Difference between Target strength & Target Mean Strength

[shazi1]

What is difference between Target Strength and Target Mean Strength ???
As for required strength (Target strength) is given as (Characterstics strength + 1.64 * S.d).. this 1.64 may vary as depending on different criteria.
I had also seen some specs defining Target mean strength with totally different calculations, i.e (0.9*Characterstics strength+2.33*S.D)

 

[BigH]

Do you know where the latter came from? I've not seen that one before and I have quite a number of references that talk about it. They certainly give different answers. If the characteristic strength is 30 MPa and the STDEV is 5 MPa, then the former gives a target strength of 37.2 MPa and the latter gives a mean target strength of 38.6 MPa. I'd be interested if anyone can provide a knowing response too!

 

[shazi1]

Kindly see attached specs (QCS 2010), according to this specification, target strength shall be calculated depending on grade of concrete.. ( which is very much practical too)... Also, they clearly referred target strength as required strength.. see clause 7.
I GUESS answer to my question is "target strength & target mean strength are similar terms only"
I am waiting if some one can answer it in more detail with reference to BS or ACI