Simply, the compressive strength can be defined as the capacity of concrete to withstand loads before failure. Of the many tests applied to the concrete, the compressive strength test is the most important, as it gives an idea about the characteristics of the concrete.
Since you are reading on the target mean strength in this article, I guess you are aware with the characteristic compressive strength.
In case you do not know the difference between characteristic compressive strength and target mean strength, read on the difference here.
Concept of Concrete Target Mean Strength
Target mean strength (Fm) is the design strength of concrete mix. This value is higher than the value that structural designer use in structural design.
This means if the design mentioned in structural drawings that concrete grade is 25 MPa [we call it concrete characteristic strength]. The mix design of concrete will not target 25 MPa. Instead, it will target a higher value [we call it target mean strength]. Why?
Because in concrete production and casting in site, there are multiple sources of variability that may cause reduction in the compressive strength of concrete. So, we increase it a little bit from the beginning as a factor of safety to account for any reductions that may occur.
Calculation of Concrete Target Mean Strength
The additional factor of safety (we call it the margin) that we add to the characteristic strength (Fc’) is 1.65 multiplied by the standard deviation (𝛔) of test results.
What you see is the curve that we usually obtain for compressive strength results. Assume we have 40 specimens from the same concrete mix that we want to get the compressive strength for.
When we plot the results for the 40 specimens, we obtained the bell curve.
The mean value on the curve is the target mean strength that we design the mix for.
At this point, I advise you to read on the difference between the characteristic compressive strength and target mean strength in case you didn’t (here).
So, the basic thing here that is essential to determine the target mean strength is the standard deviation.
Example: If I told you we need to design a concrete mix with 30 MPa characteristic strength and we have the standard deviation of results as 6 MPa. What is the target mean strength?
