In our 100-year-old co-op building in NYC, I'm replacing a failed door stop with a heavier one to prevent our (very) hefty entrance door from smashing the doorway's marble.
The door stop is intended to be bolted into the floor with a sleeve anchor very similar to this one and a 5/8" pre-drilled hole.
Unfortunately, as I drilled through the tile/concrete with a hammer drill, I discovered that the floor only extends maybe 2" down, after which it seems to be loose fill/sand. If I try to use the sleeve anchor in this hole, it looks like the "expansion" part of the sleeve will be expanding into thin air, or at best only partially in the concrete floor.
What should I do? I'm considering somehow inserting as much concrete as possible into the hole, letting it cure, then drilling and continuing as before. Unfortunately I only have this 5/8" hole for access and I'm not experienced with concrete—I'm not sure if it will be thin enough for me to get enough down there before it sets.
Clarification: this is the ground floor entry way from the street, elevated maybe 3-4’ from ground level and accessed by four steps from the sidewalk.
I guess this isn’t “the” slab of the building, but just something odd used for the entry way floor. There is no access underneath the floor.
And as to the oddness of the whole thing, hey, 1913 construction in NYC? Not sure.
Best Answer
First - what kind of construction uses a 2" slab for a floor? Trying to get more concrete down there is unlikely without breaking out part of the floor because it needs something to displace in order to do what you want. Even if you got some down there it's very unlikely to hold. Why not go to a 3/8 x 2 1/4 expansion bolt. I know you're rightfully concerned with the 2" slab holding but it should be okay unless that door is a real behemoth.
Another option is to improvise an extra large (i.e. 6"x 6") block that you can molly-bolt onto the 2" slab. What this does is take advantage of the shear strength of the molly by exerting force laterally when the door strikes the block or stop as opposed to a narrow stop that will want to tilt when it's struck exerting diagonal forces (up and down) on the 2" slab. If you go with an over-sized stop you could also strengthen it by using more than one anchor spreading out the load.