REINFORCED CONCRETE 77 



(d) No concrete having an initial set shall be used, nor shall 



the retempering of mortar or concrete (i.e., remixing 

 with water after it has partially set) be permitted. 



(e) Where the metal reinforcement occurs in several layers 



the concrete shall be deposited in equal layers and 

 rammed separately. Otherwise the concrete shall be 

 spread in horizontal layers not exceeding 4 in. in 

 slabs, or 6 in. in any case. 



(/) The concrete shall be deposited in a manner that will 

 permit the most thorough compacting, such as can 

 be obtained by working with a straight shovel or slic- 

 ing tool kept moving up and down till all the ingredients 

 have settled in their proper place by gravity and the 

 surplus water is forced to the surface. 



(g) In columns having circumferential reinforcement the 

 concrete shall preferably be introduced by means of a 

 tremie or tube, in order to prevent the ingredients 

 from sorting in layers. To prevent settlement or 

 shrinkage in freshly made columns a period of at least 

 two hours shall elapse before any girder or beam over 

 them may be commenced. 



(h) In depositing concrete under water special care shall 

 be exercised to prevent the cement from floating away 

 and to prevent the formation of laitance; when laitance 

 has formed it shall be removed before placing fresh 

 concrete. 



(i) When work is resumed concrete previously placed shall 

 be roughened, thoroughly cleansed of foreign material 

 and laitance, drenched and slushed with a mortar 

 consisting .of 1 part Portland cement and 2 parts 

 sand. 



(/) The faces of concrete exposed to premature drying should 

 be kept wet for a period of at least 7 days after 

 molding. 



334. Freezing Weather. Concrete for reinforced structures 

 shall not be mixed or deposited at a freezing temperature, except 

 under special instruction. In this case the sand, water, and 

 broken stone shall be heated, but no salt shall be added. 



Effective means shall be provided to, prevent the concrete 

 from freezing until it has thoroughly hardened. 



