Macadam

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For the Scottish family name, see McAdam (disambiguation). For the regions of imperceptible colour differences, see MacAdam ellipse.
File:Makadam.jpg
Construction of the first macadamized road in the United States (1823). In the foreground, workers are breaking stones "so as not to exceed 6 ounces in weight or to pass a two-inch ring".[1]

Macadam is a type of road construction pioneered by the Scotsman John Loudon McAdam in around 1820. The method simplified what had been considered state-of-the-art at that point. Single-sized aggregate layers of stone with a coating of binder as a cementing agent are mixed in an open-structured macadam.

Before McAdam

Pierre-Marie-Jérôme Trésaguet

Pierre-Marie-Jérôme Trésaguet is sometimes considered the first person to bring science to road 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 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.[2]

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 3 inches (76 mm), to a depth of about 8 inches (203 mm). The third layer was about 2 inches (51 mm) thick with a maximum aggregate size of 1 inch (25 mm).[3] 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. The drainage problem caused by that method were addressed by changes to road construction 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.[3]

Thomas Telford

Thomas Telford, born in Dumfriesshire Scotland,[4] was a surveyor and engineer who applied Tresaguet's road building theories. In 1801 Telford worked for the British Commission of Highlands Roads and Bridges. 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.[5]

Telford used 300×250×150mm partially-shaped 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, but with the smallest faces of the pitcher forming the upper and lower surfaces.[5]

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 150-millimetre (5.9 in) layer of stone no bigger than 60 millimetres (2.4 in) 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 these principles was a major contribution to road construction.[6]

Water-bound macadam

McAdam's background

File:John Loudon McAdam.jpg
John Loudon McAdam (1756–1836)[7]

John Loudon McAdam was born in Ayr, Scotland in 1756. In 1787 he became a trustee of the Ayrshire Turnpike in the Scottish Lowlands and during the next seven years this hobby became an obsession. As Surveyor-General of roads for the Bristol Turnpike in 1816, McAdam first put his ideas about road construction into major practice, the first 'macadamised' stretch of road being Marsh Road at Ashton Gate, Bristol. 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.[8]

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.[9]

Unlike Telford and other road builders of the time, McAdam laid his roads as level as possible. His 30-foot-wide (9.1 m) road required only a rise of three inches from the edges to the center. Cambering and elevation of the road above the water table enabled rain water to run off into ditches on either side.[10]

Photograph of Macadam Road, ca 1850's, Nicolaus, CA[11]

Size of stones was central to the McAdam's road building theory. The lower 200-millimetre (7.9 in) road thickness was restricted to stones no larger than 75 millimetres (3.0 in). The upper 50-millimetre (2.0 in) layer of stones was limited to 20 millimetres (0.79 in) size and stones 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 20 mm stone size was that the stones needed to be much smaller than the 100 mm width of the iron carriage tyres that travelled on the road.[6]

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 six ounces 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.[12]

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.[13]

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. By keeping the surface stones smaller than the tyre width, a good running surface could be created for traffic. The small surface stones also provided low stress on the road, so long as it could be kept reasonably dry.[14]

McAdam's influence

File:Fall Country Road (Macadam).jpg
Macadam country road[15]

McAdam's road building technology was applied to roads by other engineers. 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.[16] 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 "macadamised."[16]

The greatness of McAdam might be in his effective and economical construction, which was a great improvement over the methods used by his generation. McAdam's claim to fame might also be in his defeat of the resentment travellers had to the increased traffic on the roads and his advocacy of effective road maintenance and management. He emphasized that roads could be constructed for any kind of traffic. He advocated a central road authority and the trained professional official, who could be paid a salary that would keep him from corruption. This professional could give his entire time to his duties and be held responsible for his actions.[17]

Tar-bound macadam

New Macadam Road construction at McRoberts, KY: pouring tar, August 28, 1926.[18]

With the advent of motor vehicles, dust became a serious problem on macadam roads. The area of low air pressure created under fast-moving vehicles sucks dust from the road surface, creating dust clouds and a gradual unravelling of the road material.[19] This problem was approached by spraying tar on the surface to create tar-bound macadam. Later a mixture of coal tar and ironworks slag, patented by Edgar Purnell Hooley as tarmac, was introduced.

A more durable road surface, modern mixed asphalt pavement, known in the USA 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.[20] While macadam roads have now been resurfaced in most developed countries, some are preserved along stretches of roads such as the United States' National Road.[21]

The first macadam road built in the United States was constructed between Hagerstown and Boonsboro, Maryland and was named Boonsboro Turnpike Road. This was the last section of unimproved road between Baltimore on the Chesapeake Bay to Wheeling on the Ohio River. Stagecoaches travelling the Hagerstown to Boonsboro road in the winter took 5 to 7 hours to cover the 10-mile stretch. This road was built 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.[21] The second American road built using McAdam principles was the Cumberland Road which was 73 miles long and required five years of work.[21]

Due to uses of macadam as a road surface in former times, roads in some parts of the United States (as parts of Pennsylvania) are often referred to as macadam, even though they might be made of asphalt or concrete. Similarly, the term "tarmac" is sometimes colloquially misapplied to asphalt roads or aircraft runways.[22]

See also

References

  1. ↑ "1823 - First American Macadam Road" (Painting - Carl Rakeman) US Department of Transportation - Federal Highway Administration (Accessed 2008-10-10)
  2. ↑ Lay, Maxwell G (1992), Ways of the World: A History of the World's Roads and of the Vehicles That Used Them, New Brunswick, N.J.: Rutgers University Press, p. 73, ISBN 0-8135-1758-3, http://books.google.com/books?id=flvS-nJga8QC&pg=PA73&dq=world's+Tresaguet#v=onepage&q=Tresaguet&f=false, retrieved 18 June 2010 Paperback ISBN 0-8135-2691-4
  3. ↑ 3.0 3.1 Lay (1992), p.73
  4. ↑ Script error
  5. ↑ 5.0 5.1 Lay (1992), p.74
  6. ↑ 6.0 6.1 Lay (1992), p.75
  7. ↑ British Museum, Engraving by Charles Turner
  8. ↑ McAdam, John Loudon (1824), Remarks on the Present System of Road Making; With Observations, Deduced from Practice and Experience (8th ed.), London: Longman, Hurst, Rees, Orme, and Brown, Paternobter Row, http://books.google.com/books?id=a9RMAAAAYAAJ, retrieved 26 September 2011
  9. ↑ Craig, David, "The Colossus of Roads", Palimpsest (Strum.co.uk), http://www.strum.co.uk/palimps/macadam.htm, retrieved 18 June 2010
  10. ↑ McAdam (1824), p.38
  11. ↑ Photograph of Macadam Road, Nicolaus (Calif.), ca. 1850s, California Digital Library, http://content.cdlib.org/ark:/13030/kt9t1nd641/?brand=calisphere', retrieved 18 June 2010 Contributing institution: Sutter County Library
  12. ↑ McAdam (1824), pp.39-40
  13. ↑ McAdam (1824), p.41
  14. ↑ Lay (1992), pp.76-77
  15. ↑ Shari Weinsheimer (1 October 2009), Fall Country Road, Public Domain Pictures.net, http://www.publicdomainpictures.net/view-image.php?picture=fall-country-road&image=4237&large=1, retrieved 19 June 2010
  16. ↑ 16.0 16.1 John Loudon McAdam, bruzzone.org, http://www.bruzzone.org/macadam/jmca.html, retrieved 18 June 2010
  17. ↑ O'Flaherty, Coleman A (2002), Highways: the Location, Design, Construction and Maintenance of Road Pavements (4th ed.), Woburn, MA: Butterworth-Heinemann, p. 228, ISBN 0-7506-5090-7, http://books.google.com/books?id=Ren4sWQ3jKkC&pg=PA228&dq=%22whilst+mcadam%22#v=onepage&q=%22whilst%20mcadam%22&f=false, retrieved 18 June 2010
  18. ↑ SI Neg. CCC-2777. Date: 8/28/1926 Consolidation Coal Company collection, National Museum of American History (Smithsonian Institution), Likely Public Domain--http://creativecommons.org/ns#" about="http://www.flickr.com/photos/publicresourceorg/493732415"> attributionURL http://www.flickr.com/photos/publicresourceorg/ http://www.flickr.com/photos/publicresourceorg/ "license ref= http://creativecommons.org/licenses/by/2.0/
  19. ↑ Claudy, C.H. "The Right Road—and Why," The Independent, New York, Volume 99, July, August, September 1919, 228. Retrieved on 2009-11-3.
  20. ↑ Cavette, Chris, "Asphalt Paver", eNotes, http://www.enotes.com/how-products-encyclopedia/asphalt-paver/printHistory, retrieved 19 June 2010
  21. ↑ 21.0 21.1 21.2 "John Loudon MacAdam", Significant Scots (ElectricScotland.com), http://www.electricscotland.com/history/other/macadam_john.htm, retrieved 19 June 2010
  22. ↑ Stephen T. Muench, Joe P. Mahoney, Linda M. Pierce et al.,, "History", in ., WSDOT Pavement Guide, Washington State Department of Transportation, p. 2 in Module 1: Welcome and Introduction, http://training.ce.washington.edu/wsdot/Modules/01_introduction/01-2_body.htm, retrieved 19 June 2010

Further reading

External links

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