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20/20 is all you need to build a concrete pier. You might consider 30+ MPA with plastisizers if you were building a freeway bridge footing/enbuttment wall/pier/deck etc etc where outright strength and durability is the main concern (remember these structures are often stressed via cables placed in the concrete at exacting angles and the bridge must be able to flex whilst maintaining structural integrity). Main thing is add some reo (not for outright strength but to prevent cracking) and vibrate the concrete well to remove as many air bubbles as possible. Any more is really overkill for the types of loads we place on them.
Mark
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