beneficial, but others result in serious damage to the concrete by causing 

 abnormal expansion, cracking, and loss of strength (Woods, 1968). The 

 reaction that has received greatest attention and was the first to be 

 recognized involves a reaction between alkalies (Na20 and K2O), from the 

 cement or from other sources with hydroxyl, and certain siliceous con- 

 stituents that may be present in the aggregate. This phenomenon was 

 originally, and is still sometimes, referred to as "alkali-aggregate 

 reaction," but in recent years it has been more properly designated as 

 "alkali-silica reaction." 



Deterioration of concrete has occurred in certain sand-gravel aggre- 

 gates. The deterioration has been regarded as a chemical phenomenon and is 

 a reaction between the alkalies in cement and some siliceous constituents 

 of the aggregates, complicated by environmental conditions that produce 

 high concrete shrinkage and concentration by drying (Hadley, 1968) . It has 

 also been clearly demonstrated that certain carbonate rocks participate in 

 reactions with alkalies that, in some instances, produce detrimental expan- 

 sion and cracking. Detrimental reactions are usually associated with 

 argillaceous dolomitic limestones which have somewhat unusual textural 

 characteristics (Hadley, 1964) . This reaction is designated as "expansive 

 alkali-carbonate reaction." 



Other damaging chemical reactions involving aggregates include the 

 oxidation or hydration of certain unstable mineral oxides, sulfates, or 

 sulfides that occur after the aggregate is incorporated in the concrete 

 e.g., the hydration of anhydrous magnesium oxide, calcium oxide, or calcium 

 sulfate, or the oxidation of oyrite) (Mielenz, 1964). Still other reactions 

 may result from organic impurities (such as humus and sugar). Engineers 

 should be aware of these possibilities and supply corrective measures where 

 necessary. Careful testing and examination of the aggregates will usually 

 indicate the presence of such reactive impurities and their use in concrete 

 can be avoided. The alkali-silica, cement-aggregate, and expansive carbonate 

 reactions are most important. 



(b) The Alkali-Silica Reaction . This reaction can cause 

 expansion and severe cracking of concrete structures and pavements. The 

 phenomenon is complex, and various theories have been advanced to explain 

 field and laboratory evidence (Diamond, 1976) . Unanswered questions 

 remain. Apparently, reactive material in the presence of potassium, 

 sodium, and calcium hydroxide derived from the cement reacts to form 

 either a solid nonexpansive calcium-alkali-silica complex or an alkali- 

 silica complex (also solid) which can expand by imbibition of water. 



1 Laboratory Tests for Alkali-Silica Reactivity . 

 Laboratory tests should be made on aggregates from new sources and when 

 service records indicate that reactivity may be possible. The most useful 

 are: 



(a) Petrographic examination (ASTM Standard C295), 



(b) mortar bar test for potential reactivity (ASTM Standard 

 C227), and 



(c) chemical test for potential reactivity (ASTM Standard 

 289). 



84 



