WATER/CEMENT RATIO

The water/cement ratio is defined as the weight of the mixing water divided by the weight of the cement. High quality concrete is produced by lowering the water/cement ratio as much as possible without sacrificing the workability of fresh concrete. Actually water/cement ratio is an index of the strength of concrete. The strength of concrete mainly depends upon the strength of the cement paste and the cement paste strength depends upon the dilution of cement paste. In other words the strength of cement paste increases with cement content and decreases with water and air content.

For a fully compacted concrete, its strength is taken to be inversely proportional to the water/cement ratio. A typical curve of strength versus water/cement ratio is shown in Figure 1.

Figure 1 Compressive strength v/s water/cement ratio

ABRAM’S WATER/CEMENT RATIO LAW

Duff Abram carried out extensive experiments and on the basis of his experimental results he proposed a relation between the compressive strength of concrete and water/cement ratio in 1918, which is known as “water/cement ratio law”, which he presented in the following form:

Where, S = strength of concrete.

K1 and K2 = empirical constants

x = water/cement ratio

He suggested empirically the values for K1 as 14000 and K2 as 7 in F.P.S. system and K1 as 984 and K2 as 7 in M.K.S. units.

The values of K1 and K2 may depend upon the type of cement and aggregate, method of curing, age of concrete at which strength is desired and the mode of testing etc. 

Abram’s water/cement ratio law states that the strength of concrete is only dependent on water/cement ratio provided the mix is workable.

Abram’s law although established independently, is similar to a general rule formulated by Ferret in 1896. Ferret defined the strength of concrete in terms of volume fractions of cement, water and air. He expressed the compressive strength of concrete as:

Where S = strength of concrete

c, w and a = volume of cement, water and air respectively 

            K = a constant.

It may be recalled that the water/cement ratio determines the porosity of the hardened cement paste at any stage of hydration. Thus the water/cement ratio and the degree of compaction both affect the volume of voids in concrete, and this is why the volume of air in concrete is included in Ferret’s expression. 

GEL/SPACE RATIO

The influence of the water/cement ratio on strength does not truly constitute a law because the water/cement ratio rule given by Abram does not include many qualifications necessary for its validity. Some of the limitations of Abram’s water/cement ratio law are: 

• The strength at any water/cement ratio depends upon the degree of hydration of cement and its chemical and physical properties. 
• The temperature at which hydration takes place. 
• The air-content in case of air-entraining concrete. 
• Change of effective water/cement ratio. 
• Formation of fissures and cracks due to bleeding and shrinkage. 

Instead of relating the strength to water/cement ratio, it is more appropriate to relate the strength to solid products of hydration of cement in the space available for formation of these products. Powers and Brownyard have established the relationship between the strength and gel/space ratio. This ratio is defined as the ratio of the volume of the hydrated cement paste to the sum of volume of the hydrated cement and of the capillary pores.

The compressive strength of concrete tested by Powers showed that the strength of concrete bears a specific relationship with the gel/space ratio. He found the relationship to be 240 x^3 where x is the gel/space ratio and 240 represents the strength of gel in MPa. The strength calculated by Powers’ experiment holds good for an ideal case. Figure 2 shows the relationship between strength and gel/space ratio. 

Figure 2 Relationship between compressive strength and gel/space ratio.

It is pointed out that the relationship between the strength and water/cement ratio will hold good primarily for 28 days strength for fully compacted concrete, whereas the relationship between the strength of concrete and gel/space ratio is independent of age.