However, it should not be used where stray electric currents are expected 

 and should not be used in prestressed concrete because of possible stress 

 corrosion of the prestressing steel (Arber and Vivian, 1961) . Calcium 

 chloride in concrete may be expected to aggravate corrosion of embedded 

 galvanized metal and of galvanized forms that are left in place. Combina- 

 tions of metals, such as aluminum-alloy electrical conduit and steel 

 reinforcing, should not be used in concrete exposed to water. 



Calcium chloride may be especially beneficial for concrete exposed to 

 low or freezing temperatures at early ages if used as recommended in the 

 ACI Standard ACI 604-56. Calcium chloride increases the rate of early heat 

 development and accelerates the set, but lowers the freezing point of the 

 water in concrete only to an insignificant extent. 



(5) Air-Entraining Admixtures . Many materials, including natural 

 wood resins, fats, and oils, may be used in preparing air-entraining 

 admixtures. These materials are usually insoluble in water and generally 

 must be chemically processed before they can be used as admixtures. Since 

 not all such materials produce a desirable air-void system, air- entraining 

 admixtures should meet the requirements of the ASTM Standard C260. 



Air-entrained concrete containing a large number of very small air 

 bubbles is several-fold more resistant to frost action than nonair- entrained 

 concrete made of the same materials. Air-entrained concrete should be a 

 dense, impermeable mixture that is well-placed, protected, finished, and 

 cured if maximum durability is to be obtained. 



Air entrainment, while improving both workability and durability, may 

 reduce strength. Within the range of air content normally used, the 

 decrease in strength usually is about proportional to the amount of air 

 entrained. For most types of exposed concrete a slight reduction in 

 strength is far less significant than the improved resistance to frost 

 action. The reduction in strength will rarely exceed 15 percent in the 

 case of compressive strength and 10 percent in the case of flexural strength. 



In some installations of precast concrete units such as cribbing and 

 curbing, there is considerable exposure to freezing and thawing action. 

 The use of adequately prepared and controlled air-entrained concrete is the 

 best way to improve resistance to freezing and thawing. 



(6) Air-Detraining Admixtures . There have been cases where 

 aggregates have released gas into, or caused excessive air entrainment, in 

 plastic concrete which made it necessary to use an admixture able to 

 dissipate the excess air or other gas (MacNaughton and Herbich, 1954). 

 Also, it is sometimes desirable to remove part of the entrained air from a 

 concrete mixture. Compounds such as tributyl phosphate, diburyl phthalate, 

 water-insoluble alcohols, and water-insoluble esters of carbonic and boric 

 acids, as well as silicones, have been proposed for this purpose; however, 

 tributyl phosphate is the most widely used material. 



(7) Admixture to Reduce Alkali Aggregate Expansion . Test data 

 indicate that small additions of certain chemical substances may be effec- 

 tive in decreasing expansion resulting from alkali-aggregate reaction 

 (McCoy and Caldwell, 1951). Outstanding reductions in expansion of labora- 



92 



