Subgrade Preparation Essentials for Concrete Driveways

Uniformity, in both soil composition and compaction, is the key to a good subgrade—one that will provide adequate support, ensure an even slab thickness, and prevent slab settlement and structural cracking. Soft spots should be removed and replaced with good material, such as gravel or crushed rock. Many western states have expansive soils. In these conditions, 2 to 8 inches of crushed rock should be used as subgrade material, depending on the degree of expansiveness. If you are unsure about the soil characteristics in your area, consult a soils engineer.

Don't allow the concrete to be placed on bone-dry subgrade, advises the Tennessee Concrete Association. Spraying the subgrade first to dampen it will prevent it from wicking water from the fresh concrete.

Vibratory plate compactors and rammers are the most common machines used for subgrade compaction of residential driveways. Click here to read more about subgrades and subbases for concrete slabs.

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