Reinforced concrete - The Design of Prestressed Concrete Bridges - page 33

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 fi nd 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 signifi cant tensile stresses. The
Romans attempted to reinforce their concrete using bronze bars, but this was not
successful, due to the different coeffi cients 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 coeffi cient
of expansion as concrete, and which effectively gave it tensile strength. The fi rst 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 fi nding 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