Different asphalt mixtures are available for the top layer of an asphalt road. The top layer is normally called the “surface course” and the type of surface course mixture chosen depends on the requirements for the road surface.
The surface course constitutes the top layer of the pavement and should be able to withstand high traffic- and environment-induced stresses without exhibiting unsatisfactory cracking and rutting in order to provide an even profile for the comfort of the user and at the same time possessing a texture to ensure adequate skid resistance. Depending on local conditions, functional characteristics such as skid resistance, noise reduction and durability are often required for surface courses. In some cases, rapid drainage of surface water is desired through a porous surface while in other cases the surface course should be impermeable in order to keep water out of the pavement structure. As indicated, the surface layer is important for the pavement performance but no single material can provide all the desired characteristics (e.g. porous and impermeable at the same time). A wide range of surface layer products can therefore be considered appropriate depending on specific requirements:
The selection of the surface course is a matter of identifying the most appropriate material during the design process. The functional requirements can conflict. For example, noise reduction could require the use of a double-layered porous asphalt and that conflicts with the requirement of a very durable surface layer. The durability of surface layers can be improved by using higher quality materials. The higher costs of these will be compensated by the lower costs of traffic measures and user costs.
The following descriptions of the asphalt mixtures are based on the definitions that are given in the European Asphalt Standards (EN 13108 series). The text in inverted commas are the definitions use in the European asphalt Standards.
1. Asphaltic Concrete (AC)
“Asphalt in which the aggregate particles are continuously graded or gap-graded to form an interlocking structure”. Dense asphalt concrete is often used as the ‘basic’ surface layer.
2. Asphalt Concrete for very thin layers (AC-TL)“Asphalt for surface courses with a thickness of 20 mm to 30 mm, in which the aggregate particles are generally gap-graded to form a stone to stone contact and to provide an open surface texture”. This mixture is often used in France and is called BBTM (Béton Bitumineuse Très Mince).
3. Soft Asphalt (SA)“Mixture of aggregate and soft bitumen grades”. This flexible mixture is used in the Nordic countries for secondary roads.
4. Hot Rolled Asphalt (HRA)“Dense, gap graded bituminous mixture in which the mortar of fine aggregate, filler and high viscosity binder are major contributors to the performance of the laid material”. Coated chippings (nominally single size aggregate particles with a high resistance to polishing, which are lightly coated with high viscosity binder) are always rolled into and form part of a Hot Rolled Asphalt surface course. This durable surface layer was often used as a surface layer in the UK.
5. Stone Mastic Asphalt (SMA)
6. Mastic Asphalt (MA)
7. Porous Asphalt (PA)
Porous Asphalt surface
8. Double layered Porous Asphalt (2L-PA)
The top layer with a grain size 4/8 mm is about 25 mm thick and the second/bottom layer is porous asphalt with a course aggregate (11/16 mm). The total thickness is about 70 mm. Because of the finer texture at the top (that gives less tyre vibrations), it gives a better noise reduction than a single layer porous asphalt.
Double layered porous asphalt surface
9. Asphalt for Ultra-Thin Layers (AUTL)
“Asphalt for Ultra-Thin Layers (AUTL) is a hot mix asphalt road surface course laid on a bonding layer, at a nominal thickness between 10 mm and 20 mm with properties suitable for the intended use. The method of bonding is an essential part of the process and the final product is a combination of the bonding system and the bituminous mixture”.
AUTL is a mixture in which the aggregate particles are generally gap-graded to form a stone to stone contact and to provide an open surface texture. Several varieties of this layer are often used to provide a good, new noise reducing surface layer.
Difference between mixtures
Figure 1: Difference in structure between AC, SMA, BBTM and PA.
Figure 2: Difference between sand skeleton and stone skeleton mixtures
Figure 3: Structure of double- layered Porous Asphalt
Asphalt mixtures can be produced at various temperatures.
Cold mixesThese mixtures are produced with unheated aggregate and bitumen emulsion or foamed bitumen.
Cold mixtureHalf Warm Asphalt
Half-warm mixtures are: produced with heated aggregate at a mixing temperature (of the mix) between approximately 70 °C and roughly 100 °C.
Warm Mix Asphalt
The Warm Mix Asphalt (WMA) mixtures are produced and mixed at temperatures roughly between 100 and 140 °C. These mixes have properties and performance which are equivalent to conventional Hot Mix Asphalt.
Due to the lower production temperature compared to an equivalent Hot Mix Asphalt (HMA), less energy is needed for the production. It is also paved at a lower temperature than HMA resulting in lower emissions, lower exposure and improved working conditions for the crew. This lower exposure supports the goal of the European asphalt industry in reducing bitumen fumes during paving operations.For more information about WMA: Warm Mix Asphalt - EAPA Position Paper (2010)
Hot Mix Asphalt
The conventional “Hot Mix asphalt” mixtures are produced and mixed at temperatures roughly between 120 and 190 °C The production temperatures of Hot Mix Asphalt depend on the bitumen used.
More info can be found in: