Construction Aggregate: Sand, Gravel, and Crushed Stone Explained

What Is Construction Aggregate?

Aggregate is the filler material in concrete. While cement gets most of the attention, aggregate is the real workhorse—it provides bulk, strength, and dimensional stability. Without aggregate, you'd just have cement paste, which would crack and shrink as it dried.

Think of aggregate like the rocks in a stone wall. The cement paste is the mortar that holds everything together, but the rocks do the heavy lifting.

Why Aggregate Matters

• Volume: Aggregate makes up 60-75% of concrete
• Cost: It's cheaper than cement, so more aggregate = lower cost
• Strength: Quality aggregate creates stronger concrete
• Shrinkage: Aggregate resists shrinkage as concrete cures
• Durability: Hard aggregate means durable concrete

Fine Aggregate vs. Coarse Aggregate

Aggregate is classified by size into two categories:

Fine Aggregate (Sand)

Fine aggregate passes through a 3/8-inch (9.5mm) sieve. In practice, this means sand and very small particles. Fine aggregate:

• Fills the voids between coarse aggregate particles
• Improves workability (makes concrete easier to place)
• Creates a smoother finished surface
• Typically makes up 35-45% of total aggregate volume

Coarse Aggregate (Gravel or Crushed Stone)

Coarse aggregate is retained on a 3/8-inch sieve. This includes gravel, crushed stone, and larger particles. Coarse aggregate:

• Provides the skeleton structure of concrete
• Contributes most of the compressive strength
• Reduces the amount of cement paste needed
• Typically makes up 55-65% of total aggregate volume

Types of Aggregate

Natural Sand

Natural sand comes from riverbeds, beaches, and sand pits. It's formed by natural weathering and erosion over thousands of years. The particles are rounded and smooth, which makes concrete more workable but slightly weaker than mixes using angular particles.

Best for: General construction, residential driveways, patios

Manufactured Sand (M-Sand)

When natural sand is scarce, manufactured sand fills the gap. It's made by crushing larger rocks in a controlled process. The particles are angular and rough, which creates stronger bonds with cement paste but makes the mix harder to work with.

Best for: High-strength applications, areas with limited natural sand

Gravel

Gravel is naturally occurring coarse aggregate, typically rounded from water erosion. Like natural sand, the smooth surfaces make workable concrete but create slightly weaker bonds than angular materials.

Best for: Residential concrete, decorative exposed aggregate finishes

Crushed Stone

Crushed stone is made by mechanically breaking larger rocks. The angular, rough surfaces create excellent mechanical bonds with cement paste. Common types include:

• Crushed limestone: Most common, good all-around choice
• Crushed granite: Harder, more durable, better for heavy-duty applications
• Crushed basalt: Very hard, excellent for high-strength concrete
• Recycled concrete: Crushed old concrete, increasingly common

Best for: Structural concrete, heavy traffic areas, commercial projects

Aggregate Gradation: Why Size Distribution Matters

Gradation refers to the distribution of particle sizes in your aggregate. Well-graded aggregate has a good mix of sizes—large, medium, and small particles that fit together efficiently.

Well-Graded vs. Gap-Graded

Gradation Type	Description	Result
Well-graded	Good distribution of all sizes	Dense, strong concrete with minimal voids
Gap-graded	Missing certain size ranges	More voids, requires more cement paste
Uniform	All particles similar size	Very porous, used for pervious concrete

Think of it like packing a jar with marbles. If you only have large marbles, there are big gaps. Add medium marbles and they fill some gaps. Add sand and it fills the remaining spaces. That's well-graded aggregate.

Maximum Aggregate Size

The maximum aggregate size affects concrete strength, workability, and where you can use it.

Common Maximum Sizes

• 3/8 inch (10mm): Fine work, thin sections, heavily reinforced areas
• 3/4 inch (19mm): Most residential work—driveways, patios, sidewalks
• 1 inch (25mm): General construction, foundations
• 1.5 inch (38mm): Mass concrete, large footings
• 3 inch (75mm): Dam construction, massive pours

The Rule of Thumb

Maximum aggregate size should not exceed:

• 1/5 of the narrowest form dimension
• 1/3 of slab thickness
• 3/4 of the clear spacing between rebar

For a typical 4-inch residential driveway, that means maximum aggregate size of about 1.3 inches—so 3/4-inch or 1-inch aggregate is standard.

Aggregate Quality Requirements

Not all rocks make good aggregate. Quality aggregate must meet several criteria:

Hardness

Aggregate must resist abrasion and crushing. Soft aggregate creates weak concrete that wears quickly under traffic.

Durability

Aggregate must survive freeze-thaw cycles without breaking apart. Porous aggregate that absorbs water can crack when that water freezes.

Cleanliness

Dirt, clay, silt, and organic matter weaken the bond between aggregate and cement paste. Specifications typically limit these contaminants to 1-3% by weight.

Shape

Flat, elongated particles create weak points in concrete. Ideal aggregate is roughly cubical or spherical. Most specs limit flat/elongated particles to 15% of the mix.

Chemical Stability

Some aggregates react with cement paste over time, causing expansion and cracking (alkali-silica reaction or ASR). Testing identifies reactive aggregates before they cause problems.

Where Does Aggregate Come From?

Aggregate is sourced from three main places:

Natural Deposits

Gravel pits and sand quarries extract aggregate that's already the right size from natural weathering. These operations screen and wash the material to meet specifications.

Crushed Rock Quarries

Hard rock quarries blast and crush solid rock into aggregate. This gives precise control over size and gradation but requires more processing.

Recycled Materials

Crushed concrete from demolition can be recycled as aggregate. It's increasingly common as sustainability becomes important and landfill space gets scarce. Recycled aggregate typically works well for sub-base and non-structural applications.

Aggregate for Different Applications

Residential Driveways

Standard mix: 3/4-inch crushed stone or gravel with natural sand. This creates workable concrete that finishes well and handles normal vehicle traffic.

Exposed Aggregate

Decorative mixes use attractive coarse aggregate—river rock, colored stone, or specially selected gravel. The surface is washed or brushed to expose the aggregate after placing.

High-Strength Concrete

Strong aggregate matters more as concrete strength increases. Crushed granite or basalt, well-graded, with minimal flat particles creates the strongest mixes.

Lightweight Concrete

Special lightweight aggregates—expanded shale, clay, or slag—create concrete that weighs 90-115 lbs/cubic foot instead of the normal 145 lbs. Used for upper floors in buildings to reduce dead load.

How to Specify Aggregate

When ordering ready-mix concrete, you typically don't specify aggregate directly—you specify the concrete strength and intended use, and the producer selects appropriate aggregate. But understanding aggregate helps you:

• Ask informed questions about the mix design
• Understand why certain mixes cost more
• Spot potential problems (dirty or poorly graded aggregate)
• Choose appropriate finishes for your aggregate type

Common Aggregate Problems

Alkali-Silica Reaction (ASR)

Some silica-containing aggregates react with alkalis in cement, forming a gel that absorbs water and expands. This causes map cracking and popouts. Solution: test aggregate and use low-alkali cement if needed.

Freeze-Thaw Damage

Porous aggregate absorbs water. When that water freezes, it expands and can shatter the aggregate particle, creating popouts. Solution: use low-absorption aggregate in freeze-thaw climates.

Poor Gradation

Gap-graded or uniform aggregate creates concrete that's hard to place, segregates easily, and has more voids. Solution: specify well-graded aggregate meeting ASTM standards.

Frequently Asked Questions

What's the difference between gravel and crushed stone?

Gravel is naturally rounded from water erosion. Crushed stone is mechanically broken from larger rocks and has angular, rough surfaces. Crushed stone generally creates stronger concrete due to better mechanical bonding.

Can I use beach sand in concrete?

Generally no. Beach sand contains salt that can corrode reinforcing steel and cause efflorescence (white staining). It also tends to be too fine and uniform for good concrete.

What size aggregate is best for driveways?

3/4-inch maximum aggregate is standard for residential driveways. This balances strength, workability, and finish quality for typical 4-inch thick slabs.

Does aggregate affect concrete color?

Yes, significantly. Light-colored aggregate creates lighter concrete. If aggregate is exposed (exposed aggregate finish), the aggregate color dominates. For consistent color, specify consistent aggregate sources.

Why is my concrete rough with exposed rocks?

This usually means the aggregate wasn't fully embedded during finishing, or the concrete was finished too early while bleed water was still rising. It can also indicate too much coarse aggregate in the mix.

The Bottom Line

Aggregate isn't glamorous, but it's the majority of your concrete by volume. Quality, well-graded aggregate creates strong, durable concrete. Poor aggregate creates problems that no amount of cement can fix.

For residential projects, you generally don't need to obsess over aggregate specifications—your ready-mix producer handles that. But understanding the basics helps you have informed conversations and recognize when something looks wrong.

The key points to remember:

• Aggregate is 60-75% of concrete by volume
• Fine aggregate (sand) fills voids; coarse aggregate provides structure
• Well-graded aggregate with a mix of sizes creates the strongest concrete
• Angular crushed stone bonds better than rounded gravel
• Quality matters—clean, hard, durable aggregate is essential