7 days compressicve strength of concrete core is higher to 28 days.. surprising? 1

[shazi1]

Recently I come across 2 concrete core test results (tested in different dates) where 28 days strength was lower by 10 MPa to 7 days... I do not have any justification to it, only reason can be honeycomb or air voids, or low quality aggregates used for concrete.. but this cases are not applicable as coring location is very near to each other.. 6 cores were taken from panel of 800*800 sq.mm.. (for 3,7 & 28 testing)_
Can anyone suggest any other reasons for such cases r had you ever encountered such situation.

 

[Ron]

only reason can be honeycomb or air voids, or low quality aggregates used for concrete

Not true....there are other reasons this could happen. Since you are taking cores at early ages, there is likely some core disturbance either in the cores or in the panel. Coring induces vibration...vibration induces microcracking in concrete.

Was the panel fabricated from the same batch of concrete or was it randomly selected from a larger area? Is there a chance your samples represent 2 different batches of concrete?

Did you examine the cores before testing? What size cores were taken? What is the max. coarse aggregate size? Were the cores taken wet or dry? How were the cores conditioned before testing? Wet? Dry?

Did you cut all cores at the same time and then test at different ages? If so, which cores were cut first/last?

 

[yaschief]

Having been the guy on the core drill more than once in my career, I can opine on this:

1) After the core was taken did they techs allow the core to dry out or was it protected as per ACI specs? (More than once I have seen cores stacked in an empty bucket and then in the back of the truck).

2) Were the cores examined after coring but before acceptance for voids, embeds, etc. or laitance?

3) As stated above: Was the "set" taken as a unit or at different times?

4) What was spec strength? How much of a variance was there between 7 and 28 day strengths? (As you know ACI considers anything over 2500 psi to be "concrete")

 

[shazi1]

thanks for your superb reply... after reading responses here my perception changed.. below are more specific details of coring..


Core size 100 mm dia, length 100 mm.
Maximum size of aggregate : 10 mm
Sample represent 1 batch of concrete. at time of coring panel was dry and water is added at time of coring..
All cores were cut at same time, but we are not very much sure about the sequence.

strength of cores are as follows:
1) 43.79,36.31 (5 DAYS) 51.92,42.3 (7days) 30.4,31.85 (28 days)


2) 35.56,36.01 ( 3days) 38.69,33.24 (7 days) 25.36,23.08 (28 Days)

 

[shazi1]

thanks Mr. yaschief.. I had not read rules before.. i post it i diffrent forum with a thought that more people can view my post...
I deleted it from other forum.. Thanking you again

 

[Ron]

How old was the concrete when you cut the cores?

 

[shazi1]

48 hours

 

[Ingenuity]

Were the ends of the cores prepared to ensure perpendicularity and planeness?

We usually prefer to have cores sulfar-capped for testing.

 

[shazi1]

Yes

 

[IDS]

There are 12 results in all, all of which are consistent, which suggests it is not local variations in the concrete.

One possible explanation is that the early tests may have been carried out as quickly as the equipment allowed, and the 28 day tests in accordance with the specification, or at least much more slowly than the early ones.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[TehMightyEngineer]

Any possibility of temperature effecting the quality of the concrete? I seem to recall high heat and rapid curing of the concrete can cause the strengths to plateau and maybe even loose a little.

Maine Professional and Structural Engineer.

http://www.fepc.us

 

[Ron]

shazi1....not sure where you are located, but in the US, we generally follow ASTM standards. For coring, that would be ASTM C42. The recommended minimum age at time of coring is 14 days. This can vary depending on a few factors; however, 48 hours is way too early to core concrete. The disturbance of the concrete is likely a cause of your strange results. Go back and core at a later date and you'll likely find higher strengths.

 

[shazi1]

General practice is taking core after 14 days, but due to urgency of work and as consultant do not want to rely on schimdt hammer or other NDT test, the only solution was to cast a panel along with Lining of shaft and then remove panel from lining to take core (in order not to damage lining) we opt for taking core in early stages..
Requirement is to check insitu strength of concrete is 90% of required design strength.
oops.. I forgot to mention that we are also using accelerator to get early strength.

 

[hokie66]

I think the decision to take cores at such an early age resulted in your useless results. The concrete was just too immature, as Ron pointed out. Now that you have wasted these efforts, you can now go back and do it correctly, if there is still a project requirement to compare cores with cylinders. Urgency is rarely a good excuse.

 

[IDS]

Regardless of whether coring at such an early age is a good idea, I don't see how it can account for the results given. The 3-7 day results are consistent, and the 28 day results are consistent as a group, but about 40% lower than the other results.

But either way, re-coring is clearly the only way to get reliable results.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/