On Mythbusters they were once trying to drill into a concrete wall for the purposes of an experiment. But they were having a very difficult time of it because the wall was part of a former military base and was very, very tough. Much tougher than ordinary concrete. It was wearing out their concrete drilling gear.
How does one make this super concrete and how much better is it for resisting damage?
I don’t know about that concrete mixture, but I worked for a company that sold prison-grade window caulk, specially designed to be impossible to pick out by prisoners.
My guess is it’s a combination of admixture of high-tensile-strength materials, and a different (probably slower) curing process.
Those would be called Pozzolans: http://www.cement.org/basics/concreteproducts_histrength.asp
There are many ways to make concrete harder. One way is to include carbon fibers. It’s then called fiberated concrete, useful for heavy loads like driveways or foundations under stress. Another tactic is to include more cement. You can request a 4 bag mix or a 5 bag mix and so on from the vendor. I’m not sure if that is much harder to drill though. Of course, metallic particles could also be used. So it’s no myth that they are using more than sidewalk grade mix for prisons.
Concrete comes in various strengths. Mainly the proportion of Portland cement to aggregate in it. Airport runways use one of the strongest mixes.
There is something called a slump test used to test the strength of concrete. Basically, you put a sample of the concrete in a funnel or cone (narrow side up), remove the cone, and measure how far the concrete slumps down. (I think the first reference to this is from an architect in ancient Rome.)
I had a neighbor who was an inspector doing this on construction projects. Concrete truck drivers got pretty upset when he rejected a whole load as not meeting the standard. Smart contractors tried to have another project using normal-strength (like sidewalks or parking barriers) nearby, so a truck with a substandard load of concrete could be sent to that project rather than wasted.
Probably wasn’t used in the military building on the show, but Polymer Concrete is quite a bit more durable than regular concrete.
The joke in our industry is that it will never be adopted because so many municipalities need to replace their roads every 10 years just to keep people employed.
Slump has nothing to do with strength (well, given some other factors, there is some correlation, but basically nothing), it’s a measure of viscosity. Some construction projects need different levels of viscosity. Drilled shafts, for example, want high slump concrete so that while it’s curing it exerts a high lateral stress on the soil (the bearing pressure is related to the horizontal stress).
High strength concrete is generally created by different admixtures, as mentioned. I’m not too up on concrete mixtures, but fly ash, micro silica, and others are somewhat common. Different aggregates are also used. High strength gravel and sand, highly angular, different fibers and such as well. For things like radiation shielding high density concrete (metal aggregates) are often used.
Heh, when I worked in the NYC paving industry, I once heard a Superintendent brag that he’d done an (asphalt) road that had already lasted 10 years. The response from his boss - “That’s the f’n problem - we only guarantee them for three so we can do the f’n job again!”
I worked in a research lab where high strength concrete was, well, researched. It was amazing how strong you could make the stuff if you worked at it. Concrete wasnt really MY thing there, but I helped with some of the testing aspects and got to see on a nearly daily basis somebody doing something (often interesting) with it.
High strength fibers mixed in can help, particularly with tensile strength (pulling it apart strength basically). And rebar (metal rods) of course. If you look at a construction project that needs strong concrete you’ll often see that a sizable fraction of the volume is actually rebar. Chemicals can be added as well. I seem to recall latex (or some other “plastic/rubber”) being added to help prevent cracking.
One of the key things to making concrete strong is the aggregate (basically the rocks in it) you are putting in. If those rocks are weak material or fracture easily or or already fractured then your concrete will be weaker. If they are rocks that are a strong material then your concrete will be stronger.
Besides the composition of the aggregate, another key factor is the size distribution. A poor concrete would have something like say mostly pebbles a half inch across or something. A strong concrete would have half inch pebbles, and then smaller pebbles that fit into the spaces between the half inch pebbles, then smaller still pebbles to fit into the spaces still left and smaller still pebbles to fit into those spaces and so on and so on…
So, a really strong concrete would be mostly aggregate by volume, with a good and proper size distribution, with a tiny bit of cement coating the aggregate particles and filling the tiniest spaces. If you wanted to get really fancy, you could vary the aggregate composition as fucntion of size (though I don’t recall that being done).
Now, of course, there are many parameters that define the engineering characteristics of concrete. And depending on the application, you can “design” your concrete to be optimum for the important parts and not so good for the unimportant parts.
In college my roommate talked me into helping him with a project testing concrete strength. One of the additives we tested was zillions of little high-strength steel wires, about the size of sewing needles. Made that concrete pretty darned strong.
Bearing in mind that I can’t prove any of this, I’ve been told that older (say 25 year old) concrete is much harder than cured but relatively new concrete. If this is true, and I don’t know that it is, why does this happen?
I disagree. Slump is also a measure the amount of water in the concrete. Given a specified mix, more water means that there is less sand, aggregate and Portland per unit volume, which in turn guarantees a weaker mix.
One of the first lessons taught in mechanical engineering courses is that concrete dosen’t stop curing for decades and that the longer you can let it sit without applying a load to it the stronger it will be. There are standard charts available that show 30 day vs 90 vs 180 day strength and the differance is significant.
I don’t know if it’s cost-effective but if you could substitute alumina/corundum for silica sand, it would be a lot harder.
So what do we disagree about? I addressed this in my post.
Shit, given lots of other information the color or smell of the concrete could indicate strength. Doesn’t mean that looking at a pour is a strength test.
If I hired somebody to test concrete strength for a job, and they gave me slump, they’d be fired immediately.
Concrete is good in compression, but lousy in tension. The chemical bonds can’t handle pull apart loads well. That’s why things like bridges and roads use rebar - the rebar is a tension resistive element. The concrete is a composite product of rebar for tension, aggregate of lots of sizes to pack together well for compression, and portland cement for glueing. Plus other additives.
Rocketeer said:
Strong in tension, or strong in compression? I can see how the wires could work as a combination of rebar properties and aggregate properties. You embed lots of tiny metal bars that orient in all directions, so the tensile forces are resisted in all directions rather just one direction or two orthogonal directions. The tiny size lets them mix all through the mixture like aggregate, getting better bonding to more other pieces of aggregate. Better bonding means more strength.
I used to work for a company that produced this metal fiber stuff 25 years ago, so I’m quoting the company mantra from memory here. 
Metal fibers added to concrete and furnace refractories provided signifantly increased flexural strength. They are use on airport tarmacs where the weight of heavy planes causes the concrete slabs to bow directions; placing great strains on the slab. They are used in furnace refractory linings to allow the linings to expand and contact during extreme temperature cycles without spalling. Free standing dome shaped buildings without rebar or internal supports are possible due to the wonder of fiber reinforced concrete (our building and everything in it was made of this stuff).
I am actually amazed that thier is a website for this former employer of mine as it was a small family run business at the time. But the site is pretty crappy and just continues on with the same line of dribble from years ago that I’m not going to bother to link to it.
We were making domes, maybe four feet on a side, a foot or so tall, maybe an inch thick, and then testing them with both point and uniformly applied loads. So I guess we tested both compression and tension. The prof was computer-simulating the dome, and checking his results against ours.
I was kind of proud of the uniform load setup we came up with–it was a bin of sand, slightly bigger than the dome. The dome was set into the sand, upside down, and then downward load was applied to a frame holding the edges of the dome. The sand provided a fairly uniform load over the surface of the dome.
Si Amigo said:
In college we only discussed the bare basics. Road beds are essentially compressive, because the full slab is supported by the ground. Bridge spans are in tension, because you support each slab at both ends and let the middle hang.
To think about it more, road beds actually do have tension elements added - the rebar. And road beds use a lot of rebar. Hmmmm.
That’s also how bridge spans are made. They pretension the spans with rebar until they are actually curved, so that after installed and the road bed lying on them, gravity pulls the road down. The pretensioning of the rebar means that the slab is kept in compression and the load from vehicles just reduces the total stress.
As I said, I can see the tiny fibers acting as small tension elements with much better adhesion because there is more surface area to cement, and they are more finely spread, giving much more even properties in all directions. Unless the fibers are layed in directionally rather than just mixed in with the rest of the aggregate.
Rocketeer said:
Looks largely compressive. There may be some tension on the inner surfaces. Hmm.