Professor at the Department of Civil and Environmental Engineering, Massachusetts Institute of Technology (MIT), Cambridge (MA), USA
Breaking the Wall of Concrete Pollution. How Green Concrete Can Reduce the Giant Carbon Footprint of Construction.
On November 9th my father called me, “The wall fell!” His voice broke into thousands of splinters.
Thank you very much for giving me a chance to spend this memorable day with you here. My point today will be to give you the chance to have a quick speed dating with concrete and to see what green concrete is all about. Looking at the oranges that stand in front of me, my best guess is that you have only one question: what does concrete and oranges have in common? This will be my working assumption, and I am going to work with you through this assumption in the next thirteen minutes and few seconds.
Let me put concrete within the larger context we are facing today. In these times of economic crisis, we look into large-scale infrastructure renewal, particularly in the United States. People speak about a new deal; the question we raise is whether it will be green? Green means here whether it will be a sustainable development. The concept of sustainable development has three components, which we should never lose out of sight. It includes: economic growth, social progress, namely job creation- very important these days; while reducing the environmental footprint. It is with this context in mind that I will discuss materials and structures. I want to convince you that materials and structures in general, and concrete in particular, have a cultural, social and environmental meaning. It is the combination of these three elements that can be transformational, far beyond the immediate use of concrete in a particular project.
Let me put concrete then in the context of the worldwide materials consumption. The data here shows you a list of the worldwide consumption of materials. You rightly recognise that concrete, after water, is the material the most consumed on Earth. Everybody on this Earth, every person, consumes roughly one cubic metre of concrete per year. I don’t know where you stand on 9th of November 2009 with your yearly concrete consumption; you still have a few weeks to finish your cubic metre. But it is this mass consumption of concrete that defines the potential impact of concrete sciences and engineering. 25 billion tons of concrete are annually produced. There is no way that concrete could be replaced in the foreseeable future by any other material to meet our societies’ legitimate demand for housing, shelter, schools, infrastructure. Consider for instance wood: our yearly worldwide wood consumption already bypasses the ecological capacity of renewal of wood on Earth.
Now, this immense consumption of concrete comes at a non-negligible environmental price. This price results from the production of cement which requires energy, and which releases CO2 also in other forms. Roughly speaking, each ton of cement produced is associated with one ton of CO2 released into the atmosphere. Just to give you a number: one ton of CO2 is the amount of CO2 a tree absorbs over a hundred year lifetime. For comparison, consider one column here in this building. Made out of concrete, a three metre high column requires roughly one ton of cement. I made a rough calculation of this concrete building. The amount of concrete used amounts to the CO2 equivalent of a thousand trees absorbing CO2 over hundred years. This is the CO2 impact of concrete employed in this building.
Given this impact, what can we expect in 2050? Doing nothing is clearly no option. But even a change to clean heating in cement and concrete production would not curb significantly the CO2 emissions. This is why, the US Department of Energy, fostered by a bill, which went through the House of Congress of the United States, foresees by 2050 a reduction of 83% of greenhouse gas emissions related to cement and concrete production. But let us be clear: There is no way to achieve this goal without new technology and innovation.
Let me put this quest for innovation in the context of pace of concrete innovation. While concrete has been used since Roman times, the true development of concrete only started in the mid 1980s. Until then, since the invention of modern cements in the late 19th century, research and development was dedicated to developing concrete, which is highly heterogeneous at multiple scales, into an industrial product ready to be employed off-the-shelf. Then, in the mid 1980s, breakthroughs in concrete sciences discovering the sub-millimeter scale let to the development of High-Performance Concretes, which have a much higher strength (I will come back to those materials later on). In the mid 1990s, breakthrough at the sub-micrometer scale revealed that concrete was still heterogeneous, but formed of calcium-silicate-hydrates in two fundamental forms. These breakthroughs gave rise to new concrete materials, Ultra High Performance Concretes, which today have a strength like mild steel. Right now, we are discovering the atomic and electron scale. We do not know what new material products will come out from these discoveries. Let us call them a “c-crete” for a moment.
How can we make concrete “green”, a material which is grey, not green? One way of achieving this goal is by increasing the strength of the material. The idea of strength goes back to Galileo Galilei, who explained why there are no monsters on Earth: The weight of a monster increases with its volume; while the strength of body parts, bones, flesh, etc increases with the section over which the forces are transmitted from the head to the ground. Thus, since weight and volume increase with the length to the power three, while the section increases with the length squared, a monster would surely fail under its own weight beyond a certain height. This is why there are no monsters on Earth.
Let us now can turn this explanation around: As CO2 emissions increase with the volume of material employed, an increase in strength of the materials will allow us to decrease the environmental footprint by one over this strength increase. For instance, if we increase the strength by 100%, so double the strength, we reduce the environmental footprint by 50%.
How can we achieve this? We can’t achieve this in the classical way, with the 20th century top-down mindset. The classical approach recognizes a certain difficulty at a given scale, where it is solved to improve the performance at the engineering scale. As an indication, it may be surprising but true, the density of the fundamental building block of concrete, the material most consumed on Earth, was only revealed in 2007. In short, this approach dictates a pace of concrete innovation which is simply too slow to address the ecological challenge. Instead, a shift of paradigm is required to move from the classical top-down approach to a consistent bottom-up approach. This approach starts at fundamental scales of atoms and electrons, and propagates this knowledge all the way upwards to engineering scales.
Recently, a group of physicists, particular statistical physics, material scientists and engineers, actually found out that concrete, at these fundamental atomic scales, has much more in common with glass than with what one would expect concrete to look like: grey. At atomic scales, the fundamental building block, calcium-silicate-hydrate (C-S-H) is a glass deliminated in its structure by water molecules in the interparticle space.
Why is it so important to break through to this scale? Once the molecular structure is known, one is able to change this molecular structure and increase the molecular strength. This strength increase can be propagated all the way up to engineering scales to reduce the environmental footprint. By way of example, consider the atomic structure C-S-H: simulations provide a means to tilt it, squeeze it, and stretch it. The result of such simulations show that if concrete was dry, the strength would immediately increase by a factor of two, thus reducing by 50% the environmental footprint of this material most consumed on Earth! But this is just the atomic scale.
How can we bring this knowledge upwards, bottom-up, to larger scales? One way of doing is, is by probing materials at nanoscales by nanoindentation: a needle shaped probe deforms sub-micron scale particles. What we are able to do is to test some of those elementary building blocks and identify a means to translate molecular knowledge into microtexture relations.
An indentation test leaves an imprint on the surface. What we are able to assess by this technique in such materials are probability density plots of packing densities. Packing density is the concentration of particles in a granular assembly. Interestingly enough, for the material most used on Earth, concrete at this scale packs into two characteristic packing densities: in ordinary concrete, the solid exhibts a packing density of 64% as a dominating phase. This packing density corresponds to the random packing limit of spherical objects, like oranges thrown randomly in this pot.
But there is a second phase, which reaches packing densities of 74%. This packing density is the limit packing density of spherical objects packed in an ordered way, like staged oranges in a grocery shop. Take four oranges, put one in the middle, and you obtain a orange packing density of 74%.
Concrete are no oranges; but made of 5-nanometres sized nanoparticles. There is obviously no workers who put these 5-nanometre sized particles together. Fascinating enough, this material is able to generate these types of microtexture reaching limit packing densities. There is a cultural meaning to microtexture: It suffices to think about structures built of ordinary concrete: we all know such bad concrete constructions. We also know that such ordinary concrete has been, and is currently still used as a means of oppression, like the Berlin wall. Here is another wall which remains to be seen to fall.
By contrast, consider now ultra-high performance concrete materials which came out at the end of the 20th century, right after the Fall of the wall. Structures made of these materials are filigree, and use less material to achieve more. At the microscale, C-S-H particles are packed at the limit packing density of 74% like oranges. In addition, we observe a new phase appearing, which fills the space in between and which achieves a packing density of 87%. So one can achieve with those materials a denser microstructure which translates into macroscopic strength, and into a reduction of the environmental footprint.
This type of behaviour akin to oranges is not restricted to concrete only, but is a general pattern for many materials. In fact, a stiffness plot versus packing density of many materials illustrates this pattern. Concrete packs at 64% and 74%; and in between the bad and good concrete we find cow and human bones. This illustrates that the idea that nature makes everything good is, of course, not true. Nature provides us with the structures to survive. Ear bones have higher density, as have shales, compacted clays from 3 km depth important for natural gas formation in the US, Europe, etc. This characteristic packing, this nano-granular nature of concrete, bone and shale is a general pattern of hydrated nanocomposites.
Now you know what concrete and oranges have in common.
Since I still have some three minutes time, let me take you just one step further, from the materials scale to structural scales, and bring all this back to the question of sustainable development, which includes social progress. If you have ever worked on a concrete job site, you will know that concrete workers are typically at the end of the chain of command. What can materials actually do to elevate their status? A couple of years ago, I got involved in a project of the US Federal Highway Administration to design a “bridge for the future” – a bridge with the newest material possible, but still feasible in the context of infrastructure renewal. Currently, we look at replacing 150,000 of 450,000 bridges in the US highway bridge system.
The bridge we came up with is an almost classical bridge design, except for the dimensions of the bridge. The dimension of the designed deck is 7.5 cm, roughly 1/3rd to 1/4th of classical concrete bridges. This bridge is supposed to carry heavy American trucks. When the bridge was tested, I was called to Washington, and asked to stand below the bridge when a truck drove over it; a sort of evolutionary engineering. But, more importantly, I also visited the site where the bridge was actually built. It was built in Kentucky. Kentucky, for those who don’t know it - I come from Bavaria, so I can say that - it is a little bit like Oberpfalz. The principle was that if you could build such a high-tech bridge in Kentucky, you can build it everywhere. (In the most positive sense; I actually come from there.) (Laughter in audience)
The most interesting feature of the manufacturing is the monitoring of temperatures. The hydration of cement which takes place when the liquid becomes a solid, is an exothermal reaction. But that is an old story; we all know that. But there is something more to it: How can you tweak on site a material to obtain high-packing density? Temperature is key. Here is the story: What this figure shows is the temperature evolution inside the bridge structure over time. Roughly spoken, at sub-micron scales one starts out with oranges in a disordered fashion. The application of heat, actually from the exothermal sourcesof hydration, makes the particles fall in place with high packing density. This is the material secret of this high strength structure.
But the story goes on: What I witnessed on site in Kentucky was the sensation that a material could ultimately elevate the status of workers. The use of such a new material generated a collective feeling among the workers that they were part of something bigger than just a job site. This shows that materials that have an environmental impact can also have a social impact.
By way of conclusion: What is the matter with “Green” concrete? Concrete is not green, we all know that. But it is also no “c-crete”: concrete has an economic, social and cultural meaning, particularly in this age of global warming. The cultural meaning for science I see the fusion of physics and engineering to solve the next generation of environmental materials challenges. But the solving of the environmental footprint with a 20th century mindset will not be enough. Without putting materials in the larger cultural context, I am afraid we will not be able to ensure a sustainable development of concrete.
With this, let me remind you that there are not only oranges on this world, but also other fruits. Thank you very much.