Page 8 - Minutes of Proceedings of Civil Engineers Vol CXCV 1913-14 Part 1

48 WENTWoRTH-SHEILDS oN THE coNsrRUCTroN ox. [Minutes of For better work, such as reinforced concrete, the sand and gravel were graded by being passetl thlough various scr.cens. Large stones, rvhich would not pass tlrlough t f -inch rneslr, wer.e {irst screened out and reducetl in n crushcr, Thon the gllvel w:rs plssed over a {-inch screen, the portion rejectetl being classerl us stones and the remainder as sanrl. Tlre sarrd, bein6; coarse and scant in quantity, was supplemented by un equnl quantity of fine sand from the langston Ilarbour boruc)r. 'Ihc uddition of this fine sand rather improvod the cont:r'ete, ls is shown by the results of crushing-tests on l2-inch cubes given in Apporrdix II (p. 68). The concrete was formetl by mixing I part by volume of the cement, l| of the mixed sands, and 3 of stones. Only fresh woter wa,s used for reinforced concrete, as it rvas thought that salt wnter might, have a rusting action on the steel. 'l'he question of screening and regrading gra,vel for the weaker classes of concrete was considered, but although tbo practice is recommended by some authorities, it was not considered to be economical in this case, as it was thought that the extra cost of crushing and screening would not compensate for any cement which might be snved. The great advantage of regrading is that the resulting concrete is more uniform in quality and more watertight. But in the gravity walls the concrete is never stressed to its safe limit, and is not required to be specially watertight. The use of gravel in its nntural state was therefore considered to be sound oconomy for the mass and block work. Concrding.-Nearly all the concrete for the walls was mixed by four stationary machine mixers, two being placed on the east wall an