MASONRY 313 



will be liquid enough to fill the little cavities in each cinder. 

 This precaution is indispensable when the concrete is to 

 be used with steel, as otherwise the steel will be corroded by 

 the action of air reaching it through the pores in the cinders. 



Sand for Concrete. The sand used for concrete should be 

 sharp and free from loam and chemical salts, particularly salts 

 of a hygroscopic nature. The sand should not be too fine. 

 An investigation made by A. S. Cooper on the effect that the 

 size of the grains of sand has on the strength of mortar led him 

 to the conclusion that, up to a certain limit, mortars become 

 stronger as the grains of sand used become larger. However, 

 the amount of cement required to fill the voids between the 

 grains of sand is an item of importance, and increases with the 

 size of the grains themselves. It is, therefore, customary to 

 use sand with some coarse grains in it, but with enough smaller 

 grains to fill the voids between the larger ones. 



Aggregates. When concrete is to be used in a place where it 

 may have to withstand the action of fire, it is necessary that 

 the aggregate be of such nature that it will not disintegrate 

 and crumble away. Limestone and marble chips are objec- 

 tionable as aggregates, as the action of heat causes them to 

 swell, crack, and crumble to dust. Trap rock, cinder, and 

 broken brick are among the best aggregates for concrete 

 that is to be exposed to the action of fire. It should be borne 

 in mind, however, that broken brick will soon soften in concrete 

 placed under water. 



Limestone is unsafe to use in reinforced-concrete work, unless 

 special care is taken to see that the steel is well protected from 

 the stone by a layer of cement. Another material that is con- 

 sidered injurious to steel, if the latter is not coated with cement, 

 is cinders; their damaging effect is not due so much to the sul- 

 phur in them, as commonly claimed, as to their porosity, 

 However, in certain proportions in which the cinder is not so 

 predominant as in a mixture of 1 part of cement, 2 parts of 

 sand, and 3 parts of cinder the corrosive effect on the steel 

 is inconsiderable if the concrete is properly mixed. 



Proportioning Ingredients. The proper proportion of 

 ingredients for the best concrete is such that there will be enough 

 cement in the mixture to bind all the materials together, and 



