DEAIlSrAGE METHODS FOR COUNTY ROADS. 73 



rather widely. With the average batch mixer, the minimum time should not 

 be less, than about oue minute and about 14 or 20 turns of the drum, and 

 if practicable, an automatic timer or counter should be employed to see that 

 this minimum is observed. 



Z. That the subgrade is damp when the concrete is placed upon it. A special 

 hose should be provided for sprinkling the subgrade and the sprinkling should 

 be done in such manner as not to produce mud. The object of wetting the sub- 

 grade is to prevent it from absorbing the water of the concrete, and the 

 amount of sprinkling should be only sufficient to accomplish this purpose. 



3. That the concrete is placed with dispatch after it is mixed, and that it is 

 of such consistency that it may be struck off immediately and shaped without 

 danger of the layer becoming wavy or otherwise losing its shape. The con- 

 crete should be compacted as it is struck off. This may be done by using the 

 strike board as a tamper, or by some other suitable device. 



4. That no concrete is laid during freezing weather and that none of the ma- 

 terials used contain frozen lumps. Freezing of fresh concrete may cause com- 

 plete failure and nearly always impairs its strength. After concrete has set 

 for four or five days, it is not likely to be damaged by freezing. 



5. That the new concrete foundation is protected from too rapid drying out 

 by a covering of earth or other suitable material. The earth covering should be 

 at least 1 inch thick and should be spread as soon as may be without damaging 

 the concrete (ordinarily about 24 hours). The covering should remain on the 

 foundation for about 10 days and should be sprinkled repeatedly and kept moist 

 during this period. No loads "whatever should be permitted on the foundation 

 until it is 10 or 12 days old. The earth covering is of especial importance during 

 the hot, dry seasons of summer but is sometimes omitted altogether in the 

 spring or fall seasons when weather conditions are favorable. 



6. That no heavy loads are allowed on the foundation until after the road 

 crust has been completed and opened to traffic. 



Plates XI and XII illustrate the different steps involved in the 

 construction of a concrete foundation. 



SPECIAL COIJCEETE FOUNDATIONS. 



In addition to the general method described above for constructing 

 concrete foundations, several, special methods have been tried, but 

 none of these has proved of sufficient importance to warrant a detailed 

 description. In one or two instances attempts have been made to mix 

 the concrete in place on the road. According to this method, the 

 materials were spread out in proper proportions, water added, and the 

 mass mixed by means of harrowing. According to another miethod 

 the coarse aggregate is spread and rolled as for a crushed stone- foun- 

 dation, and the voids in the mass then filled, as nearly as practicable, 

 with Portland cement grout mixed very thin and poured over the 

 surface. 



A modification of the common method has beeii employed lately 

 in connection with brick pavement construction. The modification 

 consists essentially in laying the brick before the concrete of the 

 foundation has had time to set, aiid is really concerned more with the 

 road crust than the foundation. This method promises to be of im- 

 portance, but since its application is limited at present to only one 



