4 In 1816 John McAdam observed that it was the native soil that supported the weight of traffic which,when dry, would carry any weight without sinking. He advised that the native soil be made dry and a coveringimpenetrable to rain be placed over it. However, road maintenance was not given much priority due to the popularityof the railways, until the motor car superseded the horse and cart. Cars, however, accentuated the problem of dust,described by the medical journai The Lancet in 1907 as "the greatest modem plague".
5 Like so many other scientific advances, the solution came by accident. Tar mixed with stone had beenused in footpaths in certain parts of Britain in 1832, and tarred gravel was applied to roads in Nottingham in 1869,but the biggest breakthrough came in 1901. A surveyor called E. Purnell Hooley was visiting Derby Iron Worksnear Derby when he noticed a dust-free length of road produced by a burst tar barrel. The resulting pool of tar hadbeen covered with ironworks slag. Hooley experimented with blending hot slag and tar as a byproduct from thecoal industry and in 1902 patented the process produced by a company known as Tar MacAdam Syndicate Ltd. Thecompanys name was later changed to Tarmac.
6 Nowadays, blacktop materials are made up of bitumen from oil which is blended with rock, gravel or slag.A number of varieties have evolved for different uses in road construction, including hot-rolled asphalt forsurfacing major roads, dense bitumen macadam for lower layers of a road and open-textured macadam.Modern surfaces are bituminous-bound, graded stone supplied as a premix. Binders themselves have undergonetechnical developments. They are customised, ranging from soft to very hard to suit the traffic flow.
7 To accommodate higher traffic levels, either the thickness of the road must be increased or the materialsimproved. Hence the introduction within the last 10 years of heavy duty macadam in the road base which is threetimes as stiff as the dense bitumen and aggregate mix.
8 Alternatively, the structural design can be changed. For example, on an experimental reconstructionsection of the M6 at Bescot, West Midlands, the heavy duty "upside-down design" was introduced in the 1980s.Here, rolled asphalt overlays a thinner than normal road-base macadam, over a second rolled asphalt layer, all ofwhich lie on a sub-base which is again thinner than normal. This structure is thought to perform well due to thelower rolled asphalt layer being more resistant to deformation and inhibiting cracking at the bottom of the road base.
9 Another innovative idea is the use of geotextiles. In research geotextiles are being placed between the sub-grade soil and a drainage layer beneath the sub-base. The subgrade material is often clay and in the absence of thegeotextile could, over time, clog the sub-base and reduce its efficiency as a drainage layer. But geotextiles can alsohave structural uses, and could provide improved resistance to cracking arid rutting in roads.
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