Need help on durability of Reinforced Concrete

Hello all,
I'm a newly graduated civil engineer working on a structural integrity report for an existing culvert. Based on eurocode design, we found that for the exposure class (XS1), a minimum strength class of C30/37 would have been sufficient to construct it. In situ testing showed that C25/30 had been used instead.
The concrete used satisfies strength requirements, but not the durability requirements. We've been researching on how we can determine the degree of deterioration, and by extension the remaining design life of the culvert (as opposed to the intended 50 years) and any way to increase the durability of the existing culvert.
The approach I'm using is to try to understand how the Eurocode comes up with the required strength class for a certain exposure class. I've read through ec2 and can't seem to find any link to calculations or procedures that were used. I'm thinking i might be looking in the wrong place or not looking hard enough? I dunno.
So any help on reading material that could help me with a mathematical relation between the concrete class and the exposure class would be greatly appreciated.
I realize the question may not be too clear, so any questions are welcome.
Thanks in advance