Machines designed to tamp the concrete and strike it off to the required cross section are also employed for finishing. The machine is power operated and is carried on wheels that run on the side forms, and the machine moves slowly along as the tamping progresses. The concrete is tamped, struck off to shape and smoothed with the belt at one operation. This method of finishing produces denser and stronger concrete than can be produced by hand finishing methods.
Placing Concrete for Two-course Road.—The methods employed for the two-course concrete road are much the same as for the one-course road. The concrete for the lower course is placed and struck off by means of hand tools, and after that course has progressed a few feet, the upper course is placed and finished as has been described for the one-course road.
Curing the Concrete.—The setting action of cement is a chemical process, not merely a drying out of the water introduced in mixing the concrete. The chemical action is progressive for a long time, but is more rapid during the first few hours than during the later periods, and the concrete reaches about three-fourths of its maximum strength at the end of seven days. During the setting period and particularly during the first few days, plenty of water must be available to the cement.
To prevent too rapid loss of water from the concrete during the setting period, the surface must be protected from the wind and sun. This is accomplished by first covering with canvas as soon as the concrete has hardened sufficiently and by later covering with earth, to a depth of two inches. The earth covering is kept wet for about ten days and is left in place for about one month.
In some places the ponding method of curing is adopted. The surface is divided into sections by earthen dikes and the space inside the dikes filled with water to a depth of two or three inches. The water covering is maintained for two weeks or longer.
No traffic is permitted on the surface for one month, and in cold weather traffic may be kept off the surface for a longer period.
Expansion Joints.—To permit the concrete slab to accommodate itself to changes in dimension due to temperature changes, expansion joints ½ inch wide are placed about every thirty feet. These consist of a sheet of some prepared bituminous material placed in position as the concrete is poured.
Experience seems to indicate that in spite of the expansion joints, the concrete will crack more or less and many engineers think it advisable to omit expansion joints in constructing the pavement and when cracks develop to pour bituminous material into them, thus forming expansion joints.
The prevailing practice in rural highway construction is to omit the expansion joints, but they are commonly adopted in city pavements.
Reinforcing.—To minimize the cracking, either bar or wire mesh reinforcing is used in the concrete. If bars are used they are placed in the concrete as it is poured so as to form a belt around each section about 15 feet square. If the mesh type is employed, a part of the layer of concrete is placed and smoothed off and a strip of the mesh laid in place. Additional concrete is then poured on top of the mesh to bring the slab to the required thickness.