Concrete is a composite material made up of a combination of geological materials such as gravel, sand and crushed rock. The size of these materials, known as aggregates, determines the strength of the concrete. Coarse-grained aggregates, such as gravel, are used for strong concrete, while fine aggregates, such as sand, are used for normal strength concrete. The size of the coarse-grained aggregates also affects the size of the fine aggregates used to fill the voids.
The quality of the aggregate is an essential factor in determining the performance of concrete. Changes in size, weight, moisture content and gradation can alter the characteristics of concrete. The permitted percentage of harmful substances for fine and coarse-grained aggregates are listed in ASTM C 33. The more packed the particles of the aggregate are, the greater their strength and, therefore, the greater the strength of the concrete. The absorption and surface moisture of aggregates are also important factors in producing concrete that meets desired strength requirements.
Alternative sources of aggregates or additional aggregate mixtures can be considered to approach an ideal gradation that provides workability, pumpability, shrinkage reduction and economy. Good quality aggregate must be clean, hard, strong and free from absorbed harmful chemicals or other contaminants that may affect cement hydration or reduce paste-aggregate bonding. The size distribution of fine to coarse-grained aggregates plays an important role in the workability and performance of concrete. The shape of your aggregates is also important to consider when determining how much you want to place and what strength you want to achieve.
As a natural material, the aggregate may include worn or unstable particles in the supplied product.