Location: Minneapolis/St. Paul, Minnesota - Latitude 45.057 Longitude -93.074
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concrete type for piers to reinforce load bearing wall?
Where are the piers? What kind of wall (block, wood, etc.)?
Are they new piers under or adjacent to the wall? If so, just use 3000 psi concrete for poured concrete construction.
If you are talking about filling the cores of block, you use grout that is wetter (8"-11" slump) to make sure the voids are filled completely. Fill in lits to allow time for consolidation. The excess water is absorbed by the block, making an ideal curing condition.
You would get better information if there was a sketch or a photo or some dimensions provided.