Specifications for Waterproofing Floors of Steel and Concrete Bridges

SPECIFICATIONS FOR WATERPROOFING FLOORS OF STEEL AND CONCRETE BRIDGES Method. Bridge floor (whether steel or concrete) should be perfectly clean and dry and free from all dust, oil, paint, rust, and scale. Floor should then be given a coat of primer paint, applied cold with paint brushes. If work is done in cold weather, floor should be heated by spreading hot sand over it, then sweeping off sand in advance of applying paint.

This primed surface should then be given a coating of hot waterproof asphalt applied at about 425°F. with ordinary roofers' mops; over this layer of water proof asphalt shall be laid a mastic—a mixture of asphalt, sand, etc. (see below)—at least 1/2 inch thick ; this must be evenly spread and smoothed, and must be free from depressions; preferably, should be well tamped and smoothed with hot smoothing irons, care being taken that the asphalt shall not be burned.

If the mastic be omitted, it will be necessary to spread hot gravel into the waterproof asphalt while the latter is still warm.

Paint. The primer paint to be used shall contain no coal-tar products, shall not be affected by water, and must be of such quality as to make a firm bond between steel or concrete and the asphalt.

Asphalt. The waterproof asphalt used must be free from coal-tar or rosin, and must not be affected by water, and must approximate the following requirements : The melting point by capillary tube test should show that the asphalt will not start to melt before 190°F.; must not become brittle at zero ; must not volatilize more than per cent at a temperature of 350°F. for 10 hours; and must not show any sign of carbonization at 500°F. for 10 hours.

The asphalt used shall show a penetration (Dow test) within the following range : at 32°F., between 26 and 36 degrees; at 77°F., between 36 and 46 degrees; at 115°F., between 60 and 70 degrees.

Mastic. The mastic must contain pulverized natural asphaltic lime-rock equal to that known as in suitable proportions, to which shall be added at the bridge clean, fine sand or grit (preferably sand), lime stone °dust, and asphalt in suitable proportions. The

bitumen content shall be not less than 16 per cent (spe cific gravity of 1.2), must not volatilize more than of one per cent at 300°F. for 10 hours, and must not flow below 212°F, Sample and Test. Contractor shall submit a sample of finished mastic conforming to these specifications ; and in addition, the mastic shall withstand the following com pression test in a Riehle compression testing machine— namely, a sample 2 in. by 2 in. by Y?, in. should show an elastic limit of not less than 2,500 pounds applied over entire surface of sample tested.

Action of Water on Asphalts.

In view of the rapidly increasing use of asphalt and asphaltic compounds for waterproofing and other purposes of engineering construction, spe cial interest attaches to the results of a careful scientific investigation of the properties of that substance which was recently made for the pur pose of determining the relative action of water upon some of the most important brands of asphalt on the market, with reference to their use for reservoir lining. The results were sum marized and tabulated in a paper read before the Brooklyn Engineers' Club in March, 1900, by G. C. Whipple and D. D. Jackson, upon which the following abstracts are based: At one time nearly all of the asphalt used in this country was imported; now there are many American asphalts, besides numerous artificial products. These various kinds of asphalt differ in many of their charac teristics, and one might think that it is comparatively easy to determine their relative suitability for various uses; but when it is remembered that they may be mixed with each other in almost any proportion, the complexity of the problem becomes apparent. The practical tests to be applied to asphalts are largely physical. It is use ful to know their chemical composition, but it is more important to know their consistency, viscosity, elasticity, ductility, fracture, etc., and the relative importance of these tests varies with the nature of the work for which the asphalts are to be used.