I was kind of excited about Tapcons, because they let you a smaller hole than with anchors.
I’ve used Tapcons twice. The first time was to attach an outdoor guard rail to the sidewalk concrete. I drilled holes with the Tapcon bit through the rail’s base plate into the concrete, and tried to drive the Tapcons, but they stalled out about halfway through. I chatted with the Tapcon helpline, and it turns out the screws had probably bound up in the steel base plate. The bit is sized specifically to let the screws drive into concrete. In steel, the bit was a bit too small. Okay.
The second time was today, and last week. I was trying to attach a base for a Little Library to concrete (on a 4x4 post, into a plastic post base). The first time I used 3/16" by 2 1/2" Tapcons, reasoning that the pullout loads wouldn’t be big, and a narrower hole would be faster to drill. So I bought an expensive, weird 5/32" bit, the one Tapcon recommends. Well one screw broke off in the hole about halfway down. So the hole is now useless, and I have to reposition the base.
So I tried again, with 1/4" x 1 1/2" Tapcon screws. I figured the fatter screws would resist breaking better, and the shorter screws would reduce the torque on them (because the resistance increases the farther you drive them.) Again, I used a Tapcon drill bit for the holes, but 3/16" this time. Incidentaly, my M12 Fuel hammer drill had no problem drilling the holes.
And lo, my M12 impact driver (not a Fuel) wouldn’t drive the screws all the way in. It petered out about halfway through.
So I think Tapcons are very picky about depth and diameter. and even if you get it right, your typical DIY level tools might not cut it. The screws have to be short enough that your tools can snug them down, but long enough to handle the pull-out forces, and who knows what those are?
I ended up using the smallest Parawedge anchors (with the cone-shaped nut that expands a brass collar in the hole). They required a 1/4" bit (so that was a third carbide bit to purchase) but my M12 Fuel hammer drill again had no problem drilling a 3" hole (I was generous.) I tapped the anchors in with a hammer, tightened them up, and done.
Why did I bother with Tapcons? What did I do wrong?