6 REINFORCED CONCRETE CONSTRUCTION 



the strength of the concrete, if the cement paste does not entirely 

 fill the voids. The presence of clay requires very thorough mixing. 

 When gravel is used, it should be screened to separate the sand 

 and then be remixed in order that the proportions may be definite. 

 Cinders make good fireproof concrete, but are not recommended 

 by the best authorities for reinforced work. The allowable stress 

 is too low for economical use and, unless great care is taken in 

 having a wet mix and in thorough mixing, there is danger of 

 corrosion of the embedded steel due to porosity. Cinders for 

 use in concrete should not contain many, if any, fine ashes and 

 should consist of hard, clean, vitreous clinker, free from any 

 unburned coal. Concrete containing this aggregate can safely 

 be employed for filling between steel beams and for fireproofing 

 steel or iron columns, and for a concrete fill on top of reinforced 

 concrete floors and roof slabs. 



5. Consistency. Opinion differs as to the quantity of water 

 that should be employed in mixing, but it is safe to say that a 

 somewhat wet or mushy mixture should be used in reinforced 

 concrete construction. Such a mixture flows easily under and 

 around the metal reinforcement and ensures its preservation. It 

 also conforms readily to the molds and gives a smooth surface. 



Experiments show that while dry concrete carefully mixed 

 and rammed is stronger at the earlier ages than wet concrete, 

 in six months' time but little difference in strength is found. 

 Moreover, with dry mixtures there is difficulty in obtaining a 

 uniform consistency occasional batches being too dry. 



The water used in mixing concrete should be free from oil, acid, 

 alkalies, or vegetable matter. 



6. Unit for Proportioning. When proportions of the ingre- 

 dients of a concrete are specified, the specifications should state 

 whether the cement shall be measured loose, or as packed in 

 bags and barrels. The reason for this is clear when it is con- 

 sidered that loose cement occupies about 30 per cent more 

 volume than packed cement. The usual method is to specify 

 the barrel of packed cement as the unit, and to assign it some 

 definite volume the sand and stone to be measured loose. 



A barrel of Portland cement weighs 376 lb., not including the 

 barrel, and a bag of Portland cement weighs 94 lb.; in other 

 words, there are four bags to a barrel. The cement as packed in 

 a barrel occupies, on an average, a volume of about 3.2 cu. ft., 

 but as the unit adopted is an arbitrary one in any case, 3,8 cu. ft. 



