A typical concrete mix contains 60 to 80 percent sand and gravel, also known as an "aggregate." This aggregate compound is more than filler. It plays an important part in the concrete's composition. The amount of sand and gravel in a bag of concrete determines the mixture's strength and texture. In fact, when you remove sand and gravel from the concrete mixture, it becomes a completely different product.
Cement, Concrete or Mortar?
Cement consists of limestone, sand, silica sand, shale and clay. After mixing these items together, they're heated in the oven until they break down. In this state, they're ready to use as a binding agent.
Though water makes cement easier to pour and helps it to harden, cement and water by themselves don't hold together very well. In fact, cement isn't typically used without sand and gravel. The addition of sand makes cement more binding. Cement mixed with water and sand becomes mortar, the paste used to hold bricks together. Once you add gravel to the mix, it becomes concrete.
The Importance of Gravel and Sand in Concrete
Sand and gravel in concrete serve several purposes. Because they act as a filler, they also add more volume to the concrete. More volume means less air and a stronger product. The size of the gravel also helps to determine the concrete's strength. Though larger pieces of gravel produce more friction and make it harder to mix, they also make a stronger concrete.
Water also plays a part in how much sand and gravel to use. The more water you add, the weaker the mixture becomes. Adding aggregate to the mix reduces the amount of water. With less water, the concrete is stronger and less likely to shrink and crack.
Fine Aggregate vs. Coarse Aggregate
Smaller gravel particles equals a "fine" grade of concrete. Use the fine aggregate for concrete slabs or other smooth surfaces. A "coarse" aggregate means using larger pieces of gravel. Coarse concrete is helpful for sturdier projects. Concrete with a coarse aggregate doesn't produce a smooth look or feel but is stronger than a finer grade.
Most home improvement stores sell premixed concrete. As the mixture already contains sand and gravel, you only need to add water. The smaller bags of premixed concrete are perfect for do-it-yourself and small home projects. Premixed concrete is often used for fortifying fence posts or railings. A professional builder can help you with mixing and pouring concrete for foundations and other large projects.