EXPANSION AND CONTRACTION OF CONCRETE. 
11 
while the specimen containing 1.8 per cent steel contracted less than 0.01 peh 
cent, or about one-fourth of the contraction of the plain concrete specimen. 
If there "were perfect bond between the 
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concrete and the steel, this amount of 
shrinkage in the concrete would pro- 
duce a stress of about 3,000 pounds 
per square inch in compression in the 
steel reinforcing of specimen No. 549, 
and about 7,500 pounds per square 
inch in specimens Nos. 547 and 548. 
The more concrete is restrained from 
shrinking, the greater will be the ten- 
sile stress induced by the compression 
in the steel. The specimen containing 
1.8 per cent of steel contracted, roughly, 
0.0001 inch per inch of length, while 
the plain, and therefore unrestrained, 
specimen contracted 0.00035 inch. In 
other words, the steel prevented the 
concrete from contracting by an 
amount equal to 0.00025 inch per inch 
in length. Assuming the modulus of 
elasticity of this concrete to be 
3,000,000, the tensile stress produced 
by 0.00025 inch elongation would be 
3,000,000 multiplied by 0.00025, equals 
750 pounds per square inch, which 
stress, if it actually existed, would, of 
course, surely produce cracking of the 
concrete. If cracks were present in 
the specimens, they were not discern- 
ible, but they usually would be very 
small, and, moreover, it is probable 
that the slow flow of concrete pre- 
viously mentioned took place under 
the extremely gradually applied load, 
making the induced tensile stress very 
much smaller than the calculated 
amount. Reinforcing, it is seen, does 
not prevent the contraction of concrete 
due to the drying out of the moisture, 
and in view of the fact that steel and 
concrete have very nearly the same 
coefficient of expansion and contrac- 
tion, the steel can not aid in any way 
in preventing changes due to tempera- 
ture. Reinforcing can not prevent the cracking of concrete, but it does serve the 
purpose of holding the cracks together and keeping them exceedingly minute. 
SPECIMEN REINFORCED ON ONE SIDE. 
If a concrete road be reinforced with steel placed either near the top or bot- 
tom surface, any shrinkage "resulting from drying out necessarily must be un- 
equal at the two surfaces because of the restraining influences of the steel. To 
show the relative amounts of this shrinkage a specimen was made of 1:2:4 
concrete, reinforced by two i-inch round rods with their centers placed 1 inch 
