Extc is just a simple example for the use of external cables. Maybe some special mixture was used in the original setup...

The recommended way is to generate the properties for the closest concrete class or cube strength, and only adjust values known better (e.g., from a tensile or compressive or splitting test, manufacturer information, ...). When the values are generated for a very different class and some are largerly changed, it can easily happen that the resulting set is inconsistent and/or numerically problematic.