Concrete strenght 1

[Avecus]

Hi guys!
I need some help on this.
You know that the concrete class is defined by the fractal 5% value (or 0.05).
I need some clear guidelines on how to do this, with exact formulas and an example.
If I have some concrete strenght values resulted from concrete cubes with class C20/25, like: 24.1; 26.2; 30.1; 23.6; 21.5; 20.2 [MPa].
How do I calcultate de 5% fractal (fck 0.05)to see if it's above 25MPa ???

I need exactly how to calculate, because this is just an example and I have a lot of values.

Thank you.

 

[robyengIT]

it is called (at least in Europe) "characteristic strength of concrete"

 

[Avecus]

Yes, it is called that. But you didn't answer my question.

 

[robyengIT]

Google "characteristic strength of concrete" and you will find theory and formulas. After that you can do a program in excel, very easily

 

[BigH]

You can also reference Neville and Brooks' Concrete Technology.

Basically, you will do something like this: Required Mix Strength (characteristic strength) = required strength + (1.65)(stdev). Typically stdev=5 if the batch plant stdev is not known. If you need 30 MPa, then characteristic strength = 30+(1.65)(5) = 30+8.2 = 38 (I wouldn't go into the worry about the 0.2). Some books/codes acually give you the added strength to the required strength. So, you achieve 38 MPa on the average strength and only 5% would be less than 30 MPa.

 

[robyengIT]

Just to help. wake up !!

 

[Avecus]

I would like to thank you all for your answers, especially robyengIT.