A complex interplay between societal change, the development of the internal combustion engine, and the impact of World War I, led to an explosion in the number of road vehicles in the immediate postwar years—and a totally inadequate nineteenth century legacy of roads to accommodate them. Following the first International Road Congress in 1923, vast and expensive road-building programs were undertaken in the U.S. and Europe, particularly in Germany, during the 1930s. After World War II highway construction continued to grow in an attempt to keep pace with the popularity of the car for private transport.
Concrete—strong in compression but weak in tension—is not particularly satisfactory as a running surface. It can easily crack, unlike tarmac, though in the 1960s its use as a surface did become widespread. Otherwise, however, concrete became omnipresent in twentieth century road construction, and in the myriad of bridges, large and small, associated with highway networks. Because of unreinforced concrete’s limitations, nineteenth century concrete bridges were built as arches, following the usual form of bridges in natural stone, which has similar strength and weakness. Paradoxically, it was the incorporation of the ‘‘rival’’ and far more expensive material steel—equally strong in compression but also with high tensile strength—that enabled concrete thus reinforced to become the twentieth century’s most widely used material for bridges carrying road and sometimes rail traffic.
The French engineer Franc¸ois Hennebique notably developed techniques for embedding steel reinforcing rods in concrete for structural effectiveness, and had built around 100 reinforced concrete bridges by 1900. Already his designs were exploiting the material’s potential for strength and slenderness in arch designs that were much thinner than was possible with stone or mass concrete; but the first, and arguably still the greatest, designer of reinforced concrete bridges was the Swiss, Robert Maillart. Though Maillart only built in his native country and despite the fact that none of his spans exceeded 90 meters, the elegance, economy, and inventiveness of his designs, combined with their fitness for purpose and for their often sublimely beautiful locations, made him one of the most influential twentieth century bridge designers following his death in 1940.
Even more than simple reinforcement, prestressing opened up new possibilities in concrete bridge building. As so often with technical innovation, the concept—of tightening metal rods or strands within concrete to further increase its tensile strength—had been around for a long time before it became a practical proposition. That it did so is due to the observation and tenacity of another French engineer, Eugene Freyssinet, who around 1910 first observed the tendency of concrete to ‘‘creep’’ (its continuing slow shrinkage after solidification) on his first reinforced concrete bridges. In later projects he introduced the practice of jacking the arch halves apart after casting, and inserting extra concrete at the crown between them to counter the effects of the shrinkage. The scale in particular of his Plougastel Bridge, completed in Brittany in 1930 and then by far the world’s largest reinforced concrete bridge, made it necessary for him to study and evaluate the effects of creep as exactly as possible. His researches led him to the view that ‘‘locking in’’ tensile strength by incorporating steel strands in the concrete, stretched to a precisely calculated extent, was a viable structural system—and indeed would effectively produce a new building material.
Freyssinet’s first prewar attempt to mass-produce and market prestressing failed, but after World War II he successfully built six single-span prestressed concrete bridges across the River Marne in France. Prestressed concrete, either preor posttensioned, rapidly became the material of choice for some short-, most medium-, and more rarely, some long-span bridges. In pretensioning, the concrete is poured around tendons that have already been stressed against an anchor frame, this being released when the concrete has hardened so that the tensile strength locked into the strands is imparted to the concrete adhering to them. In posttensioning, the strands are threaded through voids cast into already-hardened concrete, and then tightened.
Reinforced concrete and prestressed concrete are used in several structural forms in modern bridge building. For most short single spans, the concrete is cast as an arch or a solid slab in situ (literally ‘‘on site’’) on formwork and around the mesh of reinforcing rods. For simple spans from around 16 to 20 meters, the reinforced slab is usually cast with voids to lighten the weight of the concrete. Prestressing is normally introduced when longer spans are required. Such bridges can be anything from a single span across a road to the literally thousands that comprise the 38-plus kilometer Lake Pontchartrain Bridges in Louisiana.
Beams, of rectangular or T-shaped cross-section, are prestressed and precast offsite, craned into place on supporting piers, and topped with deck slabs. For yet longer spans, sections of prestressed concrete box girder (see the entry ‘‘Bridges, Steel’’) may be are joined together between supports of up to 200 meters and more. These bridges often have the appearance of a wide, shallow arch, though rarely do they act structurally as a true arch, in which forces are carried around and down the arch and into abutments. Instead, the structure acts as a beam, with gravity creating compression forces along the top of the span and tension forces along the bottom, which the prestressing withstands. The longest-span wholly prestressed concrete bridges are, however, true arches. The archetype is the 305-meter-span Gladesville Bridge in Sydney, completed across the Parramatta River in 1964. Its design was unusual, in that it followed the same voussoir principle that the Romans used for their masonry arches, in which wedge-shaped Stones were cut to form segments of a semicircle. In the case of the Gladesville Bridge, the voussoir units are hollow prestressed concrete boxes, each precast in the shape necessary to form the giant shallow arch of the main bridge structure, from the upper surface of which the even shallower curve of the precast road deck is carried on slender upright prestressed piers.
Even longer concrete spans do exist in which a concrete deck or concrete pylons may form part of a cable-stayed or a suspension bridge. However, as all bridges in these forms also incorporate steel, always in the hangers of both types and in the cables of suspension bridges, they are discussed in a separate entry, as are all-steel suspension bridges built in the early twentieth century.
Hydroelectric Power Generation It is estimated that about 50 percent of the economically exploitable hydroelectric resources, not including tidal resources, of North America and Western Europe have already been developed. Worldwide, however, the proportion is less than 15 percent. The size of hydroelectric power plants covers an extremely wide range, from small plants of a few megawatts to large schemes such as Kariba in Zimbabwe, which comprises eight 125 megawatt generating sets. More recently, power stations such as Itaipu on the Parana River between Brazil and Paraguay in South America were built with a capacity of 12,600 megawatts, comprising eighteen generating sets each having a rated discharge of approximately 700 cubic meters per second. Hydroelectric power has traditionally been regarded as an attractive option for power generation since fuel costs are zero; operating and maintenance costs are low; and plants have a long life—an economic life of 30 to 50 years for mechanical and electrical plant and 60 to 100 years for civil works is not unusual. Small-scale hydropower schemes (typically less than 10 megawatts per site) utilize rivers, canals, and streams. Large-scale schemes generally include dams and storage reservoirs, with the option of pumped storage schemes to generate power to match demand. Pumped storage schemes are however a net energy consumer, and should not be considered as renewable projects. Small hydroelectric installations are numerous in countries such as Scotland, South America, and China, for example, and may be operated by power generation companies or privately. Although some plants have been in service since the turn of the century, a considerable number of developments took place after 1945 and up to the mid-1970s, with a few, small, run-of-river developments having taken place since then. It is likely that the investment criteria that were applied in the later years of the twentieth century were more onerous than those set previously, and this has meant that new developments became more difficult to justify. The capital cost of ‘‘green field’’ (i.e., undeveloped and particularly unpolluted land) hydroelectric developments are higher than most alternative power generation schemes. Environmental concerns, for example over the Chinese government’s undertaking of building the Three Gorges Dam, the largest hydroelectric project ever undertaken, must also be considered. In terms of a straight financial comparison, small hydroelectric plants are difficult to justify where the ‘‘competition’’ is a generating plant on a developed nationwide grid system. The existence of the National Grid in the U.K., for example, has allowed the exploitation of significant economies of scale in conventional thermal and later the combined-cycle generating plant.
