From the FHWA publication, Highway Statistics 2005:
Paved Roads surfaced with asphalt is now just over 97%. The increase in mainly due to the miles of unpaved roads being surfaced with asphalt. The raw numbers are as follows:
Asphalt is a sticky, black and highly viscous liquid or semi-solid that is present in most crude petroleums and in some natural deposits. There is some disagreement among chemists regarding the structure of asphalt, but it is most commonly modeled as a colloid, with asphaltenes as the dispersed phase amaltenesnd maltenes as the continuous phase. In U.S. terminology, asphalt (or asphalt cement) is the carefully refined residue from the distillation process of selected crude oils. Outside of North America, the product is called bitumen.
The primary use of asphalt (bitumen) is in road construction, where it is used as the glue or binder for the aggregate particles. The road surfacing material is usually called 'asphalt concrete' in North America or simply 'asphalt' elsewhere. The apparent interchangeability of the words 'asphalt' and 'bitumen' causes a lot of confusion outside of the road construction industry despite quite clear definitions within industry circles.
Mixing of asphalt and aggregate is accomplished in one of several ways:
Hot mix asphalt concrete (HMA) is produced by heating the asphalt in order to decrease its viscosity and drying the aggregate to remove moisture from it prior to mixing. Mixing is generally performed with the aggregate at about 300 °F (roughly 150 °C), and the asphalt cement at 200 °F (95 °C). Paving and compaction must be performed while the asphalt is sufficiently hot. In many states paving is restricted to summer months because in winter the compacted base will cool the asphalt too much before it is packed to the optimal air content. HMA is the form of asphalt concrete most commonly used on highly trafficked pavements such as those on major highways and airfields.
Nearly all asphalt in California is "hot mix", although much of it is starting to be replaced by warm mix asphalt.
Warm mix asphalt concrete (WMA) is produced by adding either zeolites, waxes, or asphalt emulsions to the mix. This allows significantly lower mixing and laying temperatures and results in lower consumption of fossil fuels, thus releasing less carbon dioxide, aerosols and vapours. Not only are working conditions clearly improved, but the lower laying-temperature also leads to more rapid availability of the surface for use, which is important for construction sites with critical time schedules.
Escondido Materials was the first Southern California asphalt facility to offer warm mix asphalt. Our first test of Warm Mix Asphalt was in 2006.
Cold mix asphalt concrete is produced by emulsifying the asphalt in water with (essentially) soap prior to mixing with the aggregate. While in its emulsified state the asphalt is less viscous and the mixture is easy to work and compact. The emulsion will break after enough water evaporates and the cold mix will, ideally, take on the properties of cold HMA. Cold mix is commonly used as a patching material and on lesser trafficked service roads.
Road repair crews often use "cold mix" in small patches.
For more information: http://en.wikipedia.org/wiki/Asphalt_concrete
Warm-mix asphalt has the potential to allow the producers of hot-mix asphalt pavement material to lower the temperatures at which the material is mixed and placed on the road. Reductions of 50 to 100 degrees Fahrenheit have been documented. Such drastic reductions would have the obvious benefits of cutting fuel consumption and decreasing the production of greenhouse gases.
In addition, research at the National Center for Asphalt Technology and elsewhere has shown that lowering the production temperature can drastically reduce the production of emissions. By cutting emissions, warm-mix asphalt would therefore improve conditions for workers and enhance relationships with neighbors.
Escondido Asphalt was the first Southern California asphalt facility to offer Warm Mix asphalt. Our first test of Warm Mix Asphalt was in 2006.
For more information: http://www.hotmix.org/view_article.php?ID=536