Concrete strength, aggregate and drillability
The aggregate decides, more than the grade does.
Grade is not drillability
Compressive strength is only part of the story. Drilling speed is governed heavily by aggregate hardness: flint and quartzite aggregates are brutal on carbide, while softer limestone aggregates cut comparatively easily at the same grade.
Age and curing
Concrete gains strength for a long time after nominal cure. Old concrete is frequently harder to drill than its original grade would suggest.
Reinforcement density
Heavily reinforced and post-tensioned elements constrain where you can drill at all — often more than the material properties do.
Practical consequence
Bit wear rates and drilling times vary widely between projects. Budget tooling by observation on the actual structure, not by grade on the drawing.
Questions about this
Why does the same grade drill differently on two sites?
Aggregate hardness dominates. Flint and quartzite aggregates are far harder on carbide than softer limestone aggregates at the same compressive strength.
Does old concrete drill harder?
Often yes. Concrete continues to gain strength long after nominal cure, so older structures frequently drill harder than their original grade suggests.