkage.

Report presents data on the effects of a mineral admixture such as fly ash, blast furnace slag, or silica fume on the pore structure and chloride permeability of concrete stored under various environmental conditions for a long time. Cubic concrete specimens with all surfaces coated with a polymer except for one surface were initially cured in water for 7 or 28 days, and then exposed to three environmental conditions for 1 year: in water at 20 C; in a room at 20 C, at 60 percent relative humidity; and outdoors. Cores from the specimens were investigated for the degree of hydration and the characteristics of pore structure of concretes both with and without mineral admixtures. The test involved ignition loss, mercury intrusion porosimetry, and scanning electron microscopy. The chloride permeability of exterior and interior portions of specimens was determined according to AASHTO T 277-831. The test results showed that, at the surface of concretes containing mineral admixtures, the hydration of portland cement and the pozzolanic reaction of mineral admixture were considerably depressed, and coarse pores were developed when the concrete specimens were exposed to dry environment for a long time; however, at 5 cm depth from the surface, there was little change in both the degree of hydration of cement and the pore structure. AASHTO T 277-831 data showed that both the surface layer and the interior concretes with mineral admixtures were much less permeable to chloride ions than the corresponding portland cement concrete specimens, irrespective of the curing and environmental conditions.

Reports research on the beneficial utilization of waste husk from rice production. The husk was burned in the furnace at two different temperatures, 400 and 500 C for « hr, and it was observed that all the silica obtained was amorphous at both burning temperatures. The mortars were prepared by substituting cement with husk at 10, 20, and 30 percent by weight. The ratio of (water + superplasticizer)/(cement + ash) was kept constant at 0.57 for all batches. The mortars were stored in sodium sulfate solution until the testing date after the initial 28 days normal curing in water. Compressive and flexural strength tests were carried out on the mortar specimens at 4, 8, and 12 week periods of storing in solution. It was observed that durability and strength of mortars were increased by using rice husk ash.

Three of the major uses of silica fume (microsilica) additions to concrete have been to improve mechanical properties, improve corrosion resistance by reducing permeability to aggressive anions such as chlorides, and improve concrete resistance to chemical degradation. In the last two uses, the mechanical properties are also enhanced beyond those of ordinary portland cement concretes of the same mix proportions without silica fume. Thus, the production of durable concrete often leads to an improvement in mechanical properties. Long-term resistance in accelerated laboratory corrosion testing in sodium chloride solutions is documented. It is shown that silica fume significantly lowers chloride ingress with increasing efficiency as the water-cementitious ratio decreases. A clear improvement in corrosion performance with the addition of calcium nitrite corrosion inhibitor became evident in this long-term program. It is also documented that high concrete resistivities do not necessarily prevent severe corrosion from occurring. Chemical resistance of silica fume (microsilica) concretes to numerous acids, bases, and salts is also examined. The results show significant improvements with the addition of silica fume in the time to 25 percent mass loss in cyclic and continuous ponding experiments for most chemicals. For some highly alkaline solutions, there is no improvement with microsilica. Improvements in compressive strength are documented for the mixtures used in the corrosion and chemical resistance studies. Additional mixtures were examined to determine flexural strength and modulus of elasticity. These mixtures were similar in composition to those typically used for corrosion protection. The results showed that silica fume significantly increased strengths and the modulus of elasticity. The improvement in flexural strength was greater than that expected from formulas typically used for moderate strength concretes and the increase in modulus of elasticity was less. It is hoped that the design engineer will be able to utilize the data to take full advantage of the property improvements and not merely durability or strength improvements with silica fume.