Concrete – How to patch the slab around a rough-in pipe in a difficult to access location

Quoting from "Common Questions and Answers about Frost-Protected Shallow Foundations" on page 29 of the Federal HUD guide on constructing frost proof shallow foundations dated June, 1994:

Question No. 1: How does insulation stop frost heave from occurring?

Frost heave can only occur when all of the following three conditions are preset:

1) the soil is frost susceptible (large silt fraction),

2) sufficient moisture is available (soil is above approximately 80 percent saturation), and

3) sub-freezing temperatures are penetrating the soil. Removing one of these factors will negate the possibility of fromst damage.

Insulation as required in this design guide will prevent underlying soil from freezing (an inch of polystyrene insulation, R4.5, has an equivalent R-Value of about 4 feet of soil on average). The use of insulation is particularly effective on a building foundation for several reasons. First, heat loss is minimized while storing and directing heat into the foundation soil -- not out through the vertical face of the foundation wall. Second, horizontal insulation projecting outward will shed moisture away from the foundation further minimizing the risk of frost damage. Finally, because of the insulation, the frost line will rise as it approaches the foundation.

Since frost heave forces act perpendicular to the frost line, heave forces, if present, will act in a horizontal direction and not upwards.

So you are OK as long as your non-susceptible fill does not contain much silt.

If the PVC drain indicates the age of the plumbing and the house, or at least when the slab was installed, there should be a layer of poly acting as a moisture barrier in between the concrete and gravel. Aside from that, all you need is to add the original gravel back into the hole and repair the vapor barrier and finish off the fill with a bag or two of concrete from any local building supply, big box or otherwise. A detail that would be good to do is be sure the hole cut inti the concrete floor is not all undercut. As in, the concrete should not be broken out so the hole is larger the bottom edge of concrete than the concrete at the floor level. A little undercutting is ok, but the concrete you add needs to lock in so it will not settle down away from the original concrete.