36 REINFORCED CONCRETE CONSTRUCTION 



be cracked without exposing the steel to corrosion. Concrete, 

 fortunately, contains a large proportion of lime which readily 

 absorbs carbon dioxide. For this reason, steel is effectually 

 protected by concrete if it is covered with even a thin film. 

 Experiments tend to show that concrete when well placed and 

 mixed wet, completely protects the steel in the tensile side of 

 a beam from corrosion, even when the unit stress in the steel 

 approaches the elastic limit. 



27. Shrinkage and Temperature Stresses. In reinforced con- 

 crete structures which are free to contract and expand, the stresses 

 occurring from temperature changes and from shrinkage in hard- 

 ening are due wholly to the mutual action of the steel and con- 

 crete. Of the stresses produced from these two causes, those 

 which result from hardening are the greater, but experiments show 

 that even these are not sufficient to be of practical importance. 

 In regard to the temperature stresses, they are negligible by reason 

 of the nearly equal rates of expansion of the two materials. 



On the other hand, if reinforced concrete structures are re- 

 strained by outside forces, or if they are of such dimensions 

 that they cannot be considered as sufficiently well bonded to 

 act as a unit such as long retaining walls then the stresses 

 resulting are much greater, and the tensile strength of the con- 

 crete will be reached (this will occur with a drop in temperature 

 somewhere between 10 and 20 Fahr.), thus producing cracks, 

 called contraction cracks. To prevent plainly noticeable cracks 

 due to shrinkage and lowering of the temperature, steel should 

 be inserted the amount used varying in practice from 0.2 of 

 1 per cent to 0.4 of 1 per cent, based on the cross-section of the 

 concrete. This is less than the amount required theoretically, 

 but experience shows this amount to give very satisfactory re- 

 sults where the foundations are stable. If the structure is fixed 

 in two directions, the reinforcement must be placed accordingly. 



No amount of reinforcement can entirely prevent contraction 

 cracks. The steel can, however, if of small diameter and placed 

 close to the surface, force the cracks to take place at such fre- 

 quent intervals that the required deformation occurs without any 

 one crack becoming large. No cracks will open up to be plainly 

 noticeable until the steel is stressed beyond its elastic limit. 

 The amount of steel should be such, then, that without being 

 stressed beyond its elastic limit, it will withstand the tensile 

 stress resulting from the maximum fall of temperature (usually 



