Concrete Mixing Methods

Is It Concrete... Or Cement?

It has been said that concrete is used more than any other man-made material in the world. Specifically, about 7.5 cubic kilometers of concrete are mixed each year, which is more than one cubic meter for every person on the face of the earth.

The concrete industry exceeds $35 billion annually and employs over 2,000,000 workers in the U.S. alone. What's more, over 55,000 miles of U.S. highways are paved with concrete mix.

Most people confuse concrete with cement. For example, what do you call those big cylindrical machines that workmen use when patching driveways or building sidewalks? Are they concrete mixers or cement mixers? And, what about that huge vehicle with the monstrous rotating drum that you see at a construction site? Is it a concrete truck… or a cement truck? Does one pour cement… or pour concrete?

In fact, what is the difference between cement and concrete?

Specifically, concrete is the result of combining water, cement, sand and coarse aggregate. The coarse aggregate can be composed of gravel, crushed stone or iron blast furnace slag. Cement is simply a component of concrete. It is a dry powder substance used to bind the sand to a coarse aggregate such as barite, magnetite, limonite, or iron titanate.

Other material that is cementitious in nature is sometimes added to make the final mixture more economical, as well as reduce its permeability, or perhaps increase its strength. Also, liquid chemicals can be added to the mixture to reduce the water required to pass a slump test and make the concrete flow more freely.

The density and strength of concrete are partially determined by the proportion of the ingredients, which are measured by volume or weight. It isn’t difficult to learn how to mix concrete by hand. An old rule of thumb is to use one part cement, two parts sand and three parts coarse aggregate together. Then, slowly add water until the concrete is a workable consistency.

A traditional cement mixer consists of a rotating drum mounted onto a motor driven spindle. Blades fixed to the inside of the drum help to quickly and efficiently mix the cement. These are set at the optimum angle for homogenous mixing and they aerate the mixture correctly.

Another consideration with regards to the final product relates to the horsepower of the mixer, which is predominantly influenced by whether the machine is electric, diesel or gas driven. 

Some modification may be required based on the actual coarse aggregate used for the project. The mixture must not be too stiff or too sloppy. It is difficult to form good test specimens when the concrete mix is too stiff. And, if it is too sloppy, water may separate or bleed from the mixture.

Most concrete manufactured in the United States is produced at batching plants that carefully measure the mixture and deliver it to job sites. At most of these plants, sand, aggregate, cement and water are all gravity fed from the weight hopper into the mixer trucks. The concrete is mixed on the way to the site where it is to be poured.

Sometimes, the concrete is created in a central mix drum and transferred to a transport truck. Much of the remaining concrete is manufactured into products cast in a factory. These precast products range from concrete bricks and paving stones to bridge girders, structural components and panels for cladding.

Mortar is like a softer concrete. You mix sand and cement together without any coarse aggregate. The final product is typically used to essentially glue together such things as bricks and rocks in a wall for instance.

In summary, cement is the critical element in the mixing of concrete and mortar. The performance of either end product is determined by the microstructure of composition inherent to the extent sand and water have been measured and the length of time and power by which they have been combined.