7 day concrete breaks higher than 28 days?

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