Wet and Dry Concrete

Why is it that we multiply wet volume of concrete by ~1.54 to get dry volume of concrete and wet mortar by ~1.33 to get dry mortar volume? Read on to understand the concept.

Concrete is made up of at-least 4 constituents; namely water, cement, fine aggregate and course aggregate. Sometimes more than 1 gradation of fine and course aggregate is used to attain the desired strengths. All of these constituents are batched according to ratio and mixed as per standard procedure.

But, though not plainly visible, air is entrapped in this mixture, which if not removed before setting, will give us a non homogeneous mixture which will have a high probability of leaking and non uniform strength to the mass.

Elimination of this entrapped air is done by the process called compaction. Different manual and mechanical methods can be adopted as per the requirements to compact concrete.

Also compaction rearranges the particles present in the concrete, which are initially loosely packed. Compaction helps in tight packing and interlocking of particles which gives concrete its strength.

Due to the above 2 effects of compaction, the volume of the concrete is reduced and this reduction in volume is based on the size of the constituent material particles.

For,

Course aggregate around 33% to 34%

Fine aggregate around 20%

Cement particles being extremely fine does not compact much, and so volume reduction is negligible.

Concrete:-

Made up of course aggregate, fine aggregate and cement; volume change is 34% + 20% +0%.

0.34 + 0.20 +0 = 0.54.

i.e wet volume + 0.54 wet volume = dry volume.

dry volume = 1.54 wet volume (for concrete)

I have created a table for the benefit of my reader. (Note: This table is not exact, but can be used for quantity estimation.)

Dry volume concrete calculations from wet volume.

Mortar:-

Made up of fine aggregate and cement; volume change is 20% +0%.

0.20 +0 = 0.20

But engineers add extra 10% to 13% margin for improper mixing and poor workmanship in case of mortar.

So, the wet volume + 0.33 wet volume = dry volume.

dry volume = 1.33 wet volume (for mortar)

Approximate calculations for quantity calculation of mortar.

Wall plaster ratio is 1:6

Volume of fine aggregate (Va) = 30.4 cft x ________ (Vd) = ________ cft

Volume of cement (Vc) = 4.12 bags x ________ (Vd) = ________ bags

Ceiling plaster ratio is 1:4

Volume of fine aggregate (Va) = 28.25 cft x ________ (Vd) = ________ cft

Volume of cement (Vc) = 5.764 bags x ________ (Vd) = ________ bags