Geokhan - During my time as a bridge contractor the following logic was used:

1) Concrete cylinders have "small" volume, but relatively "large" surface area.

2) Typical concrete pours made in bridge construction have the opposite, "large" volume and relatively "small" surface area.

3) To compensate for more moisture loss from cylinders than bridge placements, it's best to treat the concrete cylinders "very carefully" (spray chemicals & water tank). By doing this, concrete cylinder test results accurately represented the bridge concrete strength that received "good", but not "very careful" curing.

www.SlideRuleEra.net

SRE always has good advice . . . but cylinders cast are not really a measure of the concrete strength at site - they are an indication of the concrete's potential. If you wish to have site strength, then in situ testing would be one way to gauge the in situ strength. On thing to remember in my view is that poor consolidation of the concrete can have a major impact on the strength of the concrete placed. I have read that this can be up to and exceeding 10% as a reduction in the concrete's strength.

One source:

Compaction of concrete

Any entrapped air resulting from inadequate compaction of the plastic concrete will lead to a reduction in strength. If there was 10% trapped air in the concrete, the strength will fall down in the range of 30 to 40%.

• https://files.engineering.com/getfile.aspx?folder=4e774de1-a4b6-426c-8fa8-e

As BigH notes, the cylinders are to check the mix design, not the in situ concrete.

Geokhan - one other point - do not rely on chemicals! I had a job in India that used curing compounds and almost 50% of the concrete panels for the toll plaza had plastic shrinkage cracks (and they had immediately used a curing compound). Use WATER CURING unless it is "impossible". On another toll plaza on the same project by another contractor, he used water after curing compound and only two small slabs had plastic shrinkage cracks - less than 0.5% of the total toll plaza size. Remember WATER WATER WATER.

So nice of you all.thanks