What is Aggregate Gradation?
Concrete and asphalt products are made up of aggragetes. The aggregate size, shape, and the way particles fit have an effect on the characteristic of the building material. The aggregate gradation describes the distribution of the particle sizes in a certain sample and determines aggregate quality.
Dense Graded Aggregate (DGA)
This gradation contains a wide range of different sizes of aggregates. Voids created by larger rocks are filled by smaller rocks and fines. As a result there are fewer vois and the material compacts well. This type is typicalls used for recycled concrete aggregates, roadbase, and concrete & asphalt stone products. Mobile impact crushers are suited well for producing DGA, which makes a good compactable road-base product.
Uniformly graded aggregate
All the particle sizes are roughly the same size, which is the case in many clear stone products (e.g. #57 stone). The high void content leads to low stability and makes it difficult to compact. Water drains more easily through uniformly graded aggregates. To produce this aggregate gradation you need a screening plant to remove fines and oversize particles.
Open graded aggregates
Mostly large rocks with little fines. This results in more air voids because there are not enough small particles.Jaw crushers produce more of an open graded aggregates.
Dense Graded Aggregate (DGA)
Use: base for driveways, roads, patios, & walkways
Open Graded Aggregates
Use: bedding layer for leveling and base for hardscapes
Uniform Graded Aggregates
Use: drainage applications
Aggregate Sieve Analysis Test is Used to Assess the Aggregate Gradation
A gradation analysis (or sieve analysis) is a procedure used to assess the particle size distribution (gradation) of a granular material by allowing the material to pass through a series of sieves of progressively smaller mesh size and weighing the amount of material that is stopped by each sieve as a fraction of the whole mass. The size distribution is often of critical importance to the way the material performs in use.
How is an aggregate sieve analysis made?
- Collect A Sample
Samples should be taken directly from the discharge conveyor. If you take a sample from a pile, the material was subject to segregation in the stockpiling process and falsifies the results. This can be achieved by dragging a bucket underneath the conveyor accross with a rope. While this is a dangerous process it gives you the most accurate sample.
- Weigh the Sample to Ensure You Have Enough Material
The collected sample is weighted to ensure you have enough material. For Certain DOT specifications you need to collect a certain minimum weight of material to get to a specific DOT approved sieve analysis because the test procedure often includes the split of the material sample in several sub-samples.
- Dry the Sample in an Oven
The drying process is a very important step because the weight will be the key indicator for creating the aggregate sieve analysis. Moisture in the material adds weight falsifying the results.
- Reweigh the Sample
This is your reference weight to check if your screening results add up.
- Split the sample evenly
The dried material is put into a sample splitter that distributes the sample evenly into 2 buckets. Each bucket will be used to create a grading curve.
- Screen the Sub-Samples to Determine Aggregate Segregation
A screen tower is used to sieve the material and split up the different particles. While the coarse material stays on top the fines will fall further down in the screen tower.
- Weigh Each Particle Cartridge and Document Results
Each screened product is weight, compiled into a table, and each particle size's percentages of the total weight are computed. This percentage is then visualized in the grading curve.
The Department of Transportation Sets the Standards for Aggregate Gradations
The Departments of Transportation's primary responsibility is regulating and setting the standards for all infrastructure projects including aggregates for base and subbase in road construction and concrete and asphalt stone used in material production. This is done to ensure general safety and the longevity of the construction.
What to do when your aggregate sieve analysis is out of spec?
Are you using the right crusher type?
Are you not getting enough fines for a DGA product? Using an impact crusher instead of a jaw or cone crusher increases the amount of fines produced. If you are already using an impact crusher you can work on your impact crusher settings to get the aggregate gradadtion right.
- Rotor Speed: The rotor speed or (the speed at the tip of the hammers / tip speed) affect the intensity of the pulverization. The higher the rotor speed the higher the impact and the finer the end product.
- Crushing chamber geometry: Adjusting the closed side setting will affect the output size and gradation.
- Hammer selection: using 4 tall hammers vs 2 high / 2 low will increse the amount of pulverization because the material gets hit more often..
Are you using a screener?
Certain specs required the elimination of oversize and/or undersize particles. Mobile impact crushers feature an on-board screen and return conveyor to produce a DGA product with a capped off topsize.
- Screening attachment: The screen attachement or adjourned mobile screening plant eliminate any oversize material ensuring that 100% of the crushed material is smaller than a certain topsize.
- A mobile screening plant with minimum 2 decks allows you to produce a uniform graded product such as e.g- #57 stone.
Get our Best Advice
Join over 16,000 people who get our practical crushing & screening tips once a month.
