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Hardening of bitumen during storage, mixing and in service

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Hardening of bitumen during storage, mixing and in service

The circumstances under which hardening occurs vary considerably. During storage, the bitumen is in bulk at a high temperature for a period of days or weeks. During mixing, hot storage, transport and laying, the bitumen is a thin film at high temperature for a relatively short period. In service, the bitumen is a thin film at a low or moderate temperature for a very long time. The degree of exposure to the air of asphalts in service is important and depends on the void content of the mixture. In dense, well-compacted mixtures, the amount of hardening is relatively small, while asphalts that have a more open constitution, such as porous asphalt, will undergo significant hardening. The degree of exposure to air for surface dressings or sprayed seals is potentially much higher than is the case with dense asphalt layers, and is influenced by the mosaic achieved by the aggregate and the absorptivity of the aggregates.

Hardening of bitumen in bulk storage

Very little hardening occurs when bitumen is stored in bulk at high temperature. Data from Shell terminals shows that 3–4 kt of paving grade bitumen stored at 1508C in a 7 kt tank shows virtually no change in viscosity or penetration over a 4 week storage period. This is because the surface area of the bitumen that is exposed to oxygen is very small in relation to the volume. However, if the bitumen is being circulated and is falling from the pipe entry at the top of the tank to the surface of the bitumen, significant hardening may occur. This arises because the surface area of the bitumen will be relatively large as it falls from the entry pipe, exposing it to the action of the oxygen.

Hardening of bitumen in the pavement and on the road

As explained above, a significant amount of bitumen hardening occurs during mixing and, to a lesser extent, during hot storage and transportation. However, hardening of the binder will continue in the pavement until some limiting value is reached. This behaviour is described as ‘long term ageing’ which shows the ageing index of the bitumen after mixing, storage, transport, paving and subsequent service.