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Compressive strength of concrete with silica fume cured at high temperatures generated by heat of hydration during early age was studied. The curing temperatures simulated the site-curing conditions of structural members. Four types of aggregates and four types of admixtures were used, and a total of seven concrete samples were cured at four temperature conditions ranging from 20 to 75 C. Results indicated the following: 1) at higher curing temperatures, the 1-week strength was higher but the strength gain from 1 to 4 weeks tended to be low; 2) independent of curing temperature, the type of aggregate greatly influenced the strength, and the results were the same with all the admixtures; 3) high-temperature curing influenced concrete strength independently of the admixture and aggregate; 4) equivalent age at 20 C based on the Arrhenius equation gave a reasonable estimation of compressive strength gain.

The mechanical and structural properties of mortars containing silica fume were studied. Mortars containing 5 and 10 percent active silica additive were made. Mortars without silica fume (standard mortars) were also prepared. A first set of mortar specimens was cured entirely in water. A second set of mortars was cured in air. The third was immersed in water and then subjected to alternating cycles of storage in water and air. The results show a very close relation between the conditions of the mortars' curing and their mechanical properties. The flexural strengths of mortars containing silica fume, subjected to variable curing conditions, show periodic increases and reductions. SEM observations confirmed the relations found in the flexural strength tests.

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.

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.

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.