Engineering Wonders of the World
Volume I

420 ENGINEERING WONDERS OF THE WORLD. Fig. 4.—GRADED SAND AS USED IN CONCRETE, SHOWING VOIDS. Fig. 3.—GRADED PEBBLES AS USED IN CONCRETE, SHOWING VOIDS. spaces and, in addition, to coat the outer sur- face of the particles. In this way a solid and securely cemented mass is ensured. For the mortar he selects coarse, gritty sand, whose particles vary in size from |-inch downwards (see Fig. 4) ; and after measuring the propor- tion of voids between the grains, he adds sufficient Portland cement to fill up the spaces and to coat every particle of sand outside. Thus the mortar as well as the concrete is solid and properly cemented together. Portland cement is produced by mixing to- gether intimately chalk and clay, burning them at high temperature, and Portland grinding the resulting clinker to an almost impalpable powder. The cement so made sets rapidly when mixed with water, and becomes as hard as stone. When the three constituents of concrete have been mixed with a suitable quantity of water, and the process of setting has con- tinued for three or four weeks, the concrete is characterized by considerable tenacity and enormous resistance to pressure. Moreover, its strength and hardness continue to increase year by year for all time. In this respect concrete differs from, and is superior to, all other structural materials, which, without a solitary exception, suffer disintegration by exposure to the atmosphere. Even the hardest stone perishes in course of time, while metals such as iron and steel are rapidly destroyed by corrosive influence. No form of paint or other protective coating has been devised, and probably none will ever be devised, capable of preserving metal sur- faces permanently from corro- sion. But it is a well-authenti- Advantages . . . , of Cement. cated tact that iron or steel embedded in concrete is fully protected from injury of any kind. Among other proofs of this valuable property we may cite the case of iron clamps embedded in the walls of the Parthenon, which, when uncovered after the lapse of two thousand years, were found to be in perfect condition. Another good instance was cited at the Engineering Conference of 1907 by Mr. W. T. Douglass, M.Inst.C.E., who stated that, when