SO BULLETIN 801, U. S. DEPARTMENT OF AGRICULTURE. 
There are many waterproofing compounds on the market which 
afford various degrees of protection. Those used for concrete usually 
are designed to be mixed with the water used in preparation of the 
mortar, whereas some may be used as a brush application to the 
finished surface. While some of these preparations justify the claim 
as waterproofing, the prospective purchaser will do well to make 
thorough inquiry before investing in them. 2 
lime, 3 parts sand, 6 parts crushed stone (or such amount as may be used with 
confidence that all voids are thoroughly filled by the mortar — that is, the sand 
and cement mixture) with water to form a fairly wet mixture. This should 
be finished on top hr a mortar composed of 1 part Portland cement, one-seventh 
part hydrated lime, 2 parts sand, with water to form a freely working wet 
mortar. The concrete should be deposited oil firmly packed earth and tamped 
until thoroughly packed with water showing on top. The top-dressing of mortar 
should be applied as soon as the concrete is packed. Increased resistance to the 
passage of water from the ground is given by a very dense mixture, such as is 
secured with a liberal amount of water for mixing and also by the addition of 
the small quantity of hydrated lime. Sand, for the best results, should be of a 
mixture of fine and coarse grains, with predominance of the coarse. Under 
any conditions resistance to the passage of water is increased as the proportion 
of Portland cement used is greater. (See United States Bureau of Standards, 
Technical Paper Xo. 3: Tests of the Absorptive and Permeable Properties of 
Portland Cement Mortars and Concrete, Together with Tests of Dampproofmg 
and Waterproofing Compounds and Materials, by Rudolph J. Wig and P. H. 
Bates. 1911,) The top dressing of the richer mixture also supplies a hard, 
smooth surface which is not readily chipped by truck wheels. 
If conditions of the site warrant full protection against water, better protec- 
tion is secured by interposing between the layer of concrete and the top-dressing, 
a layer of very pliable tarred roofing felt. The edges of the widths should be 
fully lapped and cemented by tar and pitch. Where this is done the concrete 
slab must first be allowed to set and the top-dressing should be 2 or 3 inches 
thick, while the concrete slab m*ay be reduced to the same thickness. If the top- 
dressing is too thin and separated from the concrete, it is likely to buckle and 
crack. The expansion joint of tar paper in this case need extend through the 
top-dressing only. Still better protection is afforded by a thorough coating of 
hot tar or tar and pitch applied to the rough slab before the application of the 
top-dressing, but after the rough slab is dry. In this case the top-dressing need 
be only 1 inch thick. The expansion joints in this instance should extend 
through both the rough slab and the top-dressing. The latter method of retard- 
ing water is to be preferred, provided care is exercised to see that all parts of 
the surface are well covered with the tar. Where work is done carelessly the 
first method usually will be more satisfactory. Either arrangement affords 
adequate protection. However, for all favorable conditions such precautions 
are unnecessary. 
2 See United States Bureau of Standards. Technical Paper Na 3 : Tests of the 
Absorptive and Permeable Properties of Portland Cement Mortars and Con- 
crete, Together with Tests of Dampproofmg and Waterproofing Compounds 
and Materials, by Rudolph J. Wig and P. H. Bates, 1911. 
A satisfactory substitute for the waterproofing methods described (and a 
plan which well may be combined with these for unusually trying conditions) is 
