46 bulletins' 1077, U. S. DEPARTMENT OF AGEICULTUEE. 



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. Where 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, PL VIII.) The water is 

 retained on the pavement hj 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. 



