A MODERN STREET. 213 



-would hold up all ordinary traffic and that four inches would hold 

 i;i many special cases. They adopted a thickness of six inches of Port- 

 land cement concrete as a general rule, and that rule has been followed 

 in England and the United States with very general satisfaction. 



It is imperatively necessary that this foundation should present to the 

 material laid upon it an absolutely unyielding surface, for the reason 

 that the bricks or blocks being jointed and the thin sheet of asphalt, 

 while continuous, possessing very little strength in itself, either of them 

 is bound to follow a yielding foundation with disastrous results. As be- 

 fore stated, an unyielding foundation is therefore, within reasonable 

 limits, an absolute necessity; for, while the average street in our north- 

 ern cities is liable to the vicissitudes of frosts, and still more liable to 

 the vicissitudes of frequent openings, the evil effects of these vicissitudes 

 can be for the most part avoided by constructing a concrete foundation 

 sufficiently strong to form a bridge over any such weak spots of limited 

 extent, and thus hold up the surface. This very obvious requirement 

 will never be found in a concrete that is neither thick enough nor 

 strong enough. 



The action of the Commissioners of Accounts of the City of New 

 York in insisting that only Portland cement of good quality should be 

 used in the street foundations of Greater New York only confirmed and 

 was confirmed by the conclusions reached by the French engineers more 

 than a generation ago. Why such a question should have been raised 

 at this time by engineers presumably familiar with the literature of 

 their profession is not apparent. 



In constructing this foundation the contractor is reqiiired to pro- 

 ceed as follows: 



The concrete shall be composed of one part cement, three parts of sand and 

 six parts of broken stone. . . . Unless machinery be used, concrete shall be 

 mixed in batches, one barrel of cement with the requisite proportion of other 

 material, on suitable tight platforms, not less than twelve feet by twelve feet 

 in size. The cement and sand shall be thoroughly mixed dry and then made 

 into a mortar with as little water as possible, after which the broken stone, 

 having first been watered, shall be added. The whole mass shall then be 

 turned and worked until a moist resultant is obtained, with the stone uni- 

 formly distributed. 



It is very necessary that the concrete should be thoroughly set and 

 dried out before the surface should be placed upon it. It is therefore 

 ndvisable that a modern street should be constructed during dry and 

 warm weather. 



If the surface is to be of brick, a brick made especially for the pur- 

 pose is used that is uniformly burned until vitrified. The size should 

 be as uniform as possible, they should be hard enough to resist abrasion 

 and of a texture that renders them impervious to water. When they 



