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Author Topic: What explosives are used to demolish buildings?  (Read 23966 times)

Offline itsafreeman

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What explosives are used to demolish buildings?
« on: 04/02/2009 08:21:34 »
 >:(I'm having great difficulty in finding out some basic information for a personal research project.  What explosives are commonly used in controlled demolition?  Is thermate, or any variation of thermite, ever used in controlled demolition as a cutting charge?  What explosives are specifically used for pulverizing concrete? ;)

« Last Edit: 05/02/2009 14:27:44 by chris »


 

lyner

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Re: What explosives are used to demolish buildings?
« Reply #1 on: 04/02/2009 10:02:54 »
AFAIK thermit is not used as an explosive. It is a rapid reaction which can melt / weld steel but I didn't think it was fast enough to 'explode'.
 

Offline LeeE

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Offline dentstudent

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Re: What explosives are used to demolish buildings?
« Reply #3 on: 04/02/2009 13:38:00 »
Or you could have a non-explosive method. Cue the great Fred Dibnah
 

Offline lancenti

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Re: What explosives are used to demolish buildings?
« Reply #4 on: 05/02/2009 14:23:11 »
I recommend asking the Blasters:

http://www.discoverychannel.co.uk/machines_and_engineering/the_blasters/index.shtml

But if I remember right, they still use TNT.
 

Offline L_D

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What explosives are used to demolish buildings?
« Reply #5 on: 16/02/2009 23:14:37 »
>:(I'm having great difficulty in finding out some basic information for a personal research project.  What explosives are commonly used in controlled demolition?  Is thermate, or any variation of thermite, ever used in controlled demolition as a cutting charge?  What explosives are specifically used for pulverizing concrete? ;)






Because they don't need an oxygen supply, thermite and thermate are used in underwater demolitions.
 

Offline LeeE

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What explosives are used to demolish buildings?
« Reply #6 on: 17/02/2009 20:22:49 »
All explosives incorporate their own oxidser.  The only really effective way to get a proper explosion using atmospheric air is by using an aerosol, and even then, because the aerosol has to be dispersed over a great enough volume to get enough oxidiser, it's not really suitable for demolition work.  This is how fuel-air bombs work though, essentially reproducing what goes on in the cylinder head of an internal combustion engine, but on a much larger scale.

Thermite isn't really an explosive, but burns relatively slowly while generating very high temperatures.  It can be focussed and used for cutting but is more often used for welding; shaped-charges are more likely to be used for general demolition cutting tasks.  Thermite is not to be confused with Thermic Lances, which are also used for demolition  cutting, although the principle of using the rapid oxidisation of a metal to get very high temperatures is the same in both cases; Thermite typically burns aluminium whereas the Thermic Lance usually burns iron.

Thermate is basically Thermite with a few other things added to generate a flame and reduce it's ignition temperature; Thermite needs very high temperatures to ignite.  Because of the flames it produces, Thermate is used in incendiary devices.
 

Offline L_D

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What explosives are used to demolish buildings?
« Reply #7 on: 17/02/2009 23:57:13 »
Thermite isn't really an explosive..


Is nano-thermite, or superthermite classed as an explosive? The reaction is certainly quicker.

http://en.wikipedia.org/wiki/Super-thermites [nofollow]
 

Offline LeeE

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What explosives are used to demolish buildings?
« Reply #8 on: 18/02/2009 13:51:43 »
I would say not, because the reaction is still primarily just exothermic i.e. primarily producing heat.  The key feature of an explosion is the sudden increase in volume resulting from the rapid release of gases, which isn't a feature of the Thermite type exothermic reactions.

The wiki page on exothermic reactions differentiates between the two:

"Where an exothermic reaction causes heating of the reaction vessel which is not controlled, the rate of reaction can increase, in turn causing heat to be evolved even more quickly . This positive feedback situation is known as thermal runaway. An Explosion can also result from the problem." [my empthasis]

And while heat is usually a feature of explosions it's not a prerequisite; when you shake up a bottle or can of fizzy drink and then release the pressure by opening it you're creating an explosion, albeit, a rather wet one.  This is exactly the same as a volcanic explosion, except the fluid you're dealing with isn't hot.
 

Offline lightarrow

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What explosives are used to demolish buildings?
« Reply #9 on: 18/02/2009 15:40:27 »
Thermite isn't really an explosive..


Is nano-thermite, or superthermite classed as an explosive? The reaction is certainly quicker.

http://en.wikipedia.org/wiki/Super-thermites
Many of those compositions of the link react in an explosive way (not simply "very very fast").
 

Offline LeeE

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What explosives are used to demolish buildings?
« Reply #10 on: 20/02/2009 17:18:30 »
Where does it say that many of those compositions react in an explosive way in that link? 

It only states that one of the compounds listed there has a much higher pressurisation rate than the others, which are all much slower.  Having a high pressurisation rate still doesn't mean that it's volume increases by a large enough factor that it qualifies as an explosion, only that the gas that is produced is produced relatively quickly.
 

Offline lightarrow

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What explosives are used to demolish buildings?
« Reply #11 on: 20/02/2009 18:15:37 »
Where does it say that many of those compositions react in an explosive way in that link? 
You are right, the link doesn't (directly) say it, but I say it  :)
(Personal experience. One of those compositions is one of the most powerful explosives you can make. Doesn't matter how you qualify it.)

Edit: if you follow the link "pressurization rates" you find that it's translated into "Brisance" which is a term used for high explosives.

About the other compositions, think that Al-CuO when properly made (very fine, dried powders, well mixed) is almost as fast as flash powder (still by personal experience), so, if it's only after Al-MoO3 in rapidity, it's quite straightforward to conclude that this last composition is damned fast.  :)

I've found a video:
hl=it
« Last Edit: 20/02/2009 18:35:48 by lightarrow »
 

Offline LeeE

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What explosives are used to demolish buildings?
« Reply #12 on: 22/02/2009 16:13:37 »
I think you need to check the definition of 'explosion', the key feature of which, is a large increase in volume, usually due to the release of large quantities of gas, resulting in a shock wave.  This does not happen with exothermic reactions, regardless of how fast the reaction.

The wiki article to which you refer does indeed redirect from 'pressurisation rate' to brisance because the two are related.  They are not however, the same thing and 'pressurisation rate' is not translated in to 'brisance'.  Brisance is an effect of explosives but is most certainly not a term used for explosives.  If the pressurisation rate of a reaction that results in a large increase in volume is rapid enough, as is the case with explosives, it will have a brisant effect due to the shock wave produced by the rapid increase in volume.

Now the wiki article on 'super-thermites' says:

"Of the listed compositions, the Al-KMnO4 one shows the highest pressurization rates, followed by orders of magnitude slower Al-MoO3 and Al-CuO, followed by yet slower Al-Fe2O3"

Note that it just says one of these compounds has a much higher pressurisation rate than the others and specifically goes on to say that the other pressurisation rate of the others are orders of magnitude slower.  Note also, that the only reference to explosives in that article is where it is stated that MICs can be added to explosives, clearly differentiating between the two.

In the end though, it doesn't really matter how fast the pressurisation rate is if it doesn't produce sufficient gas to expand it's volume enough to cause a shock wave.

And yes, I not only followed the 'pressurisation rate' link but also looked at other relevant entries, including explosion.

I'm afraid that I'm not prepared to accept something as true just because you say so, especially when you try to claim things like 'pressurisation rate' means 'brisance', and that 'brisance', in turn, means high explosives, both of which are totally incorrect.
 

Offline rosy

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What explosives are used to demolish buildings?
« Reply #13 on: 22/02/2009 17:21:32 »
I don't think the thermite reaction gives an explosion per se, in that the reaction itself doesn't generate gases.. however, I can see that it might be useful as an explosive underwater if the reaction were easier to initiate (in a controlled fashion etc) than "conventional" nitrogen/oxygen/carbon containing explosions since it's pretty exothermic and might well cause the sudden production of large volumes of steam if set up correctly.
No idea whether it would indeed be easier to use under water, but if it's used it's presumably thought to have some advantage...
 

Offline lightarrow

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What explosives are used to demolish buildings?
« Reply #14 on: 23/02/2009 16:01:04 »
I think you need to check the definition of 'explosion', the key feature of which, is a large increase in volume, usually due to the release of large quantities of gas, resulting in a shock wave.  This does not happen with exothermic reactions, regardless of how fast the reaction.
Here I don't understand if with "exothermic reactions" you mean a specific class of exothermic reactions, because it's obvious that an exothermic reaction could be an explosive reaction.

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The wiki article to which you refer does indeed redirect from 'pressurisation rate' to brisance because the two are related.  They are not however, the same thing and 'pressurisation rate' is not translated in to 'brisance'. 
Here you can know more than me, because I'm not totally familiar with english language, especially for such technical definitions (by the way, what does "pressurisation rate" mean, instead? I can't find the italian translation for it).

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Brisance is an effect of explosives but is most certainly not a term used for explosives.  If the pressurisation rate of a reaction that results in a large increase in volume is rapid enough, as is the case with explosives, it will have a brisant effect due to the shock wave produced by the rapid increase in volume.

Now the wiki article on 'super-thermites' says:

"Of the listed compositions, the Al-KMnO4 one shows the highest pressurization rates, followed by orders of magnitude slower Al-MoO3 and Al-CuO, followed by yet slower Al-Fe2O3"

Note that it just says one of these compounds has a much higher pressurisation rate than the others and specifically goes on to say that the other pressurisation rate of the others are orders of magnitude slower.  Note also, that the only reference to explosives in that article is where it is stated that MICs can be added to explosives, clearly differentiating between the two.

In the end though, it doesn't really matter how fast the pressurisation rate is if it doesn't produce sufficient gas to expand it's volume enough to cause a shock wave.
I totally agree with you, so who tells you that Al/KMnO4 mix doesn't generate gas? Which are the reaction products (apart from Al2O3 and metallic Mn)?  :)
Anyway, if you don't believe me, then do the experiment, where is the problem?

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And yes, I not only followed the 'pressurisation rate' link but also looked at other relevant entries, including explosion.

I'm afraid that I'm not prepared to accept something as true just because you say so,
Not even I would like something of this kind. What I intended with that statement is that I haven't read somwhere or deduced in some way that Al/KMnO4 is an explosive mixture, but that I know because I made the experiment. But of course I really hope that you don't believe me just because it's me to say it... :)

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especially when you try to claim things like 'pressurisation rate' means 'brisance',
See up

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and that 'brisance', in turn, means high explosives, both of which are totally incorrect.
Wiki says:
http://en.wikipedia.org/wiki/Brisance
<<A brisant explosive is one in which the maximum pressure is attained so rapidly that a shock wave is formed, and the net effect is to shatter (by shock resonance) the material surrounding or in contact with the supersonic detonation wave created by this explosive>>
Isn't the term "Detonation" used for high explosives?

See also:
http://www.britannica.com/EBchecked/topic/265202/high-explosive
"Basically, chemical explosives are of two types: (1) detonating, or high, explosives and (2) deflagrating, or low, explosives."

http://www.chimicando.it/e-book/%5Bebook%5DHigh%20Explosives%20And%20Propellants.pdf
"Of other tests which are sometimes applied to high explosives, mention should be made of the tests for brisance".
 

Offline lightarrow

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What explosives are used to demolish buildings?
« Reply #15 on: 23/02/2009 16:08:37 »
I don't think the thermite reaction gives an explosion per se, in that the reaction itself doesn't generate gases
It depends on what you mean with "thermite reaction". If it's the same as I mean, than I agree with you, but if you mean something else, and in that "something else" definition Al/KMnO4 is a thermite mix, then it's false, because that mix DOES generate gas. Do you see which gas it is?
« Last Edit: 23/02/2009 16:16:16 by lightarrow »
 

Offline LeeE

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What explosives are used to demolish buildings?
« Reply #16 on: 23/02/2009 20:32:15 »
Hello Lightarrow

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Here I don't understand if with "exothermic reactions" you mean a specific class of exothermic reactions, because it's obvious that an exothermic reaction could be an explosive reaction.

I think that should be the other way around; an explosion could be due to an exothermic reaction, but not all explosions are exothermic.  Volcanic explosions, for example, involve hot material but the process causing the explosion does not generate heat, so it is not exothermic.  It is an explosion though, because of the large increase in volume.

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The wiki article to which you refer does indeed redirect from 'pressurisation rate' to brisance because the two are related.  They are not however, the same thing and 'pressurisation rate' is not translated in to 'brisance'.

Here you can know more than me, because I'm not totally familiar with english language, especially for such technical definitions (by the way, what does "pressurisation rate" mean, instead? I can't find the italian translation for it).

Ah - fair enough.  I apologise for being so aggressive; your English was good enough to mislead me in to believing that it was your first language.

I wouldn't say that 'Pressurisation Rate' is an especially technical term, any more so than 'breathing rate'.  'Rate' just means the speed with which something happens, in this case referring to the speed that the pressure increases.  If the pressure was decreasing, the speed with which it was decreasing could be called 'Depressurisation Rate'.

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...who tells you that Al/KMnO4 mix doesn't generate gas? Which are the reaction products (apart from Al2O3 and metallic Mn)?   :)

Heh - I'm not a chemist but, the primary purpose of the 'O' is to be the oxidiser.  Now this compound may include excess 'O', in which case it will release the excess 'O' as gas, and even if there is not excess 'O' in a thermite reaction it's likely that some 'O' will still be released as gas due to the reaction being imperfect.  The important point is though, that there will still be insufficient unoxidised 'O' released from the reaction to qualify as an explosion, at least in terms of the total amount of material used in the reaction and when compared to the magnitude of the primary exothermic reaction; if the reaction involved a huge amount of material there would be a corresponding increase in the amount of excess gas that is released and it might then be enough to qualify as a small explosion, but the magnitude of the explosion would be tiny in comparison to the magnitude of the exothermic reaction.  Thus, the predominant type of reaction, by a large degree is exothermic and not explosive.  With an explosive reaction this ratio is the other way around.

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Isn't the term "Detonation" used for high explosives?

Detonation is another word used in connection with explosions, but doesn't mean explosive [material].  Detonation is the word used to describe the explosive process.  From the wiki article:

"Detonation is a process of combustion in which a supersonic shock wave is propagated through a fluid due to an energy release in a reaction zone."

'Brisance' is the word used to describe the breaking, or shattering effect of an explosion.

Apologies once again, for misinterpreting some of what you said as being intentionally misleading.
 

Offline lightarrow

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What explosives are used to demolish buildings?
« Reply #17 on: 24/02/2009 17:00:07 »
I wouldn't say that 'Pressurisation Rate' is an especially technical term, any more so than 'breathing rate'.  'Rate' just means the speed with which something happens, in this case referring to the speed that the pressure increases.  If the pressure was decreasing, the speed with which it was decreasing could be called 'Depressurisation Rate'.
Ok, the fact wiki linked the term "pressurization rate" to the page discussing "Brisance" mislead me.

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...who tells you that Al/KMnO4 mix doesn't generate gas? Which are the reaction products (apart from Al2O3 and metallic Mn)?   :)
Heh - I'm not a chemist but, the primary purpose of the 'O' is to be the oxidiser.  Now this compound may include excess 'O', in which case it will release the excess 'O' as gas, and even if there is not excess 'O' in a thermite reaction it's likely that some 'O' will still be released as gas due to the reaction being imperfect.  The important point is though, that there will still be insufficient unoxidised 'O' released from the reaction to qualify as an explosion,
The unoxidized O is not the only source of gas in that reaction: there are K and K2O in the gaseous form too, at those teperatures.
In the case of Al/CuO instead, the unoxidized O is the only gas I can think about, (unless we also want to consider the air present in the mix' bulk) but I'm still wondering if this is the real reason of the "explosive" reaction in that case, since I made experiments with well measured stoichometric quantities and very well mixed powders and the reaction doesn't become less "explosive", on the contrary. It's something I still have not understood well.

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at least in terms of the total amount of material used in the reaction and when compared to the magnitude of the primary exothermic reaction; if the reaction involved a huge amount of material there would be a corresponding increase in the amount of excess gas that is released and it might then be enough to qualify as a small explosion, but the magnitude of the explosion would be tiny in comparison to the magnitude of the exothermic reaction.  Thus, the predominant type of reaction, by a large degree is exothermic and not explosive.  With an explosive reaction this ratio is the other way around.
But I'm wondering: if the amount of gas released is not so much, but the gas' temperature is so high that it is released with an extremely high pressure, shouldn't it be considered as an explosion too?
 

Offline LeeE

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What explosives are used to demolish buildings?
« Reply #18 on: 24/02/2009 19:00:07 »
Regarding the chemical aspect of this, I'm out of my depth, but I think the key thing you should be looking for here is a shock wave.

Although you're getting a violent reaction from your mix, I'm wondering if it's actually violent enough to qualify as a true explosion.  Many people would regard the sort of gasoline fireballs used in film special effects as explosions but strictly speaking, they're not; the shock wave produced by such ignitions are relatively weak and do little damage, which is why they're used in films instead of true explosions, which would wreck the set and require the cameras to be too far away to get the shot.

The expansion of the gas due to it's very high temperature may be a factor, but you should be able to get a good idea of what this will be by working it out; I don't know offhand what the coefficient of expansion is for the gases you're dealing with but they shouldn't be too hard to find if you're already working in the field and similarly, the sort of temperatures and burn-rates you're likely to get in the reaction.  As you also know the quantities of materials used in the reaction you should be able to get a rough idea of the magnitude and rate of expansion.
 

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What explosives are used to demolish buildings?
« Reply #18 on: 24/02/2009 19:00:07 »

 

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