In constructing an open spandrel arch bridge at St. Paul, Minn., the cornice form shown by Fig. 300, supported as shown by Fig. 301, was used. The particular feature of this form was the use of a lath and plaster lining to the lagging. This lining was used for all exposed surfaces of the bridge. So called patent lath consisting of boards with parallel dovetail grooves and ridges was used. This was plastered with cement mortar and the concrete was deposited directly against the plaster after smearing the plaster surface with boiled linseed oil. This lining is stated to have given an excellent surface finish to the concrete. It cost 55 cts. per sq. ft. for materials and labor. A section of the balustrade and cornice is shown by Fig. 302. The posts, balusters and railing were molded separately. The balusters were molded in zinc molds. At first some trouble was had in getting good casts on account of air pockets. This was largely done away with by filling the mold as compactly as possible and then driving a ¾-in. iron rod through the center vertically; this rod crowded the concrete into all parts of the mold and also served to strengthen the baluster. The baluster molds were made in two parts; this proved a mistake—three parts would have been better.
CHAPTER XXIV.
MISCELLANEOUS DATA ON MATERIALS, MACHINES AND COSTS.
The following cost data comprise such miscellaneous items as do not properly come in the preceding chapters. They are given not as including all the miscellaneous purposes for which concrete is used but as being such items of costs as were secured in collecting the more important data given in preceding sections.
Fig. 303.—Device for Drilling Green Concrete.
DRILLING AND BLASTING CONCRETE.—Concrete is exceedingly troublesome material in which to drill deep holes, and this statement is particularly true if the concrete is green. The following mode of procedure proved successful in drilling 1½-in. anchor bolt holes 6 ft. and over in depth in green concrete. The apparatus used is shown by Fig. 303, re-drawn from a rough sketch made on the work by one of the authors, and only approximately to scale. The drill is hung on a small pile driver frame, occupying exactly the position the hammer would occupy in a pile driver, and is raised and lowered by a hand windlass. By this arrangement a longer drill could be used than with the ordinary tripod mounting and less changing of drills was necessary. A wide flare bit was used, permitting a small copper pipe to be carried into the hole with the drill; through this pipe water was forced under pressure, carrying off the chips so rapidly that no wedging was possible. By this device drilling which had previously cost over 25 cts. a hole was done at a cost of less than 5 cts. a hole.
In removing an old cable railway track in St. Louis, Mo., holes 8 ins. deep were drilled in the concrete with a No. 2 Little Jap drill, using a 1¼-in. bit and air at 90 lbs. pressure. A dry hole was drilled, the exhaust air from the hollow drill blowing the dust from the hole keeping it clean. The concrete was about 18 years old and very hard. Two holes across track were drilled, one 10 ins. inside each rail; lengthwise of the track the holes were spaced 24 ins. apart, or four pairs of holes between each pair of yokes.