International Concrete Abstracts Portal

Silica fume has an accelerating effect on the early hydration of portland cement. Also, silica fume reduces the retarding effect of lignosulfates. At standard curing conditions, the contribution to strength from the pozzolanic reaction takes place primarily at 5 to 7 days. As a result, existing equations for prediction of strength development based on pure portland cement are no longer valid for concrete with silica fume. Some new equations for concrete with various contents of silica fume are presented.

High-strength and ultra high-strength cast-in-place concretes tend to contain excessive unit volumes of cement when compared with normal concrete, and since the improvement of workability relies largely on the efficiency of the air-entraining and high-range water-reducing admixtures, the properties of workability (or consistency) are different from normal concrete. With high-strength concrete, it was found that the method of mixing concrete influenced flowability, strength properties, and pore structure; details of this influence are given.

With more than 3 million underground storage tanks located throughout the U.S., and mass oil drilling, production, and transportation, leaking problems generate large quantities of petroleum-contaminated soils (PCS). With the limited availability of solid waste disposal facilities, research is needed to investigate viable reuse options for PCS. Paper presents an attempt to apply stabilization/solidification techniques to PCS to bind the hydrocarbons in a structure formed by cement, fly ash, and aggregates to produce a construction material suitable for bulk applications. An experimental program was developed to examine the potential for using PCS as a fine aggregate replacement in concrete. Two PCS types with different levels of heating oil contamination were investigated (0.11 and 0.66 percent contamination concentration by weight). For each soil type, nine mixtures were obtained by replacing sand with PCS (PCS-sand ratio of 10, 20, and 40 percent by weight) and Class C fly ash with cement (fly ash-cement ratio of 10 and 20 percent by weight). Compressive and flexural strengths, permeability (hydraulic conductivity), and leachability of benzene-to-water tests were conducted. Results indicate that the addition of PCS reduces both the compression and flexural strengths of concrete. However, the obtained strength is adequate for structural applications. Concrete containing higher PCS-sand replacement ratio develops lower strength. That strength loss increases with higher contamination concentration. Given longer curing time, the fly ash presence can reduce such loss. The permeability coefficient of PCS concrete is slightly higher than control. Fly ash addition yields a more impermeable PCS concrete. For both soil types, at 40 percent PCS-sand replacement ratio, the leachability of benzene was nondetectable after 24 hr and 10 days of casting.

Heavy damage due to alkali-aggregate reaction has been observed in concrete structure in and along the sea. An accelerated test is performed on mortar to evaluate effectiveness of fly ash for controlling alkali-aggregate reaction in the marine environment. Mortar bars using Pyrex as aggregate and cements with 0.6 and 1.1% of equivalent sodium oxide are made. The alkali content in the mixture is adjusted by adding NaOH or NaCl. Specimens are stored in distilled water, NaCl solution, and under more than 95% of relative humidity. The controlling effect of fly ash and the effect of internal and intruded chloride ion in mortar on alkali-aggregate reaction is studied by measuring the expansion of mortar. Expansion of mortar depends on the type of cement and chemical reagents used for alkali adjustment, the amount of fly ash used and the exposure condition. Even with the same equivalent sodium oxide in the mixture, mortar using NaCl for alkali adjustment shows higher expansion than mortar using NaOH. The highest expansion is revealed for mortar cured in NaCl solution. The controlling effect of fly ash also depends on the type of cement and the exposure condition.

This paper gives the results of an investigation on the performance of high-volume fly ash concrete made with ASTM Class F fly ashes from three different sources. Cementitious materials contents of 300, 370, and 430 kg/m3 were used. The percentage of fly ash used was 58 percent of the total cementitious materials content. All the concrete mixtures were air-entrained and superplasticized. A large number of concrete specimens were subjected to the determination of compressive and flexural strengths, Young's modulus of elasticity, creep strain, drying shrinkage, abrasion resistance, deicing salt-scaling resistance, and resistance to chloride-ion penetration. High-volume fly ash concrete with adequate early-age strengths and excellent later age strengths can be produced with cement and total cementitious materials as low as 125 and 300 kg/m3, respectively. The Young's modulus of elasticity, creep, and drying shrinkage of high-volume concrete are comparable to those of the plain portland cement concrete. The high-volume fly ash concrete shows excellent resistance to chloride-ion penetration and outperforms plain portland cement concrete. The total charge in coloumbs at 91 days, a measure of resistance to the chloride-ion penetration, ranges from 278 to 1078. The corresponding values for reference concrete range from 1003 to 2313. Further research is needed to explain the relatively poor performance of the high-volume fly ash concrete under deicing salt scaling and abrasion tests.