SDS actually means “slotted drive system” but to the layman is just a heavy duty machine which gives much more impact on the drill bit, which is tungsten tipped. Concrete is much more difficult than brickwork to drill through, being much denser and composed of much harder minerals.
You might get away recutting the thread if you can get hold of the gear on loan or know somebody with it, too expensive for a one off job…just a suggestion…depends how damaged they are.
You sound like me, always know what I should have done after I’ve buggered it up by pressing on regardless.
When you replace these posts in another 15 years you will have forgotten this…and do it all wrong again…I know I would.
As Barry says, Rachel. There are different composites of concrete. If it was a driveway with plain, new cement that would be easy peasy to drill through with a masonary bit, but mine is made up of concrete with pebbles and God knows what in it. I’ll give it a go with my drill but last time I did it was smoking nightmarishly!:shock:
Haha! yes Bruv, along the same wavelength you and me here.
When I get into something, I just throw away most sensible outcomes in order to get things done and dusted!
In another 15 years time I’ll surely return but being a young 52 y.o., I’m guessing there may be a wholly different bunch of folks on here by then - if anybody at all!
Yes … I see what you mean.
I have encountered different sorts of concrete and also mixed my own, occasionally
Also used the ready-mix stuff.
Not in huge quantities of course only girlie amounts.
I also did oxy-acetylene welding as a teenager at Art college … ABSOLUTELY TERRIFYING and gave us all black bits up our nostrils :shock: yug 
If the original hole is deep enough you may be able to loosen the bolt by replacing the nut temporarily and tapping the bolt into the hole which will release the locking section of the bolt from the body of the rawlbolt. Then, depending on how tight the original hole was for the rawlbolt body you may be able to move it with a screwdriver and work it out slowly until there’s enough of the body showing to get some grips on it to pull it out.
He’s already buggered the threads innit.
Do try to keep up
My first post in this thread I think.
Eminently sensible
Well… I’ve been working, I’m tired, and couldn’t be bothered wading through the whole thread. 
This thread BTW, not the one on the bolt.
Just got me thinking actually…maybe I can just welly what’s left of the bolts with a big hammer (a big hammer solves everything doesn’t it?) and smash them into the ground. At least that way the surface will be level and I won’t need to grind the heads off. First job tomorrow afternoon I think!
A club hammer.
OK, assuming you either saw off the old bolts or bash them into the ground, how do you bolt the new metal post holder to the concrete?
Won’t the old bolts obstruct where you want to drill?
We need a photograph or two.
Re: SDS drills.
Aren’t they great! I love using mine, makes light work of everything and good fun. I know, I’m a bit strange. 
No, shoogle them about. Tap em side to side, on the top, then side to side again, they will eventually pull out.
Okay, I’m gonna try this right now. Back in a mo’.
This thread has sparked an idea for me. I had a small garden bench outside my window but it was stolen one night. I’ve been reluctant to replace it but was thinking maybe I could use something similar maybe to bolt it, chain it discretely to the wall of the house.
Hurry up…we are all waiting.
Some steel garden benches and other furniture have bolt holes in their feet to fix them into the ground.
I’d be sure that you won’t want to move it in a hurry though as, like fence posts, they are not easy to sort out.
But with a garden bench like you say, Monday’s Child, perhaps you could put a chain around it and fix it to a wall hook with a padlock or something? This sort of thing?:
https://chalkboard-4585.kxcdn.com/media/catalog/product/cache/3/small_image/400x/9df78eab33525d08d6e5fb8d27136e95/c/h/chain-and-lock_2.jpg