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Presents results of investigations carried out on high fly ash content concrete (HFCC) cores removed from several structures constructed in the U.K. since 1979. Structures investigated included a road pavement, a major road viaduct, water-retaining and industrial structures, and a slipway subjected to marine exposure. Concrete properties measured after 10 years of service include compressive strength, depth of carbonation, permeability, and chloride and sulfate penetration profiles. In addition, petrographic analysis of thin sections was also undertaken. The HFCCs studied were designed considering the fly ash to be just a further ingredient in the concrete rather than as a cement replacement. This led to higher fly ash contents and lower cement contents than is generally normal practice. The structures examined were in excellent condition after 10 years. Results show a durable concrete exhibiting increases in compressive strength beyond 28 days, little evidence of carbonation, low to average permeability, and resistance to chloride penetration. In this respect, it is significant that at the marine exposure sites, the chloride concentrations decreased significantly with depth. No evidence of alkali-silica reaction was detected in spite of reactive aggregates being present in some of the concretes.

The results of a great number of trial mixes for mix design of roller compacted concrete (RCC) are presented. This particular RCC used a local fly ash, rich in lime and sulfates, which does not meet any existing specification. This fly ash's performance in concrete has been studied for some time at the Laboratory of Reinforced Concrete of Aristotle University of Thessaloniki. Recently, this fly ash was used in the construction of a large RCC dam in northern Greece. Measurements of the strength development and the elasticity of RCC mixes showed that the 80 percent (by weight) of the cementitious material could be substituted for this fly ash. Therefore, it was proven that in RCC mixes this fly ash is more effective than in conventional concrete.

Effect of residual carbon content in fly ash on the hardened cement paste properties was investigated. Mortars and concretes were prepared with three different aluminosilicate-type fly ashes of the same mineralogical composition but containing, respectively, 5, 7, and 12 percent of residual carbon. Mixtures containing 15 and 30 percent of fly ash, as replacement of cement, were studied. The microstructure was studied by means of SEM observations and EDS analysis. Pore-size distributions were determined by mercury intrusion porosimetry. Compressive and flexural strengths were measured after 2, 7, 28, and 90 days. Results indicate that the residual carbon content in fly ash does not have any detrimental influence on the microstructure and on the mechanical strength development even at the highest carbon content.

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