The objection to this method is its relatively great cost and the amount of space it occupies; and the advantage claimed for it is that the water-tightness of the diaphragm is not affected by the cracks in the wall due to settlement or to expansion and contraction.
Concrete is usually laid in warm weather, and consequently in cold weather contraction cracks are likely to appear in thin walls of any considerable length, since the resulting stress is greater than the tensile resistance of the concrete; and unless care is taken to confine these cracks to straight lines by proper contraction joints, they may seriously disfigure the face of the work. Concrete laid in cold weather is likely to expand during warm weather; but this usually does no harm, since concrete is better able to resist compression than tension. Again, concrete which sets in air shrinks 0.0003 to 0.0005 per unit of length, and that which sets in water swells 0.0002 to 0.0005, the change being greater the richer the concrete. Air-hardened concrete swells when immersed in water, and water-hardened concrete shrinks when exposed to the air. Further, the cracking of large masses of concrete is sometimes due, at least in part, to the cooling of the cement after the rise of tempera ture caused by the chemical action of setting (see 4 348).
Therefore, for these three reasons, it is customary to provide contraction joints at intervals in concrete structures, or to reinforce the concrete with sufficient steel to enable the structure to with stand the tensile stress produced by the contraction. The method of preventing contraction cracks by reinforcing the concrete will be considered in the next, chapter.
Another advantage of dividing a continuous wall into sections by vertical joints is that each section can be built by itself with a minimum liability of unsightly horizontal seams between different days' work.
1. Planes of weakness are made by building a temporary partition in the form, and casting the section thus enclosed as a single mass. When the concrete is set, the partition is removed and the new con crete is deposited against the old without any attempt to secure a bond between the new and the old, thus leaving a vertical plane of cleavage between the adjacent sections. To mask the ragged appearance of the joint, sometimes a triangular strip is nailed to the form where the partition joins the face of the form in such a manner as to leave a vertical triangular groove in the face of the wall with the plane of the contraction joint passing through its apex. Some times a sheet of tarred paper is placed in the joint between the sections to prevent a possible adhesion of the new to the ole concrete.
2. A tongue-and-groove contraction joint is made by placing vertically against the face of the temporary partition or bulkhead. a triangular or rectangular or U-shaped timber which forms a vertical groove in the end of the first section of the concrete into which the concrete of the second section is built, thus forming a concrete tongue. The advantage of the tongue-and-groove joint over the plane of weakness is to strengthen the wall against a lateral thrust applied near the joint.
3. A dowel contraction joint is frequently employed by railroads, and is made by inserting a short piece of railroad rail in the end of a section of a retaining wall and allowing the rail to project a short distance; and then when the second section is to be built against the first, the projecting ends of the rails are wrapped with paper or coated with soap or axle-grease to prevent the adhesion of the concrete. The projecting rails bind the two sections together laterally, but the concrete in the last-laid section is free to slip on the rails with temperature changes.