Reinforced Concrete
 

 

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CONCRETE

is an artificial stone, and in common with natural stone is a brittle material, strong in compression but weak in tension.  From very early times, metal has occasionally been used to strengthen, or reinforce, masonry and concrete.  The roof of one Roman bath was constructed of a coarse concrete reinforced with bronze and iron rods. Wren used an iron chain in the dome of St. Paul's to help resist lateral thrust, and J. G. Soufflot (1713-81) used wrought-iron rods embedded in masonry in the construction of the church of St. Sulpice, Paris.  Until the 19th century, however, there was no knowledge of reinforcing principles as they are understood today, and satisfactory methods of calculation were only developed towards the end of the century.

An early example of reinforced concrete was displayed in the Paris International Exhibition of 1855. This was a rowing boat designed by J. L. Lambot, a French contractor, and made of hydraulic lime concrete reinforced with a skeleton of iron rods.

In 1854, a Newcastle plasterer, William B. Wilkinson, patented a method of constructing concrete slab floors reinforced either with a network of flat iron rods or with second-hand wire ropes. His manner of arranging the steel to take tensile stresses demonstrates that he appreciated the constructional principles of reinforced concrete.

Many patents were taken out during the 19th century, only a few of which can be mentioned here. Francois Coignet patented a floor in 1855, and in following years built, among other structures, a lighthouse at Port Said and retaining walls in Paris. His son, Edmond Coignet (1850-1915), also took out various patents for the use of reinforced concrete.

Joseph Monier (1823-1906) was originally a gardener and a manufacturer of gardening tools. He conceived the idea of using tubs of concrete instead of wooden tubs for holding small trees, but finding that concrete was too brittle he made an iron network and constructed the tub by enveloping this network with mortar or concrete. This method he patented about 1867 and followed it with other uses of reinforced concrete, as for example a reservoir built in 1872. In Europe at this time, reinforced concrete construction was commonly referred to as the Monier system.

An American, T. Hyatt, carried out experiments on beams and was among the first to appreciate the basic principles of reinforced concrete construction as we understand them today. He published his discoveries in London in 1877, but his patents did not receive the attention they deserved. Experiments were carried out in Germany and elsewhere towards the end of the century which enabled K. Koenen, Morsch, Considere and others to produce mathematical design theories.

The first English textbook on reinforced concrete, by Marsh and Dunne, was published in 1904.

Another famous name associated with the development of reinforced concrete is that of Francois Hennibique (1824-1921), who constructed the first building with a complete reinforced concrete frame.  He set up a branch office in London in 1897, in charge of L. G. Mouchel, to design structures on the Hennibique system of patents.  At the beginning of the 20th century, most reinforced concrete work was done by specialist firms of this sort, since the average architect or consulting engineer had not yet acquired sufficient theoretical and practical knowledge of the new method of construction to enable them to prepare designs with confidence.  Although certain types of floor construction are still patented, most normal reinforced concrete work can now be designed and constructed without reference to patents.

THE SIMPLE REINFORCED-CONCRETE BEAM

The strength of concrete in compression is at least ten times as great as its strength in tension, and by using a material such as steel to take the tensile stresses induced by bending, full advantage can be taken of the high compressive strength of the concrete. A simple beam can be constructed by placing steel bars in a wooden or steel mould and ramming or vibrating in the wet concrete so that it completely surrounds the bars. '

On setting and hardening, the concrete shrinks slightly and grips the steel bars firmly, so that when the beam is loaded the steel and concrete bend as one unit, there being no slip (in correct design this is known as bond) between them. (Tendency to slip is due to horizontal shear.)  Ordinary mild steel is allowed to take a maximum tensile stress of 275 N/mm2.  High yield steel a maximum tensile stress of 460 N/mm2 and the accompanying elongation of the steel, although very small, is sufficient to crack the concrete in the tension zone of the beam. (Since the elongation of the steel is so small, these minute cracks are normally not detectable by eye.)

The cracking of the concrete, which causes the steel to take all the tension below the neutral layer, may be explained in this way. The elastic modulus (E) of steel is about 15 times that of concrete. But concrete fails in tension at a very low stress, the exact figure depending on the quality of the concrete.  It follows, therefore, that the concrete below the neutral axis fails in tension and is ineffective for resisting bending stress (although it is still capable of resisting shear forces). In a simple reinforced-concrete beam, the compressive stresses are taken by the concrete and the tensile stresses are taken by the steel.

The position of the neutral layer depends among other factors on the amount of the reinforcement. The resistance of a beam to bending can be varied by varying the amount of steel (and also the proportion of cement to aggregates.

If for a particular mix the percentage of steel is increased further, no advantage results because the concrete in compression will be unable to supply enough resistance to balance the high tension the steel can supply. The strength of the beam can be increased further, however, by placing steel in the compression zone of the beam to help the concrete, although this is not normally economical.  It is a method adopted when it is required to keep the beam dimensions as small as possible.

(The Elements of Structure by W Morgan)

 

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Last Edited :  10 March 2015 10:42:11