Q.: After cores have been taken in a concrete member, what do you use to fill the hole?
A.: Because such a small amount is needed, the patching material is generally hand mixed onsite. Bagged dry-mixed concrete is convenient to use and won't shrink as much as a mortar would. To further reduce shrinkage, make a stiff mix by adding as little water as possible to the dry ingredients. Hand-mixed concrete can't be air-entrained so the patch may deteriorate if it's exposed to freezing and thawing while saturated. A low water-cement ratio makes the patching material more watertight and prolongs its life.