What Is Aggregate and Why Does Gradation Matter?
The particle-size mix of an aggregate shapes how it performs, not just its size
What aggregate is
Aggregate is the general term for granular material — sand, gravel, crushed stone — used as a component in concrete, asphalt, and as a standalone base or fill material. It's typically split into fine aggregate (sand-sized) and coarse aggregate (gravel/crushed-stone-sized).
In US practice the dividing line is the No. 4 sieve (4.75 mm): fine aggregate passes it, coarse aggregate is retained on it. Aggregate is by far the largest ingredient of both concrete and asphalt, making up the majority of their volume, so its properties drive cost and performance. Recycled concrete, slag and lightweight expanded materials are used as aggregates too.
Why gradation is a separate property from size
Two aggregate samples can have the same maximum particle size but very different gradations — one might be concentrated mostly in one size range (gap-graded), while another has a smooth spread across many sizes (well-graded). A sieve analysis is the standard test used to determine this distribution.
The result is usually drawn as a gradation curve: percent passing on the vertical axis against sieve size on a logarithmic horizontal axis. A smooth, gently sloping S-curve indicates a well-graded material; a steep, near-vertical curve indicates most particles are one size.
Types of gradation
| Gradation | What the curve looks like | Behavior | Typical use |
|---|---|---|---|
| Well-graded (dense) | Smooth, spread across many sizes | Packs densely, compacts well, lower permeability | Road base, dense asphalt, concrete aggregate blends |
| Uniformly (poorly) graded | Steep — mostly one size | Many voids, drains freely, hard to compact | Drainage stone, filter layers |
| Gap-graded | Flat step where sizes are missing | Can segregate; specialized mixes | Stone-matrix asphalt, some architectural concrete |
| Open-graded | Few fines by design | High permeability | Permeable pavements, drainage layers |
Why a well-graded mix generally performs better
In a well-graded aggregate, smaller particles fill the voids between larger ones, producing a denser overall structure once compacted — this generally improves strength and reduces the volume of binder (cement or asphalt) needed to fill remaining voids. A gap-graded or uniformly-sized aggregate leaves more void space, which can require more binder or produce a weaker, more permeable result, depending on the application.
'Better' depends on the job, though. A dense, well-graded base is exactly wrong for a French drain, where you want the open voids of a uniform stone. Gradation is a design choice matched to the function — strength and stability, or drainage.
What gradation changes in concrete, asphalt and base
The same particle-size distribution affects each material in a different way, which is why each has its own grading specification.
| Material | What gradation controls | Typical problem when it is off |
|---|---|---|
| Concrete | Void content between particles, and so the cement paste needed for a given workability | Too little material passing the No. 50 and No. 100 sieves: bleeding, pumping difficulties and hard finishing. Too much fine aggregate, or a very fine sand: more paste and cost |
| Asphalt | How much void space the binder fills, and whether the mix is dense, open or gap graded | Aggregate outside the mix design's grading band changes the void space the binder must fill, so supply is checked by sieve analysis |
| Base and drainage layers | Compacted density and permeability | Too many fines in a drainage layer, or too few in a base that must compact |
Gradation and permeability
Reference values compiled by FHWA show how strongly gradation controls drainage: uniformly graded coarse aggregate typically conducts water at about 0.004 to 0.4 m/s, well-graded aggregate without fines at about 0.00004 to 0.004 m/s, and concrete sand with a high dust content at well under 0.00001 m/s. Each step down the list is a drop of a hundredfold or more.
In concrete, ACI notes that the amount passing the No. 50 and No. 100 sieves is one of the most important features of a sand's grading. Most specifications allow 10% to 30% passing No. 50 and 2% to 10% passing No. 100, and ASTM C33 allows those lower limits to drop to 5% and 0%.
Putting numbers on gradation: Cu and Cc
Engineers summarize a gradation curve with two coefficients, read from the particle sizes at which 10%, 30% and 60% of the sample passes (D10, D30 and D60):
- Coefficient of uniformity, Cu = D60 ÷ D10 — how wide the spread of sizes is.
- Coefficient of curvature, Cc = D30² ÷ (D10 × D60) — whether the middle sizes are present in the right proportion.
- Under the Unified Soil Classification System, a gravel is well graded when Cu ≥ 4 and 1 ≤ Cc ≤ 3; a sand when Cu ≥ 6 and 1 ≤ Cc ≤ 3.
Example
A sand with D10 = 0.15 mm, D30 = 0.6 mm and D60 = 2.0 mm has Cu = 2.0 ÷ 0.15 = 13.3 and Cc = 0.6² ÷ (0.15 × 2.0) = 1.2. Both are within the well-graded limits, so it would classify as a well-graded sand (SW), provided it has few fines. To read D10, D30 and D60 from a test report, see how to read a sieve analysis.
Fines: the small fraction with a big effect
Material passing the No. 200 sieve (75 µm) — silt and clay-sized particles — is limited in most specifications. A few percent helps a base compact tightly; too much makes it moisture-sensitive, lowers permeability, and in concrete increases water demand. Fines are measured by washing the sample over the No. 200 sieve (ASTM C117) before the dry sieving. When fines are plastic clay rather than rock dust, their effect is judged with Atterberg limits.
How gradation is measured
A sieve analysis shakes a dried, weighed sample through a stack of sieves from coarse to fine and weighs what each one retains. The Sieve Analysis Calculator turns those weights into cumulative percent retained, percent passing and fineness modulus. For a walk-through of reading the results, see the guide to reading a sieve analysis, and for where the test is used, Sieve Analysis Explained.
Common mistakes
- Judging an aggregate by its maximum size alone, ignoring the rest of the curve.
- Using a dense-graded base where free drainage is required, or vice versa.
- Skipping the wash step, which under-reports fines.
- Comparing a supplier's gradation to the wrong specification band.
FAQ
- What is the difference between fine and coarse aggregate?
- Fine aggregate (sand) passes the No. 4 sieve, 4.75 mm; coarse aggregate (gravel or crushed stone) is retained on it. Concrete normally uses both.
- What does well-graded aggregate mean?
- It contains a good spread of particle sizes, so smaller particles fill the gaps between larger ones. In USCS terms, a well-graded gravel has Cu ≥ 4 and a well-graded sand Cu ≥ 6, both with Cc between 1 and 3.
- Is well-graded aggregate always better?
- No. Well-graded material is best where you want density and stability, such as road base. Where you want drainage, uniformly graded or open-graded stone is the right choice.
- How is aggregate gradation tested?
- By sieve analysis — ASTM C136 for aggregates and ASTM D6913 for soils — usually after washing out the fines finer than the No. 200 sieve under ASTM C117.
Related Calculators
Related Guides
More From the Blog
Sources
ASTM C136
Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates.
ASTM C117
Materials finer than 75-µm (No. 200) sieve in mineral aggregates by washing.
ASTM D2487
Unified Soil Classification System — Cu and Cc limits for well-graded soils.
ACI Education Bulletin E1-07, Aggregates for Concrete
Significance of grading for paste demand, workability and the No. 50 / No. 100 fractions.
ACI Education Bulletin E1-07, Aggregates for Concrete — referenceFHWA NHI-05-037, Table 5-57
Typical saturated hydraulic conductivity of highway materials by gradation (after Carter and Bentley, 1991).
FHWA NHI-05-037, Table 5-57 — reference