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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.

The behavior of mortars containing fly ashes and slag in seawater has been studied under two different exposure conditions. Examined first was whether the achievement of strengths at 28 days, either similar to or higher than those of reference mortars, would lead to mortars with a durability as high as in the case of reference mortars or even higher, due to the addition of superplasticizers and the substitution of fly ashes or slag for some cement quantities. Secondly, a cement portion issued from a reference mortar was replaced by corresponding fly ash and slag quantities, the E/C ratio and the workability being kept constant, and variations of the duration of humid curing were imposed to observe their influence on the behavior in seawater. Results obtained show that: a) the criterion of strength at 28 days does not allow a guarantee of the durability in seawater; b) the direct substitution, in the mortar, of fly ashes or slag, for a certain amount of cement (known in the French regulations as nonresistant to seawater) does not improve the long-term behavior; and c) the humid curing during 7 days is, by far, the best.

The effectiveness of one ground granulated blast furnace slag, two condensed silica fumes (high-silica/low-alkali, low-silica/high alkali), and three pulverized fly ashes (low-alkali/low-calcium, low-alkali/moderate calcium, high-alkali/high-calcium) have been evaluated in the presence of two very alkali-silica reactive aggregates from Canada, a siliceous limestone and a rhyolitic tuff. Mortar bars and concrete specimens were made with various admixture contents and different cements (high- and low-alkali), and tested for expansion with the accelerated mortar bar method (ASTM C 9-P214) and the concrete prism method (CAN/CSA-A23.2-14A). The mineral admixtures were also submitted to the pyrex mortar bar method (ASTM C 441). Based on the results, the ASTM C 441 test is not recommended for assessing the effectiveness of mineral admixtures in suppressing expansion due to alkali-aggregate reaction, unless account is taken of a number of modifications concerning mix design (admixture content, water/cement ratio, alkali content, etc.) and performance criteria. Pyrex does not behave like a natural aggregate. The results from ASTM C 9-P214, using a limit of 0.1 percent expansion at 14 days, are in agreement with those from the concrete prism method, which is the most recommended test procedure. However, when testing concrete, the alkali content of the mix must always be increased to 1.25 percent of the mass of cement (Na?2O equivalent), otherwise the test is not accelerated sufficiently and low expansion will be observed in the presence of reactive aggregates, even with no mineral admixtures. The long-term effectiveness of mineral admixtures against alkali-aggregate reactions (AAR), in particular silica fume, is presently questioned by a number of workers. Therefore, it is firmly recommended that conservative limits be used when conducting laboratory tests on concrete specimens, and that the tests be extended to at least two years. The mineral admixture under study should be used in amounts such that expansion never exceeds 0.04 percent in the long term (two years or more). A more conservative, and more recommended, performance criterion is to obtain expansion in the long term that is similar to that of a control made with a low-alkali cement and containing no mineral admixture

A study was made of the effects of Saskatchewan lignite fly ash on the resistance of concrete to freezing and thawing. Concrete was made with either ASTM Types I or V cement and different percentages of fly ash with an air content of 4 to 6 percent. Performance of the concrete was evaluated by measuring the changes in its dynamic modulus and its mass. A scanning electron microscope was also used to examine the changes in the microstructure of the cement paste due to exposure to freezing and thawing. Results show that the use of high percentages of fly ash in concrete (35 and 50 percent) reduced its resistance to freezing and thawing even though it contained about 6 percent air and was cured in water for 80 days. However, concrete containing 20 percent fly ash gave satisfactory performance, provided its air content and strength were comparable to control concrete that contained no fly ash. Results from the SEM examination show that the decrease in resistance of fly ash concrete to freezing and thawing may be due to the slow migration of portlandite and ettringite crystals from the dense C-S-H zones to the air voids. Concrete with fly ash was less susceptible to the migration of portlandite, but its air voids contained more fibrous hydrates, which may have led to an increase in the past porosity.