Aggregate specification and grading criteria

A specification is not a table, it is a demand on the plant

Aggregate specification and grading criteria usually circulate around a quarry as a sheet of paper: sieve apertures down the left column, percentage passing ranges down the right. That sheet is in fact asking the plant for a specific behaviour. Material produced from the same rock by the same crusher lands on a different table as soon as the number of screen decks or the closed circuit ratio changes. The right way to read a specification is therefore to see which equipment decision each line corresponds to.

The four axes of a specification

A specification is never a single property but four independent groups of properties asked together. Performing well on one axis does not rescue another, and most disputes on site arise precisely because only the sieve analysis was checked while shape or strength was left out.

Size distribution and the d/D designation

In European practice a coarse aggregate is named by a lower sieve size and an upper sieve size, written as d/D, so that 4/20 identifies one fraction. The specification then carries two layers of rules: overall limits that apply to every producer, and tolerances around the typical grading the producer itself has declared. The second layer is often overlooked. A plant that sits inside the overall limits but drifts away from its own declared curve has still produced a non-conformity.

Particle shape

The flakiness index turns particle shape into a number. The sample is separated into size fractions, each fraction passes over bar sieves, and particles that fall through the slots count as flaky; the index is the mass passing expressed as a percentage of the total dry mass tested. A high proportion of flaky particles harms workability and strength in concrete and disturbs compaction behaviour in asphalt.

Resistance to fragmentation

The Los Angeles method measures how well a coarse aggregate resists breaking down. A prepared sample is placed in a rotating drum together with a charge of steel balls and subjected to impact and abrasion over a set number of revolutions; the coefficient is the percentage of material passing the 1.6 mm sieve at the end. A lower coefficient means higher resistance. This is a property of the deposit, not of the plant: configuration cannot improve it, though on weaker rock the number of crushing stages can be trimmed to limit the fines generated.

Fines and cleanliness

The reference method for grading combines washing with subsequent dry sieving, because dry sieving alone cannot detach the fine material clinging to coarse particles and therefore reports a distribution coarser than reality. The fines limit in a specification is consequently read together with the method used to measure it.

Translating the demand into a configuration

Every line looks toward an intervention. As the number of fractions rises, the deck count and therefore the screening area grow with it; where a narrow fraction is wanted, dedicating a separate deck gives a steadier result than running two adjacent apertures on one. Where the flakiness index comes out high, the answer is not on the screen but in the crushing circuit: lowering the reduction ratio per stage, using a tertiary machine that produces cubical product, and returning oversize through a closed circuit will move the shape. Where the fines limit is breached, dust extraction on a dry circuit may not be enough and washing enters the picture, which in turn brings a water and sludge plan of its own.

One point deserves emphasis: screening efficiency is governed less by aperture than by bed loading. On an overfed deck the material slides as a bed and particles that ought to pass never reach the surface at all. Product analysis then drifts and the producer looks for the fault in the aperture. Splitting the load across two bodies relieves this directly, which is exactly why a twin vibrating screen arrangement is chosen at high throughput.

If the sample is wrong the table is wrong

A large share of disputes originates not in the plant but in sampling. A sample taken off the side of a belt or from a stockpile cone may not represent the stream, because segregation drives coarse particles down the flanks of a cone and leaves the crest fine. Samples are taken across the full cross-section of the flow, at regular intervals, and reduced by a defined procedure afterwards. Weighing and drying steps show up in the result as well: an insufficiently dried sample lets fines agglomerate and reports a coarser distribution. Wider quarry-scale topics are collected under mining engineering.

Frequent questions

We are inside the overall limits but the customer rejected the load. Why?

Most likely the tolerance around the declared typical grading was exceeded. Overall limits alone are not sufficient; production must also stay faithful to the curve it declared.

Can a screen change fix the flakiness index?

No. A screen separates by size, not by shape. Shape is decided in the crushing stage, and the index will not move meaningfully unless reduction ratio or crusher type changes.

Why do dry and washed sieving give different results?

When fine material stays attached to coarse particles, dry sieving cannot separate it. The washed method releases that material and therefore shows the true distribution.