of polymer chemistry is helpful, but not essential; directions for curing 

 mixes are available from the resin manufacturers. Curing of PC may be 

 performed by thermal catalytic, promoter-catalysts or radiation techniques. 

 Promoter-catalyst systems are frequently best suited for PC, with curing 

 times varied, as needed, between a few minutes and several hours. Full 

 strength is attained when polymerization is completed. PC has been made 

 with epoxy, polyester, and furan resins, and more recently with sytrene 

 monomer systems. 



c. Aggregates. 



(1) Normal Aggregates . Normal aggregates consist of clean sand, 

 river -washed gravel, and crushed rock. In certain locations volcanic rock, 

 such as basalt, may be used. They should have clean, hard and uncoated 

 particles and comply with ASTM Standard C33. Other ASTM tests for concrete 

 aggregates are shown in Table 8. 



Harmful substances may be present in aggregates. These include organic 

 matter, rubbish of all kinds, silt, clay, coal, lignite, dolomitic lime- 

 stones, chalcedonic cherts, opal, cristobalite, some types of volcanic 

 glass, and pyrites. An aggregate containing these substances may be con- 

 sidered as reactive. Materials finer than the No. 200 sieve may form 

 coatings on the aggregates which weaken the bond between the aggregate and 

 the cement paste. Soft particles of aggregate affect the wear resistance 

 and durability of the concrete. 



Tests used to qualify aggregates to be used to make durable concrete 

 are as follows: 



(a) Abrasion resistance tests; 



(b) sulfate soundness tests (used for many years as an index 



of quality, however, experience has shown that it does not correlate 

 well with the actual performance of aggregates in concrete); 



(c) tests for organic impurities (on aggregates from new 

 sources) ; 



(d) laboratory freezing and thawing tests (of limited value, but 

 do furnish useful information for new source material); and 



(e) tests to determine presence of opal and chalcedony (on 

 aggregates from new sources) by making mortar bars and testing 

 them according to ASTM Standard C342 



It is an interesting fact that the water requirement of a given con- 

 sistency for concrete decreases in inverse proportion to the maximum size 

 of the coarse aggregate. For example, a 19-millimeter (P. 75 inch) size 

 coarse aggregate would require about 1.49 kilonewtons (335 pounds) of 

 water, per 0.76 cubic meter (1 cubic yard) of concrete; a 50.8-millimeter 

 (2 inch) aggregate would only need about 1.22 kilonewtons (275 pounds) per 

 0.76 cubic meter (1 cubic yard). The latter would lower the water-cement 



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