Warm mixed asphalt (WMA) can be produced using foam (or foamed) bitumen. During the WMA preparation, it is possible to produce and apply asphalt at temperatures lower than standard temperatures in hot mix asphalt (HMA), and the asphalt production temperature can be diminished by up to 25%. This temperature is between 140 ℃ and 160 ℃. In this technique, bitumen should appropriately cover the aggregates. However, it is possible to produce asphalt of high quality in terms of workability and compactness (density). This technique has been introduced as new technology. Foam bitumen is also used to fix the soil in road construction operations.
Foam bitumen is prepared by injecting a small amount of cold water into the hot bitumen with the help of a nozzle. As a result of the rapid expansion of water vapors, microscopic bubbles are formed inside the bitumen (what is bitumen) and expand its volume. Under this condition, bitumen covers the rock material easily and at a lower temperature and reduces its viscosity. This technique is also applied in older plants.
WMAs are produced by reducing the viscosity of bitumen to mix with aggregates at a lower temperature than HMAs. Foam bitumen is amongst the various techniques of reducing the bitumen viscosity, which increases the efficiency of bitumen.
Chemical additives can additionally be used to prepare WMA. These materials are added in the same manner employed for the preparation of ordinary bitumen.
Advantages of WMA include the following:
• Reducing the fuel consumption for asphalt preparation
• Less emission of pollutants and toxic odors at lower temperatures
• Elevated compactness (density) of the pavement
• Improved working conditions at the asphalt production plant
The use of WMA decreases the severity of the initial aging of bitumen. It also reduces the bitumen hardness caused by oxidation and makes the asphalt surface more flexible.
WMAs, however, come with some shortcomings. For instance, the stone materials may not be dried adequately, and a proper bond may not be formed between the bitumen and the materials due to diminished temperatures. To fix this problem, additives can be used, although they will be expensive.