Concrete Base Preparation: Gravel and Subgrade Guide

Proper subgrade and base preparation for concrete slabs. Compaction, drainage, and gravel layers.

The majority of concrete failures are caused by inadequate base preparation. A properly prepared subgrade and base layer is essential for long-lasting concrete.

Subgrade Preparation

Remove all vegetation: Grass, roots, organic matter
Excavate to proper depth: Slab thickness + base thickness
Remove unstable soil: Clay, silt, organic material replaced with gravel
Compact native soil: Plate compactor or hand tamper
Achieve uniform compaction: 95-98% Proctor density
Create slight slope: 1-2% for drainage

Base Material Options

Crushed stone (graded aggregate base): Best option, locks together
Gravel (round): Good drainage but less stable
Sand: Not recommended as base alone, lacks stability
Recycled concrete: Environmentally friendly, good drainage
Bank run gravel: Mixed sizes, decent option when crushed not available

Base Thickness by Application

Sidewalks/Patios: 4-6 inches compacted base
Driveways (passenger): 6-8 inches compacted base
Driveways (heavy/RV): 8-12 inches compacted base
Garage slabs: 6-8 inches compacted base
Shed slabs (light): 4-6 inches compacted base

Compaction Process

Spread base material in 4-6 inch lifts (layers)
Compact each lift separately: Multiple passes with plate compactor
Moisture conditioning: Slightly moist soil compacts best
Test compaction: Proctor test for critical applications
Final grade: Level to proper elevation with slope

Drainage Considerations

Slope surface away from structures: 1-2% minimum
Crown wider areas: Center higher than edges
Install drain tile: For poor drainage areas or below-grade slabs
Perimeter drains: For areas with high water table
Vapor barrier: 6-10 mil poly under slab for conditioned spaces

Pro Tips

Plate compactor rental worth the cost - hand tamping insufficient
Moist but not muddy soil compacts best - add water if too dry
Test compaction: Walk on prepared base - minimal footprints desired
Geogrid fabric: Adds stability to poor soil conditions
Save money on concrete, spend on base - cheaper in long run

Important Warnings

Never pour on frozen ground - guarantees cracking
Clay soil expands/contracts significantly - may require removal or geogrid
Inadequate compaction causes settling and uneven slabs
Skipping base to save money costs 3-5x more in repairs
Poor drainage under slab causes freeze-thaw damage in cold climates