Concrete Construction Part 65

The coating material, f. o. b. Chicago, may cost the contractor 4 cts.

The labor of application may cost the contractor 7 cts.

Administration and incidental expenses may cost the contractor 7 cts.

-------- 25 cts.

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 workmans.h.i.+p 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 55 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 5 cts. per sq. yd. for labor. Since this is for two layers of felt the labor cost was 2 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 14 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:

98,074 sq. yds. Single-Ply Felt. Per sq. yd.

Labor laying $0.05 Materials and plant 0.10 ------ Total $0.15

1,337 cu. yds. Brick in Asphalt: Per cu. yd.

Labor laying $6.32 Materials and plant 11.48 ------ Total $17.80