Reinforced concrete was in its infancy at the opening of the twentieth century, but it was very quickly adopted worldwide as an economic and versatile construction material. Employing fairly basic materials--sand, crushed stone or gravel, cement, and steel--it found use in all the existing aspects of construction, including buildings, roads, bridges, dams, reservoirs, and docks. It also served the century's new applications, such as air raid shelters and the pressure vessels of nuclear reactors. By the end of the twentieth century, concrete in its various forms--plain, reinforced, and pre-stressed--was probably the most widely used construction material in the world.
Although concrete had been used from at least Roman times, it was not until the last decades of the nineteenth century that the idea of reinforcing it was applied to construction. Until then, concrete was used as a cheaper and more versatile substitute for stone and brick masonry, with which it shared the properties of being strong in compression but weak and brittle when subject to tension or bending. Not until rods or bars of wrought iron, and later steel, were embedded in concrete was it considered worthwhile to use the resulting reinforced concrete with confidence for floor and roof slabs, beams, trusses, cantilevers, and other elements that work by bending.
Proprietary reinforced concrete systems were patented and used almost simultaneously in several countries from the 1880s, notably the U.S., France, and Germany, followed quickly by Britain, where the Frenchman Francois Hennebique had taken out a patent in 1892. His system used the plain round mild steel bar, but other systems adopted alternative profiles both to satisfy the need for originality if a patent were to be granted, and to ensure good grip or bond with the concrete. This was essential if the concrete and its reinforcement were to work together. The other key component of modern concrete--Portland cement, patented in 1824 by Joseph Aspdin--had by this time been developed as a strong and reliable product.
Guidance on the use, design, and construction of reinforced concrete quickly became available. The first textbook in the U.K., by Marsh, appeared in 1904. Subsequently, codes of practice for design and standards for materials were introduced. Subsequently, as later, research and development made major contributions to the understanding of concrete behavior through materials testing and load tests that studied the behavior of concrete structures. Many committees, learned societies, and trade associations were established during the century, providing the means of exchanging experience and understanding, both nationally and internationally.
As patents expired during the earlier decades of the century, reinforced concrete construction became freely available to contractors. This was a mixed blessing, as some lacked the skills and experience of the system providers, who took pains to ensure that their products were soundly designed and built.
A notable development was discovered by Eugene Freyssinet in 1928. He pioneered pre-stressing, using tensioned steel to precompress the whole of the concrete section, so that when loaded in service it did not develop tension stresses. The result was a more efficient member with improved durability.
The major reconstruction program undertaken in many countries in the decades after World War II made very extensive use of concrete. Inexperience, and pressures of time and money, resulted in many cases of inadequate cover of reinforcement and other defects so that subsequently, major repairs were needed to remedy the consequences of corroding reinforcement. By the end of the century, however, the issues of durability and how to achieve it were widely understood, so that any competent engineer or contractor could now be expected to build soundly in concrete.
Much research, development, and innovation was applied during the century to the essential materials used in concrete--cement, aggregates, and reinforcement.
Cements were developed for particular applications, such as rapid setting or resisting aggressive environments such as seawater. Others made use of industrial waste products such as ground granulated blast furnace slag and fly ash (the ground-up clinker from coal-burning power stations). Inevitably, some materials proved to have unwelcome side effects, such as the very quick-setting high alumina cement, which was found in the 1970s to lose strength over time, even at room temperature. This was originally thought to occur only in warm damp environments. Similarly, calcium chloride, commonly added to the concrete mix to accelerate setting, especially in cold weather, was found subsequently to increase the risk of reinforcement corrosion. Use of both materials is now restricted by codes of practice.
The most common aggregates remain sand and crushed rock or gravel. However, commercial incentives and, later in the twentieth century, environmental issues such as opposition to new quarrying for natural aggregates, led to alternatives being sought and developed. Lightweight aggregates were made cheaply from waste materials--initially clinker from coal-burning and broken bricks, and later (once again) fly ash. Fly ash, when heated, forms pellets suitable for use as aggregate. Lightweight concrete offers savings in the supporting structure, and also offers better thermal insulation than normal-weight concrete. Concern over energy conservation issues in buildings from the 1970s meant that lightweight concrete blocks have become very widely used for the inner leaf of cavity wall construction, meeting onerous building regulation requirements for thermal insulation.
With improved steel-making techniques and the need to ensure good bond to the concrete, the ribbed hot-rolled high-yield strength steel bar had displaced the plain mild steel bar to become the norm in the U.K., Europe, and elsewhere by the end of the century. Higher strength steel was needed for prestressed concrete. Forms developed and still in use included rods that could be pretensioned against the molds for precast unit manufacture, and cables or threaded bars that could be post-tensioned inside sheaths cast into the concrete, the latter approach often being used for larger beams, especially in bridges, cast in situ.
Concerns over durability led to the use of galvanized and epoxy-coated or stainless steel reinforcement, particularly on bridges and in car parks where deicing salt carried by vehicles can soak into concrete and accelerate the corrosion process. Although more expensive than plain steel, the greater initial cost may be outweighed by the potential savings from reduced--and necessarily disruptive--future maintenance and repair costs. Similar arguments apply to carbon fiber polymer-based reinforcement, which was still in its infancy at the turn of the twenty-first century.
Long-established and widely used throughout the twentieth century were asbestos sheet, corrugated asbestos, and woodwool, whose names belie the fact that all three are early forms of fiber-reinforced cement. Flat asbestos-cement sheet originated in Austria around 1900, while the corrugated form--which could span longer distances, and so was ideally suited for pitched roofs on factories and sheds--was being made in Britain by 1914. Woodwool is also believed to have originated at about this time in Austria, making use of waste timber shavings bound together with cement. Pressed into slabs, the woodwool provides lightweight roofing and walling panels with good thermal insulation properties.
Steel and polypropylene fibers have been used to reinforce concrete, particularly ground-bearing slabs. Their advocates argue that they reduce the incidence of cracking, although care is needed to obtain an even distribution of the fibers throughout the concrete mix. Glass fiber, of very light weight and capable of being molded into esthetically pleasing curves, has found use in thin glass-reinforced cement (GRC) cladding panels.
Hamilton, S.B. A Note on the History of Reinforced Concrete in Buildings: National Building Studies Special Report No. 24. Her Majesty's Stationery Office, London, 1956.
Jones, B.E., Ed. Cassell's Reinforced Concrete. Waverley Book Company, London, 1920.
Mainstone, R.J. Developments in Structural Form. Architectural Press, Oxford, 1998.
Marsh, C.F. Reinforced Concrete. Constable, London, 1904.
Neville, A.M. Properties of Concrete. Longman, Harlow; and Wiley, New York, 1981.
Sutherland, J., Humm, D. and Chrimes, M., Eds. Historic Concrete: Background to Appraisal. Thomas Telford, London, 2001.
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