Asphalt concrete

Asphalt concrete is a composite material. It is commonly used during construction projects such as airports, road surfaces and parking lots. It consists of asphalt that is used as a binder and mineral aggregate. The components are mixed together then laid down in several layers. Finally, the asphalt must be compacted.

The terms "asphaltic concrete or asphalt", "bituminous asphalt concrete" and also the abbreviation "AC" are typically used in engineering and construction literature and documents. Asphalt concrete pavements are called just "asphalt" by laypersons. That’s because they tend to associate the term concrete only with Portland cement concrete. Engineering definition of concrete is any composite material which is composed of mineral aggregate and glued together with a binder. The binders are Portland cement, asphalt. Sometimes even epoxy is used as a binder. Informally, asphalt concrete is sometimes referred to as "blacktop". Especially in North America.

Mixture formulations

Mixing of aggregate and asphalt is performed in one of several ways:

Hot mix asphalt concrete. Commonly it’s abbreviated as HMA or HMAC. This kind of mixing is produced by heating the asphalt binder to achieve decrease in its viscosity. Than the aggregate dries to remove moisture from it prior to mixing. Mixing is normally performed with the aggregate at about 150 °C (roughly 300 °F) for virgin asphalt and 166 °C (330 °F) for polymer modified asphalt. The asphalt cement has 95 °C (200 °F). Compaction and paving should be performed when the asphalt is sufficiently hot. Paving is restricted to summer months in many countries because in cold season the compacted base will dissipate the heat of the asphalt too fast before it is packed to the optimal air content. HMAC is the form of asphalt concrete which is most commonly used on pavements with high traffic like those on airfields, major highways and racetracks.

Warm mix asphalt concrete is commonly abbreviated as WAM or WMA. It is produced by adding either waxes, zeolites or asphalt emulsions to the mixture. This allows much lower laying and mixing temperatures and results in lower consumption of energy. Thus less aerosols, carbon dioxide and vapours are released. Working conditions are improved. Lower laying-temperature also tend to lead to more rapid accessibility of the surface for use. And that is important for construction of sites with critical time schedules. Usage of similar additives in hot mixed asphalt (described above) may afford to easier compaction process. Also this allows longer hauls or cold weather paving.

Cold mixture asphalt concrete is produced via emulsifying the asphalt in water with (typically) soap prior to mixing with the aggregate concrete. The asphalt is less viscous while in its emulsified state. So the mixture is easy to compact and work with. The emulsion will break after water evaporates and the cold mix will take on the properties of cold HMAC. Cold mix is typically used on lesser trafficked service roads and as a patching material.

To produce cut-back asphalt concrete the binder is dissolved in kerosene or another lighter fraction of petroleum before  mixing with the aggregate. When it is in dissolved state the asphalt is less viscous. T the mix is easy to compact and work with. When the mix is laid down to the ground the lighter fraction evaporates quickly.

Sheet asphalt or mastic asphalt concrete is produced by heating blown hard grade bitumen (oxidation) in a mixer (green cooker) until it’ll become a viscous liquid. Now the aggregate mix is added.

The bitumen aggregate mixture is matured (cooked) for 6-8 hours. When it is ready the mastic asphalt mixer should be transported to the work site where experienced workers empty the mixer. Then the machine lays the mastic asphalt content on to the destination surface. Mastic asphalt content can also be laid manually. Mastic asphalt concrete layers are typically laid to a thickness of around 20-30 mm (3⁄4–1 3⁄16 inches) for road and footpath applications. For roof and flooring applications it is laid to a thickness of approximately 10 mm (3⁄8 of an inch).

In addition to the aggregate and asphalt some additives, such as polymers, and antistripping agents may be used in conjunction to improve the properties of the ready product.

Natural asphalt concrete may be produced out of bituminous rock. These rocks are founded in some parts of the world. There the porous sedimentary rock near the surface impregnated the upwelling bitumen.

Asphalt concrete is often positioned as being 100% recyclable. Several recycling techniques have been developed to remove cracking and rejuvenate oxidized binders. Although the recycled material is generally not very smooth or water-tight and should be overlapped with a new layer of asphalt concrete. Asphalt concrete removed from a pavement is generally stockpiled for later use like a base course material. This reclaimed material is commonly known by the acronym 'RAP'. It’s used for reclaimed or recycled asphalt pavement. It is crushed to a consistent gradation and added to the mixing process of the HMA. Very few of asphalt concrete are actually disposed of in landfills. It happens that waste materials like rubber from old tires, are added to asphalt concrete like in the case with rubberized asphalt. But actually there is a concern that the hybrid material is not recyclable.

Asphalt deterioration includes potholes, alligator cracks, upheaval, rutting, raveling, , shoving, grade depressions and stripping. In rather cold climates the freezing of the groundwater underneath is often a cause of asphalt crack. It may happen even during the term of one winter. This process is called cryoturbation. Filling the cracks with bitumen temporarily fixes the cracks. But proper construction that allowing water to drain from under the road is capable to slow down this process.

Asphalt concrete pavements and especially those at airfields are sometimes called tarmac. This name was given for historical reasons. But they are not constructed using the macadam process and do not contain tar.

Performance characteristics

In terms of surface durability asphalt concrete has different performance characteristics, braking efficiency, tire wear and roadway noise. Appropriate asphalt performance characteristic can be obtained by friction coarse (FC-5) and the traffic level in categories A, B, C, D and E. Asphalt concrete is typically generating less roadway noise than Portland cement surfacing. It is generally less noisy comparing to chip seal surfaces. At higher operating speeds tire noise effects are amplified. The sound energy is generated through rolling friction. Then kinetic energy converts the energy of sound waves.