Self-healing concrete as one of the construction trends in 2022

Many industry experts believe that in the near future we will see how self-healing concrete is used on roads, buildings and houses. Since concrete is the most widely produced and consumed material in the construction industry, experts believe that by 2030, about 5 billion tons per year will be used in the United States alone.

This is partly due to the urban boom that is observed in China and India. In terms of emissions quotas, the United States accounts for only 8% of the total in this area.

The history of self-healing concrete

In 2005, bacteria producing minerals were discovered that can help eliminate microcracks in concrete. Dr. Henk Jonkers, a microbiologist from Delft University (the Netherlands), became the founder of scientific developments in the field of the production of bioconstructions that can benefit civil engineering projects.

Dmitry Sorokin takes samples from Russian soda lakes. The bacteria used to develop self-healing concrete came from a highly alkaline natural source.

Self-healing concrete could solve the problem of destruction of concrete structures long before the end of their service life. Concrete is still one of the main materials used in the construction industry, from the foundations of buildings to the construction of bridges and underground parking lots. Traditional concrete has a disadvantage, it tends to crack when exposed to stress. A healing agent that works when bacteria embedded in concrete turn nutrients into limestone has been developed at the Faculty of Civil Engineering and Geology at Delft since 2006. This project is part of a broader program to explore the self-healing potential of plastics, polymers, composites, asphalt and metals, as well as concrete. Dr. Henk Jonkers, a microbiologist specializing in the behavior of bacteria in the environment, developed self-healing concrete in the laboratory and began full-scale outdoor testing in 2011. The first self-healing concrete products began to appear on the market in 2013. It is expected that they will increase the service life of many building structures.

Why is this necessary?

Tiny cracks on the concrete surface make the entire structure vulnerable because water seeps in, destroying the concrete and corroding the steel reinforcement, which significantly reduces the service life of the structure. Concrete withstands compressive forces very well, but not tensile ones. When stretched, it begins to crack, so it is reinforced with steel to withstand tensile forces. Structures built in conditions of high water levels, such as underground basements and offshore structures, are particularly vulnerable to corrosion of steel reinforcement. Automobile bridges are also vulnerable because the salts used to remove ice from roads penetrate cracks in structures and can accelerate the corrosion of steel reinforcement. In many building structures, tensile forces can cause cracks to form, and this can happen relatively soon after the structure is built. The repair of conventional concrete structures usually involves the application of concrete mortar, which is glued to the damaged surface. Sometimes a mortar needs to be inserted into an existing structure using metal pins so that it does not fall off. Repairs can be particularly time-consuming and expensive, as it is often very difficult to access a facility for repairs, especially if they are underground or at high altitude.

An example of a badly damaged concrete support beam of a road bridge. This pillar has been corroded by reinforcement due to the ingress of anti-icing salts through microcracks formed in the concrete

How does biobetone work

Specially selected species of bacteria of the genus Bacillus, along with a calcium nutrient known as calcium lactate, as well as nitrogen and phosphorus, are added to the ingredients of concrete when it is mixed. These self-healing agents can lie dormant in concrete for up to 200 years.However, when the concrete structure is damaged and water begins to seep through the cracks that appear in the concrete, bacterial spores germinate upon contact with water and nutrients. Once activated, the bacteria begin to feed on calcium lactate. As the bacteria feed on oxygen, it is consumed, and soluble calcium lactate turns into insoluble limestone. Limestone hardens on the cracked surface, thereby compacting it. It mimics the process by which bone fractures in the human body are naturally healed by osteoblast cells, which mineralize to reshape the bone. Oxygen consumption during bacterial conversion of calcium lactate into limestone has an additional advantage. Oxygen is an important element in the corrosion process of steel, and when bacterial activity has used it up completely, it increases the durability of steel reinforced concrete structures. Two parts of the self-healing agent (bacterial spores and calcium lactate-based nutrients) are injected into the concrete in the form of separate granules of expanded clay 2-4 mm wide, which ensure that the agents will not be activated during the cement mixing process. Only when the cracks open the granules and the incoming water brings the calcium lactate into contact with the bacteria, they are activated. Tests have shown that when water seeps into concrete, bacteria germinate and multiply quickly. They turn nutrients into limestone for seven days in the laboratory. Outside, at lower temperatures, the process takes several weeks. The starting point of the study was to find bacteria capable of surviving in an extreme alkaline environment. Cement and water have a pH value of up to 13 when mixed together, it is usually a hostile environment for life: most organisms die in an environment with a pH value of 10 or higher. The search focused on microbes that thrive in alkaline environments that can be found naturally, such as alkaline lakes in Russia, carbonate-rich soils in desert areas of Spain and soda lakes in Egypt. Samples of endolytic bacteria (bacteria that can live inside rocks) were collected together with bacteria found in lake sediments. It has been found that bacterial strains of the genus Bacillus thrive in this highly alkaline environment. Back at Delft University, bacteria from samples were grown in a flask with water, which was then used as part of an aqueous mixture for concrete. Various types of bacteria were embedded in a small concrete block. Each concrete block will be left for two months to be firmly installed. Then the block was crushed into powder, and the remains were checked to see if the bacteria survived. It was found that the only group of bacteria that could survive were those that produced spores comparable to plant seeds. Such spores have extremely thick cell walls that allow them to remain intact for up to 200 years, waiting for a better environment to germinate. They are activated when the concrete begins to crack, food becomes available, and water seeps into the structure. This process lowers the pH of highly alkaline concrete to values in the range (pH from 10 to 11.5), at which bacterial spores are activated. Finding a suitable food source for bacteria that could survive in concrete took a long time, and many different nutrients were tried until it was discovered that calcium lactate is a carbon source that provides biomass. If it begins to dissolve during the mixing process, calcium lactate does not affect the setting time of concrete.

Before and after. Photos of the surface of the self-healing concrete slab. The crack is visible in the left image, and on the right white limestone filled the gap

The first full-bridge tests

Full-scale testing of new concrete structures was started in Europe in 2011. A small structure or part of the structure was created from self-healing material and has been observed for several years. Also, certain structures are equipped with some panels of self—healing concrete, and others with ordinary concrete, so that the behavior of these two elements can be compared. Is it possible to use the above-mentioned bacteria to repair existing structures? To answer this question, the University of Delft has received funding in the amount of 420,000 euros from the Government of the Netherlands. Two postdoctoral scientists will spend two years developing a self-healing system that will be applied to existing structures. Two systems are being tested during the study. In the first method, bacteria and nutrients are applied to the structure in the form of a self-healing solution that can be used to repair large-scale damage. In the second method, bacteria and food nutrients are dissolved in a liquid that is sprayed onto the concrete surface, from where it can seep into cracks. The Dutch government has allocated 450,000 euros to finance another research project, which will be carried out on concrete basement walls and pre-cast concrete floors that are vulnerable to groundwater. Meanwhile, work continues to address industry concerns about whether bacteria can survive in a dormant state for the entire life of a concrete structure. Data from soil samples taken from desert areas and stored in museums show that the soil still contains live bacterial spores after 200 years of storage. To solve other problems, laboratory tests are conducted aimed at accelerating the aging process of self-healing concrete. The tests will expose the concrete to extreme conditions to simulate the changing seasons and cycles of extreme temperatures, wetter periods and drying periods.

Disadvantages of self-healing material

There are two key hurdles to overcome if self-healing concrete is to transform concrete construction in the next decade. The first problem is that clay pellets containing a self-healing agent make up 20% of the volume of concrete. This 20% usually consists of a harder aggregate, such as gravel. Clay is much weaker than conventional aggregate, and this weakens concrete by 25% and significantly reduces its compressive strength. In many structures, this would not be a problem, but in specialized applications where higher compressive strength is required, for example in high-rise buildings, it will not be viable.

The second disadvantage is that the cost of self-healing concrete is about twice the cost of conventional concrete, which currently amounts to about 80 euros per cubic meter. According to the engineers, at a price of about 160 euros per cubic meter, self-healing concrete will be a viable product only for some building structures where the cost of concrete is much higher due to its much higher quality, for example, for laying tunnels and offshore structures, where safety is a big factor, or in structures where there is limited access for repair and maintenance. In these cases, the increase in cost due to the introduction of self-healing agents should not be too burdensome. In addition to this, if produced on an industrial scale, it is believed that self-healing concrete can significantly decrease in price. If the service life of the structure can be extended by 30%, then doubling the cost of the concrete itself will still save a lot of money in the long run.