Fri Nov 25th, 2005 at 03:56:48 AM EST
From the front page ~ whataboutbob
Most people associate modern style architecture "à la Corbs" with concrete. And you often hear "It's just a concrete building, you know..." in a pejorative way, conveying the feeling that it's some grey old thing, with some rusting rods and a lot of dust...!
So most buildings today are designed with some fashionable steel, stone, wood, glass claddings. On cheaper housing projects, it's about plastic grout in funny colors, barely hiding some concrete blocks.
Visible concrete is seldom seen, and when it does exist, nobody recognize it as such !
However, concrete is not only an excellent building material, but, on these days of sustainability, it might be one of the most high-tech material...
Concrete and cement.
The "old" concrete is the one most people know. It's a dosed mix of cement powder, fine sand and more coarser sand or pebbles (three gradings).
It was re-discovered by Louis Vicat in 1817 by sheer accident while he was dumping a missed batch of lime in the Rhone river, as usual, as lime will dissolve in water, when bargemen came ranting that he threw rocks in the river as this time they wouldn't melt...
He then rediscovered what was the "famed" Roman mortar lost knowledge in the dark ages ...!
The Pantheon's dome in Rome is made of this roman concrete, (2 century AD)
This "brand new material", cement, was first used to make a small ship hull before being used for bridges and lighthouses, then at last, for more "common" buildings (Perret, Nervi, le Corbusier among many others).
Of course for structural use it had to be reinforced with steel bars so as to withhold tension forces and not only the usual compression ones.
In 1991 Pierre-Gilles de Gennes, physicist, has the Nobel prize of physics :
"for discovering that methods developed for studying order phenomena in simple systems can be generalized to more complex forms of matter, in particular to liquid crystals and polymers"
Yves Mallier (french engineer of the Ecole Française des bétons) was looking at the TV interview of de Gennes explaining for the layman the theoretical gradings ( marbles sizes to fit best in a given space). There were not 2, 3 nor 5 grades but 4!
So that week, Mallier tested a new grading in concrete... He added a super fine grade (finer then cement) that was an industrial waste, silica ashes (fumée de silice) and tested it...! The "new" concrete was born.
Because the "holes" in the usual concrete were "filled" by the new very fine grade, the concrete was watertight (fully) and had a higher resistance to compression. Usual building concrete: 15 to 30 MPa (MegaPascal)
"New" concrete: 100 to 200 MPa with laboratory products tested to 400 MPa.
The fiber path.
On another set of thoughts and since those old adobe walls of neolithic times, some searchers (quite a lot) wondered about reinforcement of concretes.
Concrete in structural use, has to have steel rods or bars for tension forces. Without those bars, the test specimen would break sooner.
But what really happened at micro-metric scales ? Thousand of small hairline cracks appeared in the concrete, and joined to make bigger cracks that ended with the breaking of the whole test specimen.(to make it short!)
The big iron bars at, say, a beam scale didn't really stop the fine cracks but, like a giant staple held all that concrete matter together (ie. the beam "held" for a given strenght). After a tension limit it was those steel bars that played the tensile role.
Now what would happen if some mini staples were used to delay the joining of those micro cracks in the concrete matter ?
Surprise... It worked much better then the "life scale" steel bars ! It works so well that concrete made this way was a "ductile" (or tensile) material... Almost as good as a classical I beam of steel !
Fiber concrete and nanotechnology.
On one hand there was a new composition of concrete, on the other hand there was another technique to have the concrete in a ductile state... The "new" concrete was truly born !
Those "mini staples" or fibers are either organic (DuPont) or steel ones. The highest resistance is of course with the latter (till we find a cheap way of having nano-tubes of carbon or bacterium produced spider web). Sizes, sections, quantities are patented.
As most "true" building material in history that are the use of the "wastes" of the era's technology, these new concretes are "new" today because of industrial wastes recycling... The silicium ashes are from the computer processor and zirconium industry... And the small, thin, steel fibers are from the shredding of radial tires !
To give a more practical perception of this material, most concrete slab for a 6 meter span is about 20 cm thick (usual slab for housing)... While with these BFUP (Béton de Fibre Ultra Performant) you could narrow it to a thickness of less then one centimeter ! (of course it doesn't allow for acoustic and thermal inertia, those must be designed independently). If it wasn't for the sag, it could be 0.5 cm. and if it was a cupola or a shell design, there would be no sag and could stay below half a cm...
It is a tensile material... So it works well in seismic building conditions. Think of Kashmir (frame structure and earth filling), Bam (Iran) Alger (Algeria) etc...
It's dense, but you need less matter... So buildings are lighter and cost less and can optimize bad lands for foundations.
It allows for fire regulations... Much better then steel, as it keeps it's mechanical properties much longer. Compete studies have yet to be done. The only example we have is the fire in the Channel tunnel with very high temperature (oven style). This fiber concrete acted like wood, like if it was laminated with micro plies that delaminated one after the other in time.
It's watertight... So you don't need all those tar derived products (that always leak one day anyway!)
It's "easy" to make... Well, you do need an oven for cement, but it doesn't have to be carbon oriented as in a steel mill. It could even be a solar one! Most materials for cement can be readily found in any country, and the ultra fine grade can be anything finer then talc. After a Tsunami, it would be easier to build some cement plants and leave the locals to build their own architecture... (Thus providing new jobs)!
It's molded... As all concretes, and can be readily prefabricated. New additives (in old as new concretes) can give it a very "liquid" phase, allowing for "pumping" the concrete in the mold, even from quite far away (100 m) reducing drastically the noise of most works (for a better acoustical comfort in cities for neighbors as for workers).
It allows for nano technology... Such as nano LEDs, nanotubes for "breathing" through a concrete slab (as thynsulate or such), and catalyst inclusions that with UV breaks organic molecules that are the pollution "glue" (Pope's chapel in Vatican, and a Police center in Toulouse).
It allows for color and finishings... I saw a black polished sample that looked and felt exactly like those black shiny ashtrays you find in cafés ! The surface aspect can take very swallow motives (tenth of mm) and keep them durably.
And of course architectural styles, shapes and form would change... "Form follows matter!"(could be Aristotle) :-)
Of course it costs more the the "old" concrete and less then steel, but, between the rising prices of steel and the reduced volume quantities needed, in practical use it shouldn't shift much building prices.
As sated, it's mostly a French design... But the few existing built example (mostly bridges) are not in France, but in Canada and in Seoul (sigh)... As usual, we'll have to wait another 20 years before it's used commonly... Unless we get to the 100$ the barrel point :-)
Some "home made" tiles specimen...
For the braves...
A simple powerpoint in pdf
Miss Habel thesis in english (pdf)
M. Parant thesis in french (pdf)
A link to Calcia's auto cleaning concrete