Effect of water-cement ratio on strength of concrete

 

The workability of concrete increases as water content of mix increased, water lubricates the mixture of concrete. But increase in water content causes the decrease in strength of concrete .Excessive use water weakens the concrete ,produces shrinkage cracks and decreases density of concrete.

 Water occupies the space in concrete and as it evaporates it leaves voids and cracks which decrease the strength of concrete. The volume of water voids may be as much as 10 percent of the total volume of concrete. An excess of 10 percent of water may reduce the strength by about 15 percent and an excess of 30 percent of water may reduce the strength up to 50 %. Lower the water content the stronger the concrete but quantity of water must be sufficient to produce a workable mix required for particular method of compaction to be adopted for concreting.

Concrete made with low water cement ratio is unworkable. If stiff or dry concrete is used then honey-combing will result decrease in density and strength. An unworkable concrete results in incomplete compaction giving rise to air voids. If 5% Air voids exists in concrete may cause 30 % strength loss and 10 % air voids may cause as much as 50 percent strength loss.

Water Cement Ratios Adopted for different mix of Concretes.
S No	CONCRETE MIX	WATER CEMENT RATIO	QUANTITY OF WATER IN LITERS	REMARKS
1	(1:1:2)	0.42 TO .54	21 TO 27	For vibrated concrete ,the quantity of water may be reduced by about 20 %
2	(1:1.5:3)	0.52 TO 0.60	26 TO 30
3	(1:2:4)	0.58 TO 0.64	29 TO 32
4	(1:3:6)	0.68 TO 0.72	34 TO 36
5	(1:4:8)	0.90 TO 0.94	45 TO 47

Therefore there is an optimum value of water cement ratio for every mix which should only be decided after properly adopting of job mix formula, and water content has to be restricted within certain minimum limits. Concrete should be plastic to be worked around the reinforcement rods.

Sometime strength has to be sacrificed by adding more water to obtain higher degree of workability where concrete has to be placed in narrow and thin RCC sections.
The best mix is the one which gives the maximum workability with minimum amount of water .An increase in water content must be accompanied by proportionate increase of cement if strength is to be maintained.

Hydration of cement is incomplete without an adequate quantity of water, Less water impedes complete setting of cement and decreases the strength of concrete. The amount of water required to complete the hydration of cement is about 25 % of the weight of the cement.
Generally the water cement ratio for Concrete (1:2:4) mix is 0.60 ,Concrete (1:1.5:3) mix is 0.50 and for concrete mix (1:1:2) mix is 0.45.