SAND AND CEMENT 205 



commonly determined through its resistance to tensile 

 stresses, and its resistance to other forms of stress is com- 

 puted from these results. 



The tensile strength of mortar has been shown to vary 

 with the character of its ingredients, with its consistency, 

 its age, and with many other factors. In the accompany- 

 ing table is given a fair average of the tensile strength that 

 may be expected from mortars of Portland and natural 

 cements that are made in the field and with a sand of fair 

 quality but not especially prepared. 



The strength of Portland-cement mortar increases up to 

 about 3 mo.; after that period, it remains practically con- 

 stant for an indefinite time. Natural-cement mortar, on 

 the other hand, continues to increase in strength for 2 or 

 3 yr., its ultimate strength being about 25% in excess of 

 that attained in 3 mo. The strength of slag-cement mortar 

 averages about three-quarters of that of Portland-cement 

 mortar. 



The compressive strength of cement mortars is usually 

 given in textbooks as being from eight to ten times the tensile 

 strength. This value is rather high for the average mortar, 

 a ratio of from 6 to 8 being one more nearly realized in prac- 

 tice. The ratio increases with the age and richness of the 

 mortar, and varies considerably with the quality of the sand. 

 Portland-cement mortars of 1-3 mixture that are 3 mo. old 

 develop, on an average, a compressive strength of about 

 1,800 lb. per sq. in., while 12 natural-cement mortars 

 average about 1,600 lb. 



The strength of mortars in cross-breaking and shear may 

 be taken at about one and one-half to two times the tensile 

 strength, with a fair amount of accuracy. 



The adhesion of mortars to inert materials varies both 

 with the character of the mortar and with the roughness 

 and porosity of the surfaces with which they are in contact. 

 The adhesion of 1-2 Portland-cement mortar, 28 da. old 

 to sandstone averages about 100 lb. per sq. in.; to limestone, 

 75 lb.; to brick, 60 lb.; to glass, 50 lb.; and to iron or steel, 

 75 to 125 lb. Natural-cement mortars have nearly the 

 same adhesive strength as those made of Portland cement. 



