A Treatise On The Principles And Practice Of Dock Engineering

DOCK ENGINEERING. 130 of the sea-water, while the magnesia remains in solution as magnesium chloride. Although in this case there can be no further closing of the pores, yet the insoluble barium sulphate, which is formed, affords some protection and does not give rise to any increase of volume (swelling). From 2 to 3 per cent, of barium chloride does not in any way diminish the strength, as has been proved by the comparative tests of English and German cements. Frequently the strength of the mortar is increased by this addition. This substance is only to be used in the external, perfectly watertight skin of concrète ; in other words, in the 4 to 8-inch coating, com- posed of 1 cement, 1 to 2 sand, and 3 to 4 coarse gravel, flint, broken stone, &c.” Practical Notes on Mixing Concrète for Marine Work. 1. A heavy aggregate is desirable. If mixed by hand, the materials should be laid out on a platform of deals, in order to secure freedom from dirt and impurities, and covered by the cement in a thin layer. The whole should be turned over thrice dry, and as many times wet, before depositing. 2. The concrete should not be tipped from a height greater than 6 feet, or there will be a tendency for the heavier portions of the aggregate to separate from the lighter. For great depths, shoots may be employed with men stationed at the foot to shovel the mass immediately into position. The work should be well rammed and Consolidated. 3. As many rubble burrs, or stone plums, should be imbedded as the fluid concrete can adequately enclose. No two burrs should be in contact, and none should be set within 12 inches of the face of the wall. If the burrs are porous, they should be wetted before insertion. 4. The concrete should be deposited without delay after mixing, and should remain entirely undisturbed during setting. After the setting of each layer, the surface should be prepared for the reception of the next layer by picking, washing, and sweeping. In mass work, layers should not exceed 2 to 4 feet in height. 5. Concrete blocks should not be used under 14 days after mixing, and preferably the period will be extended to three or four weeks. 6. Concrete bags have a tendency to break away at the ends. Con- sequently, they should be slightly longer than the nett length required. 7. Wind screens should be provided in windy weather, otherwise the cement will be largely wasted, even if the concrete be not allowed to suffer thereby. 8. Concrete mixing should be avoided as far as possible during keen frost, except in situations where the concrete is deposited directly under water, or is soon afterwards covered by the tide. Where continuous opera- tions are essential on shore, artificial warmth from braziers and fires may be utilised to raise the surrounding temperature, and salt-water may be employed in mixing on account of its lower freezing point. An American