I’ve got 2 ‘u shaped’ cycle racks to install outside my surgery..

Need m12 bolts to fix into concrete..

What would you suggest? expanding type bolts, or ‘thunder bolts’ – i’d not heard of thunder bolts before – they look liek heavy duty self tappers…
M12 would presumably need an m10 hole or something??

DrP

Used stuff like this when we needed to fix some fence posts onto a wall;

https://www.orbitalfasteners.co.uk/en/products/fischer-fis-v360s-vinylester-resin-360ml-art-no-94405?utm_medium=google_shopping&utm_source=google&utm_campaign=google_shopping&gclid=EAIaIQobChMIlp6zzLW41QIVg7DtCh0OBw90EAYYBCABEgKZCPD_BwE

Like liquid rawlplug

EDIT: Probably over kill for you, used in our case as old wall and crumbly brick so was cracking with normal expanding bolts.

I’ve not come across thunderbolts before, had a quick google, So it’s basically a Self taping concrete screw…

Given the moment a bike rack could apply and the variability of substrates, I’d be sceptical about relying on an oversized woodscrew.

My choice would be either a good old fashioned Hilti* mechanical expanding anchor or perhaps their Resin anchor system…

*Other makes are available…

Whatever you use make sure it cannot be easily undone with hand tools, or the racks complete with bikes may go awol. Perhaps having the bolt heads below the finished surface then covering in concrete to disguise.

Concreting them is sounds the way to go.

Any type of concrete screws are only really good for static loads. They hold extremely well when the threads are perfect. As soon as there’s any flex on them they immediately start to loosen rapidly, the threads they tap are just too shallow.
If it was me I’d concrete them in directly. Failing that I’d use a resin fixing system; threaded bar and shear nuts of the biggest size possible. Make sure to clear out the holes with compressed air.
Fischer do a resin fix product that can be dispensed from a standard sealant gun, saves you buying the fancy heavier purpose built ones.

It’ll depend on what’s there. Concrete laid for a footpath might not support much at all. I’d start with a 12mm exploratory hole and make a decision from that, expanding anchors are probably the easiest (example linky)

I’d use expanding bolts, but see if you can get the type with recessed hex heads that are tightened up with an Allen Key. that way you can get a ball bearing slightly larger diameter than the Across Flats dimension of the recessed hex and hammer them into the bolt to prevent they from being removed.

Thunderbolts are great and would do the job, but not very secure.

I put my ground anchor into the concrete floor of my garage with an expanding bolt. That is quite literally never coming out again (apart from with an angle grinder)