Subgrade Concrete Preparation

Subgrade Properties

• The performance of the subgrade depends on its load-bearing capacity, moisture content and shrink-swell ratio. Soil movement from moisture in the soil and thermal expansion and contraction cause concrete to crack, lift or settle. The subgrade must be well-draining so water can't collect underneath the concrete structure and it needs to be compacted so the material remains stable under a load. Poor soils can be excavated or in some cases stabilization may be recommended.

Subgrade Materials

• The subgrade material should be aggregate composed of a mix of silt and gravel. Moisture is needed in the material for compaction, usually in the 10 to 20 percent range. Too much moisture in the material will cause it to deform under a load. Organic material in the subgrade should be excavated and replaced with high quality fill. Extremely poor soils can be stabilized by adding a cementitious or asphaltic material as a binder, such as portland cement, lime or emulsified asphalt.

Drainage

• Grade to ensure positive drainage -- away from structures toward an approved discharge point, such as a municipal storm drain. For outdoor concrete structures, such as driveways, the minimum slope is 2 percent, or 20 feet rise for every 100 feet in length. Drain to swales -- shallow depressions -- sloped to a safe discharge point, such as a municipal drain or grassed open area. If the surface ground is too flat to drain, subsurface drainage may be needed.

Subgrade Compaction

• Compaction pushes air and moisture from the subgrade material, increasing its density and therefore load-bearing capacity. Higher density makes the subgrade stronger and keeps water from flowing in and out, reducing freeze-thaw action in the material. Proctor density is the expression for its measurement; typically the required density is 95 percent maximum modified Proctor density. The density measured by soil engineers or field hand tests can also be used for more general reference.