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.