Water cement Ratio law

 W/C RATIO LAW

The lower the water-cement ratio, the higher is the strength of  concrete, provided the mix is workable".

 

S = K {C/c+e+a}²  

 

Where S = Strength of concrete

                K = constant

                c = volume of cement

                e =volume of water and

                a = volume of air

 

Limitations of water cement ratio law

1. The internal moisture conditions of concrete specimens is uniform.

2. The concrete specimens to be tested is cured under standard temperature.

3. The concrete specimens to be tested is of same age.

4. The concrete specimens to be tested is of same size.

 

The effects of these conditions on strength of concrete are :

 

1. Effect of internal moisture conditions on hydration of cement : The hydration of cement continues so long as the capillary pores are filled with water. hence to maintain the continuity of reaction the internal moisture condition should be maintained for the important period of hydration. That means curing should be done for  28 days.

 

2.  Effect of temperature on hydration of cement : At higher temperature the rate of reaction i.e. formation of gel is faster and strength develops at a faster rate. This faster development of strength does not cause full hydration of cement and hence produces poious concrete witless ultimate strength. Indian standard has specified a curing temperature of 27 + (-) 3^o C for proper hydration of cement.

 

3. Effect of age on concrete strength : Usually strength of concrete increases with age. Water cement ratio law is applicable only when the strength of concretes at the same age and compared. according to Indian Standard, 28 days strength of concrete has been specified for comparison.

 

4. Effect of size of specimen on strength of concrete : It has been found that for the same mix proportations, smaller size specimen gives greater strength than the large size specimen. Indian standard as specified a standard Cube size of 150mm side. if used 100mm size cube then strength  should be reduced by 10% so as to get the strength of 150 mm size cube. Where cubes larger than 150 mm size are used, generally no modification necessary unless otherwise specified by engineer-in-charge.