What is a grading curve?

A grading curve (also: sieve curve or particle size distribution curve) is a graph that shows how the particles in a sample of sand, gravel or soil are spread over the different particle sizes. It is the result of a sieve analysis and makes the particle size distribution visible at a glance. Where the term particle size distribution describes the property, the grading curve is the tool used to plot, read and compare it.

How is a grading curve made?

1. The sample is dried and weighed.
2. It is passed through a stack of sieves with decreasing mesh sizes.
3. The material retained on each sieve is weighed.
4. For each sieve, the percentage of the total mass that passed through is calculated.
5. These percentages are plotted against the sieve size, usually on a logarithmic scale.

For aggregates the sieving method is set out in EN 933-1. Particles smaller than 63 micrometres are too fine for reliable sieving and are determined by other methods.

How to read a grading curve

• Steep curve: almost all particles are the same size. Such uniformly graded material leaves a lot of voids.
• Gradually rising curve: coarse and fine particles in a good ratio. The small particles fill the gaps between the large ones, giving dense packing. This is called well graded.
• Flat section: a particular size is missing. This is called gap grading.

Application

Concrete technology: the grading curve of the aggregate is compared with ideal curves, such as the Fuller curve, or with limit curves. If the curve falls within the permitted design zone, the aggregate grading is suitable for the intended concrete mix.

Soil mechanics: the grading curve helps to classify a soil type and to estimate its permeability and compactability.

Practical tip

When ordering aggregate for concrete or mortar, ask the supplier for the grading curve. That shows straight away whether the material suits your mix.

Related terms

• Particle size distribution
• Particle size
• Aggregate size fraction
• Grading design zone
• Aggregates

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