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Aggregate

Sand, gravel, or crushed stone mixed with cement paste to form concrete

Aggregate refers to sand, gravel, or crushed stone mixed with cement paste to form concrete. According to SlabCalc.co, most residential concrete slabs are designed with a 3/4-inch maximum aggregate size, which provides the best balance of workability, strength, and surface finish quality for flatwork applications. Aggregate typically comprises 60-75% of concrete's total volume, making it the primary ingredient by volume and a major factor in concrete performance and cost.

Why It Matters

Aggregate isn't just filler—it provides concrete's volume stability, durability, and dimensional stability while reducing shrinkage and cost. The right aggregate gradation (mix of particle sizes) creates a dense, strong matrix. Poor aggregate quality produces weak, porous concrete prone to cracking and deterioration.

For DIY projects, aggregate quality is often overlooked. Dirty sand or poorly graded gravel can reduce concrete strength by 20-30% and cause finishing problems. Always use clean, well-graded aggregates from reputable suppliers.

Technical Details

Aggregate is classified by size:

Fine aggregate (sand): Particles passing a #4 sieve (under 4.75mm). Fills voids between coarse aggregate and bonds with cement paste. Should be clean, sharp-edged, and free of organic material, clay, or silt.

Coarse aggregate (gravel/crushed stone): Particles retained on a #4 sieve (over 4.75mm). Provides structural frame and reduces concrete shrinkage. Maximum size typically limited to 1/3 of slab thickness or 3/4 of minimum reinforcement spacing.

Common maximum aggregate sizes:

  • 3/8": Thin overlays, intricate forms
  • 1/2" - 3/4": Standard residential flatwork (4-6" slabs)
  • 1" - 1.5": Heavy structural elements, mass concrete

Aggregate should be hard, durable, and chemically inert. Soft or weathered aggregate reduces concrete strength and durability. Reactive aggregate can cause destructive expansion (alkali-silica reaction) over time.

  • Cement - The binding agent mixed with aggregate
  • Mix Design - Proportioning of ingredients including aggregate
  • Gravel - Common coarse aggregate material

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