What is the minimum tensile strength recommended for a concrete foundation that will have epoxy grout used to set rotating equipment?

You should have concrete with a minimum tensile strength of 350 psi.  This recommendation would be considered a “best practice” when using Escoweld, chockfast or most other epoxy grout brands.

Is it recommended to add additional mass to an existing concrete foundation?

When you add new concrete to existing concrete the interface will result in a cold joint and vibration will not transfer across a cold joint.  In this situation, special materials will need to be considered in order to bond with the existing concrete.  A foundation or mass can be extended with Escoweld or Chockfast materials if the correct concrete preparation is done.

How deep does my equipment foundation need to be?

The depth of the foundation should be beep enough to properly embed the anchor bolts.  This would vary based on the size of the equipment.  First, consider the mass required for the foundation.  Next, consider the 30 degree rule for a stable footprint.