The historical development of reinforced concrete

The first known use of concrete is for a floor in Israel, dated to approximately 7000 bc. The Egyptians used concrete as an infill for stone-faced walls from about the second millennium bc, and the Greeks used it as a mortar or render from about 500 bc. The earliest Roman use of concrete dates from about 300 bc, initially as a core material, between masonry facings. These early uses of concrete probably used cement made from burnt lime (quicklime).

In the second century BC, the Romans discovered that adding pozzolana to the lime produced a much stronger concrete, which could be used as a building material in its own right. This discovery allowed them to revolutionise construction by designing large span concrete domes. The two most well known and well preserved Roman concrete buildings with a domed roof are the Pantheon in Rome, Figure 1.14, built in 120 ad, and Hagia Sophia in Istanbul built in the sixth century, Figure 3.1.

Both of these buildings are in good condition, having resisted the ravages of the Mediterranean weather, and the seismicity of both sites. The most recent earthquake in Turkey destroyed many modern buildings, but left Hagia Sophia intact. Their survival is not an accident; both buildings are extremely well designed, and demonstrate great virtuosity in the knowledge and use of concrete. For instance, the domed roof of the Pantheon is made of lightweight concrete to reduce the thrust on the dense concrete walls. When they were erected, their designers had centuries of accumulated experience in the material; and it shows!

After the collapse of the Roman Empire, the knowledge of how to make and use concrete as a primary structural material was lost for many centuries. Although lime concrete was used for foundations and as a filling material for walls throughout the middle ages and the renaissance, it was not until the end of the eighteenth century

Figure 3.1 Partly cut-away isometric drawing of the sixth-century church of Hagia Sophia in Constantinople (Source: R.J. Mainstone, Hagia Sophia, Thames and Hudson 1988)

that organised experiments were conducted to improve the strength and speed of hardening of cement. John Smeaton experimented with mortars to find a material suitable for the construction of the Eddystone lighthouse, the construction of which commenced in 1756. Then in 1824, Joseph Aspdin took out a patent for Portland cement, to begin the modern development of concrete [1].

Concrete is brittle in tension, and cannot sustain significant tensile stresses. The Romans attempted to reinforce their concrete using bronze bars, but this was not successful, due to the different coefficients of thermal expansion of the two materials. Consequently, its use was limited to structures that were primarily in compression: walls, arches and domes. The great discovery of the nineteenth century was the reinforcement of concrete using iron or steel, which have virtually the same coefficient of expansion as concrete, and which effectively gave it tensile strength. The first patent in the United Kingdom for reinforced concrete was taken out in 1854 by William Wilkinson, although the technique was only used widely towards the end of the century. So, even now, we have only something over a hundred years of experience in the use of modern reinforced concrete, and despite the undoubted greater speed of learning and change characteristic of our age, we are still finding out which aggregates are safe to use, and how to control the chemical make up of cement.

Although the inclusion of reinforcement has given to concrete a vastly greater versatility, it has also brought with it the majority of the problems that the modern designer must understand if he is to make durable structures, safely and economically. The principal problem introduced by reinforcement is the corrosion of the steel leading to the deterioration of concrete structures, and to their need for maintenance.

Modern designers appear to have forgotten that concrete in compression does not need to be reinforced, and that leaving out the steel not only saves money but improves durability. The concrete side walls of the Byker tunnel, for instance, are not reinforced, Figures 17.6 and 17.7. However, there is no doubt a great future for reinforcement that does not corrode.

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Getting Started With Dumbbells

Getting Started With Dumbbells

The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.

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