# Packing factor

## What is Packing factor?

Definition of Packing factor in Construction

The packing factor of a material, is its ability to organize the particles within itself, to minimize the amount of air or voids.

In construction, this concept is extremely important when trying to understand the abilities of certain materials to compact and stabilize. For example, large stones or boulders, when used in a fill situation, will organize themselves as they are dropped into the hole, basically one time, as they are initially dropped. If these boulders are organized by a machine that is being controlled with a human operator, the boulders are placed to minimize the amount of voids between each piece. However the larger the boulders, the more spaces or voids will be present. The introduction of smaller pieces of stone, gravel and then sand allows the voids to be filled up with these smaller materials, therefore reducing the amount of air spaces in the entire mix. A properly graded construction material has a combination of larger material, together with smaller material to fill the voids.

The use of compaction equipment, which causes the materials to shift and seek their optimum packing factor creates a compacted system, with a minimum of voids. The packing factor within this material has been maximized. A very dense material basically means that all the voids within the matrix of the material have been filled, and the maximum packing factor has been reached. The term packing factor, if understood, relates to many aspects of the construction industry. This is a handy concept to understand, allowing a practical knowledge of how and why construction materials are specified for different uses. Another easy method of understanding this concept, is the popular story of a professor raising an empty glass jar and filling the glass jar with the marbles. The professor asks the class, is this class full? The class answers yes it is full of marbles. Then the professor dumps another, same size, glass jar of sand into the jar of marbles and again asks is the cup full? Again the glass answers yes. Then the professor takes a glass jar of flour of the same size and dumps that into the cup of marbles, sand and now flour. Again, is the glass full, with the answer being yes. The professor then asks the class if the cup is now as full as it could possibly get, and the class answers yes. The professor takes a glass of water and dumps that into the glass of marbles, sand, flour and now water. The moral of the story, is that there are voids in all mixtures of solids, and water being a fluid and not a solid, will find those remaining voids and fill the gaps. This story relates to many site-work processes such as stone fill, porous gravel, compaction, etc. and it all relates to the packing factor of materials.