What is an aggregate fraction?
An aggregate fraction (also called a size group) is a quantity of aggregate whose particle sizes lie between two consecutive sieve sizes, for example 4/8 or 8/16 mm. In concrete technology and road construction, aggregate is sorted into fractions to obtain a controlled particle-size distribution. The first number is the lower sieve size, the second the upper one.
Designation and meaning
A fraction is written as d/D, where d is the smallest and D the largest particle size. Common fractions include:
| Fraction | Application |
|---|---|
| 0/4 mm | Sand for mortar and concrete |
| 4/8 mm | Fine gravel |
| 8/16 mm | Coarse gravel for concrete |
| 16/32 mm | Coarse aggregate, base layers |
Importance for concrete
Combining different fractions produces a favourable particle-size distribution, shown as a sieve curve. A well-graded grain skeleton leaves little void space between particles, so less cement paste is needed and the concrete becomes stronger and easier to work. An incorrect ratio of fractions causes segregation or porous, weaker concrete.
Related terms
- Particle size
- Aggregate
- Gravel
- Sand
- Sieve curve
- Concrete mix
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