Gravel
Loose rock fragments used as coarse aggregate in concrete and as base material beneath slabs
Gravel is loose, rounded or angular rock fragments used as coarse aggregate in concrete and as base material beneath slabs. In concrete, gravel (or crushed stone) provides structural strength and volume stability. As a sub-base, gravel creates a stable, well-draining layer that prevents settlement and moisture problems.
Why It Matters
Gravel serves two critical roles in concrete construction. As aggregate, it makes up 30-50% of concrete by volume—its quality directly affects concrete strength and durability. As sub-base material, a proper gravel layer beneath slabs prevents uneven settlement, provides drainage, and creates a capillary break against ground moisture.
Skipping the gravel sub-base under a slab is one of the most common shortcuts that causes long-term problems. Without it, soil moisture migrates directly into concrete, and uneven soil compaction causes differential settlement and cracking.
Technical Details
As concrete aggregate:
- Maximum size typically 3/4 inch for residential flatwork
- Must be clean, hard, and durable
- Well-graded (mix of sizes) for optimal packing density
- Rounded gravel produces more workable concrete
- Crushed stone produces stronger concrete due to angular interlock
As sub-base material:
- 4-6 inches of compacted gravel under residential slabs
- Use clean, crushed stone (3/4" minus or road base)
- Compact to 95% density in lifts
- Provides drainage and capillary break
- Level to within 1/2 inch of target grade
Common types:
- Pea gravel (3/8"): Decorative, drainage, small aggregate
- 3/4" crushed stone: Standard concrete aggregate and sub-base
- Road base (3/4" minus): Graded mix that compacts well for sub-base
- #57 stone (1"): Drainage applications, larger structural concrete
Related Terms
- Aggregate - General term for sand and gravel in concrete
- Subgrade - Soil layer beneath the gravel base
- Slab-on-Grade - Slab type requiring gravel sub-base
Learn More
- How to Pour Concrete - Sub-base preparation steps
- Concrete Basics - Understanding concrete materials
- Concrete Calculator - Plan your project