THEORY AND PRACTICE OF CONCRETE-IRON CONSTRUCTIONS. 109 



These alterations of volume are the greater the richer 

 the concrete used, and the amount ranges for pure cements 

 from '018 to *024 inches per lineal foot, and for poorer 

 mixtures from '0036 to '0006 inch. 



Plain concrete bodies expanding and contracting without 

 outside resistance, remain free of internal stresses, but 

 when iron rods are inserted in the concrete and the latter 

 allowed to harden in the air, the rods tend to prevent the 

 contraction of the concrete, causing tensile stresses in the 

 latter and compressive stresses in the iron, the amount 

 depending on the concrete mixture and the sectional area 

 of the iron. 



Some concrete-iron prisms of square cross-section with 

 3*9 inches sides, 6*56 feet long, and with 4 iron rods inserted, 

 each *023 diameter, and of a concrete proportioned of 1 

 part cement, 2 of sand and 4 of broken metal were allowed 

 to harden in the air. The shrinkage or contraction 

 amounted to '0025 inch per lineal foot, which expressed in 

 stresses means that the iron was subjected to a compres- 

 sion of 6,527 lbs. (per square inch), and the concrete to a 

 tension of 74 lbs. per square inch. From this it is evident 

 that the two materials through the above causes may 

 become subject to considerable initial stresses which will 

 have the effect of increasing the deformations of the con- 

 struction when under loading. 



To minimise the effect caused by hardening of concrete 

 in the air, it is advisable and even necessary to keep con- 

 crete-iron constructions as damp as possible during the 

 earlier stages of hardening, the longer and more thoroughly 

 this is attended to, the better are the results achieved. 



When concrete-iron constructions harden in water the 

 opposite effect to hardening in air is produced, viz., com- 

 pressive stresses occur in the concrete and tensile in the 

 iron, the amount varying according to the aggregate of the 



