WATER CEMENT RATIO

Definition

•      Water cement ratio may be defined as the ratio weight of water to that of cement in a concrete mix

•      It is generally expressed as liter per bag of cement

Water cement ratio law

•      Established by Duff Abrahms in 1918

•      Abrams water/cement ratio law states that the strength of concrete is only dependent upon water/cement ratio provided the mix is workable.

Strength and w/c ratio relation by abrahm

•      S = A/B^x

•      where x =water/cement ratio by volume and for 28 days results the constants A and B are 14,000 lbs/sq. in. and 7 respectively.

Strength and water cement ratio

•      Strength is inversely proportional to water cement ratio

•      i.e. strength decreases as water cement ratio increases.

•      It means strength increases with increase in cement and decreases with increase in water content

•      If w/c ratio is less than 0.45, concrete is not workable and causes honey-combed structure containing large number of voids.

Fig.1 Curve showing decrease in strength as w/c ratio increases

Modified relation between strength and w/c ratio

•      It was Feret who formulated in as early as 1897, a general rule defining

•      the strength of the concrete paste and concrete in terms of volume fractions of the constituents by the equation:

•      S=K*(c/(c+e+a))²

•      where S = Strength of concrete

•      c = volume of cement

•      e = volume of water

•      a = volume of air

•      and K = a constant.

Explanation of feret equation

•      In this expression the volume of air is also included because it is not only the water/ cement ratio but also the degree of compaction, which indirectly means the volume of air filled voids in the concrete is taken into account in estimating the strength of concrete.

Fig.2 Effect of compaction included

 

Explanation of curve (fig.2)

•      It can be seen that lower water/cement ratio could be used when the concrete is vibrated to achieve higher strength

•      Whereas comparatively higher water/cement ratio is required when concrete is hand-compacted.

•      In both cases when the water/cement ratio is below the practical limit the strength of the concrete falls rapidly due to introduction of air voids.