Thursday, January 29, 2009

What is Asphalt?

Properties

Asphalt is water repellant due to the bitumen which is part of the asphalt conglomerate.

The physical properties are dependent on the temperature of the material, as at high temperatures, asphalt has a very distinctive viscous behaviour, which becomes more and more elastic with dropping temperatures.

As the physical properties are mainly dependent on the used aggregates, they can vary in a large range. Typical values are:

  • heat conductivity: 0.8 - 1.2 W/mK
  • specific heat capacity: 850 - 1050 J/kgK

Usage

The main use of asphalt is the creation of pavements like roads or airfields. Due to it's physical properties, it can be used as a robust basement as well as a high-load surface.

Manufacturing

  1. The manufacturing of asphalt is done in several steps:
  2. predose: Depending on the asphalt recipe, the single aggregate components are weighed. Usually this is done using a belt weigher, as the material has to reach the next step in a continuous flow
  3. drying: In a rotation drying drum, the material is dried in a temperature between 140 and 190 °C. After the drying, the components are sifted and divided in several silos.
  4. weighing: As the drying can change the previous grain-size distribution curve, a final weighing has to take place before adding the single components to the mixer. This also allows for quick changing of the recipe. The binder is kept in heated tanks and dosed measuring the rate of flow.
  5. mixing: The mixer can be working either continuously or at intervals. The continuously working mixer is specially used for very high volumes as it can not be adapt quickly to changing volume and recipe needs. The interval mixer is fed with the aggregates using a charging screw, and synchrounously, the binder is injected into the mixing chamber.
  6. dispatching: The mixed material is stored in a heated silo which usually has several chambers to store different recipes.

Transport

Asphalt has to reach the construction site with a reasonably high temperature as it can not be used anymore when it's too cold due to its physical properties. The transportation has to be set up in a way to ensure the right temperatures until the construction site. On short distances between the asphalt plant and the construction site, usual trucks can be used. If the ambient temperature is too low or the the construction site is too far away and it's not ensured that the temperature can be kept until the site, special trucks have to be used. The simplest way to keep up the temperature is to cover the truck with a tarpaulin.

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