7 days compressicve strength of concrete core is higher to 28 days.. surprising?
7 days compressicve strength of concrete core is higher to 28 days.. surprising?
(OP)
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.
Can anyone suggest any other reasons for such cases r had you ever encountered such situation.






RE: 7 days compressicve strength of concrete core is higher to 28 days.. surprising?
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?
RE: 7 days compressicve strength of concrete core is higher to 28 days.. surprising?
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")
RE: 7 days compressicve strength of concrete core is higher to 28 days.. surprising?
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)
RE: 7 days compressicve strength of concrete core is higher to 28 days.. surprising?
I deleted it from other forum.. Thanking you again
RE: 7 days compressicve strength of concrete core is higher to 28 days.. surprising?
RE: 7 days compressicve strength of concrete core is higher to 28 days.. surprising?
RE: 7 days compressicve strength of concrete core is higher to 28 days.. surprising?
We usually prefer to have cores sulfar-capped for testing.
RE: 7 days compressicve strength of concrete core is higher to 28 days.. surprising?
RE: 7 days compressicve strength of concrete core is higher to 28 days.. surprising?
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/
RE: 7 days compressicve strength of concrete core is higher to 28 days.. surprising?
Maine Professional and Structural Engineer. www.fepc.us
RE: 7 days compressicve strength of concrete core is higher to 28 days.. surprising?
RE: 7 days compressicve strength of concrete core is higher to 28 days.. surprising?
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.
RE: 7 days compressicve strength of concrete core is higher to 28 days.. surprising?
RE: 7 days compressicve strength of concrete core is higher to 28 days.. surprising?
But either way, re-coring is clearly the only way to get reliable results.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/