Cordless hammer drills suck. Plug in hammer drills are only slight better. Go rent a sds hammer drill from your local hardware store. It will go through it like butter. Even my Ryobi bottom of the line plugin sds will chew through concrete like nobody's business

I'm assuming you were trying to use something like a tapcon threaded anchor? I personally despise them. Even when I use the factory bits, they tend to jam and break off. I suggest removing the anchors and then drilling the holes out large enough to use glue in anchors. A SDS hammer drill will make quick work of it. Borrow or rent if needed.

What many commercial installers do, and will work for you, is to glue studs into the holes using special epoxy and then mount your post with nuts. If you are in the US, Home Depot sells the Hilti glue (link below) - which I have used and like - you would still need to drill holes and purchase some threaded rod of the correct diameter. Drill the hole a little oversized and then glue the threaded rod into the holes. Let it cure, then attach your posts. The exact length doesn't matter. I would use stainless steel threaded and stainless lockwashers and nuts to avoid corrosion problems.

It can be convenient to make a jig out of plywood and use that to check the spacing and location of the studs are correct before the glue sets.

https://www.homedepot.com/p/Hilti-HIT-1-Hybrid-Anchor-Adhesive-10-oz-Tube-3575547/303857797

SDS hammer drill and drop in anchors. Shouldn't need a pilot hole using a SDS.

It might be too late for this, but in my own experience fastening to concrete I definitely prefer wedge anchors to tapcons. If you have a nice SDS I am sure that tapcons are lovely, but for my uses I either strip the hole, snap the tapcon, or it drives perfectly. 1 in 3 worked for me so until I started used wedge anchors.

I have both conventional hammer drills and SDS. It's not the drill. Tapcons stink no matter what.

I don’t disagree. What I meant though was an SDS results in a better hole for the tapcon and more successful driving. At least that’s what I have been told. As I said I don’t have an SDS

I fell for all that, too. Tried a better drill. Bought the special Tapcon bits and drivers. Cleaned the hole like we were doing surgery.... still broke 2/3 to 3/4 of the damn things...

Titen is a better product for screws into concrete if you don't want to use wedge anchors.

Rent an actual rotohammer and use wedge anchors. If you still want to use a concrete screw, use Titen instead of Tapcons. Make sure you clean out the hole after drilling via compressed air. If you choose to use an epoxy option as one person suggested, use both compressed air and a wire pipe (tube) cleaner to clean the holes.

Oh shit, well that halts my own desire to splurge on an SDS for tapcons then. Guess I’ll have to find another use to justify it hahah

They are absolutely fantastic for making holes! Also crazy good for removing tile (with a flat chisel bit). Just pick some other concrete fastener...

This is great advice. What diameter is the best for the steel threaded rods?

They were Titan fasteners. This is great advice, thanks so much! I’m in the US and will buy the epoxy you recommended. If we use 1/4” threaded rod (since that was the size of the original fastener) do you have a recommended bit size? I’ll definitely be renting an SDS drill too because I can’t go through that again…

Generally you go 1/8" larger to make sure there is room for the epoxy all around. Check the size of the mounting holes on the posts. If they are half inch, you can increase the size of the threaded rod a bit - I would go up to 3/8" rod, and drill 1/2" holes (leaving 1/8" to fill with epoxy). You want a little "wiggle room" to get the spacing of the studs right and to fill all around with epoxy. Read the Hilti instructions - they are good - and there are some youtube videos. This is a very basic video, but shows the process (you don't need the extra step shown after using a core drill - that is something unusual) - the related videos below it are also good - https://www.youtube.com/watch?v=TsonKFh0UUw

If you are doing something that is fussy, transfer the pattern to some plywood, drill the same hole pattern in that, and hold the studs in place with two nuts (one on each side of the plywood) and make sure you can get that in and out of the drilled holes without binding up. You can even leave the plywood in place during the glue cure (just don't accidentally glue the nuts or the plywood to the concrete!).

You still have the same requirements to clean the dust out of the holes, using a small brush (I stole the brush that came with one of those reusable straws my kids had to have...) and some compressed air. Vacuuming can be part of it, but it won't get the dust out of the very bottom by itself. Sticking a thin tube down to the bottom and blowing it out with compressed air is better (but don't breath the concrete dust!).

One other thing a lot of people forget. You have to clean out the holes. Those bits don't clean out the hole like a normal drill bit in metal or wood. You need to blow or vacuum them out. You're 3" hole can be filled with debris in the bottom inch. This applies no matter which method of fastening you use. You especially want the hole clean if using epoxy.

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Take whatever your using out of the holes. Go to home depot and buy appropriate sized Redhead wedge anchor bolts. Go home and tap them all the way in with a sledge hammer. Before you hit the Redhead's back the nut off the threads a bit before hitting. Position the nut so the hammer cannot hit the threads, if the threads get mangled you have a heck of a time getting the nut on or off again. Tighten the nut down with a ratchet or wrench until you cannot tighten it anymore. 2 to 3 inch anchor bolts will more than likely be enough maintain railing post integrity if someone put their entire weight on it in any direction.

Retail hardware stores don't carry industrial epoxies, I've tried multiple retail box store epoxies and they all failed in short time (excluding JB Weld... good stuff, unfortunately they don't make concrete epoxies that I'm aware of. Unless you have access to industrial grade epoxies go with the wedge bolts.

Your probably not using the hammer drill correctly as well. The bounce part of a hammer drill needs to be allowed to bounce. People always try to push hammer drills into the concrete which does not allow the tool to do its job. The drill bit needs to bounce up and down as you drill. Most holes can be drilled quickly using only the weight of the hammer drill by itself while you're just hold it in place steering the bit.

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Drill in a 1/2" deeper than you need and don't worry about cleaning the dust out of the holes, who cares if dust is in the bottom of the hole as long as the bolt sets where it needs to set.

Incorrectly

i hate Tapcon style screws of any brand. they are prone to wringing off the heads. that said, a lil bit of bar soap applied to the threads before driving them can help greatly. if you switch to 1/4 inch wedge anchors,be aware that care must be taken pounding them in,as they are easily bent with an errant hammer strike.good luck!