A friend of mine has a hypothetical construction/demolition problem.

He needs to break through a concrete floor in an outbuilding to sink wooden fence posts in for a construction project. Up until now he's been using an electric 29 kg jack hammer to break through the concrete perfectly. However the jack hammer is gone now and there are still a few holes to sink. He considers it a perfect opportunity to actually do something useful with explosives that works out cheaper than hiring the drill again for a whole day.

Anyway he needs to know what people's opinions are on how much explosive to use and how to position it properly for the most efficient blast.

The specs are :

AN/NM is the only high velocity 'shattering' explosive he has available.

The floor is about 2 - 3 inches of non reinforced conrete sitting on about a foot of semi compacted hardcore of various sizes. This is sitting on dry clay.

He need to simply shatter the concrete over about a 12 inch diameter circle. This is what he was doing with the drill and it was a relatively simple matter afterwards to dig the shattered bits out and dig through the hardcore.

He plans to simply contain the AN/NM charge in a ziplock bag, place the charge on the floor and tamp it from above with a heavy sandbag.

So how much will he need to use to shatter the concrete? And is his thinking on the placement and containment correct? Obviously he wants to minimise the noise and use as little explosive material as he can

Thanks in advance
HVD

p.s. I'll pass all your answers on to him :)

This sounds like a job for a shaped charge if you want a clean hole. The explosive in a baggie will make a irregular hole and crack the concrete up to several meters.

Well, it's not a clean hole he wants exactly, it just to crack the conrete enough so that it can be hauled up with a pick axe/prybar etc. Once the concrete is out of the way then the hole can be dug.

If amount x of H.E. can crack the concrete over a few metres, like you say, then that's exactly what he wants but over a smaller area, so amount X/10 or something could be used. He's just looking for an approximate amount. Like 200 grams? 500 grams?

With the jack hammer he just broke the concrete up over a small area and dug the hole, he's now just looking to do the same - brak the conrete up over a small area. The edges etc don't need to perfect because the hole's going to be filled with concrete again anyway.

Cheers
HVD

Drill a hole in the concrete, and fill it with ANNM. Then do something crazy such as setting a plastic cup or small plastic bowl of ANNM ontop of the hole. center prime much as you would with a SC, and then tamp with bags of wet dirt. Now filling a drilled hole with ANNM MAY be a bad idea since pieces of debris will fly... but then again they would anyways. The idea is to throw force down into the concrete and at the same time expand it to shatter it from two angles at the same time.

No can do. If he still had the jack hammer then he'd be able to do it without the explosives. Once a hole is made you can lever up chunks and break then off - the problem is just making the hole.

There hasn't been much response to this so I'm just going to tell him to go ahead with his aforementioned method and see what happens. I'll reccommend he starts with 200 gr and work up from there i suppose.

I'll photograph what i can and post it here with details if the forum wants it.

HVD

Making a hole with a drill souldnt be TOO much of a hastle... Oh well. I'd like to see some pictures if at all possible

I know you want to have fun. But a heavy sledge and a couple of minutes work will make a hole in 2-3" of non reinforced concrete, and it probably won't blow the windows out, or otherwise damage the structure. I'd just outline the hole with a series of hammer drill hole every couple of inches and start whacking. But that would be too efficient and boring...

Surely you've heard the saying "There is no problem that cannot be solved with the application of high explosives"... :p

Then there is another...

"When you have a hammer, everything looks like a nail" ;)

EP-
The corrolary is: With enough HE, any problem will go away!

As far as the hammer thing, you should see my "tool" collection...
(damn, the ability to attach pictures has gone away again-)

Whats the point in using a explosive for construction? sure use explosives for DEMOLITION!
teehee!

Id advise just going out and renting the tool again, as it is obvious he/you dont really know what the hell he/you is/are doing. If you read the post,
http://www.roguesci.org/theforum/showthread.php?s=&threadid=3059
it shows what happens when people dont know what they are doing with explosives.

Explosives should not be messed with unless you are 100% certain you know what you are doing.

Let me be clear about this : it is virtually impossible to break through the flooring with hand tools. Trust me - we've tried very hard before going ahead and renting the jack hammer. The jack hammer got most of the work done but the jack hammer was only rented for two days and that wasn't enough time. There arn't enough holes left to warrent getting the jack hammer back again (transport and cost issues).

Drilling holes around the circumference of the hole with an ordinary masonary drill and smashing the 'disc' apart is a good idea, but I'm still keen to try explosives first and the former option will take much longer. (but it will be safer, more cost effective, get me fitter, and be about 80% less fun :-)

Anyway I'll let you all know how it goes.

Cheers
HVD

Dex-o-pan.......................................damn 30 character limit!

What is "Dex-o-pan"? Yes, I did search the forum and google. Oddly, this thread didn't come up in the search of the forum.

http://www.baustoffchemie.de/en/db/non-explosive-demolition-agent-patents/

:)

Non explosive blasting agent- The ones I've got litterature for specify a minimum slab thickness of 8", else it just pushes out of the hole rather than cracking the slab. They also specify that there must be a central hole for the expansion to take place towards in scenarios similar to the one described here- If you just start off in the middle, you'll crack the slab from side to side.

HVD, if you're still going ahead with this, I'd start out with less than 200gm - that could make quite a mess!

I have 50gm in my head, around about a film can's worth. Tamped with a sandbag this should be enough.

Let us know how you get/got on...

A good way to "cut" concrete might be a cutting torch or similar, along with a bucket of water. Wear eye protection, and ventilate well. Heat the concrete and it will "spall" all over the place. Get it really hot, then throw a bucket of water on it, and it will shatter lots off the surface.

Be careful, though. Thermite might work, too.

It should be pointed out that thermite idea is likely to end up in concrete exploding (the top ~2cm of concrete are likely to shoot off) and sending your thermite all over the place. The same is to be expected with a blowtorch, except that hot concrete will probably feel better on your skin than molten metal. Not all concrete will burst, but enough do to keep that in mind. You could of course, start a wood fire that will contain itself for a while and have a bucket of water hanging over it...

Or just use an explosive charge? Why bother with this fire /thermite nonsense. No garentee it will work, but explosives, well you can't go wrong! Besides if it blows to much away you can always fix it with more concrete, always a plus. Forgot to add, the thermite will leave nasty scorch marks on the concrete.

Originally posted by Desmikes
You could of course, start a wood fire that will contain itself for a while and have a bucket of water hanging over it...

I hope you don't mean that the fire will burn through the bucket..

Thermite might work. Wet the concrete with a water hose instead of a bucket though so that you can spray and crack just the parts wanted broken.

Of course it goes without saying that the longer and hotter the fire burns, and the colder the water is, the more the concrete will crack. If the entire thing should be cracked, setting up a 55 gallon barrel full of ice water on one side of the slab, with a very long rope tied to it would work well. After the fire dies out (very large fire) you and a friend could pull the rope quickly and spill a large amount of quite cold water over the extremely hot concrete. The length of the rope would keep you out of harms way as far as flying pieces of concrete goes. This way you don't have to worry about having a very large explosively driven concrete claymore, and you don't have to deal with the noise, and possibly pissed off neighbors. It also costs less than making explosives since water, ice, and wood to burn all come at a very cheap price when compared to AN and NM.

If you have them lying around, try using a thick cold chisel and a sledgehammer. Start by knocking the cold chisel in with a lump hammer first, then hitting it in with the sledgehammer once it's in enough to stay upright. This method is rather dependant on the hardness of the concrete, but drilling a pilot hole with a masonry drill helps. After the cold chisel is buried to a suitable depth, you can start knocking it sideways to break through the concrete.

There's the chance that you might end up with a slightly bent cold chisel :rolleyes:

Alternatively, you could just bash away at the concrete for a while. It'll break eventually. As long as the underlying clay isn't too wet, it won't absorb the impact from the sledgehammer too much. If the clay is wet, as in my back yard, you'll have a hell of a time using a sledgehammer.

EDIT - Whoops, I forgot you mentioned that hand tools had litle effect...

Is the cost of the building (if something goes wrong) more than the cost of renting the jackhammer again?

I dont think explosives are best suited here, you could crack the entire floor, or damage the walls of the room. But if you must... I'd suggest using 200-250g with several old pillow cases filled with damp sand or loose soil on top.

THErAPIST, I just didn't realize that I was being too vague and that I would have to make myself any more clear about the whole bucket over fire thing. I figured it goes without saying that you would need a rope to tilt it over� Thanks for further elaboration

HVD,

any progress? Surely you have attacked the floor again by now?

Sorry for the long delay with updates.

In short, no explosive progress. Realistic methods presented themselves, namely :

* decide to make only 2 more holes.
* borrowed heavy duty masonary drill with 10 mm SDS locking bits.
* drilled many holes right through, followed by persuasion with sledge hammer and pry bar.

Took ages, but it got the job done. Surprisingly effective actually. Before i get flamed with "Why not use that in the first place?" i should mention that the masonary drill wasn't available at the time of writing.

Cheers, HVD.