A Treatise On The Principles And Practice Of Dock Engineering

120 DOCK ENGINEERING. From the point of view of the user, the matter of greatest moment is the actual behaviour of the cement under the projected conditions. Hence the attention of engineers has been largely directed to a determination of those features which are of vital importance. The experience gained by means of numerous experiments has resulted in the sélection of four charac- teristics for purposes of comparison, viz. :_ 1. Fineness of grinding. 2. Resistance to stress. 3. Rate of setting. 4. Integrity or soundness. Fineness.—The importance of fineness is due to the fact that the coarse particles of a badly ground cement hydrate more gradually than the finer particles, and, consequently, expand at a later stage to the detriment of the work. Fine cement, again, will take more sand than coarse cement and makes a proportionately stronger concrete. It also possesses greater capa- bility of rendering the concrete watertight, which under certain conditions is imperative. Finally, the coarse particles are denser and add considerably and needlessly to the cost of carriage. Fineness is tested by sieves with meshes ranging from 1,600 to 32,000 holes to the square inch, of which standards the former is as extremely low as the latter is inordinately high. General practice at present seems to favour either a residue not exceeding 5 per cent, on a 2,500 mesh sieve, or a residue not exceeding 10 per cent, on a 5,800 mesh sieve. One caution is needful : a finely-ground cement may be obtained by supplying lie mills with comparatively soft “ clinker,” which is inferior to that which is heavily burnt. Also, there is a point at which any increase in the fineness of the cement causes additional expense without compensat- ing advantages. To prevent the use of light, underburnt clinker, the weight or the spécifie gravity of the cement is often specified. The former Ties between 100 and 120 Ibs. per bushel and the latter between 3 and 3-15, the higher values corresponding to the better samples, provided that the coarse particles (which have a high density) be sifted before weighing. A very heavy cement, however, is likely to contain an excess of lime, which, in the free State, is eminently deleterious. The weight, moreover, is not a very satisfactory criterion ; cements decrease in weight as they grow old __as much as 4 per cent, in the first month, with a total of 15 per cent, for the year. Strength.—With good Portland cement, mixed neat, a tensile strength of 500 Ibs. per square inch should be obtained at the end of 28 days after mixing 1 day in air and 27 immersed in water. Very frequently a strength of 450 Ibs. is required at the end of 7 days, but a 7 days’ test is a somewhat unreliable guide to the strength ultimately attained, as cements showmg but moderate results (say, 350 to 400 Ibs.) at the end of a week