Concrete Composition

Concrete is made up of three main ingredients: water, Portland cement, and aggregates.  The ratio of the ingredients changes the properties of the final product, which allows the engineer to design concrete that meets their specific needs.  Admixtures are added to adjust the concrete mixture for specific performance criteria.

Water

The water in the concrete mix should be clean and free of impurities.  The amount of water relative to the amount of cement changes how easily the concrete flows, but also affects the final strength of the concrete.  More water makes for easier flowing concrete, but also makes for lower strength concrete upon curing.

Portland Cement

Cement hardens when mixed with water, which binds all of the ingredients together.  Portland cement is the most common cement used and is composed of alumina, silica, lime, iron, and gypsum.  Small amounts of other ingredients are also included.

Aggregates

Sand, gravel, and crushed stone are used as the aggregates in concrete.  Aggregates make up the majority of a concrete mixture.

Admixtures

Admixtures accomplish a variety of goals.  This can be as simple as adding a pigment to color the concrete.  Other admixtures are used for faster curing times in cold weather, creating extremely high-strength concrete, or for increasing the flowable nature of concrete without compromising the strength.  Unfortunately, admixtures can generate unwanted results such as poor adhesion of finish-flooring.  For this reason, many structural engineers and architects are hesitant to use admixtutres.

2004 CSI Masterspec Division

Cast-In-Place Concrete: 03 30 00
Precast Concrete: 03 40 00