It is not certain that rapid determination of potential reactivity can 

 always be made by using the rock cylinder test, because some rocks showing 

 an initial contraction may develop considerable expansion later on. 

 Expansions greater than 0.10 percent in the rock cylinders are usually 

 taken as a warning that further tests should be undertaken to determine 

 expansion of the aggregate in concrete. Fortunately, many carbonate rocks 

 that expand in rock cylinders do not expand in concrete. 



2 Recommended Procedures to Minimize Alkali-Carbonate 

 Reactivity . Procedures that can be employed to mitigate the effects of the 

 reaction include: 



(a) Avoiding reactive rocks by selective quarrying; 



(b) dilution with nonreactive aggregates, or use of a 

 smaller maximum size; and 



(c) use of low alkali cement (probably 0.4 percent combined 

 alkali or lower) , which will prevent harmful expansions in most 

 cases; however, in pavements where sodium chloride is used as a 

 deicing chemical, this cannot be taken as certain. 



Of these measures, the first is the safest and usually the most econ- 

 omical . 



(e) Preservation of Concrete Containing Reactive Aggregate . 

 There are no known methods of adequately preserving existing concrete which 

 contains the elements that contribute to the previously described chemical 

 reactions. Water or moisture is partly involved in at least two of these 

 reactions. The destructive effects of freezing and thawing are more pro- 

 nounced after the initial stages of destruction by these chemical reactions. 

 Therefore, any practicable means of decreasing the exposure of such concrete 

 to water may extend its useful life. 



d. Admixtures . 



(1) General . An admixture is defined as a material other than 

 water, aggregates, and hydraulic cement, that is used as an ingredient of 

 concrete or mortar and is added to the batch immediately before or during 

 its mixing. ASTM Standard C494 classifies certain chemical admixtures in 

 terms of function as follows: 



(a) Type A, water-reducing admixtures, 



(b) Type B, retarding admixtures, 



(c) Type C, accelerating admixtures, 



(d) Type D, water-reducing and retarding admixtures, and 



(e) Type E, water-reducing and accelerating admixtures. 



These admixtures are discussed according to the type of materials con- 

 situting the admixture or to the characteristic effects of their use. 



88 



