Asphalt concrete
[3] It consists of mineral aggregate bound together with bitumen (a substance also independently known as asphalt, pitch, or tar), laid in layers, and compacted.
Natural asphalt (Ancient Greek: ἄσφαλτος (ásphaltos)) 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.
[4] The Procession Street of Babylonian King Nabopolassar, c. 625 BC, leading north from his palace through the city's wall, being described as being constructed from burnt brick and asphalt.
In 1829 natural Seyssel asphalt mixed with 7% aggregate, to create an asphat-mastic surface was used for a footpath at Pont Morand, Lyons, France, the technique spreading to Paris in 1835, London, England, in 1836, and the Philadelphia, USA, in 1838.
[6] A two mile stretch of a gravel constructed road, running out of Nottingham, and Huntingdon High Street, were experimentally covered is natural asphalt during the 1840s.
[6] 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 by Belgian-American chemist Edward De Smedt, at Columbia University, in 1870, obtaining a pair of U.S. patents for the material and method of hardening.
[9][10] Civil Engineer, Surveyor, and an English county Highway board member, Edgar Purnell Hooley created a process and engine to combine a synthetic, refined petroleum tar, and resin, with Macadam aggregates (gravel, portland cement, crushed rocks, and blast furnace slag) in a steam heated mixer, at 212 °F, and through a heated reservoir, conduits, and meshes, create a machine and material that can be applied to form a road surface, filing a UK patent, in 1902, for his improvement.
Different types of asphalt concrete have different performance characteristics in roads in terms of surface durability, tire wear, braking efficiency and roadway noise.
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.
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.
[24][25] With regard to structural performance, the asphalt behaviour depends on a variety of factors including the material, loading and environmental condition.
Factors that cause asphalt concrete to deteriorate over time mostly fall into one of three categories: construction quality, environmental considerations, and traffic loads.
Environmental factors include heat and cold, the presence of water in the subbase or subgrade soil underlying the pavement, and frost heaves.
Paradoxically, high heat and strong sunlight also cause the asphalt to oxidize, becoming stiffer and less resilient, leading to crack formation.
Some jurisdictions pass frost laws to reduce the allowable weight of trucks during the spring thaw season and protect their roads.
Slowly moving vehicles stress the road over a longer period of time, increasing ruts, cracking, and corrugations in the asphalt pavement.
Research has demonstrated that RAP and RAS can replace the need for up to 100% of the virgin aggregate and asphalt binder in a mix,[34] but this percentage is typically lower due to regulatory requirements and performance concerns.
[32] For this reason, asphalt mixes containing RAP and RAS are prone to exhibiting lower workability and increased susceptibility to fatigue cracking.
[38][34] Practicing proper storage and handling, such as by keeping RAP stockpiles out of damp areas or direct sunlight, is also important in avoiding quality issues.
[34][32] The binder aging process may also produce some beneficial attributes, such as by contributing to higher levels of rutting resistance in asphalts containing RAP and RAS.
[34] Research has demonstrated that the use of rejuvenators at optimal doses can allow for mixes with 100% recycled components to meet the performance requirements of conventional asphalt concrete.
Crumb rubber, generated from recycled tires, has been demonstrated to improve the fatigue resistance and flexural strength of asphalt mixes that contain RAP.
The strategies presented in the study include the use of plastics, particularly high-density polyethylene, in asphalt binders, and the use of glass, brick, ceramic, and marble quarry waste in place of traditional aggregate.
