Author Topic: Concrete screws.  (Read 3126 times)

Offline Simon Wilson

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Concrete screws.
« on: March 29, 2018, 03:21:47 pm »
Sorry for the delay in getting this report published but it's finally on that well known mine of caving information the wonderful CNCC website.

https://cncc.org.uk/fixed-aids/articles.php

Online SamT

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Re: Concrete screws.
« Reply #1 on: March 29, 2018, 03:37:16 pm »

Good work Simon  -   :thumbsup:

Online Ian Ball

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Re: Concrete screws.
« Reply #2 on: March 29, 2018, 06:18:35 pm »
 :thumbsup:


Offline nobrotson

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Re: Concrete screws.
« Reply #3 on: March 29, 2018, 06:57:11 pm »
Nice one. Do you need to use a washer with them when installing caving hangers? I assume yes...
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Offline Chocolate fireguard

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Re: Concrete screws.
« Reply #4 on: March 29, 2018, 07:50:48 pm »
 :thumbsup:
Any idea why the spread of the FBS results (12.9% of the mean) is much greater than that of the MMS ones ( 2.5% and 1.9% of their means)?
It doesn't seem to be due to  1 or 2 outlying results, although I do appreciate that it's possible to read too much into 6 tests on each size.


Online SamT

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Re: Concrete screws.
« Reply #5 on: March 29, 2018, 08:39:34 pm »
Nice one. Do you need to use a washer with them when installing caving hangers? I assume yes...
No I dont think so. Why would you out if interest? Just curious.

Online SamT

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Re: Concrete screws.
« Reply #6 on: March 29, 2018, 08:46:26 pm »
Types:

Plain washers, which spread a load, and prevent damage to the surface being fixed, or provide some sort of insulation such as electrical

Spring washers, which have axial flexibility and are used to prevent fastening or loosening due to vibrations

Locking washers, which prevent fastening or loosening by preventing unscrewing rotation of the fastening device; locking washers are usually also spring washers

Only one you might consider is a locking washer I guess but Im pretty sure the touque and friction on the bolt is enough to prevent the bolt unscrewing

Online Pete K

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Re: Concrete screws.
« Reply #7 on: March 29, 2018, 08:51:25 pm »
We've used nylon washers where the screw anchors are used with stainless plates in damp areas. They will be stripped out eventually and resin anchors placed instead but it's nice to know there won't be any nasty mixed-metal corrosion in the mean time.

Offline pwhole

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Re: Concrete screws.
« Reply #8 on: March 29, 2018, 08:54:34 pm »
I always use nylon washers with screwbolts and hanger plates now (probably the 'we' Pete's referring to!), as the bolts are always zinced in some way and the plates are always stainless. Most of the long-term maillons we've left in explorations eventually rust at the contact with the stainless. That said, I hung off two hangers on Tuesday that I installed two years ago with no washers, in a wet spot, and there was no evidence of any rust at all at the contact point. I wasn't able to unscrew them to check inside, but others I've left in long-term haven't had any at all.

Offline Mattrees

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Re: Concrete screws.
« Reply #9 on: March 30, 2018, 04:23:49 pm »
Thanks Simon,

That's really useful.

I'm using the 7.5 X 50 Multi Montis to maximise drill life. If you are doing any further tests would you mind testing a batch of these shorter anchors? I could post you some samples.

I'm expecting they would give similar figures since the file is the anchor strength rather than the rock.

Offline Simon Wilson

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Re: Concrete screws.
« Reply #10 on: April 03, 2018, 11:40:55 am »
Thanks Simon,

That's really useful.

I'm using the 7.5 X 50 Multi Montis to maximise drill life. If you are doing any further tests would you mind testing a batch of these shorter anchors? I could post you some samples.

I'm expecting they would give similar figures since the file is the anchor strength rather than the rock.

I think you're probably right. Send me some and we can find out.

Offline MarkS

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Re: Concrete screws.
« Reply #11 on: April 03, 2018, 11:44:33 am »
Super piece of work, and I hope it marks the beginning of the end for the use of throughbolts in caves. A big thanks for doing it and for writing it up.

I used some in anger for the first time this weekend.  :thumbsup:

Online SamT

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Re: Concrete screws.
« Reply #12 on: April 03, 2018, 12:43:10 pm »
I feel a little saddened that its taken this long for them to catch on!!

https://ukcaving.com/board/index.php?topic=6897.msg93205#msg93205

Online Wayland Smith

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Re: Concrete screws.
« Reply #13 on: April 03, 2018, 01:41:06 pm »
When installed correctly these are not torqued very high.

What needs to be done to prevent unfamiliar users of the rigging giving the bolts an extra tweak
"to make sure they are really tight."
Thus over stressing them with the risk of failure.  :-\

Offline MarkS

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Re: Concrete screws.
« Reply #14 on: April 03, 2018, 01:46:29 pm »
My problem with them in the past has been that I've seen/heard anecdotal evidence that they were great, but I've also seen/heard anecdotal evidence that they were bad. I'd seen little in the way of any test results with details of how they were carried out.

Offline Simon Wilson

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Re: Concrete screws.
« Reply #15 on: April 03, 2018, 03:40:19 pm »
When installed correctly these are not torqued very high.

What needs to be done to prevent unfamiliar users of the rigging giving the bolts an extra tweak
"to make sure they are really tight."
Thus over stressing them with the risk of failure.  :-\

Overtightening is a thing that a few inexperienced people have always done. I've come across bolts in Spits that have been greatly overtightened. I've even heard of people shearing them off by overtightening.

My advise to anybody wanting to use concrete screws is, if you have any uncertainty about how much to tighten them then do not use them until you have found out. They absolutely do not need to be and should never be "really tight". They should be tightened the same as a bolt in a Spit which is very lightly.

Offline droid

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Re: Concrete screws.
« Reply #16 on: April 03, 2018, 04:45:24 pm »
Is that sump drain bolt tight, ie nip up by hand then a quarter turn?
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Offline Mattrees

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Re: Concrete screws.
« Reply #17 on: April 04, 2018, 06:40:25 pm »
Is that sump drain bolt tight, ie nip up by hand then a quarter turn?

In my experience they won't cut a thread using just fingers (unless your fingers are much stronger than mine!). You need a spanner to place them, not just to nip them up at the end.

Online SamT

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Re: Concrete screws.
« Reply #18 on: April 04, 2018, 11:42:18 pm »
On the other side of the coin.. I gave some 8mm ones to a mate to equip a new climbing  route at Kinsley. The idea being to trial placement then replace with resin bolts whrn the route had been worked and the most ideal clipping points fine tuned. 

He came back saying the were shit and that he'd sheared a couple trying to get them to cut a thread.  I suspect the limestone there is bullet hard compared with the peak limestone 'cheese'. 

I've certainly struggled to get them to cut into hard red brick and sheared one.

Offline braveduck

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Re: Concrete screws.
« Reply #19 on: April 04, 2018, 11:49:36 pm »
Use them a lot in the garden/house. The secret is drilling the hole the right size ,
running drill in and out several times and making sure the hole is clear of dust.
If it gets tight when going in ,remove and and run the drill in and out a few more times.
I think they are brilliant.

Offline Simon Wilson

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Re: Concrete screws.
« Reply #20 on: April 05, 2018, 08:19:48 am »
On the other side of the coin.. I gave some 8mm ones to a mate to equip a new climbing  route at Kinsley. The idea being to trial placement then replace with resin bolts whrn the route had been worked and the most ideal clipping points fine tuned. 

He came back saying the were shit and that he'd sheared a couple trying to get them to cut a thread.  I suspect the limestone there is bullet hard compared with the peak limestone 'cheese'. 

I've certainly struggled to get them to cut into hard red brick and sheared one.

Your mate could easily be in the running for the Darwin Awards.

Kilnsey Crag is Great Scar Limestone which is a relatively uniform limestone of medium hardness (around 4 on the Mohs scale) and takes concrete screws very well, as my testing shows.

The secret is drilling the hole the right size ,
running drill in and out several times and making sure the hole is clear of dust.
If it gets tight when going in ,remove and and run the drill in and out a few more times.
I think they are brilliant.

There is no 'secret'. The manufacturers give instructons on installation. Just like any piece of gear, users should read the instructions. Anyone planning to use concrete screws should find out how to install them and only install them when they are confident that they know what they are doing. Practice in concrete before damaging our caves and crags.

In addition to what Dave said, as well as being the correct diameter and clean the hole needs to be sufficiently deep. Do not use the cheapo Chinese screws.

Offline Madness

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Re: Concrete screws.
« Reply #21 on: April 05, 2018, 12:59:01 pm »
I used Multi-Montis for investigating a raise in Slaley Sough. Only having to drill a 6mm hole made instalation very quick. You have to use a little pressure and force to get the thread to start cutting, but then you shouldn't need excessive force to drive the screw fully in. I used a 1/2" ratchet and impact socket rather than a spanner.

As Simon says, practice on a block of old concrete or house bricks before use underground. I used a piece of my garden rockery. You'll soon get the 'feel' for how much torque needs to be applied.

The only concern I've got with using them is that once loosened they need very little torque to unscrew. You can easily remove them without tools once slackened off.

Offline Simon Wilson

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Re: Concrete screws.
« Reply #22 on: April 05, 2018, 04:28:39 pm »
The only concern I've got with using them is that once loosened they need very little torque to unscrew. You can easily remove them without tools once slackened off.

They do unscrew very easily by hand once loosened but I don't think it is a problem to worry about. I have found it impossible to unscrew one by turning the hanger whilst there is a shear load. The friction of metal on rock is far greater than the friction metal on metal. Obviously normal safe practice needs to be used - Y hangs and back-ups.

Offline pwhole

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Re: Concrete screws.
« Reply #23 on: May 03, 2018, 01:40:18 pm »
I use Excaliburs, which have a very sharp thread and haven't had any issues cutting into any of the limestone or concrete I've used them on yet, even with just a hand-spanner as long as the holes are cleaned out. I've even screwed them into pure calcite (see below), but only as a temporary measure, and I wouldn't have liked to rely on those long-term. Once the bolt has been set it can be unscrewed and re-screwed by hand, more or less, once the final torque has been loosened. It does mean that you can check the bolt periodically if it's being left in for any length of time. I've deliberately left some in-situ in their original holes at the bottom of our shaft, in constant water, for over a year now, and there's no hint of any rusting. As I mentioned above, the zinc maillon/stainless hanger contact is far more likely to rust than the bolt itself.

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!
« Last Edit: May 03, 2018, 01:49:33 pm by pwhole »

Offline maxf

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Re: Concrete screws.
« Reply #24 on: May 03, 2018, 03:15:30 pm »
Thanks for testing Simon, somehow missed this thread until today.

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.

Online 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.

Online 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.