EXPANSION OF SOLIDS

With few exceptions, substances expand when heated, and very large forces may be set up if there is an obstruction to the free movement of the expanding or contracting bodies. If concrete road surfaces were laid down in one continuous piece cracks would appear owing to the expansion and contraction brought about by the difference between summer and winter temperatures. To avoid this, surfaces are laid in small sections, each one being separated from the next by a small gap which is filled in with a compound of pitch. On a hot summer day this material will often be seen to have squeezed out of the joints on account of the expansion. In the older methods of laying railway tracks gaps have to be left between successive lengths of rail to allow for expansion. Even when such gaps have been left the rails may sometimes 'creep' and close up the gaps. If this happens a rise in temperature may lead to buckling of the track.

Free movement at the rail joints is allowed for by making the bolt holes of the plates joining the tracks, slotted. In modern practice, however, railway lines are welded together to form long, continuous lengths. With this method, it is only the last fifty to one hundred metres of any length which show expansion, usually of a few centimetres. This movement is taken up by planning the ends of the rails and overlapping them. The forces set up by expansion in the remainder of the rails are, so to speak, locked up in the metal. To keep these forces to a minimum, it is usual to lay the track at a time when the temperature is midway between the summer and winter averages. This technique has been made possible by the use of concrete sleepers and improved methods of fixing the rails so that the track may withstand the thermal stresses set up in it without buckling. Allowance also has to be made for the expansion of bridges and the roofs of buildings made of steel girders. Various methods are used to overcome the difficulty, a common one being to have one end only of the structure fixed while the other rests on rollers. Free movement is thus permitted in both directions. Over a very long period of years, expansion and contraction causes 'creeping' of the lead on the sloping roofs of buildings. When heated by the sun the lead expands and tends to move down the roof under its own weight. On cooling and contracting, the force of contraction is opposed by gravity and the friction of the lead against the roof. This sets up a strain in the lead gives it a slight permanent stretch. After many years the lead stretches so much it eventually forms folds and may break.