66 • BULLETIN '724, U. S. DEPARTMENT OF AGRICULTUEE. 



concrete foundation. An inspection after failure showed that the 

 foundation of this road was made of rather poor grade concrete, and 

 Avas not uniformly of the required depth, but the completeness of the 

 failure would seem to indicate that even full 4 inches of good con- 

 crete hardly would have been sufficient. In this case the subgrade 

 conditions were fair and the road had held up under light traffic for 

 about one year, when failure was caused by a few trips of a loaded 

 motor truck. 



The maximum depth of S inches is sufficient for all ordinary sub- 

 grade and traffic conditions, provided a good quality of concrete is 

 used, and in most cases a depth of 6 or 7 inches has proved entirely 

 sufficient. 



Though the strength of a concrete slab increases rapidly with an 

 increase in depth, the conditions affecting the concrete road founda- 

 tions are so involved that no data now available furnish reasons for 

 a definite conclusion as to the rate of increase. When the probable 

 effects of an increase of depth are all considered, such as greater 

 strength as a simple beam, extra dead-weight to act against frost, 

 additional resistance to cracking under changes of temperature and. 

 moisture, additional strength to resist impact, and the lovrer per- 

 centage of scant thickness produced locally by the unavoidable ir- 

 regularities of subgrade, it is probable that a 6-inch slab foundation 

 will, give at least twice the effective service that a 4-inch slab will 

 give. 



If it be deemed advisable to employ a greater depth than 6 inches, 

 it usually is permissible to decrease the depth from the center toward 

 the edges. This is done by flattening the crown of the subgrade. 



CURBS. 



The types of road crust ordinarily requiring concrete foundations 

 also usually require curbs of some kind to protect the marginal por- 

 tions of the crust from disintegrating under traffic. The curbs may 

 be made of either concrete or stone, and, for country roads, are de- 

 signed to come flush with the road surface. Stone curbs should have 

 a depth of from 12 to 18 inches, according to the depth of the frost 

 line, and should be set before the foundation is laid. Concrete curbs 

 may be constructed either separately or in combination Avith the 

 foundation. In the former case they should have a cross section 

 similar to stone curbs and should be constructed in place ahead of 

 the foundation, while in the latter they may be simply flanges on the 

 edges of the foundation, formed of rich concrete poured before the 

 concrete of the foundation has taken its initial set. The proper thick- 

 ness for both stone and concrete curbs varies somewhat with the con- 

 ditions inA'^olved, but 6 inches is the thickness employed most com- 

 monly. Four incheiJ is the minimum- likely to prove satisfactory 



