588 



CEMENTS, LIMES, AND PLASTERS. 



most commonly applied of all tests, this action being based on the 

 assumption that the ratio between compressive and tensile strength 

 for all Portland cements is quite uniform, and that therefore varia- 

 tions in tensile strength will indicate corresponding (though much 

 greater) variations in compressive strength. This assumption is to a 

 large extent correct, and for all practical purposes may be considered 

 satisfactory. The question as to the ratio existing between the two 

 types of strength will be taken up on a later page (p. 589.) 



In Fig. 146 the results of a large series of tests on various classes of 

 cement are shown diagrammatically. The cements tested included 

 American and foreign Portlands, foreign "natural Portlands", and Amer- 

 ican natural cements, and the comparative results are quite represen- 

 tative. 



600 



400 



10 



20 30 



AGE IN WEEKS 



40 



50 



FIG. 147.* Effect of proportions of sand on tensile strength. 



The three points of most general interest in connection with tests of 

 tensile strength are (a) the decrease in tensile strength with increase 

 of percentage of sand, (6) the increase in strength with increased 

 age, and (c) the variation in strength due to differences in the character 

 of the sand. Two of these points are illustrated in Figs. 147 and 148, 

 while all three are constantly discussed in engineering publications. 



Compressive strength. The compressive strength of a cement or 

 concrete is a matter of direct practical importance, for these materials 

 are rarely subjected to any other type of strain when used in actual 



* From Johnson's " Materials of Construction", p. 571. 



