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Aggregate Properties of Sand & Gravel

Aggregates are mixed with cement, lime or other mixtures to strengthen them. Concrete alone is not very strong, but when mixed with an aggregate, such as sand or gravel, it becomes immensely stronger. Sand and gravel have some properties that make them quite suitable for use as aggregates. Sand is used primarily around homes for low-weight-bearing purposes, such as sidewalks and patios. Gravel is used for high-weight-bearing applications, such as street pavement.

Tough and Sound
- Sand and gravel are tough and hold up to the rigors of the manufacturing process, stockpiling and mixing. They also hold up during use as they are compacted and stressed. Sand and gravel adequately transmit the weight of the loads they bear to the underlying layers and the ground. Sand and gravel also remain sound in all kinds of weather conditions. They tolerate rainfall and freeze-and-thaw cycles without breaking down.
Cleanliness
- It is strange to think of sand and gravel as clean, but in the absence of clay and fine particles they adhere well to cement or other mixing agents. Vegetation, dust and other matter are removed from sand and gravel during processing. If they are left in the mix, they quickly break down and weaken the structure.
Gradation
- The difference in sizes of the particles of sand or gravel (or other materials) used as aggregates is called gradation. The different particle sizes of sand and gravel actually strengthen the mixture. If all of the particles are uniform in size, it is difficult for the particles to shift into place and form a solid bond with one another. Different sizes of sand or gravel particles are able to shift and compact themselves, forming a neat and tight bond that is absent of air pockets. This makes the structure solid and durable.
Particle Size and Shape
- The size of the aggregate material is important to its function. If it is too small, the resulting materials may be unstable. If it is too large, it is hard to work with and may separate from the other particles in the mixture. Sand does not have to be resized in manufacturing, but gravel is processed and crushed until the size of the gravel pieces meets the standards specified for the application. There are different grades of sand and gravel appropriate for different purposes.
  
  The shape of the particles is also important. If the sand or gravel is smooth it does not adhere as well as particles that are jagged from crushing. It is like trying to glue round balls together versus squares. This is why play sand or landscaping gravel, which are smooth and round, do not make good aggregates. Mixtures with crushed aggregate materials are stronger, more permanent and more expensive than particles that are not crushed. The aggregate material is considered crushed if 100 percent of the particles have two or more crushed sides.

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