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METHODS AND COST OF WATERPROOFING CONCRETE STRUCTURES.
Resistance to penetration by water is desirable in all concrete structures, and is essential in such structures as tanks, reservoirs, vaults, subways, bas.e.m.e.nts and roofs. Concrete, as it is ordinarily made, is pervious to water, hence to secure concrete structures through which water will not penetrate some method of waterproofing the concrete must be employed. Many methods have been proposed and are being used; none of these methods is without faults, the best one of them has not yet been determined, and the evidence available as to their comparative merits is biased and conflicting. For these reasons any discussion of waterproofing for concrete is at the present time bound to be unsatisfactory.
Methods of waterproofing may be roughly cla.s.sified as follows: (1) Use of mixtures so proportioned as to be impervious; (2) admixture of substances designed to produce impermeability; (3) use of waterproof coatings, washes or diaphragms. In succeeding sections enough examples of each method are given to indicate current practice; no attempt has been made to catalog all the waterproofing substances and systems being promoted--there are too many of them.
The art of waterproofing concrete is in a transition stage. Outside of the manufacturers of waterproofing material the art has received serious study by comparatively few persons. No comparative tests by independent investigators are available. Practical experience with most of the materials used has not extended over a long enough period of time to permit true conclusions to be drawn. Students of the subject are not even agreed upon the broad questions whether it is better to work toward developing an impervious concrete or toward perfecting a waterproof covering for concrete. On the minor subdivisions there is no agreement at all.
In the present state of the art one can lay fast hold to only three things. The first is that waterproofing is one component of a system of drainage; the second is that structures must, to get the best results, be designed with the fact in mind that waterproofing is a component structural element, and the third is that skilled and conscientious workmanship are essential elements in the success of all waterproofing materials and methods.
~IMPERVIOUS CONCRETE MIXTURES.~--The compounding of the regular concrete materials so as to produce an impervious concrete has been made the subject of numerous experiments. The most elaborate of these experiments were those conducted over a period of five years by Mr. Feret, of the Boulogne (France) Laboratory of the Ponts et Chaussees. Feret's experiments led him to the following conclusions:
"That in all mortars of granulometric composition the most permeable are those which contain the least quant.i.ty of cement.
"Of all mortars of the same richness, but of varying granulometric composition, those which contain very few fine grains are much more permeable. They are the more so where, with equal proportions of the fine grains, the coa.r.s.e grains predominate more in relation to the grains of medium size.
"The minimum permeability is found in mortars where the proportion of medium-sized grains is small, and the coa.r.s.e and fine grains are about equal to each other."
Mr. Feret also found that permeability decreased with time and that wet mixtures were less permeable than dry mixtures.
Tests made by Messrs. J. B. McIntyre and A. L. True at the Thayer School of Civil Engineering in 1902 gave the following results:
All the specimens composed of 1-1 mortar in the proportions of 30, 35, 40 and 45 per cent. of the whole ma.s.s were impermeable. Some of the specimens composed of 1-2 mortar in the proportions of 40 and 45 per cent. were also impermeable, as well as the 1-2-4 and 1-2-4 mixtures.
All other mixtures leaked at the high pressure (80 lbs. per sq. in.) and in a general way exhibited a degree of imperviousness in direct proportion to the proportion of mortar in them, with the lower pressures from 20 lbs. per sq. in. up as well as for the 80-lb.
Other tests confirm those cited. In general we may conclude that those mixtures richest in cement and mortar are the most impervious. It is doubtless practicable by exercising proper care to proportion, mix and place a concrete mixture which will be so nearly impervious that visible leakage will be small. The task, however, is one difficult to perform in actual construction work, and its accomplishment is never certain.
~STAR STETTIN CEMENT.~--Star Stettin cement is a Portland cement made by grinding a clinker which has been "impregnated" with substances which impart waterproofing properties to the ground product. The process is the invention of Richard Liebold, and the cement is made by the Star Stettin Portland Cement Works, Stettin, Germany. It is a.s.serted that a 1-4 fine sand mortar made with this cement is impervious. To use it the ordinary precautions adopted in the employment of Portland cement are necessary, and in addition the following: The cement must be mixed with moist instead of dry sand before the water is added; the sand should be clean, sharp and fine of grain; the mortar must be more perfectly mixed than ordinarily, and somewhat more water should be used than is ordinarily used. Perfectly even mixing is essential to the best results.
~MEDUSA WATERPROOFING COMPOUND.~--This compound is a dry powder which is mixed with the cement in proportions of from 1 per cent. to 2 per cent.
by weight, or from 4 lbs. to 8 lbs. per barrel of cement. The compound costs 12 cts. per lb., so that its addition increases the cost from 48 to 96 cts. per barrel of cement. Thorough mixing of the compound with the cement is of the utmost importance, otherwise none but the ordinary precautions in the use of Portland cement is necessary. Absorption tests on concrete blocks treated and untreated with the compound and nine months old have shown the absorbtion of treated blocks to be about one-fourth or one-fifth that of untreated blocks. The compound is made by the Sandusky Portland Cement Co., Sandusky, Ohio.
~NOVOID WATERPROOFING COMPOUND.~--This compound is a dry powder which is mixed dry with the cement in the proportion of 1 to 2 per cent. by weight or about 1 to 2 lbs. per bag of cement. The compound costs 12 cts. per pound or about from 48 to 96 cts. per barrel of cement.
Directions for making waterproofing mortar are: To 100 lbs. of Portland cement add 2 to 2 lbs. of compound and 200 lbs. of clean and sharp sand and mix the materials dry and very thoroughly. The water is then added in the proportion necessary to make a good working mortar and the mortar mixed and applied in the ordinary manner. Used as a wash 2 lbs. of compound are thoroughly mixed dry with a bag of cement. Any portion of the mixture is then mixed with water to produce a creamy grout, which is applied to a thoroughly wet surface with a brush. This compound is made by The Abbey-Dodge-Brooks Concrete Co., Newark, N. J.
~IMPERMEABLE COATINGS AND WASHES.~--The most common means employed for rendering concrete structures waterproof is to coat or wash the surface with some substance itself impervious to water or having the property of closing the pores of the surface skin of concrete so that water cannot penetrate.
~Bituminous Coatings.~--Bituminous coatings of one composition or another are among the most commonly used of impermeable coatings. The bituminous compound is used both alone and in combination with layers of a fabric of some sort to form the coating. Where bituminous coatings are used on surfaces exposed to the sun and frost attention must be given to the fact that a compound of different properties is required where the range of temperature is great than is required where this range is smaller.
Asphalt, for example, should have a flow point of 212 F. and a brittle point of -15 F. when exposed directly to sun and frost as compared with say a flow point of 185 F. and a brittle point of 0 F. when covered from the direct action of sun and frost. Another point to be kept in mind particularly in using exterior coatings is that the concrete surface must be properly prepared to receive the coating or else it will peel off. The following are examples from actual practice of waterproofing with bituminous coatings.
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 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 a.n.a.lyzing 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 5/8-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 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 a.s.sumed 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 pa.s.sage 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.
~Sylvester Wash.~--Waterproofing with Sylvester wash consists in applying alternately to the concrete surface a soap solution wash and an alum solution wash. The soap solution is applied first, and it must be applied hot and to a dry surface; the alum solution is applied second and 24 hours after the soap solution and is applied cold. This const.i.tutes one treatment. After 24 hours a second treatment may be given, and as many treatments may be given as necessary. In some cases as many as six treatments have been employed. The proportions of the solutions used in practice vary. In waterproofing the standpipe described in Chapter XXII the soap solution consisted of 12 oz. pure Castile olive oil soap per gallon of water, and the alum solution consisted of 2 oz. of alum per gallon of water. In repairing the bottom of a reservoir lined with 4 to 6 ins. of concrete the following solutions were used: lb. Olean soap to 1 gallon of water and lb.
alum to 4 gallons of water. Both alum and soap were well dissolved and the soap solution was boiled. The boiling hot soap solution was applied on the clean, dry concrete; 24 hours later the alum wash was applied cold. This treatment was repeated after 24 hours. Two men applied the solutions, using whitewash brushes, while a third man carried pails of the solution. In making the soap solution two men attended four kettles, one man kept up fires, two men carried solution to men applying it. The alum solution required fewer men, being made cold in barrels. After applying the second soap wash to the concrete slopes, the men had to be held by ropes to keep from slipping. The rope was placed around two men, who started work at the top of the slope, a third man paying out the rope. The work was done in 8 days and cost as follows:
Labor: 1,140 hours labor at 15 cts. $171.00 83 hours foreman at 30 cts. 24.90 83 hours waterboy at 6 cts. 4.98 Add for superintendence 15% 30.13 ------- Total labor $231.01 Materials: 900 lbs. Olean soap at 4-1/3 cts. $ 39.00 210 lbs. alum at 3 cts. 6.30 6 10-in. whitewash brushes at $2.25 13.50 6 stable brushes at $1.25 7.50 ------- Total materials $ 66.30 Total labor and materials $297.31
This covered 131,634 sq. ft., hence the cost of the two coats of soap and alum was $2.26 per 1,000 sq. ft., or 0.23 ct. per sq. ft.
The ordinary Sylvester wash, as described above, has been modified with success on Government fortification work as follows: To 2 gals. of water add 1 lb. concentrated lye and 5 lbs. alum and mix until completely dissolved. This is a concentrated stock solution. In use 1 pt. of solution and 10 lbs. of cement are mixed with enough water to make a mixture that will lather freely under the brush. Two coats of this wash are applied, the second at any time after the first is dry, and the first as soon as the forms are removed from the concrete. The wash should be applied to a wet surface, if the concrete is dry it should be wet down with a brush ahead of the wash.
~Sylvester Mortars.~--In this cla.s.s of coatings the alum and soap are added to the mortar which is used for facing. A successful recipe for such a mortar is given as follows: To 1 part cement and 2 parts sand add lb. of pulverized alum for each cubic foot of sand and mix these ingredients dry; then add the proper quant.i.ty of water, in which has been dissolved lb. of soap to the gallon, and mix the mortar thoroughly. Such a mortar is but slightly inferior in strength to ordinary mortar of the same proportions. In plastering a clear water well to prevent leaking a 1-2 mortar was made as follows: 1 lbs. of soap were dissolved in 15 gallons of water and 3 lbs. of powdered alum were mixed with 1 bag of cement. Two coats of plaster of an aggregate thickness of in. were applied and completely stopped the leaking. The cost of this treatment was as follows:
2 lbs. soap (with 24 gals. water) at 7 cts. $0.15 12 lbs. alum at 3 cts. 0.42 ------ Total per barrel of cement $0.57
In lining a new reservoir near Wilmerding, Pa., a mortar was made as follows: A stock solution of 2 lbs. caustic potash and 5 lbs. alum to 10 quarts of water was made in barrel lots, from which 3 quarts were taken for each batch of 2 bags of cement and 4 bags of sand. A batch of mortar covered an area 68 ft. with a 1-in. coat. The extra cost of the waterproofing was:
100 lbs. caustic potash at 10 cts. $10.00 70 lbs. caustic potash at 9 cts. 6.30 960 lbs. alum at 3, 3 and 4 cts. 34.38 60 hours mixing at 15 cts. 9.00 Freight, express and haulage 11.50 ------ Total for 74,800 sq. ft. $71.18
This gives a cost of 95 cts. per 1,000 sq. ft., or less than 0.1 ct. per sq. ft. It was found that if less than 2 parts of sand to 1 part of cement was used the mortar cracked badly in setting. Clean sand was imperative, as any organic impurities soon decomposed, leaving soft spots. Do not use an excess of potash; a slight excess of alum, however, does not decrease the strength of the mortar.
~Hydrolithic Coating.~--This waterproofing is a dry mortar composed by mixing a cementing compound with sand, and sold dry in sacks containing 96 lbs. each. The dry mortar is mixed with water to proper consistency for plastering, and is applied as a plaster to the surfaces to be waterproofed. The dry mortar is mixed with water to a grout of the consistency of thick cream and then this grout is stiffened to the proper consistency by adding more dry mortar. Thoroughness of mixing is absolutely essential. The concrete surface is prepared by picking and scoring sufficiently to get a fresh surface and washing away all chips, dust and loose material, or instead of picking in new work the outer skin may be removed by a 1 to 9 muriatic acid solution and then washed free of all acid and scrubbed with wire brushes. After preparing the fresh surface it is well wetted; in fact water soaked, so that, while not oozing moisture it will absorb no more water. The mixed mortar is then applied with a trowel in a workmanlike manner. In mixing, no more than 8 gallons of water per barrel of mortar should be used. The coatings used are 3/8 to 5/8 in. for walls and to in. for floors.
The following estimate of cost is made by the manufacturers, the E. J.
Winslow Co., Chicago, Ill. The figures are presented with the understanding that they are to be considered merely as average costs for waterproofing, without special construction, and subject to change in accordance with local conditions, and to the time of year when the work will need to be performed:
Per sq. ft.
To prepare surfaces to receive "coating" may cost the contractor 5 cts.