About Concrete Cylinder Strength

[tanvir.ce12]

Hi,

Some cylider specimen (4"x8") were casted & send them for compressive strength test after 14 days. I've used a nominal mix ratio of 1:1.5:3 for M20 grade concrete. But after crushing the strength of the three specimen is around 10.5ksi which is way higher than the target strength. What could be the reason behind that?

 

[hokie66]

Are the units mixed up? M20 refers to 20 MPa, which is a lot different than ksi. If it is 10.5 MPa at 14 days, it may be too low, but I am not accustomed to interpreting tests from that size cylinder.

 

[tanvir.ce12]

Yes, M20 represent 20MPa, which means my target strength is 3ksi but i got 10.5ksi which is very higher.

 

[hokie66]

I would agree that doesn't sound right. I was just suggesting that perhaps the cylinders broke at 10.5 MPa, not 10.5 ksi. Your mixing of the units (MPa, inches, ksi) made me suspicious.

 

[Ron]

What do your ratios represent? Is it 1 part cement, 1.5 parts water and 3 parts aggregate? Please clarify.
It is difficult to get 10.5ksi at 14 days on purpose, much less by accident. I agree with hokie66....you probably have 10.5 kPa not 10.5 ksi.

The water-cement ratio will generally dictate the concrete strength, which is usually tested at 28 days, not 14 days.

 

[tanvir.ce12]

The ratio represents 1 part cement, 1.5 part fine aggregate(sand), 3 part coarse aggregate (stone chips). All the 3 cylider broke at around 10.5ksi (around 73MPa). It seems quite unusual.

 

[dik]

as Ron noted... how much water... really critical to a mix.

Dik

 

[tanvir.ce12]

Water cement ratio was 0.42. 21L of water per bag(50kg) of cement

 

[TehMightyEngineer]

Goodness, we're lucky to hit 10 ksi with our type III cement mix. I'm also going to go with some sort of error. If the units aren't messed up then something with the testing procedure might be (incorrect calibration, something binding up in the machine, etc.).

Do you have pictures/access to the cylinders that were tested? A 10+ ksi mix will actually explode to some degree during failure, the differences in the fractured cylinder should be quite obvious between a normal strength and a high strength mix.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[jayrod12]

You can't get 73 MPa in 14 days using a standard 0.42 W/C ratio mix. Something is fishy.

 

[tanvir.ce12]

@TehMightyEngineer

One thing i can assure that the units aren't messed up. May be there was some problem in the testing procedure. Since, we sent those cylider to an university for testing, i don't have any picture during failure.

 

[hokie66]

So how do you know that the university lab didn't confuse the units? I assume you are just reading from their report. Why not ask your question of the lab? The answer should be weird to them too, if they know what they are doing.

 

[tanvir.ce12]

@hokie66
Yes, you're right.In the report, the strength is given in both ksi & MPa. I've received another report from another batch of concrete, where avg. strength is around 5ksi (after 28days of curing).

 

[dik]

Hokie, the results should have run up a flag.

Dik