506 Journal of Agricultural Research voi. xi, No. io 
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The tests indicate that the above values can be used for spans up to 
16 feet, and they probably can be used for longer spans, although no 
longer spans were tested. 
However, these conclusions were drawn from the tests of slabs under 
centrally applied concentrated loads only; and to make them properly 
applicable to the design of bridge slabs, which constantly are called upon 
to withstand concentrated loads applied near the parapet, it was recog¬ 
nized that the effect of eccentricity of loading must be determined. 
With this end in view, another large-sized reinforced-concrete slab 
has been constructed and tested recently at the Arlington Experimental 
Farm of the United States Department of Agriculture, and it is the 
object of this paper to present the results of the test. 
DESCRIPTION OF THE SPECIMEN 
The specimen was made of machine-mixed concrete in the proportions 
of i part of Portland cement to 2 parts of Potomac River sand and 4 
parts of Potomac River gravel. The sand was a good grade for use in 
concrete and the gravel was dean, wdl-graded, and free from weak 
pebbles. A rather wet mix was used, and the mixing and pladng were 
done by experienced laborers at the Arlington Farm, As in the case of 
the other slabs tested, there was no attempt to make the concrete any 
better than it would be made in the field, but efforts were directed to 
secure work thoroughly representative of that which might be obtained 
under fidd conditions. 
The slab, which was built in place and supported by concrete abut¬ 
ments, was 32 feet in width, and 16 feet in span. Its total thickness 
was 14 inches, and its effective thickness 13 inches. It was reinforced, 
in the longitudinal direction only, with l^-inch plain square rods, spaced 
53^ inches apart, the sectional area of the steel bdng 0.75 per cent of that 
of the concrete above its center of gravity. 
After the concrete was poured and while it still was soft, rows of bolts 
were set in the top surface of the slab, which, when withdrawn a few 
days later, left holes for the insertion of the plugs and feathers to be used 
for splitting off sections of the slab as the test progressed. 
