Author Topic: Concrete screws.  (Read 3967 times)

Offline PeteHall

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Re: Concrete screws.
« Reply #25 on: May 03, 2018, 03:33:34 pm »

The nasty shiny silver screwbolts we often get given at work with the purple tips are rubbish. Tried one recently in Castleton and it just jammed and snapped off - just as they did at work!

I used to get given the cheap nasty sort at work. When I first started using them, they sheared off quite regularly. I then found that doing them a bit at a time, cleaning the threads between, they seemed pretty reliable.

Still not sure I would trust a dodgy cheap bolt with my life though, even if you could get it in without breaking...
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Offline pwhole

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Re: Concrete screws.
« Reply #26 on: May 03, 2018, 03:54:34 pm »
That was the problem - we were bolting around the top (inside) of a 15m high concrete water-tower, built in the 1960s from pebbledash rubbish, and then hanging on the bolt with a Grillon to drill the next one, with another Grillon on the previous one. We had a drill that was too heavy (36V Hilti), bolts that were rubbish (so loads of failure holes and/or sheared bolts), and no rope being rigged along the bolts as we went. The second day I took my caving Makita drill and loads of Excalburs and flew round in half the time, and no confidence issues, as I know they're good. Still no traverse rope though. At the very least you can hang your tools from it instead of from yourself.

Offline andrewmc

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Re: Concrete screws.
« Reply #27 on: March 02, 2019, 12:29:58 am »
Bit of thread resurrection here:

I've been looking at the stats for concrete screws again, and your report Simon. Looking at the data from the Heco documents, the maximum allowed tension load for axial leaving for a 7.5/55mm Heco MMS is only 2kN for cracked concrete, but I can't work out what the safety factor is. That's not too disturbing given you got about 24kN for a slightly longer version. What worries me a bit more is that that length of 7.5 have the same rating for both cracked and non-cracked concrete at 3.6kN due to 'steel failure', a pattern which also appears for their longer of two lengths for each diameter. It looks like they pick the shortest length, for each diameter, where the bolts fail in shear due to steel failure making extra length irrelevant for shear loading.

Given that you didn't test a radial pull, is there a danger that they might give high strength values axially but all fail in shear at say 5kN?

I can see them being really useful for some things and much better from a conservation point of view on expedition; I'm just worried by the very low ratings. Maybe the safety factor are really high and everything is actually fine...

Offline Simon Wilson

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Re: Concrete screws.
« Reply #28 on: March 03, 2019, 05:28:26 pm »
Bit of thread resurrection here:

I've been looking at the stats for concrete screws again, and your report Simon. Looking at the data from the Heco documents, the maximum allowed tension load for axial leaving for a 7.5/55mm Heco MMS is only 2kN for cracked concrete, but I can't work out what the safety factor is. That's not too disturbing given you got about 24kN for a slightly longer version. What worries me a bit more is that that length of 7.5 have the same rating for both cracked and non-cracked concrete at 3.6kN due to 'steel failure', a pattern which also appears for their longer of two lengths for each diameter. It looks like they pick the shortest length, for each diameter, where the bolts fail in shear due to steel failure making extra length irrelevant for shear loading.

Given that you didn't test a radial pull, is there a danger that they might give high strength values axially but all fail in shear at say 5kN?

I can see them being really useful for some things and much better from a conservation point of view on expedition; I'm just worried by the very low ratings. Maybe the safety factor are really high and everything is actually fine...

You will find the most detailed information, such as safety factors used, in the ETAs. Get the ETA number and Google it or find the links to ETAs on the product webpages. They are in German but there are English translations. You need to look for the 'characteristic strength' which is a lot higher than the 'permissable', allowed' or 'recommended' load.

https://www.heco-schrauben.com/downloadcenter/downloadcenter/en/Approvals/EN_ETA_MMS-plus_15-0784.pdf

I didn't test the screws in shear because I was testing how they held in Great Scar Limestone. We don't need to test the steel or worry about its strength because the screws come from reputable companies with good quality control (Fischer and HECO) and who are happy to tell you where the screws are made unlike the other companies.

Offline andrewmc

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Re: Concrete screws.
« Reply #29 on: March 05, 2019, 01:09:59 pm »
Thanks, I hadn't found the English translations and I didn't know I was looking for the 'characteristic resistance'.

So looking at that, a 7.5-diameter anchor will fail at a shear load of ~6.1kN due to steel failure (and 13.6kN for the 10-diameter anchor)?

I think I've worked out that pryout failure is 1-2x the pullout strength depending on the length of the anchor (i.e. higher than the shear failure of the steel), using the pull-out strength (or 1-2x the concrete cone failure, which I haven't worked out).

I presume concrete edge failure isn't relevant (since we shouldn't be near edges/other bolts anyway)?

Edit: using the supplied k-factor 11.0 for cracked concrete, and the formula N = k * sqrt(concrete cubic compressional strength in MPa) * (anchor depth in mm)^1.5 (Newtons), and plugging in 25MPa concrete (to match the other things on the datasheet), I get 25kN for a 60mm anchor, so it seems concrete cone failure will still be irrelevant compared to the steel shearing?

So if you use the size 10 anchors (8mm drill bit?) in 65mm length, you get 13.6 kN shear from the steel and (in 50MPa limestone pretending to be cracked concrete) a 14.2 kN pullout (which pretty much matches), and you get to use a standard 13mm spanner and (with a bit of fiddling) 8mm hangers. Looks like that might be the way to go (possibly with longer bolts just for security in case of poor rock)...
« Last Edit: March 05, 2019, 01:20:32 pm by andrewmc »

Offline Simon Wilson

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Re: Concrete screws.
« Reply #30 on: March 05, 2019, 03:26:14 pm »
Thanks, I hadn't found the English translations and I didn't know I was looking for the 'characteristic resistance'.

So looking at that, a 7.5-diameter anchor will fail at a shear load of ~6.1kN due to steel failure (and 13.6kN for the 10-diameter anchor)?

I think I've worked out that pryout failure is 1-2x the pullout strength depending on the length of the anchor (i.e. higher than the shear failure of the steel), using the pull-out strength (or 1-2x the concrete cone failure, which I haven't worked out).

I presume concrete edge failure isn't relevant (since we shouldn't be near edges/other bolts anyway)?

Edit: using the supplied k-factor 11.0 for cracked concrete, and the formula N = k * sqrt(concrete cubic compressional strength in MPa) * (anchor depth in mm)^1.5 (Newtons), and plugging in 25MPa concrete (to match the other things on the datasheet), I get 25kN for a 60mm anchor, so it seems concrete cone failure will still be irrelevant compared to the steel shearing?

So if you use the size 10 anchors (8mm drill bit?) in 65mm length, you get 13.6 kN shear from the steel and (in 50MPa limestone pretending to be cracked concrete) a 14.2 kN pullout (which pretty much matches), and you get to use a standard 13mm spanner and (with a bit of fiddling) 8mm hangers. Looks like that might be the way to go (possibly with longer bolts just for security in case of poor rock)...

The figures they give for characteristic loads have a partial safety factor. You can calculate the strength of the steel by using the tensile strength figure in N/mm2. Even that is a minimum figure to comply with the designation of the steel.

BTW they are screws not bolts. It's a screw if it screws into something and a bolt if it has a nut.

Offline nobrotson

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Re: Concrete screws.
« Reply #31 on: September 20, 2019, 09:55:29 am »
A new technical report on concrete screws has been written by Rolf Siegenthaler of Scurion. It was presented at Sinterlaken this year, and is available to read here.
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