659. The heavy motor cars used on city and urban electric railways subject the track to very severe service. As the head of the rail must be practically flush with-the pavement on city streets, cross-ties, when used, are so far beneath the surface that they decay rapidly and their renewal entails the tearing up of the pavement. As there is not the same necessity for a cross-tie on street tracks as on railroads, since the rails are held to gage by the pavement, these objections to the cross-tie have led to the adoption of a concrete girder under each rail. The rails and ties (if ties are used) should not only rest upon the concrete, but should be imbedded in it. Track in which the rails rested upon concrete, but were not imbedded in it, has been found to yield laterally and get out of alinement, while on the other hand, if the ties rest upon earth or gravel and are filled between with concrete, the track is likely to settle, breaking the bond of the concrete.
660. The method of placing concrete beams for street railway tracks in Minneapolis was as follows:1 The rails were first spiked to cross-ties at intervals of six to eight feet, and the rail joints cast-welded. In laying the street pavement foundation of natural cement concrete, a rough groove, fifteen inches wide at the bottom and eighteen to twenty inches at the top, was left under each rail. This groove was immediately filled between ties with concrete made of one part Portland cement, two and one-half parts sand, and four and one-half parts broken stone.
1 F. W. Cappelen, M. Am. Soc. C. E., Engineering News, Oct. 14, 1897; Municipal Engineering, November, 1896.
The rails were tied together every ten feet with wrought iron tie bars, three-eighths inch by two inches, set on edge. These tie bars were rounded at the ends, threaded and attached to the web of the rail by two nuts, one on either side of the web. The rails were then spiked to the concrete beam, the temporary wooden ties removed, and the spaces left by them filled with concrete, completing the beam. As the concrete beam was eight inches thick and the rail five inches, the sub-grade was thirteen inches below the top of the rail.
On the gage side of the rail were placed toothing blocks of granite, by 9 inches by 4 1/2 inches deep, held away from the rail 1 1/4 inches by temporary wooden strips. After removing these strips, cement grout was poured into the groove to fill 2 1/2 inches over the base of the rail, the remaining 2 1/2 inches to the top of the rail being filled by asphaltic cement which remained soft enough to permit a flange groove to be made by the first car over the track. The asphalt wearing surface was laid against the rail on the outer side. Mr. Cappelen, in describing this construction, says that a rail six inches high with six-inch base should be used, with granite toothing blocks, six by nine inches by five and one-half inches deep.
The Cost per foot of rail for the concrete beam construction only, was twenty-six to twenty-seven cents, and for the filler, five cents per foot. The Cost per mile of double track, exclusive of rails and pavement, was about $8,670.00.
Somewhat similar methods have been employed in Toronto and Montreal, Canada, Indianapolis, Ind., and Scranton, Pa., Denver, Detroit and Cincinnati.
661. At Scranton, Pa.,1 the rails were laid directly on the six-inch concrete base of the pavement. This thickness was increased to twelve inches under the joints (which were reinforced by an inverted rail four feet long) and under steel cross-ties spaced ten feet centers and formed of old girder rails inverted and riveted through the flanges at the intersection. Flat steel tie bars, threaded at the ends, spaced ten feet centers, were also used here as at Minneapolis.
1 Description of the systems employed in several cities are given in Engineering News, Dec. 26, 1901.
The concrete mixing plant was mounted on a car running on the track; the materials were delivered to the machine by-hand measuring boxes, and the Drake mixer deposited the concrete directly into the trench. The total Cost per foot of track is given as $2.65, $1.17 of which was for grading, rolling, concreting and brick paving at $1.97 per square yard, and for extra concrete at joints and ties at $0.72 per square yard.
662. At Toronto, Canada, the six-inch concrete base of the pavement is increased to eight inches in thickness for twenty inches width under each rail, and the base of the latter is imbedded one inch in the concrete. A 6 1/2-inch grooved girder rail is used, with mortar rammed between the web and the adjacent paving blocks.
663. At Cincinnati the bottom of the concrete stringer is nine inches below the base of the nine-inch grooved girder rail, and the concrete is built up from three to six inches on the web, according to the thickness of the wearing surface of the pavement. The space between the upper part of the web and the adjacent paving is then filled with cement mortar, thus supporting the head of the rail as well as protecting the web from corrosion.