3G BULLETIN 801^ U. S. DEPAKTMEETT OF AGRICULTURE. 



There are maiij^ waterjDroofiiig 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. Wliile some of these preparations justify the claim 

 as waterproofing, the prospective purchaser will do well to make 

 thorough inquiry before investing in them.^ 



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 by 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 dejwsited on 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 ]jest 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 No. 3 : Tests of the Absorptive and Permeable Properties of 

 Portland Cement Mortars and Concrete, Together with Tests of Dampproofing 

 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 agalast water, better protec- 

 tion is secured by interposing between the layer of concrete and the top-dressing, 

 a layer of vei-y pliable tarred roofing felt. The etlges of the widths should be 

 fully lapped and cementetl 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 nfay 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. 



^ See United States Bureau of Standards, Technical Paper No. 3 : Tests of the 

 Absorptive and Permeable Properties of Portland Cement Mortal's and Con- 

 crete, Together with Tests of Dampproofing and Waterproofing Compounds 

 and Materials, by Rudolph J. Wig and P. H. Bates, 1911. 



A satisfactoiT substitute for the waterproofing methods described (and a 

 plan which well may be combined -with these for unusually trying conditions) is 



