Culture set in stone and asphalt
Since the construction of the first tracks for vehicles, which archaeologists have traced back to around 3500 B.C., road networks have continued to expand. Many different techniques have been developed to surface them over the ages.
Two requirements need to be met for properly built roadways. Firstly, their course needs to be charted in advance – and to this end, areas may need to be cleared and obstacles removed. Secondly, they need to be surfaced. Following the invention of the wheel, the demands on these surfaces changed. Based on excavation finds and historical depictions of vehicles and wheels, archaeologists believe this must have occurred 3,500 years B.C., i.e. no less than 5,500 years ago. Evidence of the earliest planned road construction has been found in Greece and ancient Mesopotamia, the area between the Tigris and Euphrates rivers in modern Iraq, dubbed the "cradle of civilization."
Visitors to the "Autostadt" museum at Volkswagen's headquarters in Wolfsburg can take an access road to the Ritz-Carlton Hotel. Embedded in this road is an array of road surfaces from the past that drivers can experience for themselves. Upon closer inspection, they can also find panels explaining and illustrating the construction methods used. The first surfaced roads were often built on top of existing trails and then finished with gravel. Another method covered trails with wooden planks, rather like modern boardwalks. Since the late Stone Age these have allowed humans to safely traverse swamps and marshlands. If there was no way around them, Europe's extensive moors and river valleys of the time were bridged by elevated trails supported on poles several feet high.
One of the first streets built of stone blocks was simulated by the Pergamon Museum in Berlin using a computer model of the processional road to the Ishtar Gate in Babylon. Dating from about 620 B.C., the ancient paved road – a replica of which can also be viewed in Wolfsburg – is made of white limestone blocks embedded in asphalt atop three layers of brick. This substructure fulfils an important function, absorbing the forces that vehicles apply to the top layer and therefore helping to preserve the road surface.
This layered construction was further refined by the Romans, whose roads already had a substructure up to one meter in depth. The top layer or summa crusta often took the form of artfully offset polygonal plates made of basalt, although plain gravel could also be used. The nucleus consisted of a layer of gravel sand containing crushed brick, all stabilized with Roman concrete ("opus caementicium") – a mixture of lime, tuff and brick dust that hardens when water is added. Its most famous application is the almost 2,000-year-old dome of the Pantheon in Rome. The foundations of Roman roads consisted of coarsely hewn rectangular natural stones that were laid flat or stood on their sides.
Born in 1756, John Loudon McAdam set out to increase the longevity of roads. He ultimately produced a method that used multiple layers of gravel, with the consistency of the crushed stone being coarse at the base and increasingly finer towards the top. This design was the standard in nineteenth century Europe and became known as macadam in honor of its inventor. The term tarmacadam was subsequently used to denote almost all types of road surfaces that bonded gravel with tar or bitumen – giving us the modern word tarmac. The bonding agent was needed to stabilize the road surface because the upcoming generation of motorized vehicles fractured the top layer, raising clouds of dust.
Most road surfaces today are made with asphaltic concrete in various consistencies. Asphalt is actually a bonding agent for the rock mixtures used in road construction, but the term is now synonymous with the surface as a whole. The first concrete road in the United States was built in the Ohio town of Bellefontaine in 1894 and is still in use today. The first concrete road in Austria was opened in Amstetten in 1904, only to be demolished in 1915 when the surface proved too tiring for horses. Civil engineers still use the layered construction method of Roman roads. Beneath the asphalt top surface, which is bonded hydraulically using water, lies an intermediate layer of concrete-bonded gravel atop a bed of sand and crushed rock that protects the road from frost and ice.
In the 1980s, so-called drainasphalt was developed. With its larger cavities, the open-pore concrete inside was expected to absorb the noise of tires more effectively. However, it proved twice as expensive as conventional asphalt and only lasted six to eight years. As a result, today it is found almost exclusively on highway segments where sound insulation is paramount. Trials are also underway to test low-temperature asphalt, which will reduce energy consumption and CO2 emissions during production, and colored asphalts which, for example, can be used to distinguish cycle lanes. Unchanged for over 5,000 years, however, is the primary function of the surfaces: to secure roads so that travelers can reach their final destinations safely.
Dr. Laurin Paschek is Creative Director Content at the agency 3st kommunikation and was the owner of the editorial office delta eta in Frankfurt/Main until February 2020.