312 MASONRY 



The compressive strength of cement mortar is about eight 

 times its tensile strength, and the strength of mortar in cross- 

 breaking and shear may be taken at about one and one-half 

 to two times the tensile strength. 



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

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

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

 75 to 125 Ib. Natural-cement mortars have nearly the same 

 adhesive strength as those made with Portland cement. 



CONCRETE 



Concrete consists of cement, water, sand, and large or 

 small fragments of broken stone, gravel, or cinder. The plastic 

 cement, either by itself or with the sand, is called the matrix 

 and the hard material the aggregate. 



Cement for Concrete. The cement used for concrete work 

 is almost exclusively hydraulic cement, generally Portland 

 cement. Natural cement is not so strong and reliable as Port- 

 land. It sets more quickly, but takes longer to obtain its 

 ultimate strength. It is used where economy demands it, 

 but should never be placed under water. In civil-engineering 

 work it is seldom employed, except in the form of mortar. A 

 very good substitute for Portland cement in concrete for use 

 underwater is pozzuolana cement. This cement never gets very 

 hard, but it withstands the action of sea-water even better than 

 Portland cement. It will, however, soon fail if subjected to 

 much attrition and wear. 



Water for Concrete. The wetter the concrete is the easier 

 it will be put in place, but mixtures that are too wet are not 

 so strong as medium mixtures. The quantity of water that 

 will make the best mixture is such that after the concrete has 

 been put in place and rammed, it will quake like jelly when 

 struck with a spade, and water will come to the surface. If the 

 concrete is wetter than this, the water will have a slight chemical 

 effect on the cement, and, moreover, the sand and cement will 

 tend to separate from the broken stone. 



In cinder concrete, owing to the porosity of the cinders, 

 it is necessary to use a little more water, so that the cement 



