46 BULLETIN 1077, U. S. DEPARTMENT OF AGRICULTURE. 
PROTECTING AND CURING THE CONCRETE. 
The quality of the concrete depends to a great extent upon the 
conditions under which it is cured. A concrete cured with the proper 
amount of moisture has strength and wearing qualities almost twice 
as great as the same concrete cured in the open air. Either of the 
following general methods may be used for curing: Covering the 
pavement with earth or straw, and keeping this material moist ; or 
covering the pavement with water. Until the pavement has set 
sufficiently hard so that it will not be damaged by walking upon it, it 
should be protected with a canvas covering. The canvas covering 
may be supported by wooden frames or laid directly on the concrete if 
care is taken to avoid marring the surface. (See Fig. 1, PL VIII.) 
Under ordinary weather conditions about 24 hours will be required 
for the concrete to set sufficiently hard not to be damaged by walking 
upon it. 
If an earth covering is used it should be at least 2 inches thick 
and should cover the edges of the pavement. It should be thoroughly 
watered twice each day for a period of 14 days and remain upon the 
road for at least 20 days from the time of its application. The earth 
for covering is usually obtained from the shoulders or the sides of 
the road. \Yhere earth for covering is difficult to obtain, as for ex- 
ample, where the shoulders are composed of hard compacted material, 
straw may be used, in which case the covering should be not less 
than 4 inches thick after wetting. The principal advantage in the use 
of straw is that it can be easily loaded and hauled forward for use 
again. In localities where straw can be obtained at small cost it is 
believed to be more economical than earth. 
The method of curing by covering the pavement with water is 
commonly called " ponding." (See Fig. 2, PI. VIII.) The water is 
retained on the pavement by earth dams placed across and along the 
edges of the pavement. The pavement is then covered with water to 
a depth of 2 inches. The water should be maintained on the surface 
for a period of not less than 14 days. Flooding is generally done in 
the evening when the water is not needed for the mixer. The pond- 
ing method is more positive than any other, and should be used wher- 
ever possible. It can not be used satisfactorily, however, on grades 
in excess of 3 per cent or where the earth available for the dams will 
not retain the water. 
During the period of curing the roadway should be kept entirely 
closed to traffic. If the weather conditions are favorable for rapid 
curing, as for example during midsummer, the pavement should be 
sufficiently strong to be opened to traffic at the end of 21 days. In 
cold weather a longer time should elapse before traffic is permitted 
on the pavement. 
