Colored Asphalts

After leaving Petrobras Distribuidora last year, I’m enjoying my now excessive free time and trying to revert decades of sedentarism.

So I began walking for 35-40 minutes around Maracanã, since there’s a very nice track there.

And since asphalt runs in my veins, I couldn’t help but notice that the track was done using an “asphalt binder” (the quotes are important, I will clear it bellow) in red.

I couldn’t help but notice either that the track’s surface is in very bad shape, full of potholes and raveling, even though the traffic it receives is the lightest possible: people and bikes (OK, there’s the eventual police vehicle).

The asphalt concrete used is probably the same from Avenida das Américas BRT (Bus Rapid Transit) in Rio de Janeiro, using an SMA gradation, and that failed prematurely in a dramatic way.

But in light of these failures, is colored asphalt something doable?

The answer is yes. There are a lot of successful cases out there, and several patented products from big players, like Shell, Collas and others.

So what was our problem here in Rio?

I wasn’t involved in any of these cases, so my analysis is from the viewpoint of a curious user with some understanding of the product, and should be taken only as an informed opinion.

For starters, I need to explain, although most of you probably already know it, what is asphalt from the chemical and physical point of view.

Asphalt, or asphalt binder, is a product of oil refining. It’s composed by heavy hydrocarbons with a reasonable amount of heteroatoms like sulfur, nitrogen, oxygen and even metals like iron, nickel and vanadium. It’s usually considered as a micellar colloidal system, in which the lighter hydrocarbons, called maltenes (resins, saturated and aromatic), keep the heavier ones, called asphaltenes, in suspension.

Sorry it's in Portuguese, but I think you can get the picture...

In this micellar structure, the resins and asphaltenes are very polar and rich in heteroatoms, having good adhesion to aggregates used in asphalt mixes. This, allied to the hydrophobic nature of hydrocarbons, makes asphalt an almost ideal cementitious material for paving.

There are also the viscoelastic properties, but this a subject for another article.

So, what should we expect from a colored binder?

Well, at least the characteristics of an asphalt binder, without the black color.

And that’s the problem.

What makes asphalt black is precisely the presence of asphaltenes, with their aromatic conjugated bonds and heteroatoms, that absorb practically all wavelengths of visible light. We can add pigments to it, sure, but we will not make it “colored”, we at most will make it “reddish black” or “greenish black”.

So, to achieve a colored “asphalt”, we must replace the asphaltenes with something colorless (or light colored), that allows the pigments to show off, whereas maintaining the asphalt properties on the product.

And that’s not a very easy thing to do...

It’s not possible to remove the asphaltenes from the asphalt in an economically viable way, so we can later add our miraculous “white asphaltene”.

Therefore, the colored asphalts must be something 100% synthetic, using expensive petrochemical raw materials.

The main problem is finding the adequate substitute for the asphaltenes. Luckily for us, there’s a class of products with high molecular weights, adhesive characteristics and that are mostly white, being the perfect candidates to replace the asphaltenes.

Yes, I’m talking about polymers.

A good colored asphalt must therefore be composed by resins and maltenes dispersing an appropriate polymer and colored by pigments, preferably inorganic and thermally stables, and with resistance to UV rays. And this asphaltene-free asphalt must have also all the desired asphalt characteristics, like viscoelasticity, adhesion to aggregates and resistance to oxidation.

Andy you don’t get all this overnight.

It was probably there that the asphalt used in Rio failed. It wasn’t used a commercial product from a major player, but something still experimental.

And this was the unfortunate result…

Remember the “asphalt binder”, between quotes, mentioned earlier?

The quotes are necessary because, without asphaltenes, the product can’t be considered asphaltic. And despite what it may look, this is a good thing, because now we don’t need to limit ourselves to asphalt specs when developing a colored binder, be them empirical like softening point and penetration, or not, like viscosity. We can make our own particular specs, with the characteristics we deem best suitable for the product and its performance. Who said we need to make the hot mix at 140°C? What if we can design a binder with low viscosity at 90°C – 100°C, but with a high modulus at the usual pavement temperatures and a very sharp transition from viscoelastic solid to liquid?

What if we can get excellent adhesion with polymers containing nitrogen at the right points of the molecule?

Well, last but not least we have the pigments, because how to make colored asphalt without colors right?

As I said, the pigments must be able to provide color in low dosages, meaning they must have a high covering power.

And they must be preferably inorganic, in order to be cheaper and more resistant to UV degradation from sunlight.

Depending on the amount used, one must consider its fillerizing effect.

In conclusion, colored asphalts are a whole new world. There’s a lot to be explored, even with all the advances made by the major players. We just have to do it with the right tools and methodology.