(a) Specify a low alkali cement having a maximum of 0.6 

 percent equivalent Na.20. 



(b} prohibit the use of seawater or alkali soil water as 

 mixing water; 



(c) avoid addition of sodium or potassium chloride; and 



(d) where low alkali cements are not economically available, 

 use a suitable pozzolanic material as prescribed by ASTM Standard 

 C618 and tested in accordance with ASTM Standard C441 (to 

 determine their effectiveness in preventing excessive expansion 

 due to the alkali-aggregate reaction). 



Whenever the use of pozzolanic materials is considered, it should be 

 remembered that if these materials increase water demand, they may cause 

 increased drying shrinkage in concrete exposed to drying. Increased water 

 demand results from high fineness and poor particle shape. The rate of 

 strength development in correctly proportioned pozzolanic concrete can 

 equal that of Portland cement concretes, i.e., 28 days. 



(c) Cement-Aggregate Reaction . Recent research indicates 

 that the cement-aggregate reaction is mainly a reaction between the alkalies 

 in the cement that produce high pH and abundant hydroxyl and siliceous 

 constituents of the aggregates. However, the field performance of con- 

 cretes made with reactive sand and gravels does not correlate well with 

 cement alkali content. The concrete deterioration results from moderate 

 interior expansion caused by alkali-silica reactivity, and surface shrinkage 

 caused by severe drying conditions. Evaporation at the surface of the 

 concrete causes an increase in alkali concentration in the pore fluids near 

 the drying surface, and a net migration of alkali toward this surface. 

 Under these conditions even a low alkali cement may cause objectionable 

 deterioration, particularly near the surface. This alkali distribution is 

 altered by the leaching of alkalies near the surface during periods of 

 heavy rain (Hadley, 1968). 



1_ Identification by Laboratory Tests . Although special 

 tests, such as ASTM Standard C342, have been devised to indicate potential 

 damage from this phenomenon, their reliability is doubtful. 



_2 Recommended Procedure to be Employed with Potentially 

 Deleterious Cement-Aggregate Combinations . The use of potentially de- 

 leterious cement-aggregate combinations should be avoided where possible. 

 However, if they must be used, a suitable pozzolan that does not increase 

 drying shrinkage and 30 percent or more (by weight) of coarse limestone 

 should be used with potentially deleterious cement-aggregate combinations. 

 Concrete tests should be used to determine whether the resulting combina- 

 tion is satisfactory. 



(d) Expansive Alkali-Carbonate Reactivity . Certain limestone 

 aggregates, usually dolomitic, have been reported as reactive in concrete 

 structures. There are many unanswered questions, and more than one mech- 

 anism has been proposed to explain expansive carbonate reactivity. The 

 affected concrete is characterized by a network of pattern or map cracks 



86 



