Water-cement Ratio | Properties of Water Necessary to Get Good Concrete
Describe the properties of water which are necessary to be used to get good concrete. What do you understand by the term water-cement ratio
Properties of Water For Good Concrete
1. The water used for the preparation and curing of concrete should be free from deleterious materials.
2. The presence of suspended particles of clay and silt up to 0.02% by weight of water is considered safe for concrete. According to IS 456 permits 2000 mg/l of suspended matter in water.
3. The water must be free from salts. The salts of magnesium (Mg), tin (Sn), lead (Pb) and copper (Cu) cause a reduction in the strength of concrete. ZnCl2 retards the initial setting of concrete while Pb(N03)2 is totally destructive to concrete.
Similarly, salts of sodium (Na) like sodium iodate, sodium arsenate and sodium borate etc. reduce the initial strength of concrete.
4. Seawater in concrete increases the risk of corrosion of reinforcement. Moreover, chlorides present in seawater leads to efﬂorescence.
5. Water must be free from algae as it combines with cement and decreases the bond strength between the aggregates and cement paste.
6. The amount of sugar present in the water must be less than 0.05% by the weight of the water. Higher amounts (up to 0.15% by weight) of sugar retard the setting of cement and reduces the early strength. A further increase in the sugar content in water increases the setting but reduces the 28-day strength.
A cement with average composition requires about 25% water by weight for hydration.
In addition to that, water is required to fill in the gel pores.
Thus the total amount of water needed for hydration of cement and to fill the gel pores comes out to be about 42% by weight.
Thus the minimum water-cement ratio of 0.4 (sometimes taken as 0.36) is required.
Relation Between The Water-cement Ratio And Strength
Discuss the relation between the water-cement ratio and strength.
In concrete, about 25% water by weight is required for the complete hydration of cement.
In addition to that, water is required by concrete to fill the gel pores.
The total amount of water required for cement hydration and to fill the gel pores is about 42% by weight.
It is a general fact that complete hydration of cement never takes place in concrete.
Strength of concrete increases with decrease water-cement ratio