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Macadam is a type of
road construction A road is a linear way for the conveyance of traffic that mostly has an improved surface for use by vehicles (motorized and non-motorized) and pedestrians. Unlike streets, the main function of roads is transportation. There are many types of ...
, pioneered by Scottish engineer John Loudon McAdam around 1820, in which crushed stone is placed in shallow, convex layers and compacted thoroughly. A binding layer of stone dust (crushed stone from the original material) may form; it may also, after rolling, be covered with a cement or bituminous binder to keep dust and stones together. The method simplified what had been considered state-of-the-art at that point.


Predecessors


Pierre-Marie-Jérôme Trésaguet

Pierre-Marie-Jérôme Trésaguet Pierre-Marie-Jérôme Trésaguet (15 January 1716 – 1796) was a French engineer. He is widely credited with establishing the first scientific approach to road building about the year 1764. Among his innovations was the use of a base layer of ...
is sometimes considered the first person to bring post-Roman science to
road A road is a linear way for the conveyance of traffic that mostly has an improved surface for use by vehicles (motorized and non-motorized) and pedestrians. Unlike streets, the main function of roads is transportation. There are many types of ...
building. A Frenchman from an engineering family, he worked paving roads in Paris from 1757 to 1764. As chief engineer of road construction of
Limoges Limoges (, , ; oc, Lemòtges, locally ) is a city and Communes of France, commune, and the prefecture of the Haute-Vienne Departments of France, department in west-central France. It was the administrative capital of the former Limousin region ...
, he had opportunity to develop a better and cheaper method of road construction. In 1775, Tresaguet became engineer-general and presented his answer for road improvement in France, which soon became standard practice there. Paperback Trésaguet had recommended a roadway consisting of three layers of stones laid on a crowned subgrade with side ditches for drainage. The first two layers consisted of angular hand-broken aggregate, maximum size , to a depth of about . The third layer was about thick with a maximum aggregate size of .Lay (1992), p.73 This top level surface permitted a smoother shape and protected the larger stones in the road structure from iron wheels and horse hooves. To keep the running surface level with the countryside, this road was put in a trench, which created drainage problems. These problems were addressed by changes that included digging deep side ditches, making the surface as solid as possible, and constructing the road with a difference in elevation (height) between the two edges, that difference being referred to interchangeably as the road's camber or cross slope.


Thomas Telford

Thomas Telford, born in Dumfriesshire
Scotland Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to th ...
, was a surveyor and engineer who applied Tresaguet's road building theories. In 1801 Telford worked for the
Commission of Highland Roads and Bridges The Commissioners of Highland Roads and Bridges (formally the Commissioners for Roads and Bridges in the Highlands of Scotland) was created in 1803 to take responsibility for the construction and maintenance of the long-distance roads in the Scott ...
. He became director of the Holyhead Road Commission between 1815 and 1830. Telford extended Tresaguet's theories, but emphasized high-quality stone. He recognized that some of the road problems of the French could be avoided by using cubical stone blocks.Lay (1992), p.74 Telford used roughly partially shaped paving stones (pitchers), with a slight flat face on the bottom surface. He turned the other faces more vertically than Tresaguet's method. The longest edge was arranged crossways to the traffic direction, and the joints were broken in the method of conventional
brickwork Brickwork is masonry produced by a bricklayer, using bricks and mortar. Typically, rows of bricks called ''courses'' are laid on top of one another to build up a structure such as a brick wall. Bricks may be differentiated from blocks by ...
, but with the smallest faces of the pitcher forming the upper and lower surfaces. Broken stone was wedged into the spaces between the tapered perpendicular faces to provide the layer with good lateral control. Telford kept the natural formation level and used masons to camber the upper surface of the blocks. He placed a layer of stone no bigger than on top of the rock foundation. To finish the road surface he covered the stones with a mixture of gravel and broken stone. This structure came to be known as "Telford pitching." Telford's road depended on a resistant structure to prevent water from collecting and corroding the strength of the pavement. Telford raised the pavement structure above ground level whenever possible. Where the structure could not be raised, Telford drained the area surrounding the roadside. Previous road builders in Britain ignored drainage problems and Telford's rediscovery of drainage principles was a major contribution to road construction.Lay (1992), p.75 Notably, around the same time, John Metcalf strongly advocated that drainage was in fact an important factor in road construction, and astonished colleagues by building dry roads even through marshland. He accomplished this by incorporating a layer of brushwood and heather.


Advent of macadam


John McAdam

John Loudon McAdam was born in Ayr,
Scotland Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to th ...
, in 1756. In 1787, he became a trustee of the Ayrshire Turnpike in the Scottish Lowlands and during the next seven years his hobby became an obsession. He moved to
Bristol Bristol () is a city, ceremonial county and unitary authority in England. Situated on the River Avon, it is bordered by the ceremonial counties of Gloucestershire to the north and Somerset to the south. Bristol is the most populous city i ...
, England, in 1802 and became a Commissioner for Paving in 1806. On 15 January 1816, he was elected surveyor-general of roads for the Bristol turnpike trust and was now responsible for 149 miles of road. He then put his ideas about road construction into practice, the first 'macadamised' stretch of road being Marsh Road at Ashton Gate, Bristol.A. W. Skempton (2002). A Biographical Dictionary of Civil Engineers in Great Britain and Ireland: 1500–1830. p.416. Thomas Telford, 2002 He also began to actively propagate his ideas in two booklets called ''Remarks (or Observations) on the Present System of Roadmaking'', (which ran nine editions between 1816 and 1827) and ''A Practical Essay on the Scientific Repair and Preservation of Public Roads,'' published in 1819.


McAdam's methods

McAdam's method was simpler, yet more effective at protecting roadways: he discovered that massive foundations of rock upon rock were unnecessary, and asserted that native soil alone would support the road and traffic upon it, as long as it was covered by a road crust that would protect the soil underneath from water and wear. Unlike Telford and other road builders of the time, McAdam laid his roads as level as possible. His road required only a rise of from the edges to the centre. Cambering and elevation of the road above the water table enabled rain water to run off into ditches on either side.McAdam (1824), p. 38 Size of stones was central to the McAdam's road building theory. The lower road thickness was restricted to stones no larger than . The upper layer of stones was limited to stones in diameter; these were checked by supervisors who carried scales. A workman could check the stone size himself by seeing if the stone would fit into his mouth. The importance of the 2 cm stone size was that the stones needed to be much smaller than the width of the iron carriage tyres that travelled on the road. McAdam believed that the "proper method" of breaking stones for utility and rapidity was accomplished by people sitting down and using small hammers, breaking the stones so that none of them was larger than in weight. He also wrote that the quality of the road would depend on how carefully the stones were spread on the surface over a sizeable space, one shovelful at a time.McAdam (1824), pp. 39–40 McAdam directed that no substance that would absorb water and affect the road by frost should be incorporated into the road. Neither was anything to be laid on the clean stone to bind the road. The action of the road traffic would cause the broken stone to combine with its own angles, merging into a level, solid surface that would withstand weather or traffic.McAdam (1824), p. 41 Through his road-building experience, McAdam had learned that a layer of broken angular stones would act as a solid mass and would not require the large stone layer previously used to build roads. Keeping the surface stones smaller than the tyre width made a good running surface for traffic. The small surface stones also provided low stress on the road, so long as it could be kept reasonably dry.Lay (1992), pp. 76–77


First macadam in North America

The first macadam road built in the United States was constructed between Hagerstown and Boonsboro, Maryland, and was named at the time Boonsborough Turnpike Road. This was the last section of unimproved road between
Baltimore Baltimore ( , locally: or ) is the List of municipalities in Maryland, most populous city in the U.S. state of Maryland, fourth most populous city in the Mid-Atlantic (United States), Mid-Atlantic, and List of United States cities by popula ...
on the
Chesapeake Bay The Chesapeake Bay ( ) is the largest estuary in the United States. The Bay is located in the Mid-Atlantic (United States), Mid-Atlantic region and is primarily separated from the Atlantic Ocean by the Delmarva Peninsula (including the parts: the ...
to Wheeling on the Ohio River. Stagecoaches traveling the Hagerstown to Boonsboro road in the winter took 5 to 7 hours to cover the stretch. This road was completed in 1823, using McAdam's road techniques, except that the finished road was compacted with a cast-iron roller instead of relying on road traffic for compaction. The second American road built using McAdam principles was the Cumberland Road which was long and was completed in 1830 after five years of work.


McAdam's influence

McAdam's renown is due to his effective and economical construction, which was a great improvement over the methods used by his generation. He emphasized that roads could be constructed for any kind of traffic, and he helped to alleviate the resentment travelers felt toward increasing traffic on the roads. His legacy lies in his advocacy of effective road maintenance and management. He advocated a central road authority with trained professional officials who could be paid a salary that would keep them from corruption. These professionals could give their entire time to these duties and be held responsible for their actions.


Water-bound macadam

McAdam's road building technology was applied to roads by other
engineer Engineers, as practitioners of engineering, are professionals who invent, design, analyze, build and test machines, complex systems, structures, gadgets and materials to fulfill functional objectives and requirements while considering the l ...
s. One of these engineers was Richard Edgeworth, who filled the gaps between the surface stones with a mixture of stone dust and water, providing a smoother surface for the increased traffic using the roads. This basic method of construction is sometimes known as water-bound macadam. Although this method required a great deal of manual labour, it resulted in a strong and free-draining pavement. Roads constructed in this manner were described as "macadamized."


Tar-bound macadam

With the advent of
motor vehicle A motor vehicle, also known as motorized vehicle or automotive vehicle, is a self-propelled land vehicle, commonly wheeled, that does not operate on rails (such as trains or trams) and is used for the transportation of people or cargo. The ...
s, dust became a serious problem on macadam roads. The area of low air pressure created under fast-moving vehicles sucked dust from the road surface, creating dust clouds and a gradual unraveling of the road material. Claudy, C.H.
The Right Road—and Why
" The Independent, New York, Volume 99, July, August, September 1919, 228. Retrieved on 2009-11-3.
This problem was approached by spraying
tar Tar is a dark brown or black viscosity, viscous liquid of hydrocarbons and free carbon, obtained from a wide variety of organic matter, organic materials through destructive distillation. Tar can be produced from coal, wood, petroleum, or peat. ...
on the surface to create tar-bound macadam. In 1902 a Swiss doctor,
Ernest Guglielminetti Ernest Guglielminetti (born 23 November 1862, Brig-Glis; died 20 February 1943, Geneva) was a Swiss medical doctor. Biography He studied medicine in Switzerland and received his doctorate in Berne on 1886. Then he travelled around the world a ...
, came upon the idea of using tar from
Monaco Monaco (; ), officially the Principality of Monaco (french: Principauté de Monaco; Ligurian: ; oc, Principat de Mónegue), is a sovereign ''Sovereign'' is a title which can be applied to the highest leader in various categories. The word ...
's gasworks for binding the dust. Later a mixture of coal tar and ironworks
slag Slag is a by-product of smelting ( pyrometallurgical) ores and used metals. Broadly, it can be classified as ferrous (by-products of processing iron and steel), ferroalloy (by-product of ferroalloy production) or non-ferrous/ base metals (by ...
, patented by Edgar Purnell Hooley as tarmac, was introduced. A more durable road surface (modern mixed
asphalt Asphalt, also known as bitumen (, ), is a sticky, black, highly viscous liquid or semi-solid form of petroleum. It may be found in natural deposits or may be a refined product, and is classed as a pitch. Before the 20th century, the term ...
pavement) sometimes referred to in the US as blacktop, was introduced in the 1920s. This pavement method mixed the aggregates into the asphalt with the binding material before they were laid. The macadam surface method laid the stone and sand aggregates on the road and then sprayed it with the binding material. While macadam roads have now been resurfaced in most
developed countries A developed country (or industrialized country, high-income country, more economically developed country (MEDC), advanced country) is a sovereign state that has a high quality of life, developed economy and advanced technological infrastr ...
, some are preserved along stretches of roads such as the
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., federal district, five ma ...
'
National Road The National Road (also known as the Cumberland Road) was the first major improved highway in the United States built by the Federal Government of the United States, federal government. Built between 1811 and 1837, the road connected the Pot ...
. Because of the historic use of macadam as a road surface, roads in some parts of the United States (such as parts of
Pennsylvania Pennsylvania (; (Pennsylvania Dutch: )), officially the Commonwealth of Pennsylvania, is a state spanning the Mid-Atlantic, Northeastern, Appalachian, and Great Lakes regions of the United States. It borders Delaware to its southeast, Ma ...
) are often referred to as macadam, even though they might be made of
asphalt Asphalt, also known as bitumen (, ), is a sticky, black, highly viscous liquid or semi-solid form of petroleum. It may be found in natural deposits or may be a refined product, and is classed as a pitch. Before the 20th century, the term ...
or
concrete Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most ...
. Similarly, the term "tarmac" is sometimes colloquially applied to asphalt roads or aircraft runways.


See also

* History of road transport (covers the development of road-building techniques)


References


Further reading

*


External links


McAdam's publications as available at Internet Archive
{{Road types Asphalt Pavement engineering Scottish inventions Industrial Revolution