Paved Roads (Edited from Wikipedia)

SUMMARY

Macadam is a type of road construction, pioneered by Scottish engineer John Loudon McAdam around 1820, in which single-sized crushed stone layers of small angular stones are placed in shallow lifts 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 binder to keep dust and stones together. The method simplified what had been considered state of the art at that point.

HISTORY

Pierre-Marie-Jérôme Trésaguet is sometimes considered the first person to bring post- Roman 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.

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 (a stone mixture), maximum size 3 inches, to a depth of about 8 inches. The third layer was about 2 inches thick with a maximum aggregate size of 1 inch.

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, born in Dumfriesshire Scotland, 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

1 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.

Telford used 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.

Broken stone was wedged into the spaces between the 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 6-inch (15 cm) layer of stone no bigger than 6 cm (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.

In other words, these were complex construction methods.

John McAdam

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. He moved to Bristol, England in 1802 and became a Commissioner for Paving in 1806. On 15 January 1816 he was elected Surveyor-General of roads for the Turnpike Trust, and was now responsible for 149 miles of road.

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.

2 THE MACADAM ROAD

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 30-foot-wide road required only a rise of 3 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.

Size of stones was central to the McAdam's road building theory. The lower 7.9-inch road thickness was restricted to stones no larger than 3.0 inches. The upper 2.0-inch layer of stones was limited to 0.79-inch 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 0.79-inch stone size was that the stones needed to be much smaller than the 4-inch width of the iron carriage tires 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 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 sizable space, one shovelful at a time.

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.

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.

3 Macadam Roads with Tar

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 sucked dust from the road surface, creating dust clouds and a gradual unraveling of the road material.

This problem was approached by spraying tar on the surface to create tar-bound macadam. On March 13, 1902 in Monaco, a Swiss doctor, Ernest Guglielminetti, came upon the idea of using tar from Monaco's Gasworks for binding the dust. 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) 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.

Because of the historic use of macadam as a road surface, 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.

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