Shrinkage of concrete and low strength of cylinder break

[gosai]

Hi all,
I am involved in pumping station project and during concrete pour for slab on grade for reservoir we cast set of prisms to test Linear Shrinkage. I received test results from testing company. The test results show shrinkage around 0.044, which is slightly over the specified limit in the specs.

The individual test result of one specimen between 0 and 1 day shows expansion of the concrete. The other specimen shows expansion between 4 day and 7 day reading and one specimen show no change in length between 4 day and 7 day. Is it possible to have such kind of behavior? As per the testing company all specimens were placed in controlled humidity and temperature as per ASTM

Second question related to same project. We used 4300 psi concrete. The test results from cylinders break is less than specified strength (4090 psi at 56 day break). The concrete was used for base curb. Should I ask contractor to remove concrete and replace or the strength is acceptable?

Thanks

 

[JAE]

Should I ask contractor to remove concrete and replace or the strength is acceptable?

Based on the strength, I'd say no - don't replace. Refer to this: faq507-1575

Your strength is not more than 500 psi below specified. As the EOR I'd check that the lower f'c still works with any design loads (lower f'c usually doesn't affect strength that much except for shear).

 

[JedClampett]

IMHO, strength of concrete is way overrated. It is a useful measure, however, of the quality of concrete. Unfortunately, it seems that all testing is aimed at providing a certain strength, while, as JAE says, it really doesn't affect that much of the capacity of a structure.
As far as your shrinkage tests, are you measuring between 1 and 28 days as required or 0 and 28 days? I almost never have a problem with shrinkage between 1 and 28 days.

 

[gosai]

Jed,

It is between 1 and 28 days, the final test results show shrinkage but few readings as i said show slight increase in length (between 4 and 7 days). One specimen show 1 mm increase and other specimen show 1.5 mm increase. Overall results of shrinkage looks fine. Just to know is it possible to notice increase in length between 4 and 7 day?

Thanks

 

[Ron]

Be careful with taking shrinkage or length/volume change measurements within the first few days of concrete batching.

If the aggregate, at the time of batching, was less than saturated, surface-dry (SSD) it will imbibe water, causing it to expand. This is particularly true of laboratory cured specimens.

As for strength, I agree with JAE that replacement is not indicated for that reason; however, as JC pointed out, durability will be affected...particularly since your strength was ultimately only close at 56 days, not 28.

Side note...JAE...the strength at 56 days was within the 500 psi undertolerance, but was that so for the 28 day strength, which is the evaluation criterion?

 

[JAE]

Ya - I saw that 56 day (vs. 28 day). But even so, for a strength difference of 210 psi, I assumed the 28 day was still within the 500 psi limit. I could be wrong.

Why the 4300 psi? Seems like a weird value (vs. 3000, 4000, 5000, etc.) I know they are just numbers, but 0's are good numbers.

 

[Ron]

JAE...agree...I doubt the difference would be much, particularly on higher strength concrete. Yeah, I didn't get that 4300 psi thing either. I've designed a lot of concrete mixes, but never could "fine tune" them like that.

 

[hokie66]

The OP has probably specified 30 MPa concrete, and has just converted to psi to get the attention of US contributors. Looks like he succeeded.

 

[Ron]

Geez, hokie...would you stop using common sense on us?

 

[JAE]

Yes - it drives us crazy. We are just happy clams on the bottom of the ocean, passing a bit of clam gas every now and then. Thinking of SI units is in that other ocean way over yonder.

 

[hokie66]

I place little faith in shrinkage testing or specification of low shrinkage concrete. Control of cracking is much more dependent on other factors, e.g. curing, finishing, reinforcement.

http://www.concrete.net.au/publications/pdf/Dryingshrinkagenote.pdf

 

[Forensic74]

I saw early age expansion all the time when running the prism test and the more advanced corrugated tube tests.

 

[rapt]

JAE,

I did not realise that Canada and Mexico were physically (as distinct from mentally!!) so far from USA and separated by oceans!