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 (>2.00mm)
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 Impermeable 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%) 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 |
| Dense Stone Asphalt (DSA) | 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 | 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) 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 | 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.
