Pocketbook of Useful Formulæ and Memoranda
for Civil and Mechanical Engineers

98 moleswokth’s pocket-book Portland Cement continued. 13. When concrete is used, a current, either by pumping or otherwise, will carry away the cement and leave only the clean ballast. 14. Roman cement is only about |rd of the strength of Portland, and is ill adapted for being mixed with sand. Concrete Walls. Concrete walls for houses are built of 1 of cement to 6 or 7 of broken stone, mill-cinders, burnt ballast, shingle, gravel, or slag. The sub- stance mixed with the cement must be thoroughly free from loam, mud, fine sand, clay-; or dirt of any kind. Mixture to prevent the cement from adhering to the plates, 1 lb. of yellow soap, cut in shreds, boiled and stirred until it is of the consist- ency of paint—to be applied freely with a brush to all parts before the concrete is placed in contact with them. Crushing Weight of Portland Cement in lbs. per square inch. (Grant.) About f of the crushing weights produced the first crack. — ‘ Min. Inst. Civ. Eng.,’ vol. xxv.