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I had the opportunity a few years ago to run side by side testing of everything from air content, to time strength on equal mixes at a temporary site precast operation.  The only difference in the mix or curing was the addition of fly ash in one of the mixes.  Over five hundred different test series were recorded and compared.
The only difference noted in this test, was the variation of short term strength gain with the fly ash mix.  There was no consistency to the 12, and 18 hour strengths with the fly ash mix,  but variations were not apparent at 28 days with the non fly ash mix.  Today it is normal in our area to have fly ash present in all ready mix concrete, and after some years I have not experienced any difficulty in working with fly ash in the mix.  There were rumblings from finishers in the beginning but those rumblings has subsided.
B.Davison, P.Eng.
RND Engineering (1997) Ltd.
#210 -9440 -49 Street
Edmonton, Alberta T6B 2M9
1(780) 465-8995