Aggregate

Granular materials such as crushed stone, gravel, or sand that are mixed with cement and water to make concrete or used as a sub-base beneath foundations and floors.

Aggregate is the collective term for the granular materials — crushed stone, gravel, and sand — that are mixed with cement and water to make concrete. Aggregate makes up roughly 60-75% of the volume of concrete, so it is by far the largest ingredient. It is also used on its own as a sub-base layer beneath foundations, driveways, and floor slabs.

Types of aggregate

  • Coarse aggregate — crushed stone or gravel, typically 10-20 mm in size, forming the bulk of the concrete mix
  • Fine aggregate — sand or crushed rock dust, filling the gaps between coarse particles to create a dense, strong mix
  • All-in aggregate (ballast) — a pre-blended mix of coarse and fine aggregate, commonly used for general-purpose concrete on residential projects
  • Recycled aggregate — crushed and graded demolition material (concrete, brick), increasingly used as a sustainable alternative for sub-bases

Where aggregate appears in a renovation

  • Concrete foundations — every extension, garage, or garden wall starts with a concrete foundation, which is cement plus aggregate plus water, reinforced with rebar and poured into formwork
  • Concrete floor slabs — the structural base for ground-floor rooms
  • Sub-base layers — a compacted layer of aggregate sits beneath foundations, floor slabs, driveways, and patios to provide a stable, free-draining base
  • Screed — the fine cement layer on top of a concrete slab uses fine aggregate (sand) mixed with cement

Concrete mix ratios

For homeowners, it helps to understand that different jobs need different concrete strengths, achieved by varying the ratio of cement to aggregate. Your contractor will specify the correct mix:

  • General purpose (e.g., C20) — garden paths, base layers
  • Foundation mix (e.g., C25-C30) — house foundations, retaining walls
  • Structural mix (e.g., C35+) — heavily loaded foundations, structural elements

Practical tip

If your renovation involves significant groundwork — foundations for an extension, a new driveway, or underpinning — aggregate deliveries can be large and heavy. Discuss access and storage with your contractor early. A lorry delivering aggregate needs firm, level ground, and the material takes up space on site until it is used.