16 BULLETIN 249/ U. S. ■ DEPARTMENT OF AGRICULTURE. 
atmospheric conditions it is sometimes better to spread the canvas 
immediately after the surface is finished, even at the risk of marring 
the surface slightly, than to run the risk of having cracks develop 
in the pavement. The canvas should be sprinkled until thor- 
oughly wet immediately after it is spread and should be keii>t 
wet until removed and replaced with an earth covering. Under 
ordinary weather conditions about 24 hours will be required for 
the concrete to set sufficiently hard not to be damaged by men walk- 
ing upon it while covering it with earth. The canvas should there- 
fore usually remain on the pavement about one full day. Imme- 
diately after the canvas is removed the pavement should be covered . 
with a layer of earth about 2 inches thick, which should remain on 
the pavement and be kept constantly wet for a period of about two 
weeks. During this period the roadway should be kept entirely 
closed to traffic. If the weather conditions are favorable the concrete 
ought to be sufficiently strong to withstand traffic at the end of two 
weeks. In cold or otherwise unfavorable weather the earth cover- 
ing should preferably be thicker than 2 inches and left in place for 
a longer period of time. No concrete should be laid during freezing 
weather, but if danger of freezing develops after the concrete is 
laid and before it sets, the first cover of canvas should be supple- 
mented in some way in order to prevent damage to the pavement. 
This may be done by spreading over it a layer of straw, or by using 
two thicknesses of the canvas, if this is practicable. 
The protection of the concrete is an extremely important feature 
of concrete-pavement construction. It is impossible to secure satis- 
factory results unless some such precautions as those described above 
are taken to prevent the concrete from drying out too rapidly after 
it is placed, and to insure that it sets ud under uniformly favorable 
conditions. 
THE USE OF REINFORCING STEEL. 
Probably the most satisfactory method, in point of efficiency, yet 
devised for reducing the number of objectionable cracks in concrete 
pavements is that of employing steel reinforcement. The reinforce- 
ment usually consists of woven wire or some similar material, though 
there is no apparent reason why plain round or square rods might 
not be satisfactorily used. One-quarter-inch round rods embedded 
about 2 inches above the lower surface of the pavement and spaced 
about 12 inches center to center in both directions would seem suffi- 
cient to eliminate practically all objectionable cracking, provided * 
proper joints were introduced at changes in the grade and at curves 
in the alignment. But any satisfactory system of reinforcement will 
probably add from 15 to 20 cents per square yard to the cost of the 
pavement, and this additional cost is no doubt responsible for the 
