Concrete cube testing problems 3

[Snatch]

I am working on a project in which concrete cube samples were taken. Tests were carried out at 3,7,28 days, the specified strength was 4000psi. Unfortunately the samples were not placed in a water for curing. After crushing samples at 28 days, the strength were much lower that 4000psi. Can the contractor be blamed for producinng concrete below the specified strength. Is there anyway of apply a factor to indicate what the strength would have been in an ideal condition.?

 

[RDK]

Lab curing only tests the potential of the concrete to achieve the design strength. What you are really interested in is if the concrete in the field has the necessary strength.

If there is any problem with the samples, then move on to some sort of field-testing. You could take cores of the material or use a Schmidt Hammer. In fact as far as I am concerned the samples are only to be used to determine if in situ testing is necessary.

In general if the material is to specification then the owner pays for the tests (and repairs if cores are taken) and if not to spec then the contractor pays for the tests and any necessary remedial action.


Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion

http://www.kitsonengineering.com

 

[BigH]

Well, sure the contractor can be blamed. Will the blame stand-up? That is the rub. As Rick indicated, the laboratory sampling and testing is, in reality, only a measure of the consistency of the mix to produce results that meet a specified value. If the results are erratic, then the concrete proportions are likely erratic. It is not a measure of the actual concrete strength in situ. In the lab, there are definite procedures and these are done with the idea of reproducibility from lab to lab and operator to operator. The actual strength in the field depends on level of curing and, many times forgotten, the care and attention to vibration. Poor vibration at the time of placement can lower the in-place strength by 10 to 15%.

Now, one thing to consider is - are the "non-cured" specimens similarly cured to the field? In other words, how good did the contractor cure the structure. If he didn't, then the cubes might be a reflection of the concrete in-place strength. But, of course, this will be fought.

The way I see is for you to request coring be done of the structure to confirm. Now, you will have fun with the translation of cylindrical cores to cube strengths! (See Neville's Book on Properties of Concrete. If you want, I have other data along these lines. But, in general, it is commonly accepted that the core results will not be equivalent to the laboratory specimens. It is usually applied that, given no steel is cored, that the average of three cores must be >85% and no single core less than 75% of the specified strength. Several references out there will give core strength vs time. Even after a year, the core strength will not typically be up to the specified strength.

Hope this helps - if you need some more information, advise us.

 

[napoleon]

can i ask what is the main differences between the test about indirect tensile of cylinder and the test about flexural strength test of beam?

 

[bluesman]

the indirect tensile strength (splitting strength) test measures the concretes resistance to being pulled apart, it's carried out on a standard cylinder specimen by applying a tensile force to a concrete cylinde specimen placed on its side.

flexural test measures the ability of a beam or slab to resist failure in bending. It is measured by loading unreinforced 150x150 mm concrete beams with a span three times the depth (usually 450mm).




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