Admixtures
- 1. Admixtures
- 2. Admixtures Admixtures are those ingredients in concrete other than Portland cement, water, and aggregates that are added to the mixture immediately before or during mixing
- 3. Types of admixtures 1. Air-entraining admixtures 2. Water-reducing admixtures, Plasticizer, Super plasticizer 3. Accelerating admixtures 4. Retarding admixtures 5. Hydration-control admixtures 6. Corrosion inhibitors 7. Shrinkage reducers 8. Alkali-silica reactivity inhibitors 9. Colouring admixtures 10. Miscellaneous admixtures such workability, bonding, damp proofing, permeability reducing, grouting, gas-forming, and pumping admixtures
- 4. AIR-ENTRAINING ADMIXTURES Air-entraining admixtures are used to purposely introduce and stabilize microscopic air bubbles in concrete. Air-entraining concrete is produced by using either an air-entraining cement, by adding an air- entraining admixture during batching, or a combination of these approaches. Air entrainment will dramatically improve the durability of concrete exposed to cycles of freezing and thawing.
- 5. AIR-ENTRAINING ADMIXTURES Entrained air greatly improves concrete’s resistance to surface scaling caused by chemical deicers. Furthermore, the workability of fresh concrete is improved significantly, and segregation and bleeding are reduced or eliminated.
- 6. Mechanism of Air Entrainment Air-entraining admixtures are surfactants that concentrate at the air-water interface and reduce the surface tension encouraging the formation of microscopic bubbles during the mixing process.
- 7. Mechanism of Air Entrainment Air-entraining admixtures typically have a negatively charged head which is hydrophilic and attracts water, and a hydrophobic tail which repels water. The hydrophobic end is attracted to the air within bubbles generated during the mixing process. The polar end, which is hydrophilic, orients itself towards water. The air-entraining admixture forms a tough, water- repelling film, similar to a soap film, with sufficient strength and elasticity to contain and stabilize the air bubbles. The hydrophobic film also keeps water out of the bubbles.
- 8. Entrained Air Entrained air bubbles are not like entrapped air voids, which occur in all concretes as a result of mixing, handling, and placing. Intentionally entrained air bubbles are extremely small in size, between 10 to 1000 μm in diameter, while entrapped voids are usually 1000 μm (1mm) or larger. As shown here, the bubbles are not interconnected. They are well dispersed and randomly distributed.
- 9. Water-Reducing Admixtures Normal – 5%-10% water reduction Mid-range – 6%-12% water reduction High-range – 12%-40% water reduction Super plasticizers
- 10. water reducer Water-reducing admixtures are used to reduce the quantity of mixing water required to produce concrete of a certain slump, reduce water-cement ratio, reduce cement content, or increase slump. An increase in strength is generally obtained with water-reducing admixtures as the water-cement ratio is reduced. For concretes of equal cement content, air content, and slump, the 28-day strength of a water-reduced concrete containing a water reducer can be 10% to
- 11. Mechanism of Water reducer Water reducers function as cement dispersants primarily through electrostatic and steric repulsive forces. Acidic groups within the polymer neutralize the surface charges on the cément particales.
- 12. Mechanism of Water reducer These groups bind to positive ions on the cement particle surfaces. These ions attach the polymer and give the cement a slight negative charge as well as create a layer on the surface. This negative charge and layer of adsorbed compounds create a combination of electrostatic and steric repulsion forces between individual cement particles, dispersing them, thus releasing the water tied up in agglomerations and reducing the viscosity of the paste and concrete.
- 13. Plasticizers For Flowing Concrete Plasticizers, often called super plasticizers, are essentially high-range water reducers. These admixtures are added to concrete with a low-to-normal slump and water-cement ratio to make high-slump flowing concrete. Flowing concrete is a highly fluid but workable concrete that can be placed with little or no vibration or compaction while still remaining essentially free of excessive bleeding or segregation.
- 14. Applications where flowing concrete is used (1) thin-section placements. (2) areas of closely spaced and congested reinforcing steel. (3) tremie pipe (underwater) placements,. (4) pumped concrete to reduce pump pressure, thereby increasing lift and distance capacity. (5) areas where conventional consolidation methods are impractical or can not be used. (6) for reducing handling costs.
- 15. Advantages of HRWR Admixture Significant water reduction Reduced cement content Increased workability Ease to placement More rapid rate of early strength development More effective use of cement Increased long term strength Reduced permeability
- 16. Disadvantages of HRWR Admixtures Addition cost of admixture Slump loss greater than conventional concrete Modification of air entraining admixture dosage.
- 17. Set Retarding Admixtures Used to delay the rate of setting – initial and final ◦ Offset accelerating effects ◦ Enable special placing/finishing techniques Lower early-age strength, but higher long-term strength Effects on other properties may be unpredictable Sugar, Carbohydrate, derivates, soluble zinc salts, soluble borates, citric acid, tartaric acid etc are used as a retader.
- 18. Retarders It slows down the chemical process of hydration so that concrete remains plastic and workable for longer time than the concrete without retarder. It is more useful in hot weather concreting which tends to reduce setting time due to high temperature and may not allow completion of concrete compaction and finishing work before initial set of cement. It is also used in ready mix concrete in which time required to transport the concrete mix to construction site.
- 19. Accelerators Accelerators are used in concrete to reduce the setting time to accelerate the Hardening process. It increases the rate of early strength development and hence permit early removal of formwork. It also helps in emergency repair work. It is normally used in cold weathering concrete work
- 20. Accelerators CaCl2 is an admixture is commonly used in Plain concrete work about1-2% by weight of cement. Use of CaCl2 is not recommended for R.C.C and Pre stressed concrete as it causes corrosion to steel. For R.C.C and Pre stressed concrete Nitrate such as Ca(NO3)2 and NaNO2 are preferred. It does not increase resistance to alkali aggregate reaction.
- 21. Cementitious materials /Mineral Admixtures Fly ash, Silica fume, Rice husk ash, Ground Granulated Blast Furnace Slag, Metakaolin
- 22. Fly Ash (IS 3812 - part I - 2003.) Fly ash is finely divided residue resulting from the combustion of powdered coal and transported by the flue gases and collected by electrostatic precipitator. Fly ash was first used in large scale in the construction of Hungry Horse dam in America in the approximate amount of 30 per cent by weight of cement. In India, Fly ash was used in Rihand dam construction replacing cement upto about 15 per cent.
- 24. Fly Ash In the recent time, the importance and use of fly ash in making high strength and high performance concrete. High volume fly ash concrete is a subject of current interest all over the world. The use of fly ash as concrete admixture not only extends technical advantages to the properties of concrete but also contributes to the environmental pollution control. It can be said that 7% of the world’s carbon dioxide emission is attributable to Portland cement industry.
- 26. Fly Ash There are two ways that the fly ash can be used: One way is to inter grind certain percentage of fly ash with cement clinker at the factory to produce Portland pozzolana cement (PPC). Second way is to use the fly ash as an admixture at the time of making concrete at the site. The latter method gives freedom and flexibility to the user regarding the percentage addition of fly ash.
- 27. Fly Ash Properties High fineness, low carbon content, good reactivity are the essence of good fly ash. Since fly ash is produced by rapid cooling and solidification of molten ash, a large portion of components comprising fly ash particles are in amorphous state. The amorphous characteristics greatly contribute to the pozzolanic reaction between cement and fly ash. fly ash is the spherical form of the particles which improves the flowability and reduces the water demand.
- 28. ASTM Classification for Fly Ash Class F: Fly ash normally produced by burning anthracite or bituminous coal, usually has less than 5% CaO. Class F fly ash has pozzolanic properties only. Class C: Fly ash normally produced by burning lignite or sub-bituminous coal. Some class C fly ash may have CaO content in excess of 10%. In addition to pozzolanic properties, class C fly ash also possesses cementitious properties.
- 31. Effect of Fly Ash on Fresh Concrete Good fly ash with high fineness, low carbon content, highly reactive forms only a small fraction of total fly ash collected. Coal ash are not suitable for use as pozzolan and they do not reduce the water demand. Use of right quality fly ash, results in reduction of water demand for desired slump. With the reduction of unit water content, bleeding and drying shrinkage will also be reduced.
- 32. Effect of Fly Ash on Hardened Concrete Fly ash, when used in concrete, contributes to the strength of concrete due to its pozzolanic reactivity. However, since the pozzolanic reaction proceeds slowly, the initial strength of fly ash concrete tends to be lower than that of concrete without fly ash. Due to continued pozzolanic reactivity concrete develops greater strength at later age, which may exceed that of the concrete without fly ash. Also decrease of water permeability and gas permeability.
- 33. Durability benefits of concrete It gives more resistance to the corrosion of reinforcement. Fly ash contributes to the reduction of the expansion due to alkali-aggregate reaction. In conclusion it may be said that although fly ash is an industrial waste, its use in concrete significantly improve the long term strength and durability and reduce heat of hydration.
- 34. Silica Fume It is a product resulting from reduction of high purity quartz with coal in an electric arc furnace in the manufacture of silicon or ferrosilicon alloy. It has spherical shape. It is extremely fine with particle size less than 1 micron and with an average diameter of about 0.1 micron, about 100 times smaller than average cement particles. Silica fume has specific surface area of about 20,000 m2/kg, as against 230 to 300 m2/kg.
- 35. Silica Fume as High Performance Concrete The use of silica fume in conjunction with superplasticizer has been the backbone of modern High performance concrete. The structures such as Key Tower in Cleave land with a design strength of 85 MPa, and Wacker Tower in Chicago with specified concrete strength of 85 MPa, and two Union Square in Seattle with concrete that achieved 130 MPa strength – are testaments to the benefits of silica fume technology in concrete construction”.
- 36. It should be realised that silica fume by itself, do not contribute to the strength dramatically, although it does contribute to the strength property by being very fine pozzolanic material and also creating dense packing and pore filler of cement paste.
- 37. In India, silica fume has been used very rarely. Nuclear Power Corporation was one of the first to use silica fume concrete in their Kaiga and Kota nuclear power projects. Silica fume was also used for one of the flyovers at Mumbai where,75 MPa concrete was used (1999). Silica fume is also used in construction of Bandra-Worli sea link project at Mumbai. At present, India is not producing silica fume of right quality. In India, however, the silica fume of international quality is marketed by Elkem Metallugy (P) Ltd., 66/67, Mahavir Centre, Sector 17, Vashi, Navi Mumbai-400 703.
- 38. Microsilica microsilica is initially produced as an ultrafine undensified powder at least 85% SiO2 content mean particle size between 0.1 and 0.2 micron minimum specific surface area is 15,000 m2/kg spherical particle shape. Available forms Undensified forms with bulk density of 200–300 kg/m3 Densified forms with bulk density of 500–600 kg/m3 Micro-pelletised forms with bulk density of 600– 800 kg/m3 Slurry forms with density 1400 kg/m3.
- 39. Silica Fume influence on Fresh Concrete Water demand increases in proportion to the amount of microsilica added. The addition of microsilica will lead to lower slump but more cohesive mix. The microsilica make the fresh concrete sticky in nature and hard to handle. It is reported that concrete containing microsilica is vulnerable to plastic shrinkage cracking and, therefore, sheet or mat curing should be considered.
- 41. Silica Fume Influence on Hardened Concrete
- 42. Silica Fume Influence on Hardened Concrete It has been also found out that modulus of elasticity of microsilica concrete is less than that of concrete without microsilica at the same level of compressive strength. There are some investigations indicating contradiction, particularly with reference to resistance against frost damage. With regard to whether or not, silica fume is effective for alkali-aggregate reaction, Some research workers report that it is effective, others conclude that while it is effective, addition of silica fume in small quantities actually increases the expansion.
- 43. Rise Husk Ash Rice husk ash, is obtained by burning rice husk in a controlled manner without causing environmental pollution. When properly burnt it has high SiO2 content and can be used as a concrete admixture. Rice husk ash exhibits high pozzolanic characteristics and contributes to high strength and high impermeability of concrete.
- 44. Rise Husk Ash Rice husk ash (RHA) essentially consist of amorphous silica (90% SiO2), 5% carbon, and 2% K2O. The specific surface of RHA is between 40 – 100 m2/g. India produces about 122 million ton of paddy every year. Each ton of paddy producers about 40 kg of RHA. There is a good potential to make use of RHA as a valuable pozzolanic material to give almost the same properties as that of
- 45. Ground Granulated Blast Furnace Slag (GGBS) Ground granulated blast-furnace slag is a nonmetallic product consisting essentially of silicates and aluminates of calcium and other bases. The molten slag is rapidly chilled by quenching in water to form a glassy sand like granulated material. The granulated material when further ground to less than 45 micron will have specific surface of about 400 to 600 m2/kg.
- 48. GGBS The blast furnace slag is mainly used in India for manufacturing slag cement. There are two methods for making Blast Furnace Slag Cement. In the first method blast furnace slag is inter ground with cement clinker along with gypsum. In the second method blast furnace slag is separately ground and then mixed with the cement.
- 49. GGBS Just as fly ash is used as an admixture in making concrete Ground Granulated Blast furnace Slag popularly called GGBS is used as an admixture in making concrete. Recently for marine outfall work at Bandra, Mumbai, GGBS has been used as an admixture to replace cement to the tune of 70%. Presently in India, with the growing popularity of RMC, the scope for using GGBS for customer’s tailor made requirements should also become popular.
- 50. GGBS Effect on Fresh Concrete The replacement of cement with GGBS will reduce the unit water content necessary to obtain the same slump. This is because of the surface configuration and particle shape of slag being different than cement particle. In addition, water used for mixing is not immediately lost, as the surface hydration of slag is slightly slower than that of cement. Reduction of bleeding is not significant with slag of 4000 cm2/g fineness. But significant beneficial effect is observed with slag fineness of 6000 cm2/g and above.
- 52. GGBS Effect on Hardened Concrete Reduced heat of hydration Refinement of pore structures Reduced permeability to the external agencies Increased resistance to chemical attack.
- 53. Metakaolin Considerable research has been done on natural pozzolans, namely on thermally activated ordinary clay and kaolinite clay. These unpurified materials have often been called “Metakaolin”. Metakaolin is a dehydroxylated form of the clay mineral kaolinite. Stone that are rich in kaolinite are known as china clay or kaolin, traditionally used in the manufacture of porcelain. Highly reactive metakaolin is made by water processing to remove unreactive impurities to make 100% reactive pozzolans.
- 54. Metakaolin Such a product, white or cream in colour, purified, thermally activated is called High Reactive Metakaolin (HRM). Considered to have twice the reactivity of most other pozzolans, metakaolin is a aluable admixture for concrete/cement applications. Replacing portland cement with 8–20% (by weight) metakaolin produces a concrete mix, which exhibits favorable engineering properties, including: the filler effect, the acceleration of OPC hydration, and the pozzolanic reaction. The filler effect is immediate, while the effect of pozzolanic reaction occurs between 3 and 14 days.
- 55. Advantanges Increased compressive and flexural strengths. Reduced permeability (including chloride permeability) Reduced potential for efflorescence, which occurs when calcium is transported by water to the surface where it combines with carbon dioxide from the atmosphere to make calcium carbonate, which precipitates on the surface as a white residue. Increased resistance to chemical attack Increased durability Reduced effects of alkali-silica reactivity (ASR) Enhanced workability and finishing of concrete Reduced shrinkage, due to "particle packing" making concrete denser Improved color by lightening the color of concrete making it possible to tint lighter integral color.
- 56. Uses of Metakaolin High performance, high strength, and lightweight concrete Precast and poured-mold concrete Fibercement and ferrocement products Glass fiber reinforced concrete