WATER AND MIXTURE

WATER
Responsible for the hydration reactions with the cement
Good water is essential for quality concrete.
It should be good enough to drink--free of trash, organic matter and excessive chemicals and/or minerals.
The strength and other properties of concrete are highly dependent on the amount of water and the water-cement ratio.

Generally, cement requires about 3/10 of its weight of water for hydration. Hence the minimum water-cement ratio required is 0.35. But the concrete containing water in this proportion will be very harsh and difficult to place. Additional water is required to lubricate the mix, which makes the concrete workable. This additional water must be kept to the minimum, since too much water reduces the strength of concrete. The water-cement ratio is influenced by the grade of concrete, nature and type of aggregates, the workability and durability.

If too much water is added to concrete, the excess water along with cement comes to the surface by capillary action and this cement-water mixture forms a scum or thin layer of chalky material known as laitance. This laitance prevents bond formation between the successive layers of concrete and forms a plane of weakness. The excess water may also leak through the joints of the formwork and make the concrete honeycombed. As a rule, the smaller the percentage of water, the stronger is the concrete subject to the condition that the required workability is allowed for.


Properties of Water
Water suitable for drinking
Chloride < 0.05%
Sulfate < 0.08%
Organic salts < 0.05%
Any drinkable water can be used for concrete making - Water containing more than 2000 ppm of dissolved salts should be tested for its effect on concrete
- Chloride ions not more than 1000 ppm - Sulphate ions not more than 3000 ppm
- Bicarbonate ions not more than 400 ppm
Sea water
- no adverse effect on strength and durability.
- Surface dampness, efflorescence, staining.
- Increases risk of corrosion of steel
Sea water is not recommended for reinforced concrete
Water containing acid or organic substances should be avoided.
If there is any doubt on water quality carry out
- chemical analysis
- trail mixes


EFFECT OF IMPURITIES IN WATER ON PROPERTIES OF CONCRETE
1. Suspended particles
2. Miscellaneous inorganic
salts
3. Salts in sea water
4. Acids and alkalies
5. Algae
6. Sugar
7. Oil contamination.

ADMIXTURES
Admixtures are ingredients other than Portland cement, water, and aggregates.
Admixtures are added to the concrete mixture immediately before or during mixing.


Mineral Admixtures
Pozzolanic (cement replacement)
Class C Fly ash –15 to 40% bwc
Class F Fly ash – 15 to 25% bwc
Blast Furnace Slag (cement replacement)
25 to 70% bwc
Silica Fume (cement addition)
6 to 12% bwc
ASTM C-494 and Table 6-1
Water Reducing – Type A
Set Retarding – Type B
Set Accelerating – Type C
Water Reducer-Set Retarding – Type D
Water Reducer-Accelerating – Type E
High Range Water Reducers – Type F
HR Water Reducer-Set Retarding -Type G


CHEMICAL ADMIXTURES
Accelerating admixtures:
Compounds added to cement to decrease its setting time and to improve the early strength developments - Used in cold-weather concreting - A 25% of strength gain observed at the end of three days - CaCl2 (less than 2% by weight of cement); Not recommended for cold weather concreting; Triethanolamine; Sodium thiocyanate; Acetyl alcohol; Esters of carbonic and boric acids; Silicones - Problems: Increased heat of hydration, also leads to corrosion of steel.
-such as calcium chloride, are used to increase the rate of hardening--usually during cold weather.
Retarding admixtures:
Added to concrete to increase its setting times - Used in hot weather applications - Sodium/calcium triethanolamine salts of hydrogenated adipic or gluconic acid - Problem: early strength of concrete reduced
- are used to slow the rate of concrete hardening.
-They are useful for concrete that is placed during hot weather.

Water-reducing admixtures and super plasticizers
used to reduce the amount of water used in concrete mixes - High range water reducers reduce the water required for mixing by 12% or greater - Added to improve the consistency/workability of concrete and increase the strength - Water reducers: Lignosulphates, hydroxylated carboxylic acids, carbohydrates - Superplasticizers: Suphonated melamine/naphtalene formaldehyde condensates
Air-entraining admixtures:
Allows dispersal of microscopic air bubbles (diameters ranging from 20 to 2000 μm) throughout the concrete - Decreases the freeze-thaw degradation
Foaming agents:
Vinsol resin; Sulphonated lignin compounds; Petroleum acid compounds; Alkyd benzene compounds
Used in concrete to replace part of cement or sand - When used to replace sand called as supplementary cementing materials - Added in large quantities compared to chemical admixtures.
Pozzolans:
Raw and calcined natural materials such as cherts, shale, tuff and pumice - Siliceous or siliceous and aluminous materials which by themselves possess no cementing property, but in fine pulverized form and in the presence of water can react with lime in cement to form concrete
Fly ash:
By-product of coal from electrical power plants - Finer than cement - Consists of complex compounds of silica, ferric oxide and alumina - Increases the strength of concrete and decreases the heat of hydration - Reduces alkali aggregate reaction.
Silica fume:
By-product of electric arc furnaces - Size less than 0.1μm - Consists of non-crystalline silica- Increases the compressive strength by 40-60%