What Is Base When Doing Concrete Work?

Basics and Importance

• The base course or layer is the material located immediately underneath the concrete. This base layer gives uniform and adequate support to the concrete, provides additional load distribution, contributes to frost resistance and drainage and helps to prevent subgrade soil movement. Bases must consist of a material that is able to achieve a balance between drainage and stability.

Base Specifications and Placement

• The minimum depth of the base layer is often specified by a governing jurisdiction and depends on the structure of the soil or subgrade, the purpose and amount of traffic the poured concrete will serve and the type of base material used. In most situations, the minimum base depth is about 4 inches. The base should have a relatively constant grading prior to compaction to be able to provide uniform and stable support. The base material must also be thoroughly compacted -- certain heavy-traffic projects may even have a specified minimum density.

Possible Base Course Materials

• A wide variety of materials can be used with success as bases. For most concrete applications around the home, crushed aggregate such as gravel is a suitable and common base material. Similar granular materials include crushed concrete, bank-run gravels, sands and local materials. Bases made of solely these materials are also known as unstabilized bases. For certain sites that see heavy traffic, particularly during the construction period, stabilized bases may be most suitable, as this type of base provides a more erosion-resistant base surface, prevents base consolidation and creates a stable working platform. Stabilized bases are created using hydraulic cement or asphalt.

Subbase and Subgrade

• In some situations where soil is stable and drains well, a base layer may not be necessary. The subgrade is underlying soil either native to the site or imported to improve the soil. If the site soil has a high sand or gravel content, there may be no need to import base material. In areas with poor soil, subgrade materials may need to be added to the site. Preparation for the concrete pour may also include the addition of a subbase. This course of material is located between the base and the subgrade and typically consists of lower-quality materials than the base. This layer serves to provide a working platform for construction, improve drainage and limit frost damage, as well as minimize the intrusion of fine materials from the subgrade into the base and pavement.