PFA and microsilica based concrete

[Contech2011]

I got shocker of my life to know the compressive strength @ 60 days of a 45 Mpa mix containing 35% replacement of cement with PFA (Type F C618) and about 5% of microsilica C1240, to be almost equal to 28 days strength. Only minor increase of 2 Mpa has occured @ 60 days.

I reckon the cause to be low reacting pozollonic activity of PFA. I doubt its Type F. Other factors may be temperature of the fresh concrete as casted...etc

Normally PFA concretes attain higher strengths @ later ages than early. I wait for 90 days test to one more set of the same batch.

Anybody can provide more insight on the above.

Thanks

 

[Contech2011]

any comments??

 

[Ron]

PFA will delay the strength gain. Depending on your application, the strength at 28 days should still be the defining criterion. Having the appropriate strength at 28 days is important as a design parameter.

 

[dik]

I'm surprised the strength gain is only 2 MPa at 60 days. May be a feature of the testing... ie, a high 28 day and low 60 day.

Dik

 

[Contech2011]

Loading rate, calibration was verified before testing and found okay. samples were cured in laboratory conditions. Mix has 5% silicafume, 35% PFA (Type F), and remaining Type I OPC. Microslica with HRWRA will get us initial strengths in PFA based mixes, but why is later strength gain is negligible? Has microsilica did not leave any CaoH and moisture for PFA to react?

Please solve the mystery...

 

[Ron]

One test result is not sufficient for assessment. As you noted, I would expect late strength gain. Also as dik noted, could be a testing anomaly.