WATERPROOF CONCRETE. For some purposes water-tight concrete is very important, as, for example, to keep dry an inclosed space below the water level, or for tanks, aqueducts, or sewers. In aqueducts and sewers it is required only that the leakage shall be small in comparison with the liquid conveyed; while in basements and subways it is essential not only that no water penetrate but that dampness should be prevented. In reinforced-concrete construction it is vitally important that water shall not come in contact with the steel, since it means not only the weakening of the structure by the rusting of the steel but possibly the disruption of the concrete 'itself, which is a still more serious matter.
The waterproof qualities of a concrete are tested either by deter mining the amount of water absorbed in a given time or by observing the amount of water per unit of area that flows through a given thickness in a known time under a definite head. The latter method is the better, and is the more frequently employed. For illustrated descriptions of the details of four methods of making permeability tests see: (1) Transactions American Society of Civil Engineers, Vol. lix, page 127-37; (2) Transactions American Society for Testing Materials, Vol. vi, page 334-41; (3) Bulletin No. 329, U. S. Geological Survey—Organization, Equipment, and Operation of the Structural Materials Testing Laboratories at St. Louis, Mo.,-page 76-79; (4) Engineering News, Vol. xlvii, page 517-18.
In discussing waterproof concrete, a distinction should be made between seepage through pores and leakage through cracks due to settlement or to changes in temperature. Only the former will be discussed here, the latter being considered in § 384.
In many cases the difficulty in waterproofing is increased by the failure to provide drainage; but this phase of the subject will be considered in subsequent chapters in connection with the discussion of the different structures.
of voids, while permeability depends upon the size of the voids and their inter-communication. The densest neat portland-cement mortar has from 40 to 43 per cent voids, but is absolutely impervious; while a 1 : 2 : 4 concrete made of well-assorted ingredients has only about 12 or 14 per cent of voids, and may be slightly permeable. In the first case, the voids are so small and so uniformly distributed that it is impossible after the mortar has set to displace the air in them by forcing in water; while in the second case, owing to the impossibility of getting a perfect mixture, the voids are larger and are inter-connected so as to permit the percolation of water.
The voids in concrete are due partly to entrained air and partly to the space occupied by the water. In dry concrete the air-filled voids are the larger, but in a wet concrete the water-filled spaces are the greater. Considerably more water is used in making concrete than is required for the chemical action in the setting of the cement, and consequently the evaporation of the water leaves the concrete porous. Portland cement requires for complete hydration from 12 to 14 per cent of its weight of water. The rate of hydration varies with the composition of the cement, its fineness, etc.; but from ex periments with five brands, the author concludes that usually only about 8 to 10 per cent of water enters into combination with the cement at the end of a week. The densest 1 : 2 : 4 concrete requires water equal to about 32 per cent of the weight of the cement; and this water occupies about 12 per cent of the volume of the concrete. But as only about 8 per cent of water combines with the cement within 7 days, there remains water equal to about three fourths of 12 per cent, or 9 per cent, of the volume of the concrete to be evaporated; and consequently the water-filled pores constitute about 9 per cent of the volume of the concrete. With well-mixed wet concrete the air-filled voids do not constitute more than 1 or 2 per cent of the volume; and consequently the voids in a rich, well-graded, and thoroughly mixed concrete should not exceed more than 10 or 11 per cent.