Macadam

Macadam is a type of road construction pioneered by Scottish engineer John Loudon McAdam c. 1820, in which crushed stone is placed in shallow, convex layers and compacted thoroughly.

[1] Trésaguet had recommended a roadway consisting of three layers of stones laid on a crowned subgrade with side ditches for drainage.

[2] 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.

[2] Thomas Telford, born in Dumfriesshire, Scotland,[3] was a surveyor and engineer who applied Tresaguet's road building theories.

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.

[4] Broken stone was wedged into the spaces between the tapered perpendicular faces to provide the layer with good lateral control.

Telford's road depended on a resistant structure to prevent water from collecting and corroding the strength of the pavement.

[6] 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.

[7] 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.

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.

Before it was macadamized, stagecoaches travelling the Hagerstown to Boonsboro road in the winter had taken 5 to 7 hours to cover the 10-mile (16 km) stretch.

He advocated a central road authority with trained professional officials who could be paid a salary that would keep them from corruption.

In 1902 a Swiss doctor, Ernest Guglielminetti, came upon the idea of using tar from Monaco's gasworks for binding the dust.

[19] 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 U.S. as blacktop, was introduced in the 1920s.