Practical water-tightness in concrete may be secured by using a fairly rich mixture properly proportioned. Foreign ingredients, membrane and surface coatings, or other means need not be used, except where poor workmanship is likely or where considerable damage and inconvenience may result in case of leakage. Under such circumstances the membrane treatment used in addition to a properly proportioned concrete, while the most expensive method of waterproofing, probably will give the most reliable results. This treatment consists of layers of burlap or tar paper cemented to the surface and together with tar or asphalt. Where the membrane is subject to injury it is sometimes protected by a coating of cement mortar or brick backing.

First-class workmanship and special attention to details are required to secure water-tightness. The essential requisite is that the voids be filled. A lean mixture may be made more impervious by using hydrated lime which tends to fill the voids and makes the concrete flow easily. A little more cement in the mixture would serve the same purpose. The lime should not be in excess of 10 per cent of the weight of the cement and under no circumstances should unslaked lime be used.

The materials for water-tight concrete must be well graded, so as to obtain a maximum density; that is, enough sand must be used to fill the spaces between the gravel or stone and enough cement to fill the spaces between the grains of sand. A 1:2:4 concrete will prove practically impermeable in ordinary construction, but if a head or pressure of water is to be resisted a 1:2:3 or richer mixture may be necessary. The consistency is very important. A sluggishly flowing consistency is best, for if the concrete is too wet the mortar may flow away from the stone, leaving leaky places and, if too dry, the mass may prove porous. The proportions and consistency must be accurately maintained for each batch and the concrete must be exceptionally well mixed.

It is necessary to exercise great care in the placing of the concrete. Where practicable, the structure or object should be poured in one operation to avoid leaky joints, but when this is not possible precautions should be taken to secure a tight joint between concrete of different ages. The surface of concrete which has set must be cleaned of dirt and scum down to the true concrete. This surface then should be well whetted and painted immediately with a creamy mixture of cement and water before placing the new concrete. A good plan, when discontinuing work on structures intended to hold liquids, is to embed a 6 or 8 inch strip of tin or thin sheet metal to half its width in the concrete so that the other half will project into the new concrete.

A wall thick enough to resist the stresses put upon it will generally resist percolation of water, but 6 inches may be considered as a minimum.

Contraction and expansion must be controlled to avoid the occurrence of leaks. To guard against cracks due to unequal settlement or other causes, most concrete designed for water-tightness should be reinforced. In some mass work, special contraction joints, as described on page 23 may be necessary. Rules for the use of reinforcement and contraction joints can not be given, as the requirements in each case vary with the conditions to be met.

ORGANIZATION OF THE
UNITED STATES DEPARTMENT OF AGRICULTURE

October 21, 1929

This bulletin is a contribution from

U. S. GOVERNMENT PRINTING OFFICE: 1929