310 MASONRY 



Natural-cement mortar is usually composed of 1 part of cement 

 and 2 parts of sand. This proportion is found to possess suffi- 

 cient adhesion and resistance to crushing for ordinary masonry 

 above ground. 



In the preceding table are given the quantities of materials 

 required to produce 1 cu. yd. of compacted mortar. The pro- 

 portions are by volume, a cement barrel being assumed to 

 contain 3.6 cu. ft. 



Mortar Impervious to Water. Both lime and cement mortar 

 absorb water; consequently, they disintegrate under the action 

 of frost. Impermeability of the mortar may be increased by 

 carefully grading the sand and increasing the amount of cement. 

 The addition of a small amount of lime tends to reduce the 

 volume and number of the voids and thus aids in reducing the 

 permeability. Practically impermeable mortar may be made 

 by adding to the mortar a mixture of alum and soap. The pro- 

 portions usually employed are f Ib. of pulverized alum to each 

 cubic foot of sand, and f Ib. of potash soap to each gallon of 

 water. The alum and soap combine and form compounds of 

 alumina and fatty acids that are insoluble in water. The 

 strength of the mixture is but little inferior to the strength of the 

 mortar of the same proportions. 



Strength of Mortar. The strength that mortar should pos- 

 sess is of three kinds; namely, compressive, cohesive, and adhe- 

 sive. The degree to which it should possess any one of these 

 depends on the position in which it is employed. In ashlar 

 masonry, resistance to compression is all that is required; in 

 uncoursed rubble masonry and in brick masonry, it must pos- 

 sess adhesiveness, or the capacity of adhering to the surface 

 of the stones or brick in order to prevent their displacement. 

 In masonry of all classes that may have to develop transverse 

 stresses, it must possess cohesiveness or tensile strength. 



The tensile and the compressive strength of a given mortar 

 depend on the adhesive strength of the cementing medium 

 and on the character of the aggregate. Coarse and fine sand 

 in the proportion of about 4 parts of coarse grains (^ to ft in. 

 in diameter) and 1 part of very fine grains (less than -fa in. in 

 diameter) usually produce the strongest mortar. Screenings 

 from broken stone usually produce stronger mortars than sand. 



