Reliable Concrete test 3

[ravreyes1]

The ACI Code uses cylinder test while the British standars specifies the cube test to determine the compressive strength of concrete. Which method is more reliable?

 

[JAE]

US engineer: the cylinder is more reliable.

Brit engineer: they cubes are better.

This sounds like it would take a mini-Master's thesis to compare oodles of test results and set up a statistical comparison with test breaks verses in-field core samples, or something like that.

 

[concreteguru]

The truth of course is that neither accurately reflects the strength of insitu concrete. Both standards for acceptance or rejection purposes require that the specimens be cured in a moist condition under ideal temperature i.e. 70 degrees F. This procedure will indicate the potential strength of the concrete in the structure. Strength in the structure will be more dependent on mass dimensions, curing and contractor consolidation and placing techniques. Maturity meters as per intelliRock or ASTM C1074 does a better job at providing accurate estimates of insitu strength. Very important if you want to accelerate flying forms or post-tensioning concrete.

Fred J. Croen, RSM
Engius, LLC
Boston, MA
www: engius.com

 

[BigH]

concreteguru is correct that testing in the lab for cube or cylinder strength is not an "in situ" guarantee - but that the concrete is consistent with the standard of design. In situ tests are best - but . . . Check out by searching the various threads of the site - cube and cylinder strength comparisons, etc. have been discussed several times before.

 

[SlideRuleEra]

ravreyes1 - Your question made me think of the many times that I have been asked the following: "Why do test requirements call for concrete cylinders to be 'babyed' while the in situ concrete is (more or less) exposed to the elements?"

A wise individual told me something years ago that has the "ring of truth":
The cylinders have a huge surface area to concrete volume ratio when compared to typical in situ concrete. When moisture, temperature, etc. of the test cylinder are carefully controlled, history indicates that this is a fair approximation of what the majority of a given (in situ) concrete placement experiences.

http://www.SlideRuleEra.net

 

[chollan2]

If you think about concrete, it expands, just like gas.
A perfect structure would be a sphere, like a basketball.
Well, since we can't design with spheres, there is always a trade-off of a combination of spheres and rectangles and triangles/etc.

Where is the pressure being applied? (Concrete acts the same as wood or any other material) If the pressure is applied on the side, it would emulate and arch or square. If the pressure is applied on the top, it would practically be identical, unless it is convex or concave at the top.

Other things, though rarely considered are outside forces.
Say you have pressure one 2 sides, rather than just one side?

All in all, I would have to go with the cylinder.
It emulates a arch, which is the strongest ????, but again, it is all where the pressure is applied.

Regards,

 

[jgailla]

A senior engineer I used to work for says that the cylinders are used to verify the design mix, and it is not necessarily the same strength as in the actual structure as placed. If the design mix is correct, verified by testing, any deficiencies in the actual strength in the structure should be taken care of by load reduction factors or factors of safety. As for cylinder or cube, sounds like a great research project. I would think that cylinders would have a lower standard deviation than cubes, because the pressure should be applied more uniformly and problems at edges of the cube mold would show up more often than problems with cylinders.