The following method of waterproofing with asphalt coating is given by W. H. Finley: The asphalt used must be of the best grade, free from coal tar or any of its products, and must not volatilize more than 0.5 per. cent, under a temperature of 100° F. for 10 hours. It must not be affected by a 20 per cent. solution of ammonia, a 35 per cent. solution of hydrochloric acid, a 25 per cent. solution of sulphuric acid, or a saturated solution of sodium chloride. For structures underground a flow point of 185° F. and a brittle point of 0° F. shall be required. If the surface cannot be made dry and warm it should first be coated with an asphalt paint made of asphalt reduced with naphtha. The asphalt should be heated in a kettle to a temperature not exceeding 450° F. It has been cooked enough when a piece of wood can be inserted and withdrawn without the asphalt clinging to it. The first coat should consist of a thin layer poured from buckets on the prepared surface and thoroughly mopped over. The second coat should consist of a mixture of clean sand and screenings, free from earthy admixtures, previously heated and dried, and asphalt, in the proportion of 1 of asphalt to 3 or 4 of sand or screenings by volume. This is to be thoroughly mixed in the kettle and then spread out on the surface with warm smoothing irons, such as are used in laying asphalt streets. The finishing coat should consist of pure hot asphalt spread thinly and evenly over the entire surface, and then sprinkled with washed roofing gravel, torpedo sand, or stone screenings, to harden the top. The thickness of the coating will depend on the character of the work and may vary from ¾ in. to 2 ins. in thickness.
Several firms manufacture and sell ready made priming paints and mastics for waterproofing concrete by substantially the above method. Sarco compounds made by the Standard Asphalt & Rubber Co., of Chicago, Ill., are examples. Sarco waterproofing is a compound analyzing 99.7 per cent. pure bitumen and having a range of ductility of 200° F. In waterproofing large car barn roofs of concrete in Chicago, the concrete was first swept clean and a coat of priming compound was thoroughly brushed in. On the priming coat was mopped a coat of waterproofing compound, applied hot, and covered with a layer of fine sand. The thickness of the completed coating was 1/16 in. Where a heavier waterproofing is necessary the waterproofing compound is covered with one or more ⅝-in. coats of Sarco mastic.
The following bituminous coatings have been used in waterproofing concrete fortifications by the U. S. Army Engineers:
Mobile, Ala.—The top of the concrete was covered with a thin coat of 1-2 cement mortar and given a rough trowel finish. As soon as the surface was dry it was covered with a layer of asphalt mastic 1 in. thick and rubbed down to a finish with dry sand and cement in equal parts. To prepare the mastic take 500 lbs. of Diamond T asphalt mastic, broken into small pieces, 30 lbs. of Diamond T asphalt flux, and 5 lbs. of petroleum residuum oil. When thoroughly melted add 400 lbs. clean, dry torpedo gravel previously heated. Stir gravel and asphalt until thoroughly mixed at a temperature of about 375° F.
Key West, Fla.—The top of the concrete was covered with smooth plaster, proper slope for drainage being given. Above this two layers of asphalt of an aggregate thickness of ¾ in. were applied. The composition of the asphalt was as follows: 440 lbs. rock asphalt mastic, 3 gallons coal tar, and 5 gallons silicious sand.
Delaware River Defenses.—The concrete was waterproofed with coal tar and sand. The tar was made hot and applied to the surfaces with rubber squeegees and then sanded. Joints were filled with the hot tar. A surplus of sand was left on for a few days and then swept off. One barrel of coal tar covered 2,279 sq. ft. with one coat and cost $4.25 per barrel delivered. The cost including material and labor was 0.74 ct. per sq. ft.
San Francisco Harbor.—The roof had a pitch of about 3 in 20 and was covered with an earth fill. The concrete was troweled to a fairly smooth surface, was mopped with a heavy coat of roofing asphaltum, or mastic, then covered with the heaviest grade roofing felt laid 3 ply, starting at the coping of the parade wall and made 4 ply in the gutter. On this assumed watertight surface 3-in. book tile was laid with joints normal to the gutter and cemented. The purpose of the tile was to afford a free passage for the water as soon as it met the roof. The expectations were fully realized and no water, or even a sign of moisture, has appeared in this battery, or at another of the same type since built, after a fair test of time.
The total cost of the work, including mastic, felt and tile, was 17 cts. per sq. ft. for 6,200 sq. ft. covering three roofs.
In conclusion it may be noted that any of the methods of constructing impermeable diaphragms can be used for constructing impermeable coatings.
Szerelmey Stone Liquid Wash.—This wash has been used in England for waterproofing and preserving masonry for some 20 years. It is a thin liquid compound which is applied to the surface with a brush. The stone or concrete surface is required to be dry and thoroughly clean, with all scale and loose particles removed. The standard treatment is three coats; 1 gallon of liquid is in most cases sufficient for treating (three coats) 25 sq. yds., but in exceptionally bad cases 1 gallon for 15 sq. yds. has been found necessary. The precautions necessary for the successful use of the liquid are: It must be well stirred; it must be applied to a perfectly dry, clean surface, and it must be well rubbed into the masonry. The American agency for the liquid is Szerelmey & Co., Washington, D. C.