USEFUL DATA 



The forms shall be thoroughly oiled (thin mineral oil) before the concrete is placed; 

 or the surface shall be thoroughly wetted (excepting in freezing weather). 



PLACING OF REINFORCEMENT 

 All reinforcement shall be placed in accordance with the plans furnished by the 



Engineer. 



All loose rust or scale, all adhering material, and all oil or other substance which 



will tend to destroy bonding between the concrete and the reinforcement shall be 



removed before pouring begins. 



MIXING OF CONCRETE 



Mixing shall be done in a batch machine mixer of a type which will insure uniform 

 distribution of the materials throughout the mass, and shall continue for the minimum 

 time of one and one-half minutes after all ingredients are assembled in the mixer. 

 For mixers of two or more cubic yards capacity the minimum time of mixing shall be 

 two minutes. The drum of the machine shall be operated at a uniform speed of 200 

 feet per minute. 



Unit of Measure. Measurements of fine and coarse aggregates and of cement, 

 shall be by loose volume. The unit of measure shall be a bag of cement containing 

 94 pounds net, which should be considered the equivalent of one cubic foot. 



Proportioning^. The fine and coarse aggregates shall be so proportioned with 

 the cement as to secure the ultimate compressive strength, in twenty-eight days, upon 

 which the design of the structure was based. 



The following table is recommended as the maximum ultimate compressive strength 

 of different mixtures of concrete at twenty-eight days: 



Table of Compressive Strengths of Different 

 Mixtures of Concrete 



(In Pounds per. Square Inch) 



Consistency. The materials shall be mixed wet enough to produce a concrete of 

 such consistency as will flow sluggishly into the forms and about the metal reinforce- 

 ment, and which at the same time can be conveyed from the mixer to the forms with- 

 out separation of the coarse aggregate from the mortar. A properly mixed concrete is 

 one which thoroughly sustains or supports the coarser aggregate throughout the mass 

 and which when dumped from a barrel or buggy neither breaks nor flows readily over 

 the edge. 



Retempering. The remixing of mortar or concrete that has partly set shall not 

 be permitted. 



♦The proportions here given are on the basis of separately measured aggregates. For instance; a 

 1:6 mixture refers to a mixture based on one part of cement and six parts of aggregates which were 

 measured before being combined. A standard 1:2:4 mix, therefore, falls in the above class of 1:6 mix. 



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