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Quasimodo Bolt Corrosion Investigation

Justin Lawson’s recent posting about him breaking a bolt on Quasimodo at The Steeple in Montagu raised a few eyebrows and flurry of activity to replace the entire route’s anchors.  (I considered maybe finding out what training / diet Justin was doing out in Montagu in order to emulate his superhero power).   I have done a bit of research with my engineering colleagues here at work and can share the following with you:

When Quasimodo and Gospel Express were rebolted, the bolts on Quasimodo broke much easier than its next door neighbour (Gospel Express).  They are both similar routes (ie slabs).  Visual inspection of the removed bolts shows that the Quasimodo bolts have suffered from normal wastage corrosion in the hole and they failed at the sleeve taper (ie thinnest part of the shaft).  These bolts have not suffered any Stress Corrosion Cracking (carbon steel is not susceptible to SCC, unlike stainless steel).  From what we can see the normal sleeve taper is 6.5mm in diameter.  The picture below shows a bolt that has corroded to a sleeve diameter of 4mm (GULP!).

Broken bolt from rock climb

Broken bolt from Quasimodo (with corroded sleeve at a diameter of 4mm!)

climbing_bolt_corrosion

Close up photos of the bolts from each routes

Montagu is a pretty dry area which is why carbon steel bolts do not exhibit much corrosion externally.  So what happened with Quasimodo?  This route is a slab so the bolts have been drilled at an incline into the rock.  Accordingly water was able to collect in the holes and corrosion was able to continue unabated.

But why was Quasimodo affected more than Gospel Express?  I strongly suspect the bolts in Quasimodo were of a poor quality material and may even had some inclusions.  The failure surfaces of the Quasimodo bolts exhibit some corrosion (immediate corrosion post failure or possibly corrosion of inclusions) while the Gospel Express failure surfaces look pretty healthy.

See the failure surfaces of the bolts in the picture to the right :

Everyone wants to jump on the glue-in band wagon and although glue-ins would prevent any water ingress, I really don’t think they are necessary for a dry area like Montagu.  Glue-ins are a mission and have different failure modes (e.g.  glue not mixing or people not cleaning the hole)

I was also very intrigued how Justin managed to break a bolt “easily” with a spanner. As a rule ARF has struggled to break bolts off, but like I said maybe Justin has a secret training program.  So below is a rough calculation on what the expected failure torques would be for bolts in various states of corrosion for a low grade steel (4.6):

torque chart rock climbing

The importance of material selection can be seen by comparing the ultimate tensile strengths of some other steels below:

Mild Steel (grade 4.6) – 400MPa
Stainless Steel – 600MPa
High Strength steel (grade 8.8) – 800MPa

I also did a little test with a 19 spanner to determine what kind of torque a burly young lad like myself could produce.

 

Results were:
Normal pull 20Nm
Hard pull 35Nm
Full body push 70Nm

So even a weak old man can break those bolts.

To conclude, these failures are simply caused by wastage corrosion that has been exacerbated by moist conditions in the hole and a poor quality material.

Lessons learnt from this little escapade are:

If you’re going to equip a route, do it for life and buy good quality equipment.  (I know for a fact that some high profile climbers in Cape Town are still using the cheap UPAT bolts that break).
And for my up country friends, I remember a similar issue with corrosion on Fabergé so please think carefully about not using good quality stainless steel anchors. (If you’re gonna by a rope, are you going to buy an expensive one with a good reputation and quality assurance or some cheapie from the boat shop?)

ARF recommends the following 90mm long, 10mm diameter 316 stainless steel anchors :

  • Hilti HSA-R M10x90/20/25
  • Fischer FBN 10/15+23 A4

If possible try and drill bolt holes with a slight downward incline so that water can drain out the hole.

Andy Davies (BSc Mech Eng UCT)
MCSA – Anchor Replacement Fund


Montagu Bolting Fund:

Anyone who wants to support the Montagu Bolting Fund can deposit donations into the account below – I’ve opened this savings account specifically for bolting in and around Montagu.  I will keep a spreadsheet record of what was received by whom as well as outgoing expenditures.
Please email me the deposit slip & you are also welcome to include suggestions of whether you would like the funds to go to re-bolting or new routes.
Climb ZA’s emphasis right now is to bolt “lower grade” routes at easily accessible crags as this grows the sport.

justin@climbing.co.za

Cheers,
Justin

BANKING DETAILS
Bank: Standard Bank
Account Name: Lawson
Account Number: 07 616 676 7
Account Type: Pure Save
Branch: Constantia
Branch Code: 051001

 

Bolt Corrosion Investigation

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8 Responses to Quasimodo Bolt Corrosion Investigation

  1. Paul Oct 4, 2011 at 4:51 pm #

    Thanks very much Andy
    and I take it the slight incline downward you speak of would mean that the end of the bolt (90mm deep in the rock) is just slightly higher than the bolt end protruding out of the rock ?
    cheers

  2. Justin Oct 4, 2011 at 5:56 pm #

    The only problem with that is that the hanger sits skew and is levered (so is the bolt) when loaded – more so if you’re bolting a slab!?
    My understanding was that you wanted the back of the hanger flush with the rock?

  3. paul b Oct 5, 2011 at 11:12 am #

    I think Andy means look at the rock surfaces. Often there will be a placement point where water is unlikely to collect. Yea? Don’t think you want to be placing bolts that aren’t flush with the rock.

  4. Justin Oct 5, 2011 at 1:16 pm #

    Tx Paul. I know what you mean. I just bolted another route at the Steeple which is on a slab, its about grade 8 so very hard to find such placements :)
    Let’s see how the 316 Stainless Steel bolts hold up!

  5. Mark Oct 5, 2011 at 6:18 pm #

    How do glue ins compare? I was wondering if the glue protects the bolt from water / potential rust?

  6. Grant Oct 6, 2011 at 7:08 pm #

    Thanks for that. A very sensible analysis with the required info spelled out. As one would have observed it is a moot point that the grade 8.8 has a higher strength than the stainless, if the bolt rusts/ erodes away to such small diameters.

    What sort of time span did this occur over I wonder, more as an argument for just putting in stainless steel bolts in the first place, as you recommend.

    As for the glue protecting the carbon steel, unless you are going to coat the entire bolt the corrosion notch point will just move to where the glue stops & the carbon steel is exposed.

    Maybe we should invent an attachment point that is not prone to a shearing action :-)

    Well done Arf (arf)

  7. Andy Davies Oct 9, 2011 at 7:52 pm #

    Herewith some replies:

    1] Paul as long as the hole drains the water its all good. ie inside higher than outside thread.
    2] Grant – anybody using glue-in carbon steel bolts is being silly. If you’re gonna go through the ball-ache of using glue-ins then do a proper job.

    Cheers

  8. Jayson Aug 1, 2012 at 8:35 am #

    WRT angle into the rock: it is often possible to find a small bulge on the rock such that if the bolt is placed on the front of the bulge, maybe slightly below the maximum protrusion, then you can drill at a very slight upwards angle. Then your hanger will be flush on the rock face. This also prevents potential biner-bend as the draw will hang more cleanly. (also makes it easier to find bolts sometimes…!)

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