7 day concrete breaks higher than 28 days

huangyhg

7 day concrete breaks higher than 28 days?
we have a concrete mix that we have been having problems with the 7 day breaks breaking higher than the 28 day breaks. the mix is a 6000psi mix with cement factor of 6.7 it uses admixtures of 322n, nc534, and glenium. the cyliners are being cast and stored intially in temperature controlled cure sheds then brought into the lab where they are stripped and put in a temperatured controlled room so we dont believe that cylinders are being overheated intially does anyone have any ideas on what might be causing our strength loss. thanks
find a job or post a job opening
possiblities:
1. sampling procedures? - not too likely if the first yard or so was not sampled.
2. age of concrete (mixed) before sampling - not likely, but you may be able to determine somewhat by examining batching and delivery tickets. the supplier should know when the truck left both the plant and the site.
3. concrete temperature - possible warm temperatures will accellerate early strength gain for high cement contents and have unknown efffects on admixtures. few inspectors and usually no contractors actually check temperaures.
4. cylinder making - very possible depending on the person's qualifications and experience.
5. site curing/storage  conditions - approved facilities should be equal for all. only time duration should change (weekends, etc.)
6. cylinder handling - very possible due to the weight and unstable h/t. one early tip can throw out a test. check individual cylinder breaks for the same set to look for erratics.
7. laboratory curing - not likely and conditions are probably recorded.
8. testing laboratory - is the lab certified?
9. sample preparation - possible since lower level technicians may be doing the dirty work, especially in a heavy construction season.
10. testing - possible errors (technician) due to load rate(usually regulated) or inaccurate centering causing eccentric loading (more possible on small size cylinders).
10. wrong mix design - possible but should be verified from batching reports. a wrong mix design/batch proportions could be spotted easier. an error in automatic batching would be unlikely since this is something a plant watches for cost purposes.
in an modern plant, the variable are consrolled more than they are on a construction site - one of the benfits of international cement companies and vertical integration.
just some points from an engineer that inspected and test all the way through school.
dick
just a couple of questions to put things into perspective -
1. are the 28 day breaks below specified? - if so how much?
2. how many sets of cylinders are we talking about?
3. what is the history/performance of the cylinders that following the pattern that is expected?
dick