Transparent Concrete

rakerman writes: "The Economist reports in How to see through walls that development is underway on translucent concrete, with hopes of eventually developing transparent concrete. Can transparent aluminium be far behind?"

But what about the birds?

Won't translucent structures lead to an increase rate of smack death amongst bird populations?

Re:But what about the birds?

I used to walk over a bridge with large glass windows, where there was a silhouette of a bird of prey. On the back was a caption that read "this cut-out is here to scare off small birds, which used to fly into the bridge and *subsequently* kill themselves". I often wondered why, having survived hitting the bridge, they would want to commit suicide. So perhaps they'll do the same on transparent concrete?

Re:But what about the birds?

I often wondered why, having survived hitting the bridge, they would want to commit suicide.

Embarrassment

Hmm...

Well, it's nice to see innovation within the construction sector isn't dead. Even for something that seems so off the wall as transparent (or currently, translucent) concreat can give birth to innovative new designs and possibilities from architechs.

I mean, I can just see a wall done with a bubble effect (with slighly differnt opacities in the aggitates and clear binding coumpound).

Only thing is, once transparent concreate is perfected... how are the mobsers going to get rid of bodies if they can't throw them in the foundation of a new building anymore...

How to see through walls

They're called windows, and they're usually made of a neat transparent material called glass... ;-)

Seriously, though, any slurry-based material like concrete is most likely to be opaque because microscopic structures tend to scatter light. You only need to pour a glass of milk to see this in action.

Re:How to see through walls

Windows is a trademark of Microsoft Corporation.

Please use the term "transparent exterior access devices".

Transparent aluminum

I seriously doubt we will ever see transparent aluminum. In order for it to succceed, the atoms would have to be aligned in a crystaline matrix. Such a matrix would likely create a hard, yet weak substance.

It would have much strength, yet it would fracture easily. Its called "Modulus of elasticity"; something certain steels(H11 namely) holds in spades. I doubt even if it could be produced, people would find it of much use.

Re:Transparent aluminum

Such a material already exists--in fact it predates human civilization. It's called Al2O3, or alumina, and more commonly known as sapphire (or it's chromium-doped cousin, ruby). It has a hardness of 9 out of 10 on the Mohs scale (the only harder material I know of is diamond) and is transparent in the absence of impurities. However, it is not an alloy--it's a crystalline oxide.

Metallic aluminum cannot be transparent except in thin films; this will be explained in a reply to the top-level post in this thread.

Re:Transparent aluminum

The reason you will never see transparent aluminum is not because of a lack of crystalline structure--in fact, I think metals generally are crystalline or at least have a crystalline microstructure. The reason that aluminum, and basically all metals, are opaque is the same reason that metals tend to be shiny. Because there are a lot of free electrons in metals (which is why they conduct electricity well), the electric field of light expends energy driving these free electrons (therefore metals are opaque), which in turn reradiate light back in the direction of the incident light (therefore metals are shiny). The amount of light that gets through goes as e^-ax where a is a constant and x is the thickness of the metal, so in a very thin metal film (e.g. mirrored sunglasses) you can still get some light through, but for any measurable thickness of metal (e.g. aluminum foil and anything thicker), the amount of transmitted light is negligible.

I know this is a very hand-wavy explanation, but it's hard to explain without a pretty advanced background in electromagnetics. If you want an explanation of this from a rigorous electromagnetic point of view you can try wading through Chapter 14 of Principles of Optics by Max Born and Emil Wolf, but its mostly math with very little physical intuition or explanation.

Re:Transparent aluminum

I'll try to re-state Hal-9001's post in a little different form:

Electromagnetic waves consist of oscillating electric and magnetic fields in alignment so as to be self-perpetuating. The changing magnetic field creates an electric field a little further on, and the changing electric field creates a magnetic field still further on, etc.

First consider a radar beam approaching a metal surface. The E-field will cause the free electrons in the metal to move. This transfers the energy of the beam into electron motion. And with several pages of math that I went through once and never want to again, it can be shown that the electrons move so as to create a mirror-image field, re-transmitting the beam at the angle of incidence -- in other words, a reflection.

Due to resistance to electron movement, the reflected beam will be somewhat weaker, the missing energy being absorbed as heat. If the metal is extremely thin there might not be enough free electrons to fully absorb the incident beam, so part of it passes through. In an insulating material, electrons are tightly bound to molecules, and so cannot range far enough for strong interactions with the beam, and so most of the beam will pass through (the material is "transparent" to radar). However, electrons can shift around within the molecules, which causes refraction, partial reflections, and absorption.

Things are different for x-rays, because the individual photons are pretty energetic and the wavelength (size of one photon) is close to the size of an atom. So it's more likely to be the inner electrons still bound to the atoms that wind up trying to capture the x-ray, and only rarely does this succeed -- most of the x-rays get through several inches of all but the densest materials.

Visible light photons are in-between in size, large enough to interact well with the free electrons (reflection), but small enough to also be affected by bound electrons. (Selective absorption by the bound electrons gives copper and gold their color.)

Most insulators are not transparent to visible light, except as very thin films. Most insulators (like metals) consist of irregular aggregations of tiny crystals. The interactions with the electrons bound in molecules will reflect some light, absorb some, and refract all the rest. In most insulators, the interaction varies with the polarization of the photon and the angle of the crystal; since each crystal is oriented differently, each interface between crystals refracts and reflects light in different directions, so the light that isn't reflected from the external surface is scattered and (mostly) bounces around inside the material until absorbed rather than passing through.

Most transparent materials are glasses, with no crystal structure, and so no grain boundaries to scatter the light. Single crystals may also be transparent, although it's pretty hard to grow a single crystal as big as a windowpane. Multi-crystalline insulators can be translucent if sufficiently free of the atoms or molecules that absorb light, that is if the light is scattered but not absorbed eventually it will find it's way back out of the material. Concrete could be translucent if both the aggregate and the cement were free of light-absorbing materials, but I think the price would be extremely high.

Possibly a multi-crystal insulator could be transparent if the refractive index did not depend on orientation of the crystal or polarization of the light, and if all the crystals fit together neatly and had the same refractive index. Or use glass beads for aggregate and somehow make the cement match the glass?

Metals by definition have free electrons, which strongly reflect and absorb visible light. If it's transparent, it's not a metal.

You can form Al2O3 into fairly large crystals, and maybe it could be a glass too. It's stronger and much harder than silica-based glass, so it would make a great windshield, if you didn't mind the cost of using diamonds for cutting and polishing.

Re:Transparent aluminum

I doubt even if it could be produced, people would find it of much use.

I dunno. I find it invaluable for transporting live whales in my time-travelling starship, complete with enough water to for them to move around in.

I could use steel or something, but, darn, I like to press my nose up to their enclosure during the journey.

--

Transparent aluminium

Can transparent aluminium be far behind?

It's already here [guildoptics.com], although in the form of an oxide rather than the pure metal.

Don't wait for transparent aluminum.

"Can transparent aluminium be far behind?"

Yes, transparent aluminum can be far behind. Metals like aluminum have free electrons which prevent transmission of light.

Metropolis Magazine Article, April 2001

Apparently neither The Economist nor Slashdot knows the difference between translucent and transparent. Ugh.

Anyway, this is old news. Metropolis magazine reported on the development of translucent concrete [metropolismag.com] back in April 2001.

A good weapon against terrorism...

would be to legislate that all new residential and commercial dwellings be built from transparent concrete.
Anyone refusing to demolish their existing house would be added to a database of 'potential conspirators'.
This would be quite consistent with recent 'anti-terrorist' surveillance legislation.
Also, the boom in building would boost the flagging economy.
Imagine whole neighbourhoods of people living in complete exposure, proving they're real honest patriotic Americans.

I'm going to regret this...

I wouldn't want to live in a transparent house. Think of the lack of privacy. People'd be able to see when I was in the bedroom, when I was in the bathroom... They'd be able to see all my movements.

Transparent != Translucent

There is a very big difference between "transparent" and "translucent". The former means that light passes through the material almost completely unchanged (a certain amount of distortion is okay, but the point is that you can make out what's behind it). Translucent means that light is transmitted, but it's diffuse and you can't make out what's behind the material. This concrete is translucent. It's not transparent (read the article).

Transparent building materials

The real problem with transparent building materials is that people inside want to control the transparency, just look at your own home, you got curtains and shades on all(most) windows.

A classic problem with new hightech buildings (Glass 'n metal) is climate control, its nice to be able to look outside, but if the sun is starring you right back its not that fun. So you tint the windows :)

The next problem is that in those buildings the light that gets through is not white light (sunlight) anymore, and working in that kind of buildings can cause depressions very similary to winter depressions.

Remember: Architects are just building nerds :)

Re:Transparent building materials

It's a shame that electrochromic windows [nrel.gov] haven't taken off. I first read about them in Popular Science, probably about 10-15 years ago, and if I recall correctly, they were used in a concept car by Ford (I could be mixing two Popular Science articles together), but they allow you to electrically darken and lighten windows, and they actually reflect light and heat (unlike liquid crystals, which just scatter light and heat but still let them through). I'm not sure, but they might also be wavelength-independent, i.e. reflecting all colors of light equally. The obvious barriers to their widespread adoption are probably cost and the ability to make panes large enough to use as windows.

Re:The LCD variety has been done

modern windows...have a higher R factor than a lot of walls

Not bloody likely. Even triple pane windows aren't much more than R-3 or R-4, even if you add in Low-E and all that, you don't get much more. 6" walls (USA) easily get R-38 with insulation. Maybe if you had simple plank walls your windows would be higher.

Is it really concrete?

Can material described in the article really be called "concrete [britannica.com]"?

As it is written there its only resemblance to concrete is that it consists of coarse aggregate, fine aggregate and binding agent. But this is not a recipe for concrete only - also for other materials. Also, Dr. Price's secret material can't be poured or produced on site - one the main reasons of traditional concrete popularity. It would probably find its use in form of blocks of translucent material, that could be used to enhance possibilities for architects but what Dr. Price is trying to do is another building material, which is very interesting indeed but can hardly be called "concrete".

Re:Is it really concrete?

From the story:

Technically, concrete is simply a mixture of three ingredients: big lumps of material called the coarse aggregate (such as gravel), smaller lumps called the fine aggregate (such as sand) and a binding agent, or cement, to glue it all together into a solid. So translucent concrete, in theory, should be fairly easy to make using bits of plastic or glass of various sizes, with some kind of transparent glue to act as a binding agent.

This sounds more like a composite than concrete to me. The Dictionary of Composite Materials Technology [about.com] defines a composite [about.com] as

A multiphase material formed from a combination of materials which differ in composition or form, remain bonded together, and retain their identities and properties. Composites maintain an interface between components and act in concert to provide improved specific or synergistic characteristics not obtainable by any of the original components acting alone. Composites include: (1) fibrous (composed of fibers, and usually in a matrix), (2) laminar (layers of materials), (3) particulate (composed of particles or flakes, usually in a matrix), and (4) hybrid (combinations of any of the above).

By this definition, "transparent concrete" is a particulate composite of plastic or glass, probably in a matrix of epoxy or resin. Concrete is also a composite by this definition, but despite what my civil engineering friends might try to tell me, that doesn't mean that all composites are concrete. ;-)

Edible Concrete

Edible concrete has been with us for years:

coarse aggregate - dried fruit
fine aggregate - flour
binding agent - eggs.

It proved so popular, it's got its own name - cake!

Let's face it, Marie Antoinette would've looked a bit of a dork saying "let them eat edible concrete."

Light Pollution

Buildings glowing from within? This is terrible for astronomers... the added light pollution would further ruin the viewing conditions for many great observatories.

Re:Light Pollution

Buildings glowing from within? This is terrible for astronomers... the added light pollution would further ruin the viewing conditions for many great observatories.

Maybe that wouldn't be so bad. I say, the sooner we got astronomers off the surface and up to the far side of the moon, where they belong, the better.

Glass Houses

If you lived in a transparent concrete house would you still have to refrain from stone throwing?

Remember the "clear craze" several years ago?

First we had clear Pespi.
Then we had clear deodorant.
Saturday Night introduced us to clear gravvy.

Like clear concrete was that far behind?

I know how this one started...

Architect "...are the stairs, and this is where the wi..."
Programmer "NO! NO! I will not have windows installed!"

Glass

and buildings whose windows need not be flat, rectangular panes, but can be arbitrary regions of transparency within flowing, curving walls.

Sorry, but that is already possible.
A researcher at the university of Delft has developed a way to create twisted glass allowing for twisted buildings.
A dutch article can be found here [tudelft.nl]. Take a look at the images if you don't understand the text

Blend of two transp. materials is not always ...

The explanation on how to achieve this reads a bit funny.

It seems to assume that if one mixes two transparent
components (e.g. glass grid, and some transparant matrix), the result is also transparent.

This is not true, as every high school boy that studied optics can tell you. Refraction index, surface properties etc.

It will probably be pretty hard to make a transparant material from two components, let alone keep the other properties of concrete.

Much more informative article

this april 2001 edition of Metropolis [metropolismag.com] has a pretty informative article on the man and his background.

interesting that i live in houston (concrete captial next to LA) and never have read an article on this guy.

Lens and the Sun

I can see a whole new collection of risks here, especially if there are any curved surfaces involved. At various times of the day a transparent product would focus the sun's rays into various hotspots. Some of these might be intense enough to cause burns or even fire. The lenses would not be terribly efficient, but they would be very large.

Paul.

Potential for Recycled Materials

I've calculated a possible material to use for the binding agent, that's a no brainer: Silicone (as it can be formulated for varying degrees of clarity and density)or fiberglas epoxy resins (the added bonus of this is fairly rapid curing...

However, the ecological impact is a far better thing to consider... For example, recycled bottle glass can be ground down to make both aggregate and filler (you can seperate the colored glass and use that to add a touch of color to the finished product), and ground further down, it can act as filler as well...

Considering that the majority of states in the US only have voluntary glass recycling, it might provide incentive for deposit glass bottles, not to mention finally provide a real incentive to recycle old CRT's...

Or if you're feeling daring, you can use the same optical quality sand they use for reflective road striping to give the concrete an almost luminescent quality... For added strength, use polycarbonate rods or strips in a woven lattice...

The article stated that transparent/translucent concrete can only be prepared offsite, but in theory it can be done the same way as existing concrete, just bring a lot of drums of resin or silicone to the site...

Fire resistant? Or toxic smoke as it burns?

Don't see it mentioned anywhere in the comments yet, but after reading the article, the first thing that concerned me is that the nice thing about a concrete building is that it will hold together when it catches fire, not melt, puddle, and add to the blaze with choking poisonous smoke.

Hopefully the designer is taking into account other properties besides strength.

Re:I may as well say it....

From the article:
He has visions of cities that glow from within, and buildings whose windows need not be flat, rectangular panes, but can be arbitrary regions of transparency within flowing, curving walls.

That sure sounds remarkably like Apple's philosophy...