WATERTIGHT CONCRETE 87. Permeability of Concrete.—The permeability of a wall of concrete varies with the size and shape of the aggregates, the density of the mixture, and the richness of the mortar. For given aggre gates, the densest and strongest mixture will usually be the least permeable, although the least porous concrete is not necessarily the least permeable when different materials are used.
Mortar composed of fine sand is more porous and less permeable than mortar of coarse sand mixed in the same proportins. Graded sand, with sufficient fine materials, shows less porosity and less permeability than either fine or coarse sand alone, but with any sand, the permeability of mortar decreases as the ratio of cement to sand increases.
The permeability of mortar decreases with age during the period of hardening, and mortar subject to the continuous filtration of water decreases in permeability. Messrs. Fuller and Thompson r found that the permeability of concrete decreased as the maximum size of coarse aggregate increased, and that gravel concrete was less permeable than that made with broken stone.
The use of sand cement (see Section 16) in place of Portland cement ordinarily gives a somewhat more impervious concrete, and is frequently used for the purpose. The extremely fine grinding to which the cement is subjected in preparing the sand cement is favorable to making a water-tight mortar.
carefully selected and proportioned materials and good workmanship, concrete may be made practically water-tight. To secure this result, rich mortar (at least 1 to 2) should be used, and the aggregates graded to produce a dense mix ture. The concrete should be thoroughly mixed to a plastic or soft, but not too wet, consistency, and placed carefully, eliminating joints if possible. When horizontal joints are unavoidable, the skin on the old surface should he broken and roughened before placing the cement paste to receive the new work. A thickness of 1 foot of well-constructed concrete wall may he expected to be practically water-tight, under a head of 50 feet. No wall to hold
water pressure should be less than 6 inches thick.
When a lean concrete is used for the body of the work, a thin surface of rich concrete, or of cement mortar, may he placed upon the water face; this face must be built up with the main body of the work and firmly united with it, and contraction joints must be used where cracks are likely to occur.
Tests for permeability of concrete is tested by forcing water through a block of concrete under pressure, the block being so arranged that the water can escape only by passing through the concrete. In an apparatus used by Mr. Thompson for this purpose the sides of the mold were made by two pieces of wrought iron bent to a half-circle and bolted together, these rest ing on a plank which formed the bottom of the mold until the con crete had set. The surfaces of the concrete were chipped to remove the skin, the blocks turned upside down in making the tests, and the water measured which passed through the block.
88. Integral the purpose of increasing the water-tightness of concrete, additions of other materials are some times made. There are a number of proprietary compounds on the market to be mixed with the concrete to make it impervious, known as integral water-proofing compounds. Some of these may be of value, provided they are not used in lieu of proper proportions or good work in placing the concrete, but dependence upon making meager or improperly mixed concrete water-tight by additions of waterproofing compounds is apt to end in failure.
Hydrated Lime.—The addition of hydrated lime to the cement mortar used in concrete may be useful in assisting in making the concrete water-tight. As already noted (Section 35), mortar con taining lime works easier, and is preferred by masons for brickwork (Section 60). A small addition of hydrated lime makes concrete flow more readily in placing and tends to prevent separation of the materials in handling, and it is sometimes used for this reason in concrete which is to be chuted to place.