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Asphalt is a mixture of bitumen and mineral aggregate. From these basic materials a range of asphaltic mixes can be made, each designed to solve particular problems.The properties and utility of a mix are determined by the proportions and grades of the mineral components, and by the degree of ‘filling’ with bitumen.


A complex mixture of non-volatile hydrocarbons refined from crude oil. It is a thermoplastic and highly viscous binding agent. Heat decreases viscosity and is required for mixing with mineral aggregate. Performance under load is determined by viscosity and so is strongly dependent on its temperature and the duration of the loading.

Mineral aggregate

Additions to bitumen are selected from:

Stone (>2.00mm)

Larger grains give better stability but reduce workability. Limits to layer thickness, the desired thickness and the production / installation methods are all considered in selecting grain size.

Sand (2.00mm – 0.063mm)

The stability of a mix, its voids content and requirement for binder are all determined by sand grain size.

Filler (<0.063mm)

This finer material has three functions. It fills voids in a stone / sand mix to produce a more uniform material, it combines with the bitumen to form the binder and it stiffens the binder thus reducing the risk of segregation.

Degree of filling

The mineral aggregate contains voids, so, with an increasing proportion of bitumen in the mix, the voids will be:

Underfilled Filled Overfilled
Bitumen coats and binds the various aggregate components, leaving voids unfilled.

The bitumen acts only as a binder, so the properties of the mix are determined by the stone skeleton.
Bitumen almost completely fills the voids. Both the stone skeleton and bitumen contribute to the properties of the mix.

Compaction is essential to ensure the mix is impermeable.

Bitumen almost completely fills the voids. Both the stone skeleton and bitumen contribute to the properties of the mix.

Compaction is essential to ensure the mix is impermeable.

The volume of bitumen exceeds that of the voids so its properties dominate. The mineral contributes a degree of stiffening.

These mixes do not require compaction.

Asphaltic Mixes

Industry standard and specially designed mixes are available in each category

More on the properties relevant to specific uses may be found at Applications

Material Structure and properties Applications
Bituminous Membranes  Thin, watertight layers of bitumen

Sprayed or spread manually

Usually reinforced by geotextile


Prepared on-site or supplied in prefabricated rolls
Linings for canals, banks, watercourses and reservoirs

Landfill linings
Dense Asphaltic Concrete (DAC)  A filled mixture of crushed stones or gravel, sand and filler

After compaction, voids are nearly completely filled with bitumen (void content <3%)


Stable on steep slopes

Unsuitable for application under water or in the tidal zone
Watertight facings and revetments above the mean high water level

Lining for canals and reservoirs

Cores for reservoir dams

Landfill linings  
Dense Stone Asphalt (DSA)    A gap-graded*, overfilled mix of stone, sand, filler and bitumen


Pre-fabricated for underwater applications
Bottom and slope protection

Toe constructions

Thick asphalt revetments for sea defence

Inclined or vertical cores for dams

Easily worked so appropriate for awkward situations
Asphalt reinforced geomats  Thin layer of 3-D geotextile filled with a ‘mini’ Open Stone Asphalt mixture, reinforced with rubber shreds

Open structure allows plants to grow through – vegetation cover is rapid and improves erosion resistance

Withstands waves up to ~0.3m and currents up to ~ 2.5 m/s

Prefabricated in 4.8m wide rolls up to 20 metres long

Can be placed above or below water
Lightweight erosion protection for river training and lakes/lagoons
Mastic Grout  A fully filled mixture of mastic (60 – 90%) and gravel or stone

Pourable at placing temperatures (140-180oC)
Grouting stone revetments above and below the waterline

Slab constructions
Lean Sand Asphalt (LSA)  A greatly underfilled mixture of sand and bitumen (4% of volume) with or without filler

Permeability is similar to the original sand As workable as sand when hot

Cools to form bound material resistant to erosion and wash out

Flows and settles underwater to give uniform body without cavities
A 3-D filter layer

Regulation of uneven surfaces

Filling small voids and cavities

Core material for reclamation bunds Scour protection

Easy to place so suitable for emergency measures
Marine Mastic Granules  Conventional off-shore asphaltic mastic produced by a patented process

Mastic supplied as easy-melt granules rather than dense slabs

Melts efficiently due to high surface area to weight ratio
Gap filling on concrete coated steel pipelines
Asphaltic Mastic  An overfilled sand, filler and bitumen mix (typically, 3:1:1)

Naturally dense so no compaction needed

Can be poured at working temperatures

Cools to form a viscous, flexible quasi-static mass

Impermeable Bulk can be increased by adding stone
Component for grouted stone

Component for OSA

Asphalt slabs for lining above and below water

Reinforcing stone revetments

Bed and toe protection  
Open Stone Asphalt (OSA)  An underfilled, gap-graded* mixture of mastic and limestone (other rocks may be substituted where water is acidic, as at Withens Clough Reservoir)


Strong and porous so it absorbs wave energy without damage

Can be placed on slopes up to 1:1.5

Follows all shapes and contours when placed in situ above the waterline

Can be placed underwater as prefabricated mattresses
Armour layer for river training

Armour layer for revetments in flood control and coastal protection

Scour protection for quays
Sarmac Mattresses  A material similar to dense stone asphalt, placed in a reinforced wire mesh cage and wrapped with a geotextile

Very robust and flexible
Protection for underwater pipelines and cables

Protection layer between pipes/cables at crossings underwater
Special Products  Asphalt can incorporate many different types of material to suit our client’s requirements – added materials can result in excellent insulation properties, for example

Recycled aggregate can be used, and waste materials incorporated

Asphalt mixtures can be made relatively heavy or light depending on chosen aggregate
Client/site specific

*A gap-graded material has fine and coarse but nothing in between, eg OSA has fine aggregate in the mastic element plus a 40mm stone, resulting in an open material. A well-graded material will have aggregates across the grading spectrum, ie filler, sand, fine aggregate and coarse aggregate.