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Concrete Cylinders strength gain more than 100 % at 7 days breakHelpful Member!(2) 

gosai (Civil/Environmental) (OP)
14 Dec 10 14:34
The test results for concrete cylinders for 35 MPa and 60 MPa show strength gain more than 100% at 7 days break. Any one can focus on the issue what factors can cause concrete cylinders to gain more  than specified strength at earlier than 28 days age. Is there any testing issue?

The lab is using curing tanks for curing. The temperature of water in curing tank ranges between 21 and 25 degree celsius. The ends of the cylinders capped using sulper

Thanks to all    
henri2 (Materials)
14 Dec 10 14:43
More than 100% of what or from what? What were the actual 7-day test results for the 35 MPa and 60 MPa mixes and while you are at it, provide results of earlier and later ages.
Helpful Member!  Ron (Structural)
14 Dec 10 14:54
Not entirely unusual to see 100 percent of f'c at 7 days.  Occasionally you get the "perfect storm" of batching, cylinder fabrication, curing and testing...

Watch the results closely for a while to see if this repeats.  If so, check the testing first, then the batching.

Curing temperature, at 25C is just slightly high...that can cause early strength gain if it is consistent.
Helpful Member!  BigH (Geotechnical)
15 Dec 10 6:33
Curing temps in India are 25 - a bit higher than ASTM and CSA.  I'd like to know how much cement was put in?  Maybe the mix was batched to achieve the 28-d strength at 7 days because of a construction issue - early loading? etc.  We just don't have enough facts . . .
gosai (Civil/Environmental) (OP)
15 Dec 10 8:50
Henri2 The strength I mentioned is specified and mentioned on the delivery ticket from supplier.

About curing temperature CSA recommend curing temperature should be 23 plus or minus 2 C. When the cylinders were tested for 28 days the strength gain is 40 % higher than 7 days strength (If strength is 40 MPa @ 7 days than @ 28 days strength is 57 MPa)

My only concren is do I need to ask testing lab to review there testing procedure or not?

Thanks to all
Ron (Structural)
15 Dec 10 13:05
It depends on how many of these test results are like this.  If many of them are high, then start with the lab and then the concrete supplier.

Just keep in mind that testing labs do more through their procedures to reduce the apparent strength of concrete than to increase it; however, I have investigated lab results where there was actually no test performed...just made up and reported falsified results.  That's rare, but it does happen.

I would agree with BigH that there is likely some "juicing" of the cement content to get higher early strengths.

Do the batch tickets match the mix design?
bb29510 (Geotechnical)
21 Dec 10 22:02
I see 100% in seven days very often, its usually a hot mix placed on the dry end of the slump. By placing dry you increase the strength mix so it could be batch as a 3000 but less water lower water cement ratio, its closer to a 5000.

point I'm making, I see it everyday, its nothing abnormal. I have seen 100% in 3 day breaks. It just a good mix

ICC special inspector, Structural masonry, Reinforced concrete.  

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