Asphalt concrete
Asphalt concrete is a composite material commonly used to surface roads, parking lots, airports, and the core of embankment dams. Asphalt mixtures have been used in pavement construction since the nineteenth century. It consists of mineral aggregate bound together with bitumen, laid in layers, and compacted.
The American English terms asphalt ''concrete, bituminous asphalt concrete, and bituminous mixture are typically used only in engineering and construction documents, which define concrete as any composite material composed of mineral aggregate adhered with a binder. The abbreviation, AC, is sometimes used for asphalt concrete but can also denote asphalt content or asphalt cement'', referring to the liquid asphalt portion of the composite material.
History
Natural asphalt has been known of and used since antiquity, in Mesopotamia, Phoenicia, Egypt, Babylon, Greece, Carthage, and Rome, to waterproof temple baths, reservoirs, aqueducts, tunnels, and moats, as a masonry mortar, to cork vessels, and surface roads.The Procession Street of Babylonian King Nabopolassar,, leading north from his palace through the city's wall, was described as being constructed from burnt brick and asphalt.
Natural asphalt covered and bonded cobblestones were used from 1824 in France as a means to construct roads, as were moulded asphalt cobbles or blocks, formed from rammed natural rock asphalt.
In 1829 natural Seyssel asphalt mixed with 7% aggregate, to create an asphat-mastic surface, was used for a footpath at Pont Morand, Lyon, France, the technique spreading to Paris in 1835, London, England, in 1836, and Philadelphia, United States, in 1838.
In 1834, John Henry Cassell & Company of Poplar, London, a pitch and varnish supplier, obtained an English patent for a method to surface roads with a layer of tar, covered by a layer of macadam, and sealed with a layer of tar and sand, and marketed the surface "lava stone for paving and waterproofing"; soon after being contracted to surface the approach road to Vauxhall bridge, and a road in Millwall, London.
In 1837, R. T. Claridge obtained a similar English patent, substituting Seyssel asphalt as the binder, having seen it employed in France and Belgium; he would subsequently form the Claridge's Patent Asphalte Company, in 1838.
A stretch of a gravel constructed road, running out of Nottingham, was experimentally covered in natural asphalt in 1840. This experiment was repeated in 1845 in Huntingdon High Street.
A macadam road surfaced with asphalt was constructed in 1852, between Paris and Perpignan, France, using Swiss Val de Travers rock asphalt.
In 1869, Threadneedle Street, in London, was resurfaced with Swiss Val de Travers rock asphalt.
A process to surface a packed sand road through application of heated natural asphalt mixed with sand, in a ratio of 1:5, rolling, and hardened through the application of natural asphalt mixed with a petroleum oil, was invented in 1870 at Columbia University by Belgian-American chemist Edward de Smedt, who obtained a pair of U.S. patents for the material and method of hardening.
Civil engineer, surveyor, and an English county highway board member, Edgar Purnell Hooley, created a process and engine to combine petroleum bitumen with macadam aggregates in a steam heated mixer, at, and through a heated reservoir, conduits, and meshes, create a machine and material that can be applied to form a road surface. He filed a UK patent, in 1902, for his improvement. Hooley founding a UK company to market the technology, where the term tar macadam, shortened to tarmac, was coined, after the name of his company Tar Macadam Syndicate Limited, derived from the combination of tar and macadam gravel composite mixtures.
Mixture formulations
Mixing of asphalt and aggregate is accomplished in one of several ways:; Hot-mix asphalt concrete : This is produced by heating the bitumen binder 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 for virgin asphalt and for polymer modified asphalt, and the asphalt cement at. Paving and compaction must be performed while the asphalt is sufficiently hot. In many locales paving is restricted to summer months because in winter the base will cool the asphalt too quickly before it can be packed to the required density. HMA is the form of asphalt concrete most commonly used on high traffic pavements such as those on major highways, racetracks and airfields. It is also used as an environmental liner for landfills, reservoirs, and fish hatchery ponds.
; Warm-mix asphalt concrete : This is produced by adding either zeolites, waxes, asphalt emulsions or sometimes water to the asphalt binder prior to mixing. This allows significantly lower mixing and laying temperatures and results in lower consumption of fossil fuels, thus releasing less carbon dioxide, aerosols and vapors. This improves working conditions, and lowers laying-temperature, which leads to more rapid availability of the surface for use, which is important for construction sites with critical time schedules. The usage of these additives in hot-mixed asphalt may afford easier compaction and allow cold-weather paving or longer hauls. Use of warm mix is rapidly expanding. A survey of US asphalt producers found that nearly 25% of asphalt produced in 2012 was warm mix, a 416% increase since 2009. Cleaner road pavements can be potentially developed by combining WMA and material recycling. Warm Mix Asphalt technology has environmental, production, and economic benefits.
; Cold-mix asphalt concrete: This is produced by emulsifying the bitumen in water with an emulsifying agent before mixing with the aggregate. While in its emulsified state, the bitumen 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 an HMA pavement. Cold mix is commonly used as a patching material and on lesser-trafficked service roads.
; Cut-back asphalt concrete: Is a form of cold mix asphalt produced by dissolving the binder in kerosene or another lighter fraction of petroleum before mixing with the aggregate. While in its dissolved state, the bitumen is less viscous and the mix is easy to work and compact. After the mix is laid down the lighter fraction evaporates. Because of concerns with pollution from the volatile organic compounds in the lighter fraction, cut-back asphalt has been largely replaced by asphalt emulsion.
; Mastic asphalt concrete, or sheet asphalt: This is produced by heating hard grade blown bitumen in a green cooker until it has become a viscous liquid after which the aggregate mix is then added. The bitumen aggregate mixture is cooked for around 6–8 hours and once it is ready, the mastic asphalt mixer is transported to the work site where experienced layers empty the mixer and either machine or hand lay the mastic asphalt contents on to the road. Mastic asphalt concrete is generally laid to a thickness of around for footpath and road applications and around for flooring or roof applications.
; High-modulus asphalt concrete, sometimes referred to by the French-language acronym EMÉ : This uses a very hard bituminous formulation, sometimes modified, in proportions close to 6% by weight of the aggregates, as well as a high proportion of mineral powder to create an asphalt concrete layer with a high modulus of elasticity. This makes it possible to reduce the thickness of the base layer up to 25% in relation to conventional bitumen, while offering as very high fatigue strengths. High-modulus asphalt layers are used both in reinforcement operations and in the construction of new reinforcements for medium and heavy traffic. In base layers, they tend to exhibit a greater capacity of absorbing tensions and, in general, better fatigue resistance.
In addition to the asphalt and aggregate, additives, such as polymers, and antistripping agents may be added to improve the properties of the final product.
Areas paved with asphalt concrete—especially airport aprons—have been called "the tarmac" at times, despite not being constructed using the tarmacadam process.
A variety of specialty asphalt concrete mixtures have been developed to meet specific needs, such as stone-matrix asphalt, which is designed to ensure a strong wearing surface, or porous asphalt pavements, which are permeable and allow water to drain through the pavement for controlling storm water.
Roadway performance characteristics
Different types of asphalt concrete have different performance characteristics in roads in terms of surface durability, tire wear, braking efficiency and roadway noise. In principle, the determination of appropriate asphalt performance characteristics must take into account the volume of traffic in each vehicle category, and the performance requirements of the friction course. In general, the viscosity of asphalt allows it to conveniently form a convex surface, and a central apex to streets and roads to drain water to the edges. This is not, however, in itself an advantage over concrete, which has various grades of viscosity and can be formed into a convex road surface. Rather, it is the economy of asphalt concrete that renders it more frequently used. Concrete is found on interstate highways where maintenance is highly crucial.Asphalt concrete generates less roadway noise than a Portland cement concrete surface, and is typically less noisy than chip seal surfaces. Because tire noise is generated through the conversion of kinetic energy to sound waves, more noise is produced as the speed of a vehicle increases. The notion that highway design might take into account acoustical engineering considerations, including the selection of the type of surface paving, arose in the early 1970s.
With regard to structural performance, the asphalt behaviour depends on a variety of factors including the material, loading and environmental condition. Furthermore, the performance of pavement varies over time. Therefore, the long-term behaviour of asphalt pavement is different from its short-term performance. The LTPP is a research program by the FHWA, which is specifically focusing on long-term pavement behaviour.