Concrete Construction: Methods and Costs

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 class 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 ¾ lb. of pulverized alum for each cubic foot of sand and mix these ingredients dry; then add the proper quantity of water, in which has been dissolved ¾ 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: 1¼ 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 ½ in. were applied and completely stopped the leaking. The cost of this treatment was as follows:

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 6×8 ft. with a 1-in. coat. The extra cost of the waterproofing was:

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 ⅜ to ⅝ in. for walls and ½ to ¾ 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:

The lowest price yet asked for work was 20 cts., and the highest, 55 cts., these two prices representing the opposite extremes of conditions that different jobs will present.

Cement Mortar Coatings.—Rich cement mortar mixtures offer considerable resistance to penetration by water and when well made may be used with a fair degree of success to waterproof ordinary concrete. European engineers make wide use of mortar coatings for waterproofing tanks and reservoirs and appear to have good success with them. The experience in this country is that no great reliance can be placed on them, where the pressures are at all large. Records of work done show both successes and failures, with no apparent reason for either so far as composition of mortar or quality of workmanship goes. A rich mortar plaster will reduce leakage, and may prevent it entirely, but it is uncertain how far it will prove water tight.

Oil and Paraffin Washes.—The theory of the use of oil and paraffin washes is that the material soaks into the concrete and closes the surface pores against the penetration of water. Paraffin has been quite widely used for preserving stone masonry walls for buildings. It is applied hot, and in the best practice is applied to a dry heated surface. Concerns doing such work on buildings have portable devices for heating the masonry. Oil is sometimes applied hot but is more often flushed onto the surface and allowed to soak in as it will.

IMPERMEABLE DIAPHRAGMS.—The most generally employed method of waterproofing concrete structures, with the possible exception of painting and coating methods, is to embed in the wall, roof and floor slabs a diaphragm that is impervious to water. Such diaphragms are usually composed of layers of waterproof felt or paper cemented together and to the concrete by asphalt, coal tar pitch or patented cementing compound. Another construction consists of a layer of asphaltic compound between two layers of cement mortar. In some cases also the combination felt and cementing compound diaphragm is further strengthened by placing it between layers of mortar. In wall work the diaphragm is frequently applied to the face of a single layer brick wall and the concrete filled against it. The brick wall may be further waterproofed by laying the brick in hot asphalt instead of in mortar.

Within the last few years a number of firms have devoted their efforts to producing special fabrics (felts or papers) and special cementing compounds designed to be used with the fabrics for waterproofing concrete. These fabrics and cements are in most cases superior in toughness, flexibility, ease of application, etc., to the ordinary roofing and waterproofing fabrics designed originally for general building purposes.

Long Island R. R. Subway.—In constructing the Long Island R. R. subway the roof was waterproofed according to specifications as follows: After the roof concrete was crowned, brought to a smooth surface and thoroughly dried, it was swabbed over with hot melted "medium hard" coal tar pitch to an even thickness of not less than 1-16 in. Immediately upon the first coat of pitch and while it was still melted was laid a covering of single-ply roofing felt, with the sheets lapping 4 ins. on all cross joints and 12 ins. on longitudinal joints. This felt was in turn mopped with pitch, and upon that again was laid another layer of roofing felt, which was given a final coating of pitch. The pitch used was of a grade somewhat softer than that used for roofing purposes, or such as would soften at a temperature of 60° F. and melt at a temperature of 100° F. The felt used consisted of pure wood paper pulp or asbestos pulp, which had been thoroughly treated and soaked in refined coal tar and which weighed for single ply at least 15 lbs. per 100 sq. ft.

After the waterproofing with pitch and felt had thoroughly hardened it was plastered over with a trowel with a 1-in. layer of Portland cement mortar, laid in uniform squares, in every respect similar to the plaster on top of granolithic pavement. The dimensions of the squares were 5×5 ft. Their purpose was to take up expansion and contraction in the coating.

During the year 1903, there were laid 9,056 sq. yds. of the waterproofing described. The labor cost of placing the two layers of felt and the three coats of pitch was as follows: 206 days labor at a cost of $498 (or an average of $2.41 per day) for the 9.056 sq. yds., which is equivalent to 5½ cts. per sq. yd. for labor. Since this is for two layers of felt the labor cost was 2¾ cts. per sq. yd. of single layer. The labor cost of mixing and placing the 1-in. mortar covering was as follows: It required 589 days at a cost of $1,306 (or an average of $2.22 per day) to place 9,056 sq. yds., which is equivalent to 14½ cts. per sq. yd. The total cost of labor for two layers of tar felt and the layer of cement mortar was, therefore, 20 cts. per sq. yd.

New York Rapid Transit Subway.—The waterproofing consisted of alternate layers of asbestos felt and asphalt laid on the concrete and covered with concrete. A coat of hot asphalt was laid on the concrete and on this a layer of felt, then another coat of asphalt and another layer of felt, and so on until the required number of layers of felt, from 2 to 6, were laid with asphalt between and on top and bottom. Natural asphalt containing not less than 95 per cent bitumen was specified. The felt was required to weigh 10 lbs. per 100 sq. ft. In constructing sidewalls the alternative was allowed of placing the waterproofing layer between a 4-in. outside wall of brick laid in asphalt and the concrete lining. On two sections of the work the actual cost of waterproofing was as follows:

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