C35/45 concrete: higher strength does not replace a durability specification
C35/45 means 35 MPa characteristic cylinder strength and 45 MPa cube strength. It may appear on more demanding structural or exposure-controlled work, but the class alone cannot tell you whether a concrete is suitable for chlorides, freeze-thaw, aggressive ground or a particular working life.
Start with the deterioration mechanism
For chloride-bearing, marine, deicing or freeze-thaw environments, identify the actual exposure classes and project requirements first. Those requirements can affect maximum w/c, air entrainment, cement/combination selection, cover and other properties. Do not use “C35/45 = aggressive exposure” as a universal shortcut.
Cement combinations are part of UK durability design
BS 8500-1:2023 expanded UK provisions for lower-carbon cements and combinations and revised aspects of durability specification. This is one reason older internet tables can be misleading: cement choice and exposure guidance evolve, while a simple C35/45 label does not capture them.
For a live project, use the current project-specified standard tables and approved cement/combination rather than copying a legacy CEM I recipe.
Low w/c still has to be placeable
A more restrictive w/c can make concrete less naturally fluid. Pumping and congested reinforcement may require careful aggregate grading, paste volume and water-reducing admixture rather than added water. Compatibility and consistence retention should be verified with the actual binder and delivery time.
Worked troubleshooting: strength is good, surface durability is poor
If strength cubes pass but an exposed surface scales or deteriorates, raising the strength class may miss the cause. Review exposure specification, air system where applicable, finishing practice, curing, early-age weather and deicer/chemical exposure. Durability is a system outcome, not just a compressive-strength number.