structural

[cvlenv]

good day to all...
we are doing the construction of a water tank...we are on the phase where we are constructing the RC walls...
this is the case: our inspector required us to get beam samples for the retaining walls and do the test for the modulus of rupture...
my question : is it necessary?
fyi : we haven't established the parameter for the modulus of rupture...
thanks and more power...

 

[Ron]

Some inspectors are under the mistaken impression that any concrete in flexure needs the modulus of rupture for its design. That is generally only true of unreinforced concrete (there are some exceptions). Retaining walls are reinforced concrete, typically designed using the compressive strength of the concrete.

If your specifications do not require testing for the modulus of rupture and the structural engineer did not use it for the design of the retaining wall, it is unnecessary to test for it. If you have no specification for it, then you also have no acceptance or rejection criteria upon which to base an interpretation of the results.

 

[strucguy]

Aren't water tanks designed as leak proof structures (no cracking what so ever)? In that case isn't Modulus of rupture is an important parameter for maintaining quality in the construction of the tank?

 

[steellion]

Modulus of rupture is proportional to SQRT(f'c) in the ACI code. I'm not sure how much additional benefit you would get from an MOR test rather than backing out the number from standard cylinder breaks.

 

[cvg]

concrete is not leak proof, only resistant based on good design and QC. Unless the tank is properly designed and detailed including sufficient steel in the tension face at the proper spacing, adequate cover over steel, waterstops, and good quality concrete and finishing among other details, increasing the flexural strength of the concrete may make no difference in it's leak resistance.

 

[Ron]

Considering the rarity of sampling and testing beams these days, the better test results would be gained from compressive strength testing.

Beam testing is finicky...if the technician doesn't know how to properly fabricate the beam in the field and the lab tech isn't familiar with the nuances of testing beams in the lab, then the results won't mean much anyway.

While I agree with strucguy that it can be used, the MOR is only one aspect of testing for QA and does not do a great deal for maintaining the quality of construction. There is a good liklihood that the mix design used for the tank construction was not a design based on MOR, but compressive strength. There's a difference.

 

[bigmig]

Concrete that doesn't crack is really acting like unreinforced concrete, meaning every concrete tank design using this assumption would be mega thick with no bar. Obviously this isn't the case. So in other words, I don't think you need an fr value.

We are just finishing the final touches on a project that has been drawn out for a year by a debate over spec on a concrete tank. The project was a concrete tank that would not stop leaking. Upon start of the project, I would have disagreed with cvg, but my opinion now is they are very very difficult to make water tight, but not impossible.

In hind sight make sure your specs are water tight, because if your tank leaks this will be where everyone pulls out the guns. Have descriptive leak tests, time periods of testing defined, definitions of dampness...anything you can think of to make sure everyone involved knows what is considered a "leak proof tank".

The quality control is difficult for you to enforce despite your best efforts, unless you are the concrete sub, so again, cover yourself with a good spec.

And by the way, yes we did get it to stop leaking. Water tight concrete is possible.

 

[cvlenv]

many thanks to all of you!...

one thing, is there any code(s) that would technically back-up these concepts and would also practically specify when are test specimens for rupture are required?...

thanks again and more power....

 

[Ron]

ACI 350 addresses these issues, including maximum crack widths.

 

[bigmig]

Control of Cracking in Concrete Structures ACI 224R-89

Specifications for Structural Concrete Buildings, ACI 301-89
Chapter 16 "Testing".