Macadam is a type of road construction pioneered by the Scotsman
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...

 John Loudon McAdam
John Loudon McAdam
John Loudon McAdam was a Scottish engineer and road-builder. He invented a new process, "macadamisation", for building roads with a smooth hard surface that would be more durable and less muddy than soil-based tracks....

 in around 1820. The method simplified what had been considered state-of-the-art at that point. Single-sized aggregate
Construction Aggregate
Construction aggregate, or simply "aggregate", is a broad category of coarse particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates. Aggregates are the most mined material in the world...

 layers of stone with a coating of binder as a cementing agent are mixed in an open-structured macadam.

Pierre-Marie-Jérôme Trésaguet

Pierre-Marie-Jérôme Trésaguet
Pierre-Marie-Jérôme Trésaguet
Pierre-Marie-Jérôme Trésaguet 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 large stone covered with a thin layer of smaller stone...

 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.

Trésaguet had recommended a roadway consisting of three layers of stones laid on a crowned subgrade
In transport engineering, subgrade is the native material underneath a constructed road, pavement or railway track. It is also called formation level.The term can also refer to imported material that has been used to build an embankment....

 with side ditches for drainage. The first two layers consisted of angular hand-broken aggregate, maximum size 3 inches (76.2 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 inches (25.4 mm). 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 interchangably as the road's camber
Cant (road/rail)
The cant of a railway track or a road is the difference in elevation between the two edges...

 or cross slope
Cross slope
Cross slope is a geometric feature of pavement surfaces; the transversal slope [%] with respect to the horizon. It is a very important safety factor. One task is to make water run off the surface to a drainage system, as Cross Slope is the main contributor to Pavement Drainage gradient. Very low...


Thomas Telford

Thomas Telford
Thomas Telford
Thomas Telford FRS, FRSE was a Scottish civil engineer, architect and stonemason, and a noted road, bridge and canal builder.-Early career:...

 born in Dumfriesshire
Dumfriesshire or the County of Dumfries is a registration county of Scotland. The lieutenancy area of Dumfries has similar boundaries.Until 1975 it was a county. Its county town was Dumfries...

Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...

 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
Holyhead is the largest town in the county of Anglesey in the North Wales. It is also a major port adjacent to the Irish Sea serving Ireland....

 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 300 × 250 × 150 mm 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
Brickwork is masonry produced by a bricklayer, using bricks and mortar to build up brick structures such as walls. Brickwork is also used to finish corners, door, and window openings, etc...

, 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 150 millimetres (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
Gravel is composed of unconsolidated rock fragments that have a general particle size range and include size classes from granule- to boulder-sized fragments. Gravel can be sub-categorized into granule and cobble...

 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.

McAdam's background

John Loudon McAdam was born in Ayr
Ayr is a town and port situated on the Firth of Clyde in south-west Scotland. With a population of around 46,000, Ayr is the largest settlement in Ayrshire, of which it is the county town, and has held royal burgh status since 1205...

, Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...

 in 1756. In 1787 he became a trustee of the Ayrshire Turnpike in the Scottish Lowlands
Scottish Lowlands
The Scottish Lowlands is a name given to the Southern half of Scotland.The area is called a' Ghalldachd in Scottish Gaelic, and the Lawlands ....

 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.

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 30 feet (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.
Size of stones was central to the McAdam's road building theory. The lower 200 millimetres (7.9 in) road thickness was restricted to stones no larger than 75 millimetres (3 in). The upper 50 millimetres (2 in) layer of stones was limited to 20 millimetre (0.78740157480315 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
A carriage is a wheeled vehicle for people, usually horse-drawn; litters and sedan chairs are excluded, since they are wheelless vehicles. The carriage is especially designed for private passenger use and for comfort or elegance, though some are also used to transport goods. It may be light,...

 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 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.

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. 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.

McAdam's influence

McAdam's road building technology was applied to roads by other engineer
An engineer is a professional practitioner of engineering, concerned with applying scientific knowledge, mathematics and ingenuity to develop solutions for technical problems. Engineers design materials, structures, machines and systems while considering the limitations imposed by practicality,...

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 "macadamised."

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.

Tar-bound macadam

With the advent of motor vehicle
Motor vehicle
A motor vehicle or road vehicle is a self-propelled wheeled vehicle that does not operate on rails, such as trains or trolleys. The vehicle propulsion is provided by an engine or motor, usually by an internal combustion engine, or an electric motor, or some combination of the two, such as hybrid...

s, 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. This problem was approached by spraying tar
Tar is modified pitch produced primarily from the wood and roots of pine by destructive distillation under pyrolysis. Production and trade in tar was a major contributor in the economies of Northern Europe and Colonial America. Its main use was in preserving wooden vessels against rot. The largest...

 on the surface to create tar-bound macadam. Later a mixture of coal tar
Coal tar
Coal tar is a brown or black liquid of extremely high viscosity, which smells of naphthalene and aromatic hydrocarbons. Coal tar is among the by-products when coal iscarbonized to make coke or gasified to make coal gas...

 and ironworks slag
Slag is a partially vitreous by-product of smelting ore to separate the metal fraction from the unwanted fraction. It can usually be considered to be a mixture of metal oxides and silicon dioxide. However, slags can contain metal sulfides and metal atoms in the elemental form...

, patented by Edgar Purnell Hooley as tarmac
Tarmac is a type of road surface. Tarmac refers to a material patented by Edgar Purnell Hooley in 1901...

, was introduced.

A more durable road surface, modern mixed asphalt
Asphalt or , also known as bitumen, is a sticky, black and highly viscous liquid or semi-solid that is present in most crude petroleums and in some natural deposits, it is a substance classed as a pitch...

 pavement, known in the USA as blacktop
Asphalt concrete
Asphalt concrete is a composite material commonly used in construction projects such as road surfaces, airports and parking lots. It consists of asphalt and mineral aggregate mixed together, then laid down in layers and compacted...

, 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
Developed country
A developed country is a country that has a high level of development according to some criteria. Which criteria, and which countries are classified as being developed, is a contentious issue...

, some are preserved along stretches of roads such as the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

' National Road
National Road
The National Road or Cumberland Road was the first major improved highway in the United States to be built by the federal government. Construction began heading west in 1811 at Cumberland, Maryland, on the Potomac River. It crossed the Allegheny Mountains and southwestern Pennsylvania, reaching...


The first macadam road built in the United States was constructed between Hagerstown
Hagerstown, Maryland
Hagerstown is a city in northwestern Maryland, United States. It is the county seat of Washington County, and, by many definitions, the largest city in a region known as Western Maryland. The population of Hagerstown city proper at the 2010 census was 39,662, and the population of the...

 and Boonsboro, Maryland
Boonsboro, Maryland
Boonsboro is a town in Washington County, Maryland, United States, located at the foot of South Mountain. It nearly borders Frederick County and is proximate to the Antietam National Battlefield...

 and was named Boonsboro Turnpike
Toll road
A toll road is a privately or publicly built road for which a driver pays a toll for use. Structures for which tolls are charged include toll bridges and toll tunnels. Non-toll roads are financed using other sources of revenue, most typically fuel tax or general tax funds...

 Road. This was the last section of unimproved road between Baltimore
Baltimore is the largest independent city in the United States and the largest city and cultural center of the US state of Maryland. The city is located in central Maryland along the tidal portion of the Patapsco River, an arm of the Chesapeake Bay. Baltimore is sometimes referred to as Baltimore...

 on the Chesapeake Bay
Chesapeake Bay
The Chesapeake Bay is the largest estuary in the United States. It lies off the Atlantic Ocean, surrounded by Maryland and Virginia. The Chesapeake Bay's drainage basin covers in the District of Columbia and parts of six states: New York, Pennsylvania, Delaware, Maryland, Virginia, and West...

 to Wheeling on the Ohio River
Ohio River
The Ohio River is the largest tributary, by volume, of the Mississippi River. At the confluence, the Ohio is even bigger than the Mississippi and, thus, is hydrologically the main stream of the whole river system, including the Allegheny River further upstream...

. 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. The second American road built using McAdam principles was the Cumberland Road which was 73 miles long and required five years of work.

Due to uses of macadam as a road surface in former times, roads in some parts of the United States (as parts of Pennsylvania
The Commonwealth of Pennsylvania is a U.S. state that is located in the Northeastern and Mid-Atlantic regions of the United States. The state borders Delaware and Maryland to the south, West Virginia to the southwest, Ohio to the west, New York and Ontario, Canada, to the north, and New Jersey to...

) are often referred to as macadam, even though they might be made of asphalt or concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...

. Similarly, the term "tarmac" is sometimes colloquially misapplied to asphalt roads or aircraft runways.

See also

  • History of road transport
    History of road transport
    The history of road transport started with the development of tracks by humans and their beasts of burden.- Early roads :The first forms of road transport were horses, oxen or even humans carrying goods over tracks that often followed game trails, such as the Natchez Trace. In the Stone Age humans...

     – covers the development of road-building techniques
  • Crushed stone
    Crushed stone
    Crushed stone or angular rock is a form of construction aggregate, typically produced by mining a suitable rock deposit and breaking the removed rock down to the desired size using crushers...

External links

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