Self-Healing Concrete Solutions

The City of Chicago spends around US$200 million every year in repaving roads and sidewalks that are damaged during the winter, according to the Chicago Department of Transportation. Smart materials could help build longer-lasting infrastructure with potentially large savings for cities and nations. Self-healing bio-concrete (or self-healing concrete), first developed by Henk Jonkers and Eric Schlangen at Delft University of Technology in the Netherlands, includes in its composition a type of bacteria that refills cracks with new concrete, closing them and thus repairing the damaged surface.

Conventional concrete

Concrete is the most widely used construction material today. Romans started using concrete for their enduring architectural wonders around 200 BCE. Cement is manufactured from a variety of ingredients, including limestone and clay. The ingredients are finely ground and mixed and heated in a rotating kiln at a high temperature (up to 3,000 degrees Fahrenheit). The raw feed chemically changes to produce a pellet-sized product called clinker. The clinker, with a small quantity of gypsum, is then ground to a very fine powder called Portland Cement or cement. When cement and water are mixed, a crystallization process occurs. Then aggregates (sand and gravel) are added, and the hardening reaction binds the mix into concrete.

Admixtures are chemicals added to the mix to achieve special purposes. For example, to increase workability at the same water content, or to change the setting time or to compensate for the shrinkage of concrete when it hardens. They can also increase strength, improve resistance to freeze-thaw damage, reduce permeability and inhibit corrosion of steel reinforcement. For example, retarders can temporarily prevent hydration from taking place, slowing the reaction and allowing the concrete to be workable during the placing process. To help infrastructure resist freeze-thaw damage, “air entraining” admixtures are commonly used to stabilize air bubbles created during mixing. This provides space for water to expand and contract, increases the resistance of concrete to freeze-thaw damage and avoids premature cracking.

Environmental concerns

More than 10 billion tons of concrete are produced worldwide each year, requiring vast amounts of natural resources. In the United States alone, 84 million tons of cement were produced in 2017, according to a United States Geological Survey report. The production of one ton of Portland cement releases approximately one ton of CO2 into the atmosphere, and the world production of cement accounts for seven percent of the world’s carbon dioxide emissions every year. Concrete use is expected to soar because of building booms in developing countries such as China, which consumed more cement from 2011 to 2013 than the quantity used in the United States in the entire 20th century.

Cement production is also energy intensive, requiring about four Giga Joules per ton despite improvements in energy efficiency. The concrete industry also uses about one billion cubic meters of water each year worldwide, an enormous quantity that can be a particular burden in locations where water is not abundant. Finally, the demolition and disposal of concrete structures has a negative impact as recycling the construction debris is difficult and contributes significantly to landfilled solid waste.

Concrete cracks

While concrete can handle significant compressive loads, its tensile and flexural load carrying capacity is limited. Application of tensile loads, expansion and contraction, freeze-thaw damage or deterioration of the steel reinforcement can cause concrete to crack. Cracks can appear because of the shrinkage experienced when water not used for hydration gradually evaporates from the hardened mix. Concrete can also undergo volume changes due to thermal effects, and this expansion and contraction can lead to cracks.

Freeze-and-thaw cycles leave cracks in concrete roads, and when salt is spread to defer ice formation, it enters the concrete and damages the steel reinforcement. Some cold-climate cities are beginning to use sand instead of salt to avoid this problem. Cracks can result in problems such as water leakage, frost damage and reinforcement corrosion. Repairing concrete, in addition to being costly, causes significant inconvenience to residents. Construction-related traffic congestion affects people’s everyday lives. Are there longer-lasting alternatives to traditional concrete that construction and facility managers can use?

Self-healing bio-concrete

Traditional concrete has always been produced with inert materials. However, “self-healing bio-concrete” changes this paradigm. Inspired by biology, this concrete is made of “living materials” and can heal itself, like trees or human skin. It can autonomously repair itself after crack formation, with no or limited human intervention, and that can dramatically reduce maintenance and repair costs. This product is a sustainable solution that improves the lifespan of buildings, bridges and roads.

The concept of self-healing concrete was first developed by Carolyn Dry, an architecture professor at the University of Illinois, Urbana-Champaign, in the early 1990s. Now, it appears that the first commercial products are in sight. Microbiologist Hendrik M. Jonkers developed a “self-healing concrete containing bacteria” that earned him the European Inventor Award 2015. Subsequently, more healing products based on bacteria and fungi have been developed, including a repair mortar for patching cracked concrete and a liquid-based repair system that can be sprayed to seal cracks that have already occurred in traditional concrete structures.

How does self-healing work?

Self-healing concrete introduces a healing agent into the concrete mixture as another type of aggregate. The healing agent consists of three components: the active organism (bacteria or fungi) that acts as a catalyst, a mineral that acts as “food” for the organism and capsules that are containers. The capsules need to survive the mixing and casting of the concrete and should remain intact until the organisms’ action is needed to close the cracks. Spores of limestone-producing bacteria like Bacillus pseudofirmus, which are alkali-resistant, are well adapted for concrete applications. They produce endospores with low metabolic activity, are able to resist high mechanically and chemically induced stresses and can remain dormant for up to 200 years inside the concrete.

When concrete cracks and water and air reach the capsules, the environmental conditions are favorable for the spores to germinate and grow into active bacterial cells. The bacteria then feed on the calcium lactate and form limestone, sealing the cracks. This healing happens in just three weeks. Calcium lactate is “food” for the bacteria and does not affect the material properties when mixed in the concrete. Biodegradable plastic is used for the capsules as it has to be strong enough to survive the mixing phase while bonding well with the surrounding hydrated cement paste. It is also able to break when cracks form to release the bacteria and nutrients. In addition, the capsules need to be inert to the chemicals and additives in concrete and to the healing material they are carrying. This has been achieved by the immobilization of the bacterial spores and the calcium lactate in expanded biodegradable plastic particles. They represent a structural element of the concrete and a protective matrix for the self-healing agent.

Current self-healing bio-concrete has proved to be able to close cracks up to any length, provided they have a width of 0.03 inches or less, according to the European Patent Office. Research efforts are underway to increase the width, which enables more opportunities for application.

Apart from cracks, there is another important surface flaw in concrete called “scaling degradation,” which results in “spalling.” This appears during freeze/thaw cycles in the presence of de-icing salts. Test results showed 50 percent less scaling with the self-healing concrete compared to untreated samples. Like concrete, self-healing bio-concrete is non-flammable and non-explosive. By making concrete more long-lasting, the need for cement is reduced.

Bio-concrete challenges

It is difficult to change the mindset of contractors who have worked for years with regular concrete. It might be hard to convince companies to invest in self-healing bio-concrete as they might anticipate more expenses for material, installation and maintenance. However, self-healing materials can be added as another aggregate in the concrete mix. Companies can proceed as if it were traditional concrete.

Self-healing bio-concrete currently has a production cost that is almost twice that of regular concrete. This is driven by the calcium lactate nutrient for the bacteria (European Patent Office). Active research is underway to develop alternative nutrients that would reduce the cost to a level closer to regular concrete, between US$100 and US$120 per cubic meter. However, concrete often accounts for only a small percent of the total cost of construction projects. The higher initial cost of self-healing materials can be recovered in three to four years.

Chicago, a city of 2seasons: winter & construction

The majority of Chicago’s roads are made of composite pavement, which consists of different layers of materials bonded together: a top asphalt layer, a Portland cement concrete base, a coarse aggregate base and the natural subsoil. The main challenge with these streets is the surface layer cracks during the winter because of freeze-thaw damage. Due to extreme weather conditions, Chicago has a paving season that starts in the spring as soon as temperatures allow construction work.

In fiscal year 2014, the city budgeted US$276,272,834 for street maintenance activities. Resurfacing and reconstruction accounted for 90 percent of those funds and pothole filling was the rest. A Chicago Sun-Times analysis stated potholes are a real issue, as complaints rose 14 percent in the first two months of 2018 compared to the same period the previous year. In 2013, the city paid an estimated US$181,217 on 754 damage claims, or about US$240 a claim. However, the estimated damage to vehicles due to potholes averages over US$300 per vehicle per year.

Can bio-concrete help?

Self-healing bio-concrete seems perfect for residential streets, allowing a longer service life and avoiding problems with potholes. Models using National Ready Mixed Concrete Association (NRMCA) and CDOT data, along with reasonable assumptions, suggest an initial investment of nearly US$2.5 billion is needed to pave 4,000 miles of Chicago’s residential streets with self-healing bio-concrete. This can be divided into a multi-year program.

As the city needs less repair and maintenance activities, construction work will be reduced, and this will save time and frustration for citizens. A 50 percent decrease in construction-related delays can save citizens more than US$600 million in estimated time value of money. In addition, self-healing bio-concrete would reduce the transportation of asphalt needed for repairs and save about 1.20 billion gallons of diesel fuel nationally and reduce CO2 emissions by an estimated 13.8 million tons each year.

Constructing a sustainable future

Smart, self-healing construction materials open a variety of paths toward sustainable construction and maintenance of facilities. Volume production of these materials is in the near future, and more applications can drive costs down and fund further research. It is time for the public and private facility owners and managers to move from conventional techniques and invest in materials that are better for the environment and have lower operating expenses. These forward-looking industry professionals can then tell the roads and other facilities, in all seriousness, to “heal thyself.”