Concrete aggregate is a material which is mixed with cement to create concrete which is hard, strong, and long-lasting. There are a number of different types of concrete aggregate, with contractors selecting their aggregate on the basis of the type of job being performed. If you live in an area with concrete structures, you may be able to see some examples of aggregate in action.
Using aggregate makes concrete much stronger, with the aggregate acting as a type of reinforcement. The aggregate increases the lifetime of the concrete, and makes it more durable. Contractors can choose from fine aggregates, made from small particles of material, and coarse aggregates, which are made with large chunks. Companies which produce concrete aggregate usually grade their products by size, allowing contractors to order from a spectrum of sizes.
In order for aggregate to be effective, it must be strong. Weak aggregate materials will weaken the resulting concrete, which is not desirable. Aggregate must also be hard, so that it keeps its shape without deforming in the concrete. Finally, it must be clean, which in the construction sense means that it is free of chemicals, clays, and various leached materials which could interact with the concrete and interfere with the way it sets.
The texture of the aggregate is also a consideration. Coarse, porous materials will suck up water during the mixing process, requiring the addition of more water to compensate. This in turn throws off the water to concrete ratio, requiring the contractor to add more concrete to avoid weakening the mixture. Smoother aggregates do not require additional water, and they are less likely to cause cracks when the concrete sets.
Sand is a common example of a fine aggregate. Rocks and crushed glass can be used as large aggregates. Natural aggregate materials are quarried and crushed down to size. Some companies use recycled materials, including metal, with the goal of keeping usable materials out of landfills, although the concrete aggregate may be slightly more expensive because it needs to be thoroughly cleaned before use.
Some contractors like to use exposed concrete aggregate in their projects, making the aggregate visible with a variety of construction techniques. Concrete with exposed aggregate is designed to be pleasant to look at, and it may be made with aggregate which is decorative as well as functional. This technique is often used for walkways and concrete furniture to add more visual interest to what would otherwise be a bland gray expanse.