Highway, heal thyself

The Biomimicry Column

Highway, heal thyself

Sustainable road materials — some of them designed to be "self-healing" — are getting a boost as researchers seek alternatives to concrete.

On a recent drive along the dense highway corridor from New York to Boston, an hour-long traffic jam quickly put the brakes on a bright, sunny morning.

“It must be an accident," I thought.

But when I was finally able to squeeze through the one lane still open, I realized the real source of the delay: Two men were shoveling cold asphalt out of the back of a dump truck as it trundled along ahead of a flashing police car.

They were throwing the granular bits into all the winter potholes they could spot and counting on all of us motorists to tamp it down. Along the roadsides, I could see where at least a quarter of these fragments actually ended up.

I was incredulous, but it got me thinking about materials and some clever innovations already developed that might have prevented this costly delay for thousands of commuters, not to mention the damage done by the potholes in the first place.

Beyond the issue of cost is that of safety — and it is not trivial, as recent events in Nepal have shown. Providing less vulnerable materials at less cost could save lives.

The potential environmental stakes are also high. From the reduction of greenhouse gases and use of energy in the production of concrete to the elimination of maintenance and stretching material lifespans, a new world of benefits could evolve from smarter and safer structures.

Can biology help make 'dumb materials' smarter?

One of these innovations is a mixture created by Erik Schlangen and his civil engineering team at Delft University of Technology in the Netherlands. They have added another ingredient to the traditional composite of bitumen and aggregate: steel.

The steel’s purpose is not structural, however. Thin strands of it serve as heat sinks, like the barista’s trick of putting a metal spoon into a glass before adding a hot liquid. The spoon’s lower thermal capacity means that its temperature will rise faster than the brittle glass, and thus prevent it from shattering.

When an induction heater is passed over the asphalt, the metal reaches temperatures high enough to melt the asphalt and fill any cracks. The asphalt then hardens and seals up the crack when the heat is removed.

Schlangen has installed a 400-meter test section on a Dutch road and has been encouraged by a technology that is still in the development stage. He estimates that a pass with their so-called “healing machine” every four years will double the lifetime of the average road, saving a lot of money in capital improvement costs alone.

While not exactly “self-healing,” as people still have to run the heat source machine, the Dutch asphalt example is part of a growing group of smart materials that will increase the durability and resilience of our structures.

Nature has provided the models for several of these innovations, but it is important to recognize the limits of the translation from the biological to the technological.

As in many natural processes, a complex transfer of information drives these everyday miracles. For example, when you cut your skin, your body responds immediately.

Some of the millions of microscopic nerve endings in the skin have signaled the body to constrict the blood vessels in the area of the wound. Enzymes leaked from the torn blood vessels initiate the flow of platelets to the site to plug the tear and form a fibrin net with clotting proteins. This net eventually will form the familiar scab over the wound.

Once the bleeding is controlled, the body sends white blood cells to combat any foreign objects. When flow and infection are controlled, fibroblasts are sent to the site to produce the protein collagen to rebuild the skin structure beneath the scab, including the essential oxygen-carrying blood vessels.

Most engineers applying self-healing concepts to some of our “dumbest” materials, asphalt and concrete, are not trying to emulate the details of a complex biological process, but control simple foundational reactions or structures. It is probably useful to distinguish between self-sealing and self-healing.

The natural metaphor from our example above would be the forming of the scab over the wound, as opposed to the regeneration of the skin underneath. True self-healing materials, however, need structural integrity to actually be restored.

Cracking the case of unsustainable concrete

Concrete is a mixture of water, Portland cement and aggregate. While very strong in compression, the substance is quite weak in tension — which is the reason that steel rebar is often added for reinforcement. Materials such as fiberglass also have been successfully added to aid in local tensile strength. 

Despite these improvements, cracking still occurs and most of the history of concrete use has entailed the control of this cracking, rather than its prevention.

When water gets into cracks, it serves to accelerate their growth, particularly if freezing and thawing are part of the local regime. Wedging by stray particles and chemical intrusion of substances such as the chlorides from saltwater are also causes of concern.

At the University of Illinois at Urbana-Champaign, engineers are investigating the use of a common soil bacteria to create a biomineral when triggered by a change in oxygen and pH caused by a crack.

This is bio-utilization as well as bio-inspiration, and the Bacteria pasteurii are seeded into the concrete mix, inactive until prompted to produce calcium carbonate, much as they might do naturally in limestone.

At Ghent University, researchers are looking at three material additives to achieve self-healing: bacteria, hydrogels and polymers. In the Ghent process, bacteria are triggered by water intrusion to precipitate the biomineral.

The bacteria are attracted to the micro-nutrients in the water and their subsequent metabolizing produces the calcium carbonate that seals the crack.

The researchers have looked at the use of hydrogels in combination with the bacteria to aid biomineralization. Hydrogels can be made from many materials but are distinguished by their ability to absorb water, often many hundreds of times their own weight.

They are networks of polymer chains that are cross-linked and have hydrophilic groups. From a phase perspective, they are interesting because they can be categorized as neither solid nor liquid, but have properties of both; they contain 50-90 percent water but cannot be dissolved.

As a result, the gel will exist and behave somewhat like a solid because of the three-dimensional crosslinking within the polymer chains. The blood clots I had mentioned above are examples of hydrogels, as are cartilage, the vitreous humor of the eye, tendons and the mucus in your body.

In the Ghent concrete mix the hydrogels, both protect the bacteria from the stresses of mixing and pouring and collect the intrusive water for the bacteria to feed on.

The Ghent researchers are also investigating how to employ existing synthetic polymer surface sealants on the inside of the concrete and are experimenting with different types of breakable capsules. Challenges include achieving an even distribution and developing a vehicle that can withstand the forces of mixing and pouring, but then breaks as a result of cracking.

Borrowing from the body

In perhaps the most direct study of nature to improve concrete, a member of Schlangen’s lab at Delft has been inspired by the structure and behavior of bone.

Besides being a living tissue, bone is a classic example of a hierarchically structured composite material, and the Delft team has attempted to translate these attributes to a porous concrete network.

At the macro scale, the engineers sought to mimic the two basic types of bone structure: hard (cortical) and soft (cancellous). They then distilled into three steps the process they had observed for bone healing: inflammation; bone production; and bone remodeling.

The resulting prototype was a sandwich of hard, self-compacted concrete surrounding an interior layer
of porous concrete through which a two-part epoxy could be injected.

The porous layer was first filled with a PVA mix that would dissolve after all the lifts had been compacted and cured, leaving the voids. They then notched the outer layer in a test and injected the epoxy into the middle porous layer and got a successful end to the crack propagation.

Unlike many of the other techniques, this method would rely on sensors and actuators, rather than a physical reaction of the materials, to employ a response.

But this approach presents some unique challenges, precisely locating breaks and adjusting pressure and viscosity of the epoxy from the right outlet for full penetration being among them.

These are just a few tactics being pursued to put some life into our roads and buildings. They will have a wide impact if costs can be reduced and manufacturing techniques scaled up.

Five major tactics currently are employed in the industry: chemical encapsulation; bacterial encapsulation; chemicals in brittle tubes; custom admixtures; and fibers. Broadly speaking, the benefits of more sustainable alternatives to old-school concrete could include more resilient infrastructure, better safety, cost savings and increased environmental sustainability.

If society is to produce things that are more efficient and less costly in time, energy and materials, then a biomimetic approach often has been quite useful. When it comes to the way we build our roads, nature's single biggest point of inspiration just might be self-healing.