Chapter IV Concrete Making Materials Admixtures

Transcription

1 CIV415 CONCRETE TECHNOLOGY Chapter IV Concrete Making Materials Admixtures Assist.Prof.Dr. Mert Yücel YARDIMCI Spring, 2014/2015 Advanced Concrete Technology - Zongjun Li 1

2 Admixtures Historically, an admixture is almost as old as concrete itself. The Romans used animal fat, milk, and blood to improve their concrete properties. Although these were added to improve workability, blood was a very effective air-entraining agent and might well have improved Roman concrete durability. In more recent times, calcium chloride was often used to accelerate the hydration of cement. The systematic study of admixtures began with the introduction of air-entraining agents in the 1930s, when it was accidentally found that cement ground with beef tallow (grinding aid) had more resistance to freezing and thawing than a cement ground without beef tallow. Nowadays, as we mentioned earlier, admixtures are important and necessary components for modern concrete technology. Advanced Concrete Technology - Zongjun Li 2

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4 Admixtures definition & classification An admixture is defined as a material other than water, aggregates, cement, and reinforcing fibers that is used in concrete as an ingredient, and added to the batch immediately before or during mixing. Air-entraining agents Chemical admixtures Mineral admixtures Advanced Concrete Technology - Zongjun Li 4

5 Air-entraining agents (ASTM C260): to improve the frost resistance of concrete. Kimyasal katkılar (ASTM C494, BS 5075 and EN 934-2): Any chemical additive to the concrete mixture that enhances the properties of concrete in the fresh or hardened state. Reducing the water demand of mixture (water reducers) Controlling the setting time and strength gain rate (accelerators and retarders) Viscosity modifying agent (adjusting the viscosity of paste) Shrinkage reducing chemicals Alkali-silica mitigating admixtures Mineral katkılar (ASTM C494 and BS 5075): consists of finely divided solids added to concrete to improve its workability, durability, and strength. Fly ash, slag and pozzolans are important categories of mineral admixtures. Miscellaneous admixtures such as latexes, corrosion inhibitors, and expansive admixtures. Advanced Concrete Technology - Zongjun Li 5

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8 Water-reducing admixture They are used to reduce the water content of a concrete mixture while maintaining a given constant workability. The resultant effect of reduced water content is increased strength and durability of concrete. Water reducers may also be employed to plasticize the concrete, i.e., to make concrete flowable. In this case, the water content (or water-to-cement ratio) is held constant, and the addition of the admixtures makes the concrete flow better, while the compressive strength (which is a function of the water-to-cement ratio) is not affected. Another use of water reducers is to lower the amount of cement (since water is proportionately reduced) without affecting strength and workability. This makes the concrete cheaper and more environmentally friendly, as less cement is consumed. Water reducers are classified broadly into two categories: (1) normal and (2) high range. Advanced Concrete Technology - Zongjun Li 8

9 Water-reducing admixture The normal water reducers are also called plasticizers, while the high-range water reducers are called superplasticizers. Normal water reducer (Plasticizer) Reducing water demand by 5 10% Lignosulfonate salts of sodium and calcium High range water reducer (Superplasticizer) Reducing water demand by 15 40% Sulfonated naphthalene formaldehyde (SNF) polycarboxylic ether (PCE) Lignosulfonates (normal and sugar-refined), and SNF-based water reducers work on the mechanism of lowering zeta potential, which leads to electrostatic repulsion to separate cement particles from flocculation, thus releasing the entrapped water by cement particle clusters Advanced Concrete Technology - Zongjun Li 9

10 Water-reducing admixture Polycarboxylic ether-based water reducers are polymers with backbone and side chains. The backbone gets adsorbed on the surface of the cement grains, and the side chains cause dispersion of cement grains by steric hindrance. This phenomenon relates to the separation of the admixture molecules from each other due to the bulky side chains, demonstrated in Figure Steric hindrance is a more effective mechanism than electrostatic repulsion. The dosages of water reducer for the liquid form can refer to either the solid content or liquid mass and is expressed as weight percentage of the binder. Advanced Concrete Technology - Zongjun Li 10

11 Water-reducing admixture Superplasticizers (SPs) are used mainly for two main purposes: to produce high-strength concrete at a w/c ratio in a range of ; to create flowing concrete with high slump flow in the range of 500 to 600 mm, the self-compacting concrete. Another benefit is that a lower w/c ratio would lead to better durability and lower creep and shrinkage. The major drawbacks of superplasticizers retarding setting (especially in large amounts) causing more bleeding entraining too much air. Advanced Concrete Technology - Zongjun Li 11

12 Water-reducing admixture The most common problem in the application of water reducers in concrete is incompatibility with cement and aggregate, which refers to the abnormal behavior of a concrete due to the superplasticizer used. Common problems: Shorten setting time, delayed setting time, rapid slump loss, improper early-age strength development. These issues in turn affect the hardened properties of concrete, primarily strength and durability. Compatibility between cements and superplasticizers is affected by many factors, including cement composition, admixture type and dosage, and concrete mixture proportions. 12

13 Water-reducing admixture The C 3 A content or, more specifically, the C 3 A to SO 3 ratio has a profound effect on the compatibility between cement and SPs. When the C 3 A content of cement is high and the sulfate availability is low, superplasticized concretes experience high rates of slump loss. When there is less C 3 A available, higher amounts of SPs tend to adsorb on C 3 S and C 2 S, resulting in a reduction in the rate of strength development. Advanced Concrete Technology - Zongjun Li 13

14 Water-reducing admixture Most of the chemical admixtures are in liquid form. The solid content varies between 30% - 50%. The remaining is water. The additional water added coming from the superplasticizer should be deducted from the mixing water amount. Kg/m3 Cement 460 Water mm sand mm crushed stone mm crushed stone 357 Superplasticizer 4.60 TOTAL 2412 If the solid content of the superplasticizer is 30% Mixing water should be ( *0.70) Advanced Concrete Technology - Zongjun Li 14

15 Shrinkage-reducing admixture Shrinkage-reducing admixtures (SRA) provide a significant technical approach to reduce the drying shrinkage of concrete. The main mechanism of SRA in reducing drying shrinkage of concrete is that the SRA lowers the surface tension of the pore solution and subsequently reduces the stresses in the pore solution that are directly proportional to the surface tension. With the reduction of the driving stress, the drying shrinkage can be reduced. Advanced Concrete Technology - Zongjun Li 15

16 Setting-control admixture Setting-control admixtures are used to either extend or shorten the plastic stage of concrete to meet the special requirements of the construction of concrete structures. If the admixture is used to extend the plastic period, it is called a retarder. If the admixture is used to shorten the plastic period, it is called an accelerator. Some of the admixtures are designed to retard (I and IV) or accelerate (V) the setting. On the other hand, there are some admixtures presenting dual-role depending on the dosage. Advanced Concrete Technology - Zongjun Li 16

17 Setting-control admixture Retarding admixtures: Mainly used to offset fast setting caused by ambient temperature, particularly in hot weather; As a Setting control of large structural units to keep concrete workable throughout the entire placing period; to meet the requirement of long transportation time from the concrete plant to the construction site. For example, for the construction of building built up in a very far place from the concrete plant, the concrete has to be transported by trucks (mobile mixers). In this case, 5 to 6 h or even longer may be required. Accelerators: It is mainly used when the more quick setting is required. (plugging leaks in swimming pools, water tanks, and pipelines; emergency repairs for highways, bridges, airport runways, and tunnels, etc.) Winter construction in cold regions. Soluble inorganic salts, such as calcium chloride, are by far the best known and most widely used accelerators. A side effect of using chloride, however, is that it induces corrosion of the reinforcement in concrete structures. Nowadays, calcium nitrate is used instead of calcium chloride. Advanced Concrete Technology - Zongjun Li 17

18 Air-entraining admixture Air-entraining admixtures entrain air in the concrete. Surface-active agents having two poles The entrained air is produced by admixtures that cause the mixing water to foam, and the foam is locked into the paste during hardening. Commonly, carboxylic acid or sulfonic acid groups are used as to achieve the hydrophilicity, while aliphatic or aromatic hydrocarbons are used for hydrophobility. The agents are concentrated at the air water interface, the hydrophilic side with water and the hydrophobic side with air. The surface tension is lowered so that bubbles can form more readily and then stabilize once they are formed. Advanced Concrete Technology - Zongjun Li 18

19 Entrapped air void Entrained air voids are different from entrapped air voids. Entrained air voids are formed on purpose, while an entrapped air void is formed by chance when the air gets into the fresh concrete during mixing. Entrapped air voids may be as large as 3 mm; Entrained air voids usually range from 50 to 200 μm. Advanced Concrete Technology - Zongjun Li 19

20 Advantages of air-entraining admixtures The workability of concrete can be improved because air bubbles act as lubricants in a fresh concrete. Subsequently, the water amount can be reduced for a targeted workability. The ductility of concrete can be improved, since the air bubbles generated by the air-entraining agent provide more room for deformation to occur. The permeability of concrete can be improved due to the effect of air-entraining agent in enclosing the air bubbles. The impact resistance of concrete, can be improved as the air bubbles provide more deformation. Finally and most importantly, the durability, especially the ability to resist freezing and thawing of concrete, can be significantly improved by adding an air-entraining agent. Advanced Concrete Technology - Zongjun Li 20

21 Spacing factor in air-entrainment The smaller the spacing factor (which is defined as the average distance from any point in the paste to the edge of a nearest entrained air bubble), the more durable the concrete. For the case of a spacing factor greater than 0.3 mm, the air entrained has a small or negligible effect on durability. Advanced Concrete Technology - Zongjun Li 21

22 The volume of air required to give optimum durability has been found to be about 4 8% by volume of concrete. The actual fraction depends on the maximum size of aggregate: the larger the aggregate size, the lower requirement for air content. Advanced Concrete Technology - Zongjun Li 22

23 Mineral Admixtures Advanced Concrete Technology - Zongjun Li 23

24 Mineral Admixtures finely divided siliceous materials. added to concrete during mixing in relatively large amounts. Industrial by-products are the primary source of mineral admixtures. silica fume, slag, fly ash, and metakaoline. Advanced Concrete Technology - Zongjun Li 24

25 Silica fume a by-product of the silicon metal and ferrosilicon alloy industries. very fine spherical particles consisting of noncrystalline silica. silica fume is also called condensed silica fume or microsilica. Size 0.50 µm 99% 0.20 µm 95% 0.10 µm 70% 0.05 µm 20% Smaller than Advanced Concrete Technology - Zongjun Li 25

26 Silica fume The surface area around 200,000 cm 2 /g. This value is typically around 3,700 cm 2 /g for normal Portland Cement. Presents very high puzzolanic activity. The normal percentage of silica fume used to replace Portland cement is from 5 to 15%. However, it creates handling problems and increases the water requirement in concrete appreciably, unless superplasticizer is used. Advanced Concrete Technology - Zongjun Li 26

27 Size distribution of silica fume Advanced Concrete Technology - Zongjun Li 27

28 Puzzolanic reaction of silica fume A denser concrete microstructure can be achieved and a high compressive strength can be reached. Pozzolanic reaction is defined as Cement + water = C S H (Tobermorite gel) + Ca(OH) 2 pozzolan + calcium hydroxide + water = Calcium Silicate Hydrate (secondary) Silica fume is a very active pozzolanic material and readily reacts with CH and water to form secondary C S H. The process can further reduce the porosity and permeability in concrete as well as the possibility of chemical reaction of CH with other ions to Advanced Concrete Technology - Zongjun Li 28 form harmful products.

29 Metakaolin High-reactivity metakaolin (MK) is one of the recently developed supplementary cementing materials for high-performance concrete. It is produced by calcining purified kaolinite clay in a specific temperature range (650 to 800 C) to drive off the chemically bound water in the interstices of kaolin and destroy the crystalline structure, which effectively converts the material to the MK phase, an amorphous aluminosilicate. Unlike industrial by-products, such as silica fume (SF), fly ash, and blast-furnace slag, MK is carefully refined to lighten its color, remove inert impurities, and control its particle size. Advanced Concrete Technology - Zongjun Li 29

30 Metakaolin The particle size of MK is generally less than 2 μm, which is significantly smaller than cement particles, though not as fine as SF. It is typically incorporated into concrete to replace 5 20 % of cement by mass. Silicon dioxide and aluminum oxide are the two main components in MK and little CaO exists in MK. Similar to silica fume, metakaolin improves concrete performance by the packing effect and by reacting with calcium hydroxide to form secondary C S H. Advanced Concrete Technology - Zongjun Li 30

31 Advantages of metakaolin MK was found to improve concrete properties while offering good workability. Concrete modified by MK requires less water-reducing admixture than that modified by SF to achieve a comparable fluidity. MK is particularly effective in reducing the rate of diffusion of sodium and chloride ions. The diffusion coefficient can be reduced by about 50% when 8% MK is added into the concrete mixture. MK and SF had similar functions in improving strength of concrete, reducing free drying shrinkage, and chloride diffusion rate. Advanced Concrete Technology - Zongjun Li 31

32 Fly ash Fly ash (pulverized fuel ash) is a by-product of an electricitygenerating plant using coal as fuel. Fly ash can be divided into two categories according to type of coal burned (ASTM C618) Bituminous coal class F is called lowcalcium fly ash class C is called highcalcium burning lignite or sub-bituminous coal. Advanced Concrete Technology - Zongjun Li 32

33 Fly ash The size distributions of fly ash are slightly smaller than those of Portland cement with more than 50% under 20 μm. Advanced Concrete Technology - Zongjun Li 33

34 Fly ash Fly ash can be used to produce a so-called blended cement. In this case, fly ash is milled with clinker and gypsum in the last procedure in cement production. Fly ash can also be used to produce modified concrete by adding it to concrete during the mixing process. Advanced Concrete Technology - Zongjun Li 34

35 Fly ash The incorporation of fly ash into concrete has certain advantages and some disadvantages. Incorporation of fly ash into concrete can improve the workability due to the spherical shape and glassy surface of fly ash particles. Incorporating fly ash into concrete can reduce the hydration heat of fresh concrete and is good for mass concrete structures. By replacing cement with fly ash, the cost of concrete can be reduced, since fly ash costs low than cement. Because fly ash is an industrial by product, it lowers the energy demand in producing concrete. 35

36 Fly ash Fly ash concrete are low early age strength and longer initial setting time due to the low reactivity of fly ash (it is not favorable in cold-weather concreting). The normal replacement of cement by fly ash is around 25 to 30% by weight. Advanced Concrete Technology - Zongjun Li 36

37 Slag Slag is a by-product of iron or steel production. For iron production, blast furnaces are used and for steel production, either a basic oxygen furnace or an electrical furnace. The slag produced in iron production is different from that from steel production. Currently, the slag used in the concrete industry is mainly the slag from iron production, and its full name is ground, granulated blast furnace slag or GGBS. Advanced Concrete Technology - Zongjun Li 37

38 GGBFS CaO and SiO 2 are the two main components, and the CaO content in slag is quite close to that of high-calcium fly ash. Ground granulated blast-furnace slag and high-calcium fly ashes are similar in mineralogical character and reactivity. Besides a similar content of CaO, both are essentially noncrystalline, and their high-calcium glassy phases have a similar order of reactivity. Advanced Concrete Technology - Zongjun Li 38

39 GGBFS Compared to low-calcium fly ash, which usually does not make any significant contribution to the strength of Portland cement concrete until after about 4 weeks of hydration, the strength contribution by high-calcium fly ash or granulated blast-furnace slag may become apparent as early as 7 days after hydration. Similar to fly ash, slag can also be used in both cement and concrete production. In cement production, slag can be milled with clinker and gypsum to produce Portland slag blended cement. In concrete production, up to 50% of cement can be replaced by slag. Slag-modified concrete shows an improved slump retention as well as both early and long-term strength Advanced Concrete Technology - Zongjun Li 39

40 Benefits of using mineral admixtures in concrete Advanced Concrete Technology - Zongjun Li 40