A directly visible consequence of concrete crack formation is water leakage, while the durability of the material may be indirectly affected.
Fortunately concrete has a built-in ability to repair cracks, mainly by formation of limestone deposits on the crack surface, sealing the crack entry. The limitation of this ability determines the maximum crack widths that can be blocked by the concrete itself, autogenously. In case this system can be extended, by formation of additional minerals such as limestone, larger cracks may be able to automatically seal. One of the ways to produce additional stone-like material is through a process occurring in nature: bio-mineralization. In the case of concrete, a way to promote mineral formation is by using bacteria that metabolically convert nutrients and form minerals in the process. Bacteria and its nutrients can be mixed in with concrete or mortar to build in additional healing capacity in new materials. Alternatively, bacteria and its nutrients can be applied on existing concrete surfaces in order to seal cracks against leakage or water intrusion, or to densify the surface as a way to enhance resistance against freeze-thaw damage and salt scaling.
If we look at concrete structures, the material consists not only of concrete, but also of reinforcement. Behavior of concrete structures is dependent on the interaction of the reinforcement inside and the concrete matrix. Generally speaking, the concrete takes up the compressive load and the reinforcement of the tensile stresses. Tensile stresses can be expected due to the structural design, but also can appear due to deformations of the concrete material, as in the case of shrinkage.
In order for the reinforcement to activate and start functioning, the concrete matrix needs to be cracked. In practical guidelines it is recommended to limit crack widths, either based on aesthetics, durability requirements or water tightness. In an effort to retain water tightness of structures, crack-width limitations are quite stringent. Often additional reinforcement is applied to control the width of cracks, without a structural need, adding extra cost to building the structure. Allowing larger crack widths may reduce the addition of deformation-controlling reinforcement.
The application of dormant bacteria to a concrete mix matrix can allow the bacteria to survive the mixing process and yield immediate and inherent benefits to the final concrete setting’s ability to self-heal, such as the above Ecuadorian irrigation canal cross-section.
A new concrete cure
Maximum crack widths for water-bearing structures are generally based on the autogenous healing ability of concrete, which is the capacity for the concrete to repair its own cracks. This autogenous healing mechanism is primarily based on calcium carbonate formation, due to a reaction of one of the products of cement hydration, calcium hydroxide or Portlandite, with carbon dioxide from the environment. This reaction also is known as concrete carbonation. Internal carbon dioxide production in cracks may enhance mineral formation and internal crack blocking. Carbon dioxide in the alkaline surrounding concrete becomes carbonate, which may deposit with present calcium elements in the form of calcium carbonate and limestone. Carbon dioxide can be produced from the microbial metabolic conversion of nutrients containing carbon, such as sugars and organic salts. By incorporating bacteria and the specific nutrients, the healing capacity of concrete can be enhanced autonomously.
The easiest way to apply bacteria and its nutrients to concrete is by preparing a liquid and spraying it on a porous or cracked concrete surface. Upon mineral formation, the surface becomes again densified. This method is applicable for repair and maintenance work. More challenging is to incorporate bacteria and its nutrients from the moment mortar or concrete is made, so that the repair occurs automatically.
The easiest way to apply bacteria and its nutrients to concrete is by preparing a liquid and spraying it on a porous or cracked concrete surface.
For this application, particles can be developed that consist of dormant bacteria and its nutrients. Bacteria in a dormant state, similar to plant seeds, can survive the harsh conditions of concrete and remain viable for prolonged periods of time. The particles can be mixed in with the sand fraction of mortar or concrete, so that the particles are widely distributed through the matrix, which is packed with healing potential. Upon cracking of the hardened concrete matrix, the particles are hit and the ingredients can be released. Water disperses the ingredients in the surrounding crack volume and bacteria can activate, grow out into a colony and start converting the nutrients into crack-blocking minerals.
Since the biological ingredients are dispersible in water, particles may disintegrate during the wet stage of the mixing process. In order to prevent this, the particles can be treated so that the ingredients are protected during the mixing stage. This can, for instance, be done by applying a protective barrier to the particles in the form of a coating. Since part of the particle may still be released during mixing, it is important that the selected nutrients do not interfere with concrete hardening or affect functionality of other additives, such as a superplasticizer. In order to limit the effect, nutrients can be selected that show negligible influence on cement hydration and strength development.
The permeability of the cracks before and after application of the repair system is monitored by measuring the amount of water leaking through the crack in a specified timeframe.
In order to check the functionality of the system, main tests must indicate the retention of water tightness. Mortar or concrete samples can be either split or bent to the point a specific crack width is reached. Upon addition of a water layer or water pressure to the crack, the sample starts to leak. The amount of water that leaks through the crack at this point determines the initial water flow rate. After a healing regimen, the amount of water flow through the crack is again measured and the decrease in water flow can be followed in time.
For healing, the bacteria-based system needs the presence of water, which can be delivered in various ways, such as through immersion in water, undergoing wet-dry cycles, or placement in a humidity room or in outside conditions. Other interesting results may be obtained by testing properties of surface densification, such as capillary sorption or freeze-thaw salt scaling.
Outdoor trials are being executed mainly on existing structures, using the liquid system for maintenance and repair and mortar with bacteria- and nutrient-containing particles for patch repair. Main applications tested are parking decks, where cars drive in water and de-icing salts. Other applications are at water-retaining walls, where a hind-lying water head leads to water seeping through porous parts and cracks.
Behavior of the repaired material can be compared to the situation before application and in comparison to a control repair in the form of a commercially available material. In order to check for water-tightness retention of water-bearing cracks, the amount of drops of water per minute can be counted over time and compared to control areas without the addition of bacteria-based material. Other than functionality, the way of application is of importance, which should be similar to alternative methods.
For structural concrete, a first trial project was executed by producing part of an Ecuadorian irrigation canal out of concrete with bacteria and nutrients. Given the severe daily temperature changes and long stretches of canal, the concrete generally ruptures and therefore leaks, leading to the farmers in the valley not receiving their precious water. In close collaboration with local parties, a mixture was developed using locally available materials, with the addition of particles containing bacteria and nutrients. In time the performance of the canal section will be monitored.
In the future, more extensive application in new concrete structures is expected, especially for water-retaining structures. Think, for instance, of reservoirs, tunnel linings and retention walls. More specifically, areas sensitive to leakage problems may be covered, such as near dilatation joints.
In order to extend the applications range, the general idea is to add the bacteria-based particles to commercially used materials—mortar and concrete. In this way the main characteristics of the concrete are governed by currently known mixtures, with addition of an extra function in the form of automatic blockage of cracks in watery conditions. In order to enable a large-scale application, additional determination of properties is required according to the norms and guidelines known in practice. R&B