THE ASPHALT – The the most viscous liquid on Earth – Part 2 – How Asphalt can become a “smart” material?

“Smart Materials” already exist, but we still have a lot learn more about them. In the category of such materials of course I must mention the “Shape Memory” alloys and “Functionally Graded Materials”. Yet more recently something new has attracted a huge interest: “The Self-Healing materials”. What I consider super interesting here is the fact that actually these materials are there for centuries, we just didn´t know how to use them. One such material which can indeed have self-healing effect is the Asphalt. In this post as Part 2, I want to tell you some other new facts about this amazing extremely viscous liquid.

Living on a fluid planet, the one thing we can be certain about is change: sea levels are rising, the Earth´s mantle is flowing, moving the continents, volcanoes erupt, creating new lands and destroying others; hurricanes, typhoons and tsunamis continue to batter our coastlines, reducing cities to rubble. In the face of this future it seems only rational to build our homes, roads, water systems, power stations and indeed airports – all the stuff we rely on to live a dignified and civilized life – withstand damage. This stuff needs to be strong and tough to survive earthquakes and floods, yes, but it´d be even better if we could design our infrastructure so it would repair itself, allowing our cities to be more nimble and resilient in the face of environmental change. This may sound far-fetched, but, in fact, it´s what biological systems have been doing for million of years. Consider a tree: if it´s damaged in a storm, it can repair itself by growing new limbs. Likewise, if you cut yourself, your skin heals itself. So then: Could our cities become similarly self-healing?

In 1927 Professor Thomas Parnell of the University of Queensland conducted an experiment to see what would happen to black tar if it was left to settle in a funnel. What he found was that, over days, it behaved like a solid, staying just where he put it. But over months and years it started to creep and behave like a liquid. Indeed, it flowed down the funnel and started to form droplets. The 1st drop fell in 1938, the 2nd fell in 1947, the 3rd in 1954 and so on, with the 9th drop falling in 2014. This is surprising behavior from a material that seems so solid when you drive over it in your car. That´s asphalt, but asphalt is just tar mixed with stones. So what´s going on?

FIGURE 1. – The University of Queensland pitch drop experiment (taken in 1990 two years after the 7th drop anf 10 years before the 8th drop fell)

Tar is a much more interesting material than anyone initially thought – materials scientists included. Extracted from the ground or produced as a by-product of crude oil, it seems to be nothing more than boring, black sludge. But in reality it´s a dynamic mixture of hydrocarbon molecules that formed over millions of years from the decayed molecular machinery of biological organisms. The decay products are complex molecules which although not part of a living system anymore, nevertheless self-organize themselves within tar, creating a set of interlinked structures. At normal temperatures the smaller molecules inside the tar have enough energy to move through its internal architecture, which gives the material fluidity. So tar is a liquid, albeit a very viscous one: it is 2 billion times more viscous than peanut butter, which explains why Professor Parnell´s tar has taken so long to drip through the funnel.

The Asphalt self-healing, mainly restoring mechanical properties of asphalt through crack closing, is an ability conferred by the asphalt component bitumen. Correlation between self-healing rate and bitumen viscosity and the influence of filler content, filler type, temperature, amount of bitumen and bitumen modification suggests that the closing of cracks is a flow process. Hence, it is mainly assumed that bitumen is responsible for the recovery of diminished or lost properties due to damage.

FIGURE 2. – The Asphalt Self-Healing process

Tar´s characteristic pungent smell comes from molecules which contain sulphur, an element often associated with smelly organic substances. When you walk or drive past engineers laying down a new road surface, you´ll see and smell them heating up tar, which gives the molecules more energy to move, and thus to flow. But the extra energy also allows more of the molecules to evaporate into the air and so the material becomes smellier, just as drinks become more aromatic when they are heated up. A smelly liquid might seem an idiotic thing to build a road with, but engineers add stones to the material, creating a composite substance: part liquid and part solid -similar, in fact, to the structure of peanut butter, which is made of a lot of ground-up pieces of peanut all held together by an oil. The strength and hardness of the stones supports the weight of vehicles driving over the asphalt and also helps the road resist damage from exposure. Cracks do sometimes open up if the forces exerted on the road get to be too high, but they do so between the stones and the tar that´s gluing them together. This is where the liquid nature of tar comes to the rescue: the tar flows in and reseals these cracks, allowing the road to repair itself and last far longer than a purely solid surface ever would.

Of course, as road users, you´ll have noticed that there is a limit to their self-repairing properties: roads eventually do get old and start to disintegrate. Temperature is partially to blame. If the temperature gets below, say, 20°C, then the liquid tar becomes so viscous that it cannot reflow and heal the cracks as they appear. And beyond that, over time, oxygen from the air reacts with molecules on the surface of the tar and alters their properties, again making it more and more viscous, and less and less able to seal up cracks. Over time , the road skin will change colour and become less fluid, just as your skin becomes less flexible and drier as you age. This is when you´ll see small potholes form, which, unless tended to, grow and grow and eventually destroy the road surface entirely.

Scientists and engineers across the world are busily developing strategies for increasing the life of roads and thus reducing traffic jams. In the Nederlands, a group of engineers is studying the effect of incorporating tiny microscopic fibres of steel into tar. This does not alter the mechanical properties of the road much, but it does make them more powerful?. When the material is exposed to an alternating magnetic field, electric currents flow inside the steel fibres, heating them up. The hot steel, in turn heats up the tar, making it locally more fluid, allowing it to flow and heal any cracks. Essentially, they´re supercharging the self-healing properties of tar, and also countering the challenges of winter´s cold temperatures. The technology is being tested on stretches of motorway in the Nederlands now, using a special vehicle that applies a magnetic field to the road as it drives along. The idea is that, in the future, all vehicles could be fitted with such a device, so anyone driving on a road would also be revitalizing it.

Another way to address tar´s natural loss of fluidity is to replenish its lost volatile ingredients – the molecules that make it flow. The easiest way of doing that would be to apply a special kind of cream on to the road surface – essentially a moisturizing cream, just like the ones we use on our skin. A more sophisticated version of this method is being tested by a group in Notthingam University, led by Dr. Alvaro Garcia. They put microcapsules of sunflower oil into tar. These remain intact inside the material until microcracks form, causing the capsules to rupture. The oil, once released, locally increases the fluidity of the tar and thus promotes flow and self-healing capabilities. The results of their studies show that cracked asphalt samples are restored to their full strength two days after the sunflower oil is released. This is a dramatic improvement. It is estimated that this has the potential to increase the lifespan of a road from 12 years to 16 years with only a marginal increase in cost.

A new research has been recently done by a group of scientists at the Institute of Making (a multidisciplinary research club based at University College London- U.K.), where they are working on technologies that can help repair asphalt efficiently once the cracks have already got bigger: they´ve started 3D printing tar.

FIGURE 3. – The 3D printing process

As shown in Fig.3 during the 3D printing process, a print head converts a solid to a liquid (often by heating), and squirts, it out in a predetermined pattern in an x-y plane. Once cooled, this creates a single solid layer. Then the printing platform is moved down and another layer in a different pattern is printed. Printing hundreds of layers this way creates a whole object.

3D printing is a relatively new way of making and repairing objects. Thousands of years ago, printing was invented in China as a process of transferring ink on to a page via a wooden printing block. The rest of the world caught on and innovated, giving us a world of books, newspapers and magazines – an information revolution. But all that is a 2D printing. 3D printing takes the approach one stage further; instead of printing a thin, 2-dimensional layer of liquid ink on to a page, 3D printing allows you to print many 2-dimensional layers of liquid on top of one another, each one solidifying before the next is applied, ultimately constructing a 3D object. Of course you don´t need to use ink to make a 3D print. You can use any material that can transform from a liquid into a solid. Just look at bees. This is exactly how bees make their extraordinary hexagonal honeycombs. When they are between 12 and 20 days old, worker bees develop a special gland for converting honey into soft wax flakes. They chew up the wax and deposit it layer by layer to make the honeycomb. Wasps use the same trick to make their nests, chewing wood fibres and mixing them with saliva to create paper houses for their larvae.

FIGURE 4. – Bees were using 3D printing to build their honeycombs long before humans happened upon the technique

Human 3D printing technology is now catching up with the bees and wasps. Plastics, for instance , can be squirted out of a printer, layer by layer, to create solid objects even more complex than honeycombs. We’re now looking at a strange project to develop an asphalt roadway 3D printing device. But will it work?  Advanced Paving Technologies proposes what they say is a new method of paving using 3D technologies. But wait, we already have machines that pave roads very well, what is the problem being solved here?

APT claims the issue is in the method of paving, which assumes a relatively flat surface, upon which fresh asphalt is applied. Normally, streets requiring repair are in fact no where near flat, as they’re full of potholes and other damage. They have an irregular surface. The standard practice today is to scrape off sufficient irregular material to produce a flat surface to apply the asphalt. APT says this two-step approach is time consuming and expensive. That’s probably true, but how do they solve the problem?

They propose to pave directly on top of the potholed surface, thus eliminating the scraping stage. Here’s how it works: at the front end of the paver, a 3D scanner examines the surface to determine its shape. Then, the magic occurs: a variable screed automatically adjusts the paving depth to match the detected potholes, ensuring a perfectly flat roadway surface. Sounds good, but will this actually work? Hmmm… scientists dealing with this challenge are very suspicious. First, repeated applications of this process will just make the roadway thicker, eventually flowing over the curbs! It’s very likely you’d have to scrape material down anyway eventually, perhaps even after only one or two 3D asphalt applications. 

The second problem they see is more devious: the potholes are formed by weakened material on the surface, typically by pounding, but also erosion by water and ice – and mostly by ice freezing and expanding in cracks. As a result, you’d most likely find the areas immediately surrounding potholes to be quite weak and ready to break up. You cannot build a road on top of that weak material; it would quickly deteriorate. You’d also have to remove any loose material before 3D asphalting as well. 

FIGURE 5. – How Potholes Form

So then how to design, build and test an asphalt 3D printer? The main difficulty encountered is that asphalt behaves as a non-Newtonian liquid when moving through the extruder. By definition a non-Newtonian liquid is a fluid that does not follow Newton´s law of viscosity, i.e., constant viscosity independent of stress. In non-Newtonian fluids, viscosity can change when under force to either more liquid or more solid. Ketchup, for example, becomes runnier when shaken and is thus a non-Newtonian fluid. Most commonly, the viscosity (the gradual deformation by shear or tensile stresses) of non-Newtonian fluids is dependent on shear rate or shear rate history. Thus, the rheology and pressure in relation to set temperature and other operational parameters showed highly non-linear behaviour and made control of the extrusion process difficult. This difficulty was overcome through an innovative extruder design enabling 3D printing of asphalt at a variety of temperatures and process conditions.

However by using the 3D printing the scientists demonstrate the ability to extrude asphalt into complex geometries, and to repair cracks. The mechanical properties of 3D printed asphalt are compared with cast asphalt over a range of process conditions. The 3D printed asphalt has different properties from cast, being significantly more ductile under a defined range of process conditions. In particular, the enhanced mechanical properties are a function of process temperature and scientists believe this is due to microstructural changes in the asphalt resulting in crack-bridging fibres that increase toughness. The advantages and opportunities of using 3D printed asphalt to repair cracks and potholes in roads are therefore evaluated.

Asphalt (bitumen) composites are the most common material used to surface roads, with 95% of UK roads paved with asphalt mixtures. Its success is due to a combination of factors that have been widely studied: it creates a safe and robust road surface for driving when combined with stone aggregates and appropriate polymer binders; road surfacing can be carried out rapidly and without complex machinery; it has good acoustic properties and so muffles the sounds of traffic; it is robust, repairable and indeed self-repairs. However asphalt composites do degrade over time due to the effects of road usage, oxidation, loss of volatiles, moisture damage, and various other factors. This degradation leads to increased stiffness of the road surface, cracks forming, stripping, ravelling, loss of aggregate, and development of pot-holes. Despite a stipulated minimum lifetime of 40 years, the resurfacing of roads is estimated to cost £2 billion per year in the UK alone. Increasing the life of roads has the potential to reduce environmental and financial costs associated with road closures and the congestion they cause. One approach taken to increase the life of asphalt roads has been to enhance their self-healing properties.

Environmental considerations have led to much interest in the use of recycled materials such as rubber and plastic in asphalt composites. These materials are often inexpensive, being currently considered as waste, but the asphalt mixing phase is energy intensive and the cost of transport is also a factor. So when looking to increase the environmental sustainability of infrastructure maintenance, materials and repair processes should ideally be optimised together. In the future, many of these demands could be met by autonomous vehicles which repair locally on demand. In the case of asphalt roads, this optimised preventative approach to road maintenance would need to focus on the early stages of road degradation when small cracks form on the road surface. These cracks allow water ingress and grow rapidly during freeze-thaw cycles through the de-bonding of aggregates to form potholes. Once formed it is very hard to stop the growth of these potholes which cause significant vehicle damage and so lead to shortening of the usable lifetime of the road.

So in this case we must describe a 3D printing technology that could be attached to an autonomous vehicle or drone, and used to repair small cracks before they turn into potholes is. 3D printing is a method by which objects can be fabricated layer by layer from a CAD model of the object. By scanning a road surface the negative shape of the crack can be obtained and processed into a 3D model. This information can then be processed and passed to a 3D printer, which can then print exactly the correct amount of material to conform to the crack shape and volume, thus repairing the crack. 3D printing technology has previously been used to repair spall damage in concrete road surfaces . But more recently has been demonstrated that it is also possible to 3D print asphalt into a crack to restore the road surface.

FIGURE 6. – Asphalt 3D Printing System Design

By selecting the hardest grade of bitumen, 10/20, the scientists hoped to match the in-use material properties as much as possible. The asphalt pellets were formed from larger pieces of asphalt by low temperature casting (below 150 °C) into a machined mould to obtain millimeter scale pellets. Asphalt is a substance made principally of long-chain hydrocarbon molecules in a colloidal structure of aphaltenes and maltenes with complex rheological properties. Above a threshold temperature, typically between the range of 30–70 ◦C, it behaves as a Newtonian fluid. Below this threshold, it undergoes shear-thinning. The rheology in this regime has been studied in detail and shows an Arrhenius-like behaviour. (Arrhenius equation is a formula for the temperature dependence of reaction rates). Much work has been done to study the effect on rheology of polymeric binders that are added to asphalt, these behave as viscoelastic components. In the face of this complexity, the scientists chose not to try to model the regime of rheology under different shear stresses as the asphalt pellets travelled through our extruder, but instead they aimed to find the optimum processing variables by carrying out a systematic empirical investigation of the extrusion process through a number of design iterations (such as design parameters of the extrusion nozzle, heating chamber, extrusion screw and process parameters such as temperature, power limit and torque limit).

FIGURE 7. – Asphalt 3D Printing Design Parameters

The results were remarkably good. Firstly, basic 3D printing experiments were carried out in which three single lines of asphalt of length 100mm were printed with 1mm layer height using an aperture width of 2mm. These experiments were performed using a range of print-head temperatures between 100 and 150 °C. Temperatures between 125 °C and 135 °C were found to be optimal to create a continuous extrusion of asphalt with a consistent line width. The 3D printed specimens showed up to 9 times the ductility of cast samples but had similar fracture strengths of around 2 MPa. The toughness of the moulded and 130 °C printed samples were found to be 10.2±7.1 and 24.1±7.2 J/cm2 respectively.

Observation of the fracture surfaces provides some explanation for the difference in ductility between the cast asphalt and the 3D printed asphalt. When the printed samples were cracked, a brown substance was revealed which was dotted throughout the sample cross section, that in many cases stretched out to bridge the crack, see Fig. 7. This brown phase and crack bridging effect were not observed in the cast test samples. Using X-ray photoelectron spectroscopy (XPS), the elemental composition of the brown phase was compared to the bulk. No significant differences were found in the composition, both being hydrocarbons with trace amounts of silicon and sulphur.

With these experiments the scientists have successfully managed to design, build and test an asphalt 3D printer capable of printing small objects and repairing cracks in asphalt. The main difficulty they encountered is that asphalt behaves as a relatively low melting point non-Newtonian liquid when the material is moving through the extruder as it is heated up, and then in between the extruder tip and deposition surface, as it cools down. Although polymers used in filament-fed 3D printers are generally non-Newtonian too, their simpler extruder system makes flow control much easier. Flow through the auger screw extruder used in asphalt case created a more complicated regime of rheology and pressure in relation to set temperature and other operational parameters which showed highly non-linear behaviour and made control of the extrusion process difficult. The functional constraints of some of the process variables affected the ability to print asphalt, for instance, the rotation speed of the auger screw is linked to the print speed (the extrusion multiplier is programmed to double the rotation speed if the print speed is doubled in order to deposit material at the same rate). The print speed was also limited by the materials properties of the auger screw (they used the high temperature SLA resin), since the low fracture strength of this resin limited the torque they could apply. The aperture affects the resolution of the printer, but again, low fracture strength of SLA resin limited their ability to reduce aperture size since it led to high pressures and resulted in mechanical failure. It is hoped that future designs with metal parts will allow us to explore a greater range of extrusion rates and print resolutions.

The impact of 3D printing on mechanical properties is interesting because it allows us to print a more ductile asphalt. There is a significant increase (up to 900%) in elongation to fracture for the printed samples. A possible explanation of this increased ductility lies with the appearance of a crack bridging component in the samples. It is hypothesised that the brown phase precipitated throughout the sample is composed of a lighter saturated fraction of the asphalt that has coalesced due to size dependent mobility conditions during the heating, screw mixing and/or extrusion process. Small amounts of softer components coalesce naturally in asphalt, but usually at scales of around 1–10 µm. Here, the components are around 20–100 µm in diameter. This means that the 3D printing process at this scale seems to avoid the degradation of its mechanical properties that can arise from leaving molten asphalt static for a long time. The 3D printing process seems to create a composite structure comprising of large concentrations of the brown phase dotted throughout the asphalt (as seen in Fig. 7 (c)), giving the material a higher toughness than cast asphalt. This would be advantageous to any crack repair scenario since sites of cracks on roads are often areas of increased stress or wear, and so depositing material with enhanced ductility could prolong the life of the repair.

With the correct parameters, is possible to modulate the asphalt properties through changing print temperature fairly rapidly over a small 10–15 °C range. Furthermore, the feed and pellet system make it relatively straightforward to add other materials such as small microaggregates or nanomaterials (initial test prints with 10% 10nm diameter titanium dioxide nanoparticles have been successful), and then vary the composition of the feedstock during printing to create more complex, functionally graded infrastructure materials with a wider range of properties.

It is believed that these improved and tunable material properties of 3D printed asphalt, combined with the flexibility and efficiency of the printing platform, offers a compelling new approach not just to the maintenance of road infrastructure, but by attaching it to a drone, opens up a new way to repair hard to access structures such as the flat roofs of buildings and other complex structures. The advantage of this is not only in being able to cut costs – other repair methods often requiring the erection of scaffolding and the closure or shutting down of infrastructure to gain access – but also the repair can initiated earlier before large scale deterioration has occurred. The development of such repair drones would have implications both for the way in which city infrastructure is repaired but also for the economic model that underpins it.

At the moment, much of city infrastructure is built to fail and then be replaced, with the capital costs of construction dominating the design parameters. Infrastructure designed to be continually monitored and repaired by fleet of drones promises to be a different model which could have economic benefits to society.

For instance, such an approach has the potential to be used for roads. If road degradation is continually monitored then small cracks can be repaired before they turn into potholes. By intervening at this early stage and repairing the crack autonomously using 3D printing the road surface might be preserved for longer. Such approaches have been explored for concrete road surfaces for spall damage repair. It was already demonstrated that the 3D printing of asphalt using a drone is possible. The next stage of developing this technology involves understanding the effect of environmental variables such as road temperature, air temperature, the local chemistry, interface with aggregate, as well as more comprehensive testing such as cyclic loading of repaired crack roads.

FIGURE 8. – Prototype road-repair drone in action

The materials science of repair is not the only consideration in the application of this technology. Identification and detection of crack morphology, especially in the case of complex-shaped cracks will be an important challenge. Automated computer vision systems are currently being explored to address this issue. The use of gantry systems versus the employment of 6-axis robotic systems is another design issue that is pertinent in the area of automated construction and repair. Although 6-axis systems have more
flexibility; for the repair of sub-cm small cracks in a horizontal road surface, a simple gantry system, such as the one I´ve talked about in this article, may well prove effective enough.

To sum up, the highlights of Asphalt 3D printing are the following:

  • With these experiments the scientists have successfully created a technique to 3D print asphalt — which is the first of its kind.
  • the 3D printed asphalt is more ductile than cast asphalt.
  • the changes in mechanical properties are related to the microstructural changes in asphalt that occur during 3D printing.
  • the mechanical properties of 3D printed asphalt depend on process conditions, this can be advantageous allowing toughness to be tailored to the repair.
  • the technique has the potential to be used on autonomous vehicles or drones to autonomously repair roads and complex infrastructure.

In conclusion it is now possible to build roads using the 3D printer capable of printing asphalt. It has been demonstrated that this technology can be used to 3D print asphalt into complex geometries, and to repair cracks. The mechanical properties of 3D printed asphalt are different from cast asphalt, showing up to 9 times the ductility of cast samples with similar fracture strengths. The increased ductility is due to microstructural changes in the asphalt which result in crack-bridging fibres that increase toughness. The material properties of 3D printed asphalt are tunable, and combined with the flexibility and efficiency of the printing platform, this technique offers a compelling new design approach to the maintenance of infrastructure.

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