

HIGHWAY BRIDGE FLOORS. 112m 



The timbers for the sub-floor shall be surfaced on one side and one edge, and shall not vary 

 moiv than ^ in. from the specified thickness. The timbers shall be laid with the surfaced side 

 tluwii with tight joints, and shall be fastened to the outside spiking strips with two 6-in. lag screws 

 ;it i-.irli end of each plank, and to the intermediate stringers with two spikes in each stringer, the 

 length of the spikes to be at least twice the thickness of the floor planks. The fellow guard shall 

 be bolted to the stringers with |-in. bolts spaced not more than 5 ft. centers. 



WEARING SURFACES FOR HIGHWAY BRIDGE FLOORS. The wearing surface of a 

 highway bridge floor should satisfy the usual conditions for a pavement and in addition should 

 not have an excessive weight; as an increase in dead load on the bridge increases the necessary 

 amount of steel in the floor supports and the trusses and increases the total cost. The most 

 common wearing surfaces will be briefly described. 



Concrete. A concrete wearing surface is laid on top of the concrete slab by the Illinois High- 

 way Commission as follows: The wearing surface shall have a thickness of not less than 4 

 inches. The lower 2 in. of the wearing surface shall be made of concrete mixed in the proportions 

 of one part Portland cement, 2 parts clean sand and 4 parts clean gravel or broken stone that will 

 pass a I J-in. ring. The concrete shall be thoroughly mixed in a batch mixer to a jelly-like consis- 

 tency and shall be placed immediately on the sub-floor slab. Upon this concrete layer shall be 

 immediately laid a 2-in. layer of mortar made by mixing one part Portland cement and 2 parts of 

 clean, coarse sand. The mortar shall be mixed to a jelly-like consistency in a batch mixer and 

 shall be immediately placed upon the freshly laid concrete. Before the mortar has begun to set 

 it shall be finished off with a wood float, and before it has hardened it shall be roughened by brush- 

 ing with a stiff vegetable brush or broom. 



The concrete slab and the concrete wearing surface are commonly laid in one operation, 

 the wearing surface being finished up as for a concrete pavement. 



Creosoted Timber Blocks. The blocks shall be made of prime sound long-leaf yellow pine 

 or Oregon fir and shall contain no loose knots, worm holes or other defects, and shall be well manu- 

 factured. No wood averaging less than 6 rings to the inch, measured radially from the center of 

 the heart shall be used. The blocks shall have a depth as specified, but the depth shall not be less 

 than 3 in. The blocks shall be from 6 to 10. in. long. The width shall be from 3 to 4 in., but the 

 blocks in any contract shall have the same width. A variation of -fa in. in depth and J inch in 

 width will be permitted. The width shall be greater or less then the depth by not less than J in. 

 The blocks shall be impregnated with creosote oil by the full cell process. The creosote oil and the 

 method of creosoting timber blocks shall be the same as specified for creospted timber. All creo- 

 soted timber blocks shall contain not less than 16 Ib. of creosote oil per cubic foot of timber. 



Laying Creosoted Timber Blocks. When the creosoted timber blocks are laid on a creosoted 

 timber base, a layer of tar paper shall be laid on the timber base. When creosoted timber blocks 

 are laid on a concrete floor slab, a layer of dry cement mortar made by mixing dry one part of 

 Portland cement and four parts of clean dry sand shall be spread on the dry floor slab. The cement 

 cushion shall be rolled to a thickness of | in. As the blocks are laid on the concrete slab the sand 

 and cement shall be moistened by sprinkling and the blocks shall be laid before the cement has 

 had time to set. The blocks shall be laid at right angles to the length of the bridge in parallel 

 lines, with the grain vertical. The blocks shall break joints at least 3 in. Two lines of blocks 

 shall be laid next to the curb with the long dimension of the block parallel to the bridge, and the 

 remainder of the blocks shall be laid at right angles to those blocks. The blocks shall be laid with 

 open joints, J-in. open joints transversely, J-in. open joints longitudinally. Expansion joints not 

 less than I in. thick the full depth of the block shall be provided along each curb, and transverse 

 joints not less than \ in. thick shall be provided every 50 ft. in length of the bridge. These joints 

 shall be kept closed until the blocks are all laid, and the space is then to be filled with a bituminous 

 filler. After the blocks have been laid they shall be tamped or rolled to firm bearing. All defect- 

 ive, broken, damaged or displaced blocks shall be removed and replaced with sound blocks. All 

 joints and expansion joints shall then be filled to a depth of two-thirds the depth of the block with 

 a satisfactory bituminous filler. The filler shall not be brittle at o F. nor flow at 120 F. The 

 filler shall be applied at a temperature of not less than 300 F. After the first application has 

 set the joints shall be filled to the proper height with a second coat. Joints shall be filled only in 

 dry weather, when the temperature is not less than 50 F. Before the second coat has hardened 

 a layer of sand J in. thick shall be spread on the surface and shall be swept into the joints. 



Bituminous Wearing Surface Floors. Bituminous wearing surface floors may be laid on a 

 creosoted timber sub-floor or on a concrete sub-floor. 



