SPECIFICATIONS FOR NATURAL CEMENTS. 



283 



Strength. The tensile strength required is also highly variable in 

 different specifications. These variations are shown in the summary 

 below. 



TABLE 138. 

 STRENGTH REQUIRED BY VARIOUS SPECIFICATIONS. 



* In this specification the mortar mixture is 1 cement, 3 sand. 



Product actually delivered. Specification requirements are made 

 for the purpose of preventing the use of poor material. They must 

 therefore be set low enough to include all the good cement. For this 

 reason the cement supplied to any important work is usually found, 

 when tested, to be considerably better in every way than is required 

 by the specifications. This fact is well brought out by the following 

 tables, which give the experience of two important pieces of work. 



TABLE 139. 

 CEMENT SUPPLIED TO NEW YORK RAPID-TRANSIT SUBWAY, 1900-1901. 



