Author Topic: What cement?  (Read 718 times)

Offline mole02

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What cement?
« on: September 19, 2019, 07:19:26 pm »
Hi all, having never used cement/concrete underground, I would appreciate any advice?
Having to cement boulders back in to whence they came to fill a hole.
Do you just use standard 4:1 cement or is a concrete mix better?
They won't be near water but obviously the cave is a damp environment so anything special need to be done?
Many thanks in advance, Mark

Offline Pitlamp

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Re: What cement?
« Reply #1 on: September 19, 2019, 08:22:11 pm »
I've used concrete and also standard sand and cement in a show cave situation (e.g. to repair paths); it goes off fine, just as normal. Without seeing your job I'm tempted to suggest concrete is more appropriate than just sand and cement.

Two bits of advice I'd offer; firstly, if it needs to be strong, get a bag of nylon fibres to mix in. All builders merchants should have this. (It must be very well mixed of course, to ensure even distribution of all components of the mix.) Depending on the nature of the job you might also consider incorporating some steel reinforcing. Some rudimentary shuttering may be advisable if the surface is steep, or the mix may well sag and therefore not be in the right place.

Secondly, in a cave you usually need to protect it from drips by covering with a tarp or polythene sheet, preferably for (at least) several days. If there are any drippers they'll make big holes in the concrete. (Full curing of concrete takes about 4 weeks - it's a chemical reaction and not just a case of water evaporating.)

Hope that helps - I'm sure others will offer additional advice.

Offline alastairgott

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Re: What cement?
« Reply #2 on: September 19, 2019, 09:20:49 pm »
If you're using sand at all, Grey sharp sand rather than red sharp sand (builders) ;)

(unlike squirrels grey is better in this case)
Scraps and Maps, Scraps and Maps, Squawk, pretty polly...

Offline mrodoc

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Re: What cement?
« Reply #3 on: September 19, 2019, 11:14:08 pm »
Currently we are using basalt gravel and sand with cement in a 2+1+1 ratio and it is working fine. Why basalt? Because there are piles of it near our dig.

Offline Pitlamp

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Re: What cement?
« Reply #4 on: September 20, 2019, 07:10:35 am »
Are those basalt chips fairly angular? If so, that should certainly help with the strength of the concrete.

Offline pwhole

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Re: What cement?
« Reply #5 on: September 21, 2019, 01:11:39 am »
If you're using sand at all, Grey sharp sand rather than red sharp sand (builders) ;)

(unlike squirrels grey is better in this case)
No-one wants taramasalata on limestone  :-[

Recently I had to mix some concrete at the bottom of our shaft and realised we'd run out of aggregate. Casting around I realised that there was abundant 'aggregate' all around me in the form of mining gravel waste, so scraping up a few handfuls, and picking out the big lumps gave me a lovely sparkly blend of calcite, fluorite and baryte fragments, with plenty of limestone chippings for good measure. Sadly trowelling it down left no hint of the rich mixture inside, but it felt good to be using the materials available - and it should certainly be strong enough.

Offline mole02

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Re: What cement?
« Reply #6 on: September 21, 2019, 07:15:30 am »
Ah that's great advice, many thanks fellas.
It does need to be strong so I think I'll go down the concrete route maybe with some fibres? (Never heard that before but sounds logical). Going to need a fair bit so probably some steel reinforcing also.
Many thanks once again, Mark  :clap2:

Offline The Old Ruminator

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Re: What cement?
« Reply #7 on: September 21, 2019, 10:10:07 am »
Are those basalt chips fairly angular? If so, that should certainly help with the strength of the concrete.

P9170038 by Nicholas Chipchase, on Flickr


We have scaffolding and mesh behind the wall which has been added to make things look more natural. Having over burdened Vurley with 100m of scaffolding we are trying in our present dig to make things more aesthetically pleasing. The basalt sand is quite fine. A concrete mix is used for packing but sand and cement is needed for courses. The basalt sand/cement dries the same colour as limestone.

Offline TMan

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Re: What cement?
« Reply #8 on: October 03, 2019, 10:28:55 am »
A 6l curver drum full of cement fits exactly a 1.5 liters bottle of water. Very portable and waterproff, for smaller applications at the end of universe....cheers.