1. Improved Fresh Concrete Properties
Workability Concrete is easier to place and responds better to vibrations to fill forms and voids more completely. Hence, better compaction. Ease in paving The extended setting time of Fly Ash – based concrete allows it to be properly compacted and finished with lesser energy and without problems, even in the hottest climate. 2. Reduced Permeability Reduced water demand, increased density and long term pozzolanic action of Fly Ash which ties up free time results in fewer bleed channels and decreases permeability. Lower permeability offers resistance to Chloride ion penetration and prevents corrosion of reinforcement.3. Reduced Heat Of Hydration The delayed pozzolanic reaction between Fly Ash and lime generates less heat, resulting in reduced thermal cracking. When Fly Ash is used to replace Portland cement, Fly Ash decreases the total amount of heat generated. It also delays the peak rate of heat generated by which time the green concrete has considerably higher strength to resist thermal cracks.4. Improved Pumpability More paste volume and spherical shape of Fly Ash particles aids easy pumping of concrete.5. Improved Sulphate and Chloride Resistance Reduced permeability of concrete results in reduced entry of sulphate and chloride ions in concrete. By consuming the Calcium Hydroxide in course of pozzolanic action it checks conversion of Monosulphoaluminate into Ettringite.
The use of Fly Ash as a partial replacement of cement significantly reduces the co-efficient of chloride diffusion of concrete. Two reasons are suggested: (i) the smaller pore and capillary structure of Fly Ash concrete and the ongoing pozzolanic reaction which makes the pores even smaller and capillaries discontinued, ensuring a reduced quantity of chlorides to diffuse through the concrete, (ii) due to an increased amount of calcium aluminate which is a product of the pozzolanic reaction of fly ash with the cement, more chlorides are chemically bound in the cement matrix rendering them harmless. Chloride Ingress The denser nature of Fly Ash concrete greatly reduces the amount of Chlorides penetrating the concrete structure and therefore the extent of the attack is lessened, extending the life span of the structure. Results of the research show clearly that, both in an accelerated and real life environment, Fly Ash concrete is superior to a plain cement concrete of the same strength with up to 60% reduction in corroded area in the case of a 35M Pa concrete with 25mm cover in an accelerated environment over a period of six years. This phenomenon is even more emphasized when the cover is increased to 44mm where the steel reinforcement in the 30% Fly Ash concrete only showed 2% corrosion against a 100% corrosion of the steel in the OPC concrete after six years, although the correlation between accelerated environment and real life exposure has not been fully established yet, it is thought that a one year accelerate exposure in the FAMCET translates to five years in real life. 6. Reduced Risk of Alkali Aggregate Reaction Fly Ash combines with alkalis from cement that might otherwise combine the silica from and potential alkali reactive aggregates causing destructive expansion.7. Increases Long Term Strength Fly Ash continues to combine with free lime released during hydration of cement thereby increasing structural strength over a period of time. Upon filling the voids Fly Ash densifies the matrix, increasing points of contact, this increases the tensile strength of concrete. Hence there is an increase in flexural strength.The weakest link in the concrete micro structural chain is the aggregate matrix transition zone, this happens because at that transition zone the number of surface irregularities allow for collection of water, therefore, this is the prime site for the formation of Calcium Hydroxide crystals. These crystals are oriented and fail easily. On addition of Fly Ash the interface is strengthened because the Calcium Hydroxide is consumed in pozzolanic activity and there results the cementitious binding gel C-S-H. 8. Better Concrete Finish More paste volume and lesser bleeding and segregation of concrete during placement and compaction leads to better finishing and texturing.9. Reducing Bleeding and Segregation Improved cohesiveness and reduced water demand of Fly Ash concrete reduces bleeding and segregation. Fewer bleed channels decreases porosity and improve resistance of concrete against chemical attack.10. Reduced Shrinkage The largest contributor to drying shrinkage in water content. The lubricating action of Fly Ash reduces water content and thereby drying shrinkage. |
0 comments:
Post a Comment