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Coarse Aggregate

Larger stones or gravel retained on the #4 sieve, providing bulk and strength to concrete

Coarse aggregate consists of larger stones or gravel retained on the #4 sieve (4.75mm opening), typically ranging from 3/8 inch to 1.5 inches. Coarse aggregate provides bulk, economy, and dimensional stability to concrete. It occupies 30-45% of concrete volume and significantly affects strength, shrinkage, and economy.

Why It Matters

Coarse aggregate costs less than cement, so maximizing coarse aggregate content within workability limits reduces cost. Larger aggregate reduces shrinkage—stones don't shrink, only paste shrinks. Strong, durable aggregate produces strong, durable concrete. Conversely, weak or reactive aggregate limits concrete quality regardless of cement quality.

For residential concrete, verify aggregate source produces durable, non-reactive stone meeting ASTM C33. Most established suppliers provide quality aggregate, but verify—especially if using unknown sources or on-site materials.

Technical Details

Size designations:

  • #57 stone: 3/4" to #4 (most common for structural concrete)
  • #67 stone: 3/4" to #4 (similar to #57)
  • #8 stone: 3/8" to #8 (thin sections)
  • Nominal maximum size typically 3/4" or 1" for structural

Maximum size considerations:

  • Should not exceed 1/3 slab thickness
  • Should not exceed 3/4 spacing between rebar
  • Larger aggregate more economical but less workable
  • Smaller aggregate for thin sections, congested reinforcement

Aggregate properties:

  • Strength: Should be 2-3× concrete strength target
  • Absorption: Less than 2-3% typical, affects water demand
  • Soundness: Resists freeze-thaw cycles
  • Reactivity: Non-reactive with cement alkalis
  • Shape: Rounded better for workability, crushed for strength

Aggregate quality testing:

  • Gradation (ASTM C136)
  • Soundness (ASTM C88)
  • Absorption (ASTM C127)
  • Freeze-thaw (ASTM C666)
  • Alkali reactivity (ASTM C1260, C1293)

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