Concrete Strength for 200 kg/m3 cementitious

[BigH]

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 . . .

 

[Ron]

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.

 

[BigH]

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.