usually most strongly developed in areas of the structure where the concrete 

 has a constantly renewable supply of moisture, such as close to the water- 

 line in piers, from the ground behind retaining walls, beneath road or 

 sidewalk slabs, or hy wick action in posts or columns. A distinguishing 

 feature from alkali-silica reaction is the general absence of silica gel 

 exudations at cracks. Additional signs of the severity of the reaction are 

 closed expansion joints with possible crushing of the adjacent concrete 

 (Hadley, 1964). 



1 Identification by Laboratory Tests . The most useful 

 laboratory tests are discussed below. 



a Pe trographic Examination . This examination of 

 aggregates may be used to identify the features of the rock. The presence 

 of all or any dolomite in a fine-grained carbonate rock makes it desirable 

 to perform the rock cylinder test (ASTM Standard C586) . This is recommended 

 whether or not the texture is believed to be typical, and whether or not 

 insoluble residue including clay amounts to a substantial part of the 

 aggregate. As expansive rocks are recognized from more areas, the more 

 variable the textures and compositions appear to be. 



b Expansion of Concrete Prisms Test . This test is 

 performed with prisms made of job materials and stored at 100 percent 

 relative humidity at 23° Celsius, (73° Farenheit) , or (in order to accel- 

 erate the reaction) the prisms may be made with additional alkali and 

 stored at elevated temperature. The comparison is usually made with the 

 expansion of prisms containing a nonreactive control aggregate. 



c Petrographic Analysis . A petrographic analysis of 

 the concrete can confirm the type of aggregate present and its character- 

 istics. Distress that has occurred in the aggregate and surrounding matrix, 

 such as microcracking and macrocracking, may be observed. Reaction rims, which 

 do not necessarily signify harmful results, may be observed in certain 

 aggregate particles and may be identified as negative or positive by acid 

 etching. Secondary deposits of calcium carbonate, calcium hydroxide, and 

 ettringite (calcium sulfoaluminate) may be found in voids within the con- 

 crete; however, there are no deposits of silica, hardened or in gel form, 

 associated with the suspect aggregate pieces. 



d Other Laboratory Tests . Additional tests on alkali- 

 carbonate reaction include identifying by visual observation sawed or 

 ground surfaces. X-ray examination of reaction products is also sometimes 

 useful . 



e Criteria for Judging Reactivity . Several criteria 

 are available for judging the reactivity of aggregates. These include 

 definitive correlations between expansions occurring in the laboratory in 

 rock cylinders or concrete prisms and deleterious field performance which 

 have not yet been established. The factors involved are complex and include 

 the heterogeneity of the rock, coarse aggregate size, permeability of the 

 concrete, and seasonal changes in environmental conditions in service, 

 principally availability of moisture, level of temperature, and possibly 

 the use of sodium chloride as a deicing chemical. 



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