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Concrete Strength for 200 kg/m3 cementitious

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BigH

Geotechnical
Dec 1, 2002
6,012
All,

I have a 200 kg/m3 (337#/yd3)concrete mix for 20 MPa (cube) - or 16 MPa cylinder requirement at 90 days (using 25% fly ash substitution). One of our batch plants is producing an average of 46 MPa (cube - or 37 MPa cylinder). This seems very high to me as a 300 kg/m3 (505#/yd3) mix is only producing 52 MPa (42 MPa).

Any thoughts - other than they picked up a 300 kg mix specimen for the 200 kg mix. We have fairly consistent results for over 500 samples. On another batch plant - same mix designs and same aggregates, we are getting 27 MPa (22 MPa) for the 200 kg mix.

Appreciate your thoughts on this - btw, the compression machine was calibrated in February and this does not seem to be the culprit . . .
 
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BigH...sounds like a batch weight issue assuming cylinders were not mixed up. Have the batch weights been verified? Even different cement fineness wouldn't show that much variation.
 
We are trying to work through some of this - some of the batches use a Type 43 cement and others Type 33 (it's a British thing - having to do with the fineness of grinding). The concrete batch plant operator says it might have to do with sieving the +40 mm aggregate - leaving too much "paste" in that which passes. That's one problem in that no standards really address how to treat large aggregate concrete - sieving through a 40 mm site sieve is inherently problematic and not representative. BTW, the w/c ratio is 0.5.
 
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