178 STEEL. RAILWAY BRIDGES. CHAP. IV. 



recommended that guard timbers be used on all open-floor bridges, also that guard rails be used 

 on all bridges, and that the guard rails should extend at least 50 ft. beyond the end of the bridge. 

 For additional details see Chapter VII, "Timber Bridges and Trestles." 



Details of a ballasted floor with a reinforced concrete slab deck, and a ballasted floor with a 

 timber deck, as designed and used by the Chicago, Milwaukee & St. Paul Ry. are given in 

 Fig. 24. The reinforced concrete slabs are made either at the bridge site or at some other con- 

 venient location and are hoisted into place after the concrete has gained sufficient strength. 



The Chicago, Burlington & Quincy R. R. uses reinforced concrete slabs for a ballasted deck 

 on deck girders that differ from the Chicago, Milwaukee & St. Paul slabs in Fig. 24, in the following 

 details. The reinforced concrete slabs are 14 ft. long in place of 13 ft.; and are 5 ft. wide in place 

 of 3 ft. 7 in. The top of the slabs and the edges of the slabs are painted with tar paint (made of 

 1 6 parts coal tar, 4 parts Portland cement, and 3 parts kerosene). The edges of the reinforced 

 concrete slabs are beveled and after the slabs are laid the joint between the slabs is packed with 

 oakum for a depth of I in. at the bottom and the remainder of the joint is filled with I to 3 Portland 

 cement mortar. Where the reinforced concrete deck, is placed on a deck girder with cover plates, 

 a strip of No. 22 gage lead 3 in. wider than the cover plate is placed on top of the cover plate and 

 forced down over the rivet heads. After the slabs have been put in place and blocked up to the 

 proper elevation the space between the lead sheet and the slab is filled with I to 3 Portland cement 

 mortar. The minimum thickness of the mortar joint is one'inch. Cinders or slag are not used 

 for ballast on reinforced concrete slab decks. 



A standard reinforced concrete floor for a through plate girder bridge as designed by the 

 Chicago, Burlington & Quincy R. R. is shown in Fig. 25. The concrete is 1:2:4 Portland 

 cement concrete. The upper surface of the concrete slab is painted with coal tar paint, the same 

 as the deck slabs. Zinc sheets, No. 22 gage and 8 in. wide are placed on the tops of the floorbeams. 



A steel plate ballasted floor on a through riveted truss bridge is shown in Fig. 41. 



WATERPROOFING BRIDGE FLOORS. The problem of waterproofing bridge floors is a 

 difficult one and has been worked out in great detail by the engineers of many railroads, and by 

 the American Railway Engineering Association. For a very full discussion of the problem, see 

 the proceedings of the American Railway Engineering Association, especially Volume 14, 1913, 

 and Volume 15, 1914. The following extracts from the report of a committee of the American 

 Railway Engineering Association presented at the annual meeting of the society in March, 1914, 

 are of value. 



The methods of waterproofing are stated as follows: 



"The ordinary methods of waterproofing are. 



" (i) Coatings: (a) Linseed oil paints and varnishes, (b) Bituminous; asphalt and coal tar. 

 (c) Liquid hydrocarbons, (d) Miscellaneous compounds, (e) Cement mortar. 



" (2) Membranes: Felts and burlaps in combination with various cementing compounds. 



" (3) Integrals: (a) Inert fillers, (b) Active fillers. 



" (4) Watertight concrete construction." 



The conclusions reached in the report are as follows: 



" (i) Watertight concrete may be obtained by proper design, reinforcing the concrete against 

 cracks due to expansion and contraction, using the proper proportions of cement and graded aggre- 

 gates to secure the filling of the voids and employing proper workmanship and close supervision. 



" (2) Membrane waterproofing, of either asphalt or pure coal tar pitch in connection with felts 

 and burlaps, with proper number of layers, good materials and workmanship and good working 

 conditions, is recommended as good practice for waterproofing masonry, concrete and bridge floors. 



" (3) Permanent drainage of bridge floors is essential to secure good results in waterproofing. 



" (4) Integral methods of waterproofing concrete have given good results. Special care is 

 required to properly proportion the concrete, mix thoroughly and deposit properly so as to have 

 the void-filling compounds do the required duty; if this is neglected the value of the compound is 

 lost and its waterproofing effect is destroyed. Careful tests should be made to ascertain the 

 proper proportions and effectiveness of such compounds. Integral compounds should be used 

 with caution, ascertaining their chemical action on the concrete as well as their effect on its 

 strength; as a general rule, integral compounds are not to be recommended, since the same results 

 as to water-tightness can be obtained by adding a small percentage of cement and properly grading 

 the aggregate. 



