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.
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ORGANIZATION OF THE
UNITED STATES DEPARTMENT OF AGRICULTURE