Screening separates aggregate into particle size fractions (source - Powerscreening)

Aggregates are essential for civil engineering construction. They are hard, granular, mineral materials, used on their own or with the addition of cement, lime or bituminous binders. They are utilised across all forms of infrastructure and are a critical component in roads, concrete structures, and coastal protection. In this blog we look at the different types of aggregate and their uses. We will also cover the increased use of manufactured and recycled aggregates.

There’s plenty of ground to cover here. Use the links below to jump to specific sections of interest:

• What are the different types of aggregate?
• Primary versus secondary aggregates?
• What are aggregates used for?
• How are aggregates classified?
• Types of aggregate and their uses
• Mechanical stabilisation of aggregate layers

 

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What are the different types of aggregate?

Aggregates fall into three types based on where the minerals are sourced from: natural, manufactured, or recycled. Materials include sand, natural gravel, crushed rock, Type 1, recycled asphalt, crushed concrete, and Type 6F2. Common specifications for aggregate in UK highways and construction include Type 1, Type 6F2 and Type 6F5. 

Read on to discover more about these aggregate categories – and the ways in which the various aggregate materials can be used.

Natural aggregates

These are composed of rock fragments that are mined, quarried, or dredged and used in their natural state or processed by crushing, washing, and grading by particle size. This group includes crushed rock, sands, and gravel.

Manufactured aggregates

These are produced from an industrial process, often as a by-product of the primary process, where mineral material undergoes change involving thermal or other modification. Such aggregate products include iron and steel slags, and ash. Some material may undergo secondary processing. An example would be lightweight aggregates produced from fly ash, a by-product from coal-fired power stations that is heated to high temperatures to create hard, rounded pellets.

Recycled aggregates

These result from the reprocessing of materials previously used in construction. Examples are, building demolition waste, used railway ballast, and reclaimed asphalt arising from road resurfacing works.

Primary versus secondary aggregates

It is also common practice in the UK to categorise aggregates as either primary or secondary aggregates.

Primary aggregates

These are natural minerals extracted specifically for use as aggregates in construction. This category includes all natural aggregates; sands, gravels, and crushed rock extracted from quarries.

Secondary aggregates

This category includes manufactured and recycled aggregate, plus aggregate obtained as a by-product of other mineral extraction operations, such as colliery spoil.

What are aggregates used for?

Aggregates may be used on their own or in a composite, where aggregate particles are bound together with cement, lime, bitumen, or resin. Humankind has utilised aggregates for infrastructure and to construct our built environment since the bronze age. Massive volumes of aggregate are consumed by today’s construction sector, with applications in roads, railways, bridges, buildings, and other structures.


Tensar InterAx has optimised geogrid geometry to enhance the stabilising effect on the aggregate to provide a mechanically stabilised layer

Road foundations 

The lower layers of a road pavement are typically constructed from unbound aggregate materials compacted in layers. For a capping layer, ‘lower grade’ aggregate may be incorporated over weak soils. The main support layer, the subbase, is constructed from a good quality aggregate selected to provide a stiff foundation. In some cases, a structural base layer of high quality, carefully graded aggregate, will be used above the subbase to provide a dense, strong, stiff layer to support the asphalt or paved surfacing.

Rail trackbed

Ballasted track construction may incorporate a sand layer at the base to reduce water pumping. The foundation layer, the sub-ballast, will typically be a graded aggregate, performing a similar function to a road subbase. The ballast layer, of single size aggregate, supports the sleepers (rail ties) and distributes load down to the foundation. Visit our railway track reinforcement page to discover more.

Asphalt

More correctly termed asphalt concrete, this material so familiar to us all, provides the smooth road surface on which we drive. Asphalt is distinct from tarmac, and comprises a mix of aggregate particles bound together with bitumen. Other components may be included to modify and improve the asphalt properties.

Concrete

Familiar to us all, concrete has been around as a construction material since 6500 BC. Aggregate particles are bound together with a cementitious paste to form a rock-like material. What we call concrete today is typically produced using Portland Cement. Modern concrete can have extremely high compressive strength. It typically comprises 60%-80% aggregate by weight. 

Reinforced concrete, where steel is incorporated to provide the necessary tensile strength, is the backbone of the construction industry. It is used in foundations, tunnels, drainage works, buildings, dams, and bridges. In fact, just about every construction project will incorporate concrete somewhere. High quality, crushed rock aggregates are required for concrete production.

Drainage

Aggregates with limited fine particle size content will have large contiguous voids when compacted. This allows water to pass freely through the aggregate. Drainage aggregates are used to create pathways for water and other liquids. Trenches filled with drainage aggregates are constructed alongside roads and rail lines to lower groundwater levels and remove stormwater. 

Drainage aggregates are installed under paved areas to collect, store, and slowly release stormwater in SUDS systems. A layer of aggregate below the membrane liner in a waste containment lagoon will collect any leakage and channel this to collection systems. Aggregate drainage layers are found across all infrastructure works. In many cases, the boundary between soil and drainage aggregate will be protected by a geotextile separator/filter to prevent contamination of the aggregate and maintain flow capacity.

How are aggregates classified?

As with all construction products, aggregates must be classified and have characteristics relating to specific functions and applications. Aggregates are typically defined by origin or source, particle shape, particle size, and density. 

Origin

The different aggregate types and their origin has been discussed. The chart below summarises the various aggregate sources, separating primary aggregates (crushed rock and sand gravel) from secondary aggregates (recycled, manufactured, and mineral extraction by product).

Table 1 - Classification of construction aggregate by their origin or souce.

Shape

The shape of aggregate particles influence how they interact under load and how densely they pack together. Angular particles, typical in crushed rock aggregate, will interlock and have a higher shear strength compared to rounded particles, which more easily slide against each other. For drainage, aggregates with rounded particles, such as river gravels, will be advantageous as they have a greater void content (the space between particles) compared to aggregates with interlocking angular particles. For concrete use, aggregates with rounded particles give better workability, while angular aggregates lead to higher strength concrete.

Density

The density of aggregate materials will directly relate to the weight. For some applications, lightweight aggregates have an advantage. For example, earthworks structures constructed over weak soils can lead to bearing failure of the foundation soil. The use of lightweight aggregates reduces the bearing pressure and reduces the potential for failure. Similarly, the use of lightweight aggregates can significantly reduce the settlement of earth structures. Lightweight aggregate has been used as a structural fill in TensarTech reinforced soil walls to reduce settlements.


Construction of the A47 Longthorpe Footbridge where this TensarTech structure required complex geometry and lightweight fill to create wheelchair friendly shallow access ramps within a very restricted space.

Size

Particle size and particle size distribution (the proportion of soils in each size range) have major influence on shear strength, compacted density, and permeability of the aggregate. As a result of this, particle size distribution parameters for aggregates are specified during road construction projects, highlighting the lower and upper bound limit for each particle size.

Concrete 

The particle size used in concrete mixes will vary, but typically it will comprise a mix of fine aggregate (<4.75mm size) and coarse aggregate (usually 9.5mm-37.5mm size). A coarse aggregate of 20mm size is most commonly used for reinforced concrete, with a larger 40mm size for mass concrete.

Road subbase

The compacted subbase layer must be dense, with a high shear strength to distribute load onto the formation. The aggregate will comprise a mix or particle sizes designed to create the necessary compacted density and shear strength. Typically, particle size will range from 31.5mm down to 1mm, with only a small fraction in the silt/clay range (<0.063mm). In the UK, particle size distribution for highway works is defined in the Manual of Contract Documents for Highways Works, Volume 1 Series 800. 

Types of aggregate and their uses

Having looked at the common sources or aggregates and their classification, we can take a closer look at specific aggregate types and their uses.

Sand 

Sand is obtained from open pits or by marine dredging. Sand results from the weathering and erosion of rock. Accumulated deposits are formed by the action of wind, glaciers, and rivers. Most sand particles are durable fragments, rich in silica. 

Different types of sand

Sand sources can be classified as either superficial or bedrock deposits:

• Superficial deposits, laid down by rivers in the last two million years, are found in the bottom of river valleys or former riverbeds, or forming terraces higher up the valley sides. These sources are clean with low silt and clay content. Superficial deposits laid down by glacial meltwaters are often associated with boulder clay and contain a higher silt and clay component.
• Bedrock deposits occur as bedded formations from the Permian to Paleogene geological eras. 

 

Sand screening plant - separating by particle size (source Centristic)

Sand size

Sand for construction is defined by particle size rather than mineral composition. 

Table 2 - Sand particle size range

Uses of sand

Sand aggregates are an essential component of concrete mixes and cement mortars. Sand is also used as a filter medium, as a levelling layer beneath block paving, and incorporated in some asphalt mixes. 

Natural Gravel

Similar to sand, gravel is also obtained from open pits or by marine dredging. Gravel particles are durable rock fragments that result from the weathering and erosion of rock. Accumulated deposits of gravel, often in combination with sand, are formed by the action of wind, glaciers, and rivers. 

Different types of gravel

• Gravel sources can be classified as either superficial or bedrock deposits:
• Superficial deposits, laid down by rivers in the last two million years, are typically clean deposits with low silt and clay content. They are found in the bottom of river valleys, or former riverbeds.  Superficial glacial deposits are often associated with boulder clay and contain a higher silt and clay component.
• Bedrock gravel or sand/gravel deposits occur as bedded formations, often loosely compacted and easily excavated.
• Marine deposits are an important source of gravel in the UK. They are superficial deposits laid down by rivers and glaciers that have subsequently been submerged by post glacial rises in sea level.

 

Gravel size

Gravel is a coarse aggregate. It can be further classified into fine, medium, and coarse sizes.

Table 3 - Gravel particle size range

Uses of natural gravels

Natural gravels are used as a fill material, as backfill to buried structures and in capping layers as part of a road foundation. When washed and graded by particle size, natural gravels can be utilised in concrete mixes and as a drainage aggregate. After crushing and grading, natural gravel can be used as a road subbase.

Crushed rock

Crushed rock aggregate is now the most common source of primary aggregate in the UK. After quarrying, the rock is crushed and separated by particle size. The various particle size fractions can then be combined in the correct proportions to meet specific grading requirements for the different applications. One quarry source can then supply suitable aggregate mixes for use in concrete, subbase, asphalt, drainage, and other applications.

Uses of crushed rock aggregate

The strength and angular shape of crushed rock aggregates make it ideally suited for high strength concrete mixes, road subbase, rail ballast, asphalt concrete, and surface dressing applications.

Type 1 aggregate

It is very common to see reference to Type 1 aggregate. It is a well understood and readily available aggregate mix. Type 1 refers to a specific aggregate specification found in the UK Manual of Contract Documents for Highway Works – Volume 1, Specification for Highway Works. Series 800. The particle size distribution (grading curve) is smooth and broad, with a maximum particle size of 63mm and 99% passing the 31.5mm sieve size, The silt/clay content is limited to a maximum of 9%. The smooth broad grading curve ensures a high compacted density. 

Uses of Type 1 aggregate

Type 1 aggregate is a premium material. It is used to provide a strong stable foundation layer for roads. It may also be specified for working platforms and unpaved access roads.

                                         

Figure 1 - Type 1 Aggregate                   Figure 2 - type 6F2 Recycled Aggregate                                                                                             

Recycled asphalt

Road resurfacing often entails planning off the upper layer of road surface prior to overlaying with new asphalt. The plannings are a useful source of aggregate

Asphalt arisings from resurfacing are valuable aggregate resource (source MGL Group)

Uses of recycled asphalt

Asphalt planings (or arisings) are suitable for recycling into new asphalt. At the batching plant, the recycled component is incorporated into the asphalt mix in place of a proportion of primary aggregates. Asphalt arisings are also useful as road subbase material. The UK Specification for Highway works includes a specification for Type 4 aggregate (asphalt arisings) for use as unbound aggregate in roads.

Crushed concrete

Concrete from demolition can be crushed and separated into particle size fractions, then re-combined in the appropriate proportions for specific applications.

Uses for crushed concrete aggregate

Crushed concrete aggregate can be used as an alternative to crushed rock aggregates in concrete and the unbound layers in road construction. It can also be used for asphalt concrete although consistency may be an issue due to variations in the source of crushed concrete.

Type 6F2 and Type 6F5 aggregate

Type 6F2 and Type 6F5 refer to selected granular fill material, including recycled aggregates, defined in the UK Manual of Contract Documents for Highway Works – Volume 1, Specification for Highway Works. Series 600.
Type 6F2 is made up of crushed demolition material obtained from the project site, comprising concrete, brick, and mortar fragments. Grading requirements are defined in Table 6/2 of Series 600. The maximum particle size is 125mm with a minimum 80% passing the 90mm seive size. The particle distribution is defined and tested in accordance with BS 1377:Part 2. Any material with this classification imported to site is classified as Type 6F5
Type 6F5 is a selected granular fill material imported to site. The grading requirements are similar to 6F2 but are defined in Table 6/5 of Series 600 and tested in accordance with BS EN 13285. The material can be made up or aggregate from one or more sources, including; natural aggegate,crushed concrete, crushed brick, incinerator bottom ash, slag, and mine waste.

Uses of Type 6F2 and Type 6F5 aggregate

These gradings are very common as a fill material for piling mats and working platforms to support cranes and other plants. They are also used as a capping layer in road foundations over weaker soils, partially replacing more expensive subbase aggregate.

Mechanical stabilisation of aggregate layers

Mechanically stabilised aggregate - using Tensar InterAx geogrid

As we learned above, compacted aggregate layers form the foundation of most road pavements and access tracks. Well compacted aggregate layers distribute the loading and reduce the bearing pressure on the subgrade. The strength and stiffness of an aggregate layer can be increased by incorporating a stabilisation geogrid, such as Tensar InterAx geogrid.

The aggregate particles interlock with the geogrid apertures and are confined by the geogrid. The particle confinement prevents movement and rotation of the particles, stabilising the aggregate and increasing shear strength. This in turn, increases the bearing capacity of the layer. This is known as a mechanically stabilised layer (MSL).

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