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Materials
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.
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 (>2.36mm)
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 (2.36mm - 0.075mm)
The stability of a mix, its voids content and requirement for binder are all determined by sand grain size.
Filler (<0.075mm)
This finer material has three functions. It fills voids in a stone / sand mix to produce a more uniform material. Together with the bitumen, it forms the binder. By increasing viscosity 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 |
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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. |
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
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Thin, watertight layers of bitumen
Sprayed or spread manually
Usually reinforced by geotextile
Impermeable
Prepared on-site or supplied in prefabricated rolls |
Linings for canals, banks, watercourses and reservoirs
Landfill linings |
Dense Asphaltic Concrete (DAC)
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A filled mixture of crushed stones or gravel, sand and filler
After compaction, voids are nearly completely filled with bitumen (void content <3%)
Impermeable
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
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Dense Stone Asphalt (DSA)
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A gap-graded*, overfilled mix of stone, sand, filler and bitumen
Impermeable
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
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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
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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)
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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
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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
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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
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Open Stone Asphalt (OSA)
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An underfilled, gap-graded* mixture of mastic and limestone (other rocks may be substituted where water is acidic, as at Withens Clough Reservoir)
Permeable
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
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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
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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.
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