A sub-base is a layer of material between the paved surface and the bare ground below. The sub-base provides a foundation for a solid paved surface, such as asphalt or concrete. Workers lay sub-base material down and compact it with machinery. The compacted area should mimic the future pavement's surface, exhibiting a leveled layer that does not have more than a 10 mm difference of height along its entire installation.
Sand And Gravel
Sand and gravel typically comes from stone deposits near rivers and creeks. The moving water breaks the stones apart, creating small gravel and fine sand. According to Paving Expert, sand and gravel mixtures used for sub-bases are considered Type 2 unbound materials. The material is not held together by tar or cement, but relies on natural interlocking between granules during compaction for creating a sturdy pavement layer. However, the sand and gravel mixture has a uniform granular size throughout its volume, creating less adhesion between the granules. As a result, sand and gravel sub-bases are weak compared to a crushed concrete layer.
Concrete consists of cement paste and aggregates, such as crushed stone, gravel or sand. Crushed concrete, a Type 1 unbound material, provides granules that are varied in diameter from the crushing process. As a result, crushed concrete has a mixture of large and fine materials mixed together, generating a strong interlocking during sub-base compaction. The large granules create strength and stability, whereas the fine particles fill in any voids between the larger materials. The resulting sub-base is stronger than a sand and gravel configuration.
Sub-Base Installation Considerations
Some contractors will quote driveway installation pricing without a sub-base, generating an overall cheaper cost that may convince the homeowner to hire the company. However, paving a driveway over bare ground can develop problems in the future. The paved surface may exhibit premature cracking, or even pockets of sinking, without the supportive sub-base. Homeowners should also be aware that the sub-base does not require extremely firm compaction by the machinery. There should be some space within the sub-base substrate for movement of rainwater for adequate drainage from beneath the paved surface.