• Image

    Credit: Joe Nasvik

    The ultimate performance of a concrete slab depends on the type of soil materials underneath, soil density, moisture content, and the flatness of the surface the concrete rests on. There are many types of compactors, each working better on some soil types than others.
Methods used for preparing the ground for concrete placement depend on the job. The types of projects include foundations, industrial and commercial floors, roads and highways, and other exterior slab-on-grade hardscapes. The construction sequence starts with the removal of surface vegetation and topsoil, excavation of high areas of grade, surface preparation and fill placement, followed by concreting. Each successfully completed step makes it possible to continue to the next phase. The reverse is also true. For example, when soils are excavated from one location and are used for fill in other areas without proper compaction, the resulting settlement over time destroys even the best concrete work. This is also the case when foundations and utility trenches are backfilled without compaction and concrete elements placed on top.

Proper soil preparation is very important to the performance of a project. This includes the removal of weak materials from the surface, sub-grade evaluation, the selection of suitable materials for use as fill, proper compaction, control of moisture, and providing flat uniform surfaces for concrete placement. Normally, grading contractors bring the site to rough grade through excavation, fill placement, and compaction while concrete contractors cover fine grading and compaction of the upper few inches in their contracts. Both parties should be included in preconstruction meetings to agree on specifications and details.

  • Image

    Credit: Multiquip

    A gas-powered rammer can compact both cohesive and mixed soil. Rammers are designed for use in trenches, around retaining walls, and when solidifying bases for concrete slabs, roads, and bridge columns.

Soil basics

The ground under concrete must have the proper engineering characteristics to support anticipated loads without experiencing bearing capacity failure or excessive settlement. For instance, a concrete foundation must support the loads associated with columns or bearing walls without shifting or settling more than can be tolerated by the building's frame. The ground under an industrial floor slab must support the weight of forklifts and materials stored on the slab. Structural engineers specify the load that the ground must support for every application. Then geotechnical engineers sample the soil on the site to determine if it can meet the load support requirements. If it can't, there are several options available to improve support capability. Soil can be removed and replaced with suitable materials, aggregates can be mixed with the soil to improve strength and compressibility, or layers of more suitable materials can be placed above weaker soils to better distribute applied loads. In general the denser a soil is, the more load it can carry. Achieving the proper level of density is accomplished through the control of moisture within the soil and proper compaction.