Bitumen and asphalt

11_B-Asphalt07.jpgBitumen is a crucial component of asphalt - the most widely used material for constructing and maintaining roads in the world. There are over 4,000 hot mix asphalt plants in Europe alone, producing some 300 million tonnes of asphalt per year.

Asphalt is typically a mixture of approximately 95% aggregate particles and sand, and 5% bitumen, which acts as the binder, or glue. The viscous nature of the bitumen allows the asphalt to sustain significant flexibility, creating a very durable surface material.

There are many different types of asphalt, each with its own combination of different amounts and type of bituminous binder and mineral aggregate, and each type of asphalt has performance characteristics appropriate for specific applications. Thus, for each application there is a suitable asphalt mixture available.

Asphalt is totally recyclable and recycling has increased significantly in recent years. Quantities recycled directly back into road surfaces vary from country to country, but can be as high as 70%.
Asphalt is routinely milled and re-laid along with fresh materials, saving money and preserving non-renewable natural resources.

Built to last

Modern asphalts roads, with a structurally solid base course and protective replaceable surface wearing course, are now designed to last for over 40 years and with correct surface maintenance they can, and do, last even longer. The structure should be able to withstand exposure to traffic and the environment in such a way that structural distress mechanisms are minimised.

Structural design

A typical asphalt road construction is multi-layered in form, comprising bitumen-bound and unbound materials. Essentially,
the lower indigenous subgrade layer is covered by a bound or unbound sub-base, providing drainage and frost protection for the subgrade, and a road base  layer upon which the asphalt layers are laid as a final surface coating. The structural design of a pavement relates to the ability of the road to carry the imposed loads without the need for excessive maintenance.

 Pavement structure.jpg

 An asphalt road is constructed in layers for optimum load distribution, and allows the stress and resultant strain from the
vehicles above to be transmitted through the road structure, which then spreads and lessens with depth. In order to achieve this, stronger and consequently more expensive materials are used in the upper levels, with relatively low strength materials being used in the lower layers. It is also important that a good bond is achieved between all of the layers to ensure the road structure acts as a single structural entity with good bearing capacity. Additionally, the nearer the surface of the road the flatter the profile must be, as an uneven surface will be uncomfortable for vehicle occupants and will wear more quickly. Each time a vehicle hits a bump, it creates a dynamic loading up to three times the static loading that would be imposed by the vehicle and therefore is significantly more damaging.

Structural layers

The asphalt layers consist of three tiers - a surface course, a binder course and an asphalt base course - and together these consitute the top layer of the road structure.

There are a wide range of surface course products available, and these wearing mixtures must be designed to have sufficient
stability and durability to withstand the appropriate traffic loads and the detrimental effects of environmentally-induced stresses - such as air, water and temperature changes - without exhibiting cracking, rutting or other failure modes. Their usage also depends on specific requirements, local conditions and functional characteristics, such as traffic levels, skid resistance, noise reduction and durability. In some cases, rapid drainage of surface water is desired, while in other cases the wearing course should be impermeable, to keep water out of the road structure.

The binder course is an intermediate layer.
It is designed to reduce rutting and withstand the highest stresses that occur about 50-70 mm below the surface course layer. Binder mixtures typically use a large aggregate size (19-38 mm) with a corresponding lower asphalt binder content to produce a combination of stability and durability.

The asphalt base course mixtures have a maximum aggregate size (up to 75 mm) and an even lower asphalt binder content, providing adequate durability since this layer is not exposed to the environment.

The road base course is perhaps the most important structural layer, and is specifically designed to effectively
distribute traffic and environmental loading, to ensure that underlying unbound layers are not exposed to excessive stresses and strains. The road base course should also exhibit long-life characteristics, ensuring that fatigue of the structure is resisted for as long as possible and no damage develops.

The sub-base and subgrade layers constitute the foundations of the road structure, and since the formation
and sub-soil often comprise of relatively weak materials, it is of utmost importance that the damaging loadings are effectively eliminated by the layers above. These sub-base layers consist of unbound materials, such as indigenous soil, crushed or uncrushed aggregate, or re-used secondary material.

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