| It is estimated that about 80% of concrete produced in North America contains one or more types of chemical and/or mineral admixtures. The present program deals with the use of two such admixtures, namely, lignite fly ash and condensed silica fume, in high strength concrete. A comprehensive study was undertaken on the compressive strengths, shrinkage, long-term creep, frost and sulphate resistance of high strength concrete containing various levels of fly ash replacements (0 to 80%) and a fixed percentage of silica fume (10%). The cement used was CSA Type 10 (ASTM Type 1) normal portland cement. No air-entraining agent was added to the majority of the mixtures, however, for the frost resistance tests air-entrainment of 4-8% was used in some of the mixtures to compare its frost resistance with non air-entrained concrete mixtures. Optimum water/binder ratio was fixed ar a low value of 0.27 from preliminary testing for strength and workability considerations and superplasticizer was used to give proper workability. Aggregate/cement and coarse/fine aggregate ratios were maintained at 5 and 1.22, respectively. The test program consisted of compressive strength tests at various ages on concrete cylinders; drying shrinkage tests at room temperature on concrete cylinders; long-term (3 or 12 months) creep tests of sealed and unsealed concrete cylinders at room (70;The results indicated that up to 60% fly ash replacement with 10% silica fume, showed superior or similar 28 and 56 days compressive strengths when compared to the 100% cement control mixture. Fly ash + silica fume concrete indicated lower shrinkage and long-term creep. Creep increased with increase in temperature due to physico-chemical processes which were confirmed by microstructure analysis using the Scanning Electron Microscope (SEM). An empirical model for long-term creep of high fly ash + silica fume concrete was suggested. Replacement of cement by up to 35% fly ash and 10% silica fume, indicated enhanced frost resistance, without any air-entrainment. The addition of 8% air-entrainment in the 20% fly ash + 10% silica fume mixture increased the durability factor by about 10%. For the 50% fly ash + 10% silica fume mixture, the frost durability fact.or was found comparable to that of the 100% cement control mixture and air entrainment did not increase the frost durability factor appreciably. Sulphate resistance of 103% CSA Type 10 cement concrete was found satisfactory, however, with increasing fly ash contents (up to 50%), the sulphate expansion was suppressed. A study of matrix morphology and microstructure bonding, using the scanning electron microscope, helped to explain the observed results in a comprehensive manner. |
