Concrete expands and contracts with changes in temperature, causing
cracks to appear. Contraction cracks occur in thin sections exposed to
wide variations in temperature and are common in sidewalks; therefore,
large stretches of concrete should not be laid without breaks or spaces
to allow for the changes in size. The spaces should be filled with tar or
some similar material that will yield or give when the concrete expands.
A joint like that shown in Figure 17 is frequently used for thick walls.
A section of the wall is poured and before the next is poured the
abutting end is covered with tar and paper, the thickness of the covering
depending upon the length of the section and the exposure. Sidewalks and
similar work, when not cast in alternate blocks, should have a one-fourth
inch space left at intervals of 40 feet. The joint may be filled with tar
paper or tar. Steel is used to take care of contraction in long or high
walls and water-tight work. Important structures in which temperature
reinforcement is necessary should be designed by one experienced in
concrete design.
[Illustration: Fig. 17.--Expansion joint showing rebate form removed and
filler in place.]