The Density of Concrete for a Garage Floor

Density Basics

• The density of concrete is measured by removing all water or air particles, leaving just the actual pure concrete. The greater the number of air or water pockets, the less dense the concrete, hence making it weaker. Given this method, the densest concrete is about 103 lbs. per cubic foot and is the highest structural grade. The process of "compaction" adds density to concrete because the particles are "squeezed" or "shaken" out of the concrete sample. The concrete that is left over is very dense. A fresh batch of cement might not be very dense, weighing maybe 60 lbs. per cubic foot, since it is filled with water and air. When that same batch is vibrated in a special bin, it will leave a denser but physically smaller amount of pure concrete. This implies that cement should be measured by mass, not volume. That specific grade of concrete weighing 103 lbs. per cubic foot would be an expensive and limited-use sort -- not the type that would serve a garage. The type of concrete density used to make a floor for a storage building or garage might be between 60 and 80 lbs. per cubic foot of concrete.

Light Structural

• Lightweight concrete, sometimes called light structural grade concrete, contains pumice to reduce density. This is in addition to the higher air and water content in this kind of structural concrete. In some cases, lightweight concrete will also contain foam that acts as an insulating layer. Other lightweight models might also contains chemicals that serve to waterproof the concrete. These would be the best for a garage, since it is sitting directly on the soil grade — usually on top of gravel — and is not as subject to the effect of shifting loads as the residence.

Aggregates

• Light structural concrete as well as “plain mass” concrete would be ideal for a garage foundation. The latter uses a stone aggregate rather than pumice. The stone aggregate is added to the concrete mold as filler. The rougher and larger the pieces of stone, the rougher the finished product. Since a garage, if it is worked in, might be absorbing many fluids and chemicals, a rough, no-slip surface might be ideal. Garage-grade concrete would use more sand and gravel than stronger concrete used for large buildings, dams or foundations.

Water Issues

• If the garage is not going to be heated, the water content of the cement can become an issue. The more water present, the less dense the concrete, and also the more prone it is to freezing and cracking. The result would be either a denser concrete that will not freeze or, as is more common, a form of cement called “pervious concrete” that uses no aggregates at all. It is a porous concrete that permits water to run through it rather than being absorbed into it. This might be perfect for heavily used garages that are not heated most of the time. Pervious concrete would not freeze and would not absorb the spilled fluids common to an automotive garage. This particular concrete is very light.