International Concrete Abstracts Portal

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.

SP-132 Published in two volumes...The first volume contains papers dealing with fly ash and natural pozzolans. The second volume consists of papers dealing with condensed silica fume and ferrous and non-ferrous slags.

In the third year of a research project on roller-compacted concrete pavements, a test section was cast during the summer of 1989, using 13 different mixtures. Five types of binder (ASTM Types I, I + slag, I + fly ash, a blended silica fume cement, and a blended silica fume cement + fly ash) were used to prepare these mixtures. To verify whether a proper air bubble network could be obtained, two different air-entraining admixtures were utilized. Approximately half of the mixtures were air-entrained. Half of the test section was moist-cured for 14 days and a white curing compound was sprayed on the remaining portion. Samples representative of all mixtures and all curing conditions were taken from the pavement after 28 days. The air-void characteristics of all concretes were determined in accordance with ASTM C 457, and the salt scaling resistance of all combinations (of the type of mixture and the type of curing) was evaluated using ASTM C 672 on both rolled and sawn surfaces. Results indicate that it is extremely difficult to entrain air in this type of concrete. In accordance with previous results, good scaling resistances were obtained with the silica fume concretes cured with a membrane.

Expansion of mortar bars with and without selected pozzolans exposed to saturated calcium hydroxide and sodium chloride solutions at 50 C has been measured up to 20 weeks of exposure. The mixes contained a Danish low-alkali sulfate-resistant cement or a French high-alkali cement, and inert quartz sand added 2 and 6 percent of synthetic cristobalite and varying amounts of pozzolans. Additions were 5, 15, and 25 percent fly ash; 3, 5, and 7 percent silica fume; 5 percent silica fume plus 15 percent fly ash; and 35 percent slag. Both fly ash and silica fume were found to prevent deleterious alkali-silica reactions. The amount of pozzolan necessary to prevent expansion increased with the amount of reactive aggregate and varied with the type of pozzolan. Additions of 5 percent fly ash or 3 percent silica fume were enough to suppress deleterious reactions in the mixes with 2 percent synthetic cristobalite during the period of testing. Twenty-five percent fly ash or 5 percent silica fume plus 15 percent fly ash were found to prevent deleterious reactions in the mixes with 6 percent synthetic cristobalite until 15 weeks of exposure.

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.