72 BtTLLETTN 724, U. S. DEPARTMENT OF AGRICULTURE. 



grade from being more or less damaged. Where hauling over the 

 prepared subgrade is permitted it always tends to produce a lack of 

 uniformity by increasing the degi'ee of compactness of the parts 

 traversed, and this may result easily in unequal settlement with con- 

 sequent cracking of the foundation and road crust. 



If the localit}^ of the construction is one where rains are heavy and 

 frequent, or in a season having such rains, judgment should be exer- 

 cised in placing the materials on the subgrade very far in advance 

 of the time they are needed for use, because under such conditions 

 the subgi'ade may become saturated under the material piles and not 

 dr}'^ out before the concrete is placed. 



According to present practice concrete for road foundations is 

 almost invariably mixed in a machine mixer. Hand mixing can be 

 afforded only where the work to be done is inconsiderable and rarely 

 produces as uniform concrete as machine mixing. In this discus- 

 sion, therefore, it will be assumed that a machine mixer is employed 

 in all cases. To accomplish satisfactory work under the conditions 

 met ordinarily in foundation construction the concrete mixer should 

 possess the following features: (1) It should be of the batch type, 

 so that uniform proportioning of the material may be assured; (2) 

 it should be fitted with a traction attachment so that it may be 

 moved along the road under its own power as the work progresses; 

 (3) it should include a device for distributing the concrete. This 

 device may consist of a boom with a bucket traveler, a chute, or a 

 revolving tube. The boom and bucket is to be preferred generally 

 because it enables a greater area of foundation to be placed from one 

 position of the mixer than either of the other devices, and conse- 

 quently machines so equipped require less moving than others. 



Sometimes the concrete is mixed in a stationary mixer and hauled 

 to its place in the foundation, but this method is objectionable be- 

 cause it permits a separation of the component materials to take 

 place between the time of mixing and the time of depositing. If a 

 uniformly dense concrete is to be produced it must be deposited in 

 the foundation immediately after it is mixed. Failure to secure 

 uniform density impairs the strength of the concrete, and, in extreme 

 cases, may cause cracks to develop in the foundation, thus greatly 

 lessening its power of distributing stresses due to concentrated 

 loads. 



The principal precautions essential to observe in mixing and plac- 

 ing concrete in a road foundation are to see — 



1. That the amount and time of mixing is sufTicient to secure a thorough 

 distribution of the different materials through the mass and complete hydration 

 of the cement. Tests have shown that increasing the time a batch of concrete 

 remains in the revolving drum of a mixer, within reasonable limits, has very 

 much the same effect as increasing the proportion of cement. The efficiency 

 with which mixers work, as expressed in the strength of concrete, varies 



