The building where I live has continuous steel guard rails running next to the front stairs and then around a wheelchair ramp. They are bolted into the concrete at their bases. On one side, the bolts rusted out, so there was nothing holding that rail down at the base of the stairs. It didn't move, though, because the rest of the rail was still tied down at about six places. I told the HOA I would fix it with those blue concrete screws (Tapcons). A Tapcon rep told me they would work fine, as long as I didn't reuse the old anchor holes, but drilled new ones.
So I sanded off most of the rust on that mounting plate, and started drilling holes. I drilled 3/16" holes in the steel plate and concrete, using first an ordinary twist bit for the steel (maybe 1/8" thick) and then a masonry bit for the concrete. I used the prescribed Tapcon masonry bit of the right size (they say use 3/16" bit for 1/4" screws) and drilled to the prescribed depth (about 1 1/2"). Then I started driving the 1 1/4" screws.
And I couldn't get a single one to seat fully! I stalled out a 1/2 corded drill, even when driving a screw, backing it out, then redriving. I tried a socket wrench and managed to twist the heads off three of the screws. They are all now standing proud and awkward. The plate is fasted to the concrete though, because the screws self-tapped into both the concrete and the plate.
What did I do wrong? Should I have made larger holes in the steel plate? Should I have blown out the dust in the holes?