Breaking through a concrete wall easily

[fawk3s]

Hi !

So recently I came across this interesting phenomenon that you can break through a strong concrete wall pretty easily by weakening it with certain holes drilled through it, following Hooke's law.
I had heard about it before, but have never really thought about it. But now that I am, I became interested in what kind of forces make the wall strong, and how do the holes weaken it?
I drew a picture to help understand a tad better:
[PLAIN]http://img411.imageshack.us/img411/5872/hookewall.jpg

I have a small theory, but I am not sure its correct, thus is why I am asking.
I figured that above and below the holes, the the stress on the wall is smaller than elsewhere, because below and above the hole the wall isn't "compressed". Making the wall easier to break in those points. Pic:
[PLAIN]http://img827.imageshack.us/img827/2638/hookewall2.jpg
(The red lines indicate a lower stress level, and the green lines are just for illustration that the whole thing forms a certain "X" in a way.)

It is also said that drilling these holes compromises the walls load carrying capacity.

So I am interested in how certain forces work together there to make this possible. Feel free to annihilate my idea and bring out your own.
Im not so much after the math which tells where to drill the holes, but if you can include it, I'd really appreaciate it.

Thanks in advance !

 

[Naty1]

I don't know the physics involved, but the traditional way to get a round hole in a concrete wall is to drill a circle of small holes at the circumference desired.

I'd sure be interested to know why the pattern presented here would require less effort than a circular pattern...my gut tells me "unlikely"...
but then I sometimes get indigesition, too...

 

[fawk3s]

Im not talking about the traditional way. I've just heard that by drilling these few holes, its easier to beat the wall in, say, with a sledgehammer or something.

 

[pallidin]

Say, fawks, I'm by no means an expert but I think i see where your going with this:
With holes being drilled, the impact of a sledgehammer allows for fractured concrete material to go into those holes, further weakening the total structure and, more importantly, allowing numerous secondary fractures to form from a single blow of a sledgehammer that would otherwise not occur.

 

[fawk3s]

Thank you for your answer pallidin. It seems fair enough and pretty logical, and I'd go along with it and that idea of mine posted above, but I am afraid this is not the case in this situation, because it is said that this scenario has a lot to do with Hooke's law of elasticity, and that the holes must be drilled in specific points. So I think there might be even more to it, but I just can't put my finger on it. Not sure though, but that's why I posted this problem anyway.

 

[Integral]

I would bet a grid of re-bar inside that concrete wall would nullify this theory in a heart beat.

 

[fawk3s]

Thats actually a pretty good observation. But I think a couple of guys from China actually tested it, and succeeded. Dont think there were any metal rods in the wall they tested it with though.

 

[Studiot]

I can't see any connection to Hooke's law.

Concrete is a brittle (elastic) material. As such it is subject to 'fast fracture' or rapid crack propagation.
The lines of holes provides lines of weakness for the cracks to propagate along, when struck with a hammer at right angles.
Paradoxically the holes (presumably being drilled rounded) also form barriers to further crack propagation so allow the cutting out by drilling a pattern as described by another poster.
Yes I agree the presence of rebar would alter matters, but the concrete would still tend to crack along these lines.

 

[JDługosz]

Season 1 of "Prison Break"?

 

[fawk3s]

Yees... You caught me. =D

But I got interested after reading that some chinese guys approved it.

 

[mugaliens]

Concrete is horribly poor in strength when it comes to tension. In compression, it's very good, hence most such fortifications involving prestressed concrete tensioners.

Even so, drop a 1,000 lb bomb with a 6-inch radius nose onto a hardened concrete structure, and it will penetrate the structure. If someone's not playing nice and decided to "fortify" the structure, simply up the weight and or reduce the diameter. Designs of many more thousands of pounds at mach+ speeds have been on the drawing boards for years, though the only realized weapon, the 2,000 lb BLU-109, travels about 6' through steel-reinforced concrete. Although the much publisized variant, the BLU-118/B, is only good for somewhat more than six feet, these "bunker-busters" will easily penetrate steel-reinforced concrete, not to mention solid rock.

It would not be difficult to design and deliver a 60,000 lb, nuclear-tipped weapon capable of decimating a target buried beneath literally miles of steel-reinforced concrete, though it may take multiple lobs on the target to finish the job. :)

 

[mugaliens]

I would bet a grid of re-bar inside that concrete wall would nullify this theory in a heart beat.

You are largely correct, and the greater the tension and the more even the distribution of htat tension, the more correct you become! In fact, pre-tensioned concrete slabs are key elements of most overpasses, so much so they've become a common element in most custom home designs.

 

[mr. magneto]

hi.
the x that u have made indicates the line of maximum stress of the rectangular cross section whose corners r the holes at the end.. when u drill in, u actually reduce the stress. so there is no holding up there in the part of the concrete and when u strike it , it generally breaks out in a pattern, just the rectangular cross section. i still need to verify it though.

 

[hihiip201]

I really don't see how this is related to hookes law.

I mean, the goal here is not to yield or distort the concrete wall but to break it (fracture), and as many have pointed out concretes are brittle which means they are poor materials to be put into tension, and according to one of the lab we did (also many resources where you can find online), the yield strength , elastic zone, and young's modulus of concrete are not as easily found as metals.

if you look at the SS curve of most concretes, the 'elastic' zone are far from linear, hence i really don't see any resemblance of hooke's law here (I believe hooke's law stated force and displacement varies linearly, and theory of elasticity takes that to the next step which states stress and strain varies linearly in elastic region).

I think it has to do with stress concentrations. whenever you have holes in a wall, stress increases around the holes (higher than if you don't have holes),as studiot have pointed out, the high stressed cracks propagate along the lines of the holes once it is smashed, and cracks would probably be left along the X if you put the concrete debris together after you break the wall. (The reason for cracks to propagate is because: if you start with holes, high stress induce cracks, and those cracks would induce high stress around it and introduce more cracks once you apply stress )

as far as why they have to be X, my guess is it has to do with how you want the cracks to propagate, you can imagine having the cracks alone your green line to begin with the concrete would break easily.

and once the center concrete is smashed outward, then it should be cake to break the ones around it.As for palidin's reply, I'm skeptical, I don't think the holes will get smaller if you put the concrete debris back together after you smash it.

do correct me if i am wrong.