Particle size distribution states in what proportions the different particle sizes occur in a mixture. It is also called grading or gradation. It is determined by sieve analysis: the material passes through a series of sieves with progressively finer mesh, and the amount retained on each sieve is weighed. The result is plotted as a grading curve.
Continuous or gap graded
- Continuous grading — every fraction is present, from fine to coarse. The small particles fill the space between the large ones, leaving little void space. This produces the densest, strongest concrete.
- Gap grading — one or more fractions are missing, leaving a lot of void space. Sometimes that is a defect; sometimes it is exactly the intention, since no-fines concrete and pervious concrete owe their function to a gap-graded mix.
Why it governs concrete quality
Grading directly controls the water demand, and water demand controls the strength:
- Too little fine material — the mix is harsh and lean, the particles bond poorly and the concrete segregates easily. When placed, the coarse particles sink and the cement paste stays on top.
- Too much fine material — the total surface area to be coated becomes very large. More cement paste and more water are needed to wet everything. More water means a higher water-cement ratio, and so lower strength, more shrinkage and more cracking.
- Optimal grading — as much contact as possible between particles of differing sizes, with minimum void space. That gives the greatest strength for the least cement.
Workability depends on it too: a well-graded mix can be placed and compacted without adding water.
Beyond concrete
- Ground investigation — particle size distribution determines the name and the properties of a soil: permeability, settlement, compactability.
- Sub-base and capping layers — a well-graded sand or blended granulate compacts far better than a single fraction.
- Asphalt and mortars — the same logic: the particle structure determines void content, binder demand and strength.
Practical tip
Order aggregate by size fraction and, if in doubt, ask the supplier for the grading curve. Sand that looks fine but is heavily single-sized (dune sand, for example, with virtually one particle size) is not suitable for concrete.
Related terms
- Aggregate size fraction
- Particle size
- Fraction
- Concrete mix
- Consistence class
- Soil type
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