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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.

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.

Presents results of an investigation dealing with the long-term strength of silica fume concrete. Three series of concrete mixtures with and without silica fume were made with water-cementitious ratios from 0.25 to 0.40. The replacement level of portland cement with silica fume was kept constant at 10 percent. Test specimens were cast from each mixture to determine the compressive and flexural strengths of concrete at up to 3.5 years under both water-curing and air-drying conditions. The test specimens were also subjected to the determination of microstructure, carbonation, and weight changes with time. It is concluded that, under water-curing conditions, both the control and silica-fume concretes show gain in strength with age, with both concretes reaching similar strength levels after 3.5 years. However, continuous air-curing adversely affects the long-term compressive strength development of both types of concrete. This effect is considerably more marked for silica-fume concrete than for the control concrete, especially at w/c + sf of 0.30 and 0.40.

It is well known that curing concrete at elevated temperatures reduces the final compressive strength. The reduction depends on the temperature regime as well as the concrete composition. This program was based on recent data indicating that concrete containing condensed silica fume suffers less strength loss if a strength of about 10 MPa is reached at 20 C before heating. In this investigation, concrete characteristics were w/c + s = 0.30, 0.45, and 0.60 with and without 8 percent condensed silica fume. The temperature regime was to transfer specimens at 40 and 60 C, after delay times at 20 C. The delay times corresponded to strengths of about, 0, 3, 6, 9, 12, and 16 MPa. After 6 days, all specimens were cooled to 20 C and tested at 28d. The results show that the delay period had no significant influence on the final strength, except for the specimens with zero delay. The rest suffered some strength reduction compared to 20 C references, about 15 percent for w/c + s = 0.60, and less than 10 percent for the others. The reductions at 60 C were slightly greater than at 40 C. Concretes containing condensed silica fume generally suffered the smallest strength reductions.

In normal strength concretes, the compressive strength is limited by the strength of the binder and the binder-aggregate bond. In high-strength concretes, however, the binder strength and the bond may be fully comparable to the strength of the aggregate. This fact may lead to the conclusion that the strength of high-strength concretes may be improved by replacing an ordinary aggregate type with a high-strength aggregate. A number of aggregate types have been combined with high-strength binders to evaluate the impact of the aggregate strength on concrete compressive strength. The significance of the aggregate strength has been compared with the effect of the cement type and the use of silica fume. According to the obtained results, the impact of the aggregate strength on the strength of high-strength concrete is limited, compared to the binder type, while the difference in E-moduli between the different aggregate types is fully reflected in the concrete E-moduli. This contradiction is explained by a hypothesis based on stress concentrations due to the difference in rigidity between the binder and aggregate.