Clitheroe
Shap
Horwich, Bolton
Chorley
Horwich, Bolton
Clitheroe
Shap
Horwich, Bolton
Chorley
Horwich, Bolton
Two aggregates can both be strong and well produced but perform very differently in the same road. What matters is whether their properties suit the pavement layer, traffic loading and conditions they will face.
Pavements have supporting layers beneath their visible surfaces, each with a different job to do. Each layer therefore needs an aggregate suited to its role.
Making the right choice can help the completed road remain stable and perform reliably for longer.
There is no single aggregate that is best for every road or pavement. Selection should begin with where the material will be used, as the requirements at the surface are very different from those deeper within the structure.
For an asphalt surface course, look first at the performance required rather than choosing by rock type.
Two important measures are Polished Stone Value (PSV), which indicates resistance to polishing, and Aggregate Abrasion Value (AAV), which reflects resistance to surface wear.
A higher PSV and lower AAV indicate better performance in these respective areas.
Greater polishing resistance is particularly important on busy roads and where vehicles frequently brake or turn, such as junctions, roundabouts and approaches to pedestrian crossings. At particularly high-risk locations, specialist high-friction surfacing may be required.
The aggregate must also meet the specified requirements for grading, particle shape and flakiness. Suitability should be based on test results for the individual source rather than an assumption that a particular rock type will perform well.
The aggregates in the binder course and base are not directly exposed to tyres, so polishing resistance is less important. Their main role is to help the asphalt withstand repeated loading without deforming or breaking down.
A strong, properly graded aggregate with suitable particle shape helps create a stable mixture. Resistance to fragmentation becomes increasingly important on roads carrying frequent heavy vehicles, while the aggregate must also form a reliable bond with the chosen binder.
In unbound layers, the priority is to create a stable foundation that can be compacted evenly and continue distributing loads after the road opens.
Where a dense load-spreading layer is required, a well-graded aggregate with controlled fines will normally be more appropriate. If the layer has been designed to provide drainage, a more open-graded material may be needed instead. These products should not be substituted for one another simply because their particle sizes appear similar.
The aggregate should also be strong enough to resist breaking down during compaction and construction traffic. Its maximum particle size must suit the proposed layer thickness and the equipment being used.
In a concrete pavement, aggregate forms part of the slab itself. Selection therefore affects how the concrete behaves during placement as well as the strength and durability it develops after hardening.
A suitable blend of coarse and fine aggregate helps the particles pack together efficiently. This can improve workability and reduce the amount of cement paste needed, helping to limit shrinkage and the movement of water through the finished concrete.
Absorption must be understood so that the mix water can be controlled accurately. The aggregate should also be durable and chemically compatible with the other concrete constituents, as reactive material can contribute to expansion and cracking over time.
Where the concrete surface is exposed directly to traffic, resistance to abrasion and polishing may also be important. These properties must be considered alongside the surface finish or texturing used to provide skid resistance.
Before selecting an aggregate, consider how the pavement will be used. A lightly trafficked access road and a busy distribution route may have similar layers, but frequent heavy vehicles place very different demands on them.
Junctions and roundabouts also experience concentrated stress as traffic brakes, accelerates and turns.
Conditions during construction matter too, as plant and delivery vehicles may travel over partially completed layers. Once the road is open, effective drainage is essential to prevent trapped water from weakening the structure. Freeze–thaw exposure may place further demands on porous materials.
The intended service life should also inform selection, particularly where future maintenance would be difficult or disruptive.
Aggregate selection is not about achieving the highest result in every test. It means identifying which properties matter in a particular pavement layer and choosing accordingly.
Aggregate grading describes the balance of particle sizes. In dense unbound layers, smaller particles fill the spaces between larger ones, supporting compaction and load distribution. An unsuitable or inconsistent grading can result in uneven support and movement.
Drainage layers require a more open structure, while asphalt and concrete need gradings suited to their mixture designs.
Aggregate must resist breaking down during processing, compaction and repeated traffic. Fragmentation can change its grading and produce additional fines, affecting stability and drainage.
Within asphalt or concrete, weak particles may also limit the performance of the mixture. The objective is to meet the pavement’s structural requirements rather than simply choose the strongest available stone.
Find out more about how aggregate density and strength affect structural performance.
Angular particles can provide interlock in unbound layers and asphalt, while rough surfaces can support bonding with bitumen or cement paste.
However, very angular material may require more effort to place or compact, and excessive flaky or elongated particles can be vulnerable to breakage. The most suitable shape therefore depends on the layer and construction method.
Read more about how aggregate shape influences compaction and stability.
Polishing gradually smooths exposed particles, while abrasion wears material away. Both can reduce the texture and skid resistance of a trafficked road surface.
These properties matter most on busy roads and where vehicles frequently brake or turn. For aggregates buried within the pavement, strength and resistance to fragmentation are generally more relevant.
Water absorption describes how much water can enter an aggregate’s pores. In concrete, this can influence workability and mix-water control. In asphalt, moisture can affect the bond between the aggregate and binder.
Pore structure also influences resistance to wetting, drying and freezing. The required weathering resistance should therefore reflect the conditions in which the pavement will be used.
Controlled fines help dense-graded sub-base materials compact, but the same quantity could restrict drainage in an open-graded product. Dust and clay may also interfere with bonding in asphalt or concrete.
The important distinction is whether the fines form part of the intended grading or are unwanted contamination. Find out more about fines content in aggregates and why it matters.
Concrete aggregates must be chemically compatible with the other mixture constituents. Certain reactive materials can contribute to alkali–silica reaction, which causes expansion and progressive cracking when moisture is present.
This risk cannot be judged from rock type alone. Suitability should be confirmed against the concrete specification and appropriate information for the individual source.
Visible deterioration can provide clues about what is happening within the pavement:
These symptoms do not prove that aggregate selection is responsible. Drainage, construction, ground conditions, binder performance and maintenance should also form part of any investigation.
Not necessarily. Durability must be judged against the aggregate’s intended position and purpose. High polishing resistance adds little value in a buried layer, while the strongest available stone may not provide the grading, workability or other properties the application requires.
There is also no universal aggregate lifespan. Performance depends on the source, pavement design and conditions in service.
Recycled or secondary aggregates may be suitable where their properties and permitted use satisfy the relevant specification, while unnecessarily over-specifying primary material can add cost without extending the pavement’s life.
Appearance and rock type are not enough to demonstrate suitability. The applicable product standard depends on where the aggregate will be used:
These standards contain different property categories, so stating that a product meets a standard does not confirm that it is suitable for every application. The project specification must identify the categories and any additional highway requirements that apply.
Declared grading, representative test results and production-control records provide supporting evidence. Our guide to aggregate testing methods used in UK construction explains how these properties are assessed.
That suitability must then be protected through production, storage and delivery. Segregation, contamination or loading the wrong product can compromise an otherwise appropriate aggregate.
Consistent quality control and material traceability help connect the material delivered with the product originally specified.
Pavement longevity depends on choosing aggregates that suit each layer and the conditions the completed road will experience. Explore Armstrongs’ aggregate range or contact our team to discuss the requirements of your project.
There is no single best aggregate for every road. The right product depends on the pavement layer, traffic, exposure conditions and applicable specification.
Grading influences compaction, void content and load distribution. It must suit the layer, as dense supporting materials and free-draining aggregates require different particle-size distributions.
Repeated tyre contact can gradually smooth exposed aggregate. Sufficient polishing resistance helps the surface retain wet-weather skid resistance.
No. Strength is only one requirement, and selecting the strongest stone will not compensate for unsuitable grading, shape or durability.
Breakdown can result from crushing during construction, repeated traffic, abrasion, weathering or freeze–thaw exposure. The main risks depend on where the aggregate is used.
Yes, where their use is permitted and their properties meet the relevant specification. Suitability must be assessed for the intended layer rather than assumed from the material’s origin.
BS EN 13043 applies to bituminous mixtures and surface treatments, BS EN 13242 to unbound and hydraulically bound materials, and BS EN 12620 to concrete aggregates. Project-specific requirements may also apply.
Water can affect the stability of supporting layers, the bond between aggregate and asphalt binder, and the durability of porous aggregates exposed to freezing conditions.
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