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 

 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- 



0<tt 



061 



081 



Oil 



091 



OSI 



OVl 



0£t 



OJI 



on 



001 



s 



§06 



?08 

 til 



<0L 



09 



OS 



0* 



0£ 



■02 











I 











f 











I 





^ 







b 





+ 





• 













o 



















) 







+ 













• 











a 



o 













o 





- 





. 





















' V 



J 









» /! 



J- 

 J 









/( 



• 





/o 



® 



Wo 







/v9 

 / / 



0/ 



o/ 



Of// 



'/I 











1 1 

 fa 









Iip 



? 

















-(- j 











• A/ 



/© 









f + 









— o — 



d> o © <3 o 

 NOU.DVaj.NOD ±»Nfl 



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 



