Chapter 6 : Admixtures
Admixture are materials added to the concrete to improve it’s properties most used to improve the quality of the concrete or as an alternative to using special type of cement.
Additives and admixtures are not the same. Admixtures are materials added to the concrete in the mixing stage while additives are add to the cement at the manufacturing stage.
Water
reducers (ASTM Type A):
-Also called plasticizers, these admixtures are used for three purposes:
-The water reducers contain
surface-active agents. When added to concrete these agents are absorbed on the
cement particles giving them a negative charge, which lead to a repulsion
between the particles and results in stabilizing their dispersing. In addition,
the negative charge causes the development of a cover of water oriented
molecules around each water thus separating the particles and because the water
is no longer restrained by the cement particles and it becomes available to
lubricate the mix which increases the workability.
-The reduction of water when water reducers are used varies between 5-15%.
-The air content is increases because the surface-active agent lowers the surface tension of water and also makes it harder to the air bubbles to stick to the cement particles. Sometime it is necessary to use Air-detraining agent to avoid over-air entrainment.
-Setting is retarded.
-The rate of loss of workability is not reduced (the higher the initial workability, the higher the rate)
-Early age strength is increased because of the increased surface area available for hydration as a result from the cement particles dispersing.
-Long-term strength is increased because of the more uniformed distribution of the dispersed cement throughout the concrete.
-Does not improve the cohesiveness of the concert like the air-entrainment agents.
-Also available with as water reducing and retarding admixture (ASTM Type D) and water reducing and accelerating admixture (ASTM Type E).
-With time the admixture is
removed from the concrete by being incorporated into the hydrated materials
without changing the properties of those materials.
Superplasticizers
(ASTM Type F):
-A more effective type of water reducers. Also known as high range water reducers.
Superplasticizers are (sulphonated melamine formaldehyde condensates) or (sulphonated naphthalene formaldehyde condensates) the latter is more effective in dispersing the cement particles with some retarding effects.
-The dispersing action happens when the adsorption of the sulphonic acid on the surface of the cement particle, causing them to become negative in charge and repulse from each other.
-The dosage is usually higher than water reducers with higher reduction in water content.
-Less side effects (less bleeding at higher workability, more cohesive)
-Don’t entrain much air because they don’t lower the surface tension of water
-Can raise the slump from 75mm to 200mm for a constant w/c ratio.
-Used to produce higher workability (flowing concrete) or higher strength concrete as much as 100 MPa. As it can reduce the water content by 25-35% about twice as much as water reducers.
-Gives better result when very angular, flaky or elongated aggregates are avoided and the fine aggregate raised by 4-5% but exert high hydrostatic pressure of the formwork.
-High rate or slump loss after 30-90 minutes the workability is back to its original degree.
-Adding a additional dose of superplasticizer to concrete mixed with the same admixture after it lost its workability maybe cause segregation.
-Don’t affect the setting expect cement with a very low C3A content.
-The use of
superplasticizer with air entraining agent could reduce the entrapped air, and
improve the air-voids system. Some modified superplasticizers are available
which are compatible with air entraining admixture.
-Also available as superplasticizer and set-retarding admixture (ASTM Type G)
Accelerators
(ASTM Type C):
-Accelerate the hardening
or development of early strength.
-Not intended to affect the setting time (in practice they do!).
S-et-accelerators (quick setting) admixtures, which reduce the setting, time like sodium carbonate (washing soda), which causes a flash set usually, used in shotcrete. These admixtures lower the strength other set-accelerators include: aluminum chloride, potassium carbonate, sodium fluoride, sodium aluminate and ferric salts.
-The most common accelerators is calcium chloride (CaCl2), accelerates the early strength development.
-Accelerators are helpful when concrete is to be placed at low temperatures (2-4 C°) or when it’s loaded at an early age.
-Calcium chloride acts as a catalyst in the hydration of C3S and C2S or reduces the alkalinity of the solution, which increases the hydration of the silicates. Also delays the hydration of C3A a little.
-The greater the rate of hydration of the cement the more are the effects of the accelerators especially in Type I and Type III cements.
-Not to be used with high alumina cement.
-Also raises the 28-days strength a little.
-The typical dosage is 1-2% as a fraction of the cement content. A dosage of 1%of CaCl2 gives the same effect as a rise in temperature of 6 C° and an overdose can cause flash setting.
-Better results when it’s dissolved in the mixing water to ensure better distributing, especially if it was in a flaky not granular shape (dissolves slowly). The flakes are CaCl2.2H2O and each 1.37 g of it equals 1 g of CaCl2.
-Can cause corrosion of
steel and reduces the resistance to Sulphate attack especially in lean mixes
and increases the risk of alkali-silica reactivity. When reactive aggregates
are used. also increases shrinkage and creep and reduced the resistance of air entrained
concrete to freezing and thawing at later age. However, it increases the
resistance of concrete to erosion and abrasion.
-When acceleration without the steel corrosion is preferred, chloride-free accelerators could be used which are based on calcium formate blended with corrosion inhibitors. This kind of accelerator got lower effects than calcium chloride.
- With time the admixture is removed from the concrete by being incorporated into the hydrated materials without changing the properties of those materials.
Set-retarders
(ASTM Type B):
-Delays the setting of concrete
-Useful when concrete is casted in hot temperatures also in preventing the formation of cold joints.
-Delays the hardening of the cement.
-Set-retarders include: sugar, carbohydrates, derivatives, soluble zinc salts and soluble borates.
-Usually used when also water reducing (ASTM Type D).
-A dose of 0.05% of sugar by mass of cement will delay the setting time by 4 hours. When dosage up to 0.2-1% the concrete can no more set.
-The setting time is increased by delaying the addition of the admixture because once some of the C3A has hydrated, it no longer absorb the admixture so more is available for the calcium silicates.
-The admixture improves the crystal growth so that here is a more efficient barrier to further hydration.
-Reduces the early strength but later, the rate of strength development is higher, the long-term strength is not affected.
-Increases the plastic shrinkage because the plastic stage is extended no effect on drying shrinkage.
-With time the admixture is
removed from the concrete by being incorporated into the hydrated materials
without changing the properties of those materials.
Air-entraining
admixtures:
-Admixtures used to entrain air bubbles into the concrete to protect it from freezing and thawing.
-When mixed with water, air-entraining admixtures produce separate bubbles cavities which when hardened become incorporated within the paste.
-These admixtures contain surface-active agents, which lower the surface tension of water to ease the formation of bubbles. They do this by concentrating on the air/water interfaces and have water-repelling and water-attracting properties which are responsible for the dispersion and stabilization od the bubbles.
-Those bubbles are separate from the capillary voids system and will never be filled with hydration products as the hydration only happen in water filled voids.
-Air-entraining agents includes:
-A typical dosage is 0.005-0.05% of the cement weight premixed with the mixing water.
-Should provide a minimum air volume of 9% o of the volume of the mortar with less than 0.25 mm between air bubbles.
-Air-entraining admixtures reduces the required water content, increase the workability and improve the cohesiveness of the concrete.
Pozzolans:
-Pozzolans are siliceous or siliceous and aluminous materials, which have no cementing, affect. But when finely divided and in the presence of moisture, will react with lime CH from the hydration of the cement at ordinary temperatures to compounds with cementitious properties.
-The original pozzolans are volcanic ashes. Pozzolans include pumicite, opaline shales and cherts. Also calcined diatomaceous earth, burnt clay and fly ash (PFA: pulverized fuel ash)
The pozzolanic activity index is the ratio of the compressive strength of the mixture with a specified replacement of cement with pozzolans to the strength of a mix without replacement.
-The most common artificial pozzolan is fly ash (PFA) obtained by mechanical means from the flue gases of furnaces in coal-fired power stations. PFA particles are spherical and of at least the same fineness of cement.
Fly ash can be used as:
-PFA is used to replace a
portion of the cement content and can reduce the water content at least to 95%
-Concrete with PFA gains strength slowly and requires longer curing periods.
-Low early strength but a higher long-term strength.
-Slower hydration therefore reduces the rate of heat generation
-PFA got a lower specific gravity (1.9-2.4) than the cement so replacement by weight will increase the cementitious materials volume. PDF can be added without replacing the cement content when early strengths are required.
-Fly ash increases the percent of fines thus reduces permeability, increases workability and improves the pumping of the concrete.
-Fly ash reduces bleeding because it reduces the required water content to produce a specific workability.
Silica
Fume:
Silica fume is a byproduct of producing silicon metal or ferrosilicon alloys. Silicon metal and alloys are produced in electric furnaces. The raw materials are quartz, coal, and woodchips. The smoke that results from furnace operation is collected and sold as silica fume available in wet or dry forms.
Silica fume consists primarily of amorphous (non-crystalline) silicon dioxide (SiO2). The individual particles are extremely small, approximately 1/100th the size of an average cement particle. Because of its fine particles, large surface area, and the high SiO2 content, silica fume is a very reactive pozzolan when used in concrete.
Silica fume is available in wet or dry forms. Among it’s uses, Silica fume is added to the concrete during the mixing stage to produce very high strength in excess of 15,000 psi (around 104 N/mm2) and also to produce more durable concrete (for example Silica-fume concrete with a low water content is highly resistant to penetration by chloride ions or other chemical attacks) as basically the silica fume particles tend to fill the capillary voids in the hardened cement paste lowering it’s permeability.
Silica fume is not to be used to replace some of the cement content as it is more expensive than cement itself, this practice is not that common. (A better way is using less expensive admixtures like fly ash) you should note that silica fume concrete needs less effort in the finishing process as the micro fine particles increase the workability of the concrete.