I finally got round to putting up the handrail I mentioned in a post a
week or two ago. The company I bought it from modified their standard
design after I sent them photos where it was to be fitted. It came with
eight M6 x 100mm Thunderbolt concrete screw anchors.

As many others here agree, Bosch multipurpose drill bits are great for
drilling into hard material. I've used them for drilling into concrete
lintels with rebars and they went through pretty easily. For the
handrail posts I needed to drill a 6mm hole at least 100mm deep, so
bought a couple of new drill bits, 6 x 150mm. They had hex shanks, so at
best would do only 120mm, and the flutes didn't extend all the way up.
They are /misdescribed/ he
https://www.screwfix.com/p/bosch-hex-shank-multi-construction-drill-bit-6-x-200mm/490jf.
As the reviews note, they are not 200mm long, but only 150mm. That was
ok for what I needed. The first four holes were in brick. The bits went
through very quickly, and it was easy to screw in the concrete anchors.

But the Bosch bits met their match with the next four holes, which had
to be made in 150mm thick C30 concrete. I could get only one hole 100mm
deep. Two got to 90mm, and the last one only about 75mm. I had been
drilling with an old 550W B&D hammer drill, so thought it best to get
out the 850W SDS Plus drill. This made no difference; the drill bit
remained stubbornly at 75mm depth. When I removed it from the hole I was
amazed. Photo here for comparison with drill bit used once:
https://ibb.co/mS8sfDj
End of blunt drill bit in close-up:
https://ibb.co/8xJs3kk

What in hell can do that to a tungsten carbide tip? I've seen them with
the cutting edge blunted after extended use although the "V" shape is
still there, but /never/ seen one where the carbide bit has been
completely flattened back to the shank.

I bought a small pack of M6 x 75mm concrete screws and put two of those
in with the 100mm one I could get in. For the other one I used a Dremel
with a disc cutter to take it down to 50mm, and I'm sure it didn't go in
properly and has smoothed some of the thread it cut. All were very
difficult to screw in, even using a 300mm Tommy bar. They seem, however,
to have fixed the handrail post pretty well.

Those concrete screws really are hard steel, and it took some effort
with the Dremel to cut one off. However, this is what the cut-off end
looks like after trying to cut a thread in that hard concrete (unused
screw for comparison):
https://ibb.co/KGdkk59
I have my doubts that the smoothed thread grips as well as one might
think. No doubt that some of the screw has all its thread complete near
the top, and that grips better, but are they really suited for use in
very hard concrete with lots of flints and stones?

--

Jeff