Self-healing concrete: definition, mechanism and application in different types of structures

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Российский университет дружбы народов, Москва

SELF-HEALING CONCRETE: DEFINITION, MECHANISM AND APPLICATION IN DIFFERENT TYPES OF STRUCTURES

Alhalabi Z.Sh., Dopudja D.

Аннотация

Бетон - это один из наиболее часто используемых строительных материалов. Однако он также является одним из основных производителей углекислого газа (CO2), который способствует разрушению нашей окружающей среды. Кроме того, содержание бетонных конструкций требует огромных расходов каждый год. Трещины различных размеров образуются во всех бетонных конструкциях. Их необходимо устранять вручную, что сокращает срок службы конструкции. Именно поэтому самовосстанавливающийся бетон (СВБ) является революционным строительным материалом, который способен решить все эти проблемы и, безусловно, является строительным материалом будущего. Поэтому нам нужно понять его свойства и то, как работает этот механизм, как он повлияет на архитектурные проекты в будущем, и как будет использоваться для создания полезных и эстетичных зданий и сооружений.

Ключевые слова: самовосстановление, бетон, строительные материалы, умный материал, трещины, механизм, ремонт, дизайн, архитектура.

Abstract

Concrete is one of the most used building materials. However, it is one of the major producers of carbon dioxide (CO2) which is directly contributing to destroying our environment. Not to mention that enormous costs are being spent each year to maintain concrete constructions. Cracks of various sizes form in all concrete constructions which need to be sealed manually shortening the life of a particular construction. On the other hand, Self-healing concrete (SHC) is a revolutionary building material that has the solution to all these problems and is definitely the building material of the near future. Therefore, we need to understand its properties and mechanism and foresee how it impacts the architectural designs of the time to come, which standers are needed to create useful and aesthetic buildings and constructions.

Keywords: self-healing, concrete, building material, smart material, cracks, mechanism, repair, design, architecture.

Introduction

The word concrete is originated from the Latin word “concretus” which means condensed and hardened [1]. The earliest use of cement is dated back to twelve million years ago [2], while the early use of concrete-like building material is dated back to 6500 BC. [3]. However, it wasn't formed as concrete until later during the Roman Empire.

As revolutionary as it was and still is, modern concrete (Lime-based) has a short lifespan caused by the formation of cracks shortening the longevity of a particular construction. Many researchers have been attempting to improve concrete in order to get a better longevity among many other things. That's how the concept of self-healing finds its way to concrete. There are two main areas of research when it comes to developing this kind of concrete; the natural way of hydrates to seal cracks over time, and the artificial way to seal cracks which needs a man-made intervention. The main purpose of such work is to increase concrete's durability, which will have a huge positive impact on both the environment and economics.

On the other hand, it might also improve the architectural designs by forcing new design methods and hence, change the shape of internal spaces so that it serves many functions and provides flexibility.

Definition of Self-healing

A self-healing material is described as a material that is capable of repairing itself back to the original state. The concept of self-healing concrete (SHC) that happens over time (autogenic) has been noticed for over 20 years. It can be observed in many old structures which have remained standing for long periods of time in spite of the fact that they have limited maintenance. This observation concludes that the cracks heal when moisture interacts with non-hydrated cement clinker in the crack. Nevertheless, in present-day constructions the cement is lowered as a result of modern construction methods. Hence, the amount of available non-hydrated cement is less and therefore, the natural healing effect is reduced.

The principal phases of the natural healing ability are the inflammation and hydration of cement pastes; followed by the precipitation of calcium carbonate (CaCO₃), and lastly the obstruction of flow paths as a result of the deposition of water impurities or the movement of some concrete bits that get detached throughout the cracking process [4, P. 19]. Many factors are considered in the natural way of healing, such as; temperature, degree of damage, freeze-thaw cycles, the age of the concrete and the mortar state.

As for the artificial way to repair cracks in concrete, which is man-made self-healing process was first invented in 1994. The main method and first approach was to use a healing agent (adhesive) which is encapsulated inside a micro capsule, once a crack forms, it causes the micro capsules to break, releasing the healing agent, hence healing the crack. The adhesives can be stored in short fiber or in longer tubes (Nishiwaki et al 2006, Joseph 2008, Joseph et al. 2008) however, more effective mechanisms were later approached by researchers at Cardiff University, the University of Cambridge, the University of Bath, and Korea Institute of Construction. In this article two of the main approaches - that seem promising and distinguished - will be tackled briefly alongside the advantages and disadvantages of using this kind of concrete, which will soon be inevitably used worldwide.

Main Approaches and Their Mechanism

There are many approaches to create smart concrete and enhance its properties while reducing the cost of overall use of the material. Many of these approaches were dedicated to create SHC; two of the main approaches have proved to be efficient and easy to use.

Bacteria-Based Healing Process

Also known as Bio-Concrete; this kind of concrete uses a simple process to close the formed crack. The main mechanism is achieved by making a concrete mixture that contains (i) a precursor like calcium lactate (Ca(C3H5O2)2) and, (ii) bacteria planted in micro capsules (or just added to the mixture) that will later germinate, once the water reaches the crack. As soon as the bacteria germinate, they produce limestone (CaCo3) caused by the multiplying bacteria. Dr. Richard Cooper of Bath's Department of Biology & Biochemistry says that incorporating bacteria in concrete adds a double layer shield in order to prevent corrosion in steel. Not to mention that it employs oxygen present which would then benefit the process of steel corrosion [5].

The bacteria which are applied in this kind of concrete are Spore-forming and alkali-resistant bacteria. Bacteria from this group are the most suitable as they are spore-forming and can live for more than 200 years in dry conditions [6 - p.102]. Therefore, using bacteria as a healing mechanism is one of the best mechanisms to produce this kind of concrete because of its sustainable organic properties.

Shape Memory Polymers

New smart materials (SMP) that are capable of returning to their initiative state by changing back their form upon applying a stimulus. [7, P. 2034] This mechanism employs both the autogenic and autonomic principles. It uses a man-made system to increase the natural autogenic healing and seal cracks in concrete. This kind of polymers is semi-crystalline polymers that have a predefined shape memorized in their structure that later helps the polymers to go back to their original state.

When a crack occurs, the system will be triggered, hence, the shape memory polymer within the crack gets activated through heating which can be in the form of direct heat, or an electrical current. As soon as it's activated, the shape memory effect or shrinkage takes place, and due to the restrained nature of the tendon, a tensile force is generated, hence the crack closes on itself. After that, the autogenous healing starts taking place.

Factors That Affect the Use of Self-Healing Concrete

There are many factors that intervene with the usage of this kind of concrete. As it is noticed; it is not yet used in all new constructions as it is still being under development. Recently Self-healing bacteria-based concrete has been successfully tested on a full-scale in the University of Bath in the UK [8, P. 6-8]. However, the cost of using it is still not determined as it is hard to predict a full cost. The cost efficiency is one of the most important factors and will determine whether the material will have limited usage restricted to spots that are hard to fix and important constructions such as bridges and highways.

Other than cost, long-term efficiency is one of the important factors as well alongside the size of the formed cracks which must not exceed 150 millimeters of depth to establish an ideal result.

All in all, some factors that will definitely determine whether SHC will be used as a replacement of concrete are; the economical factor, long-term efficiency, prospect suppliers and safety factors.

Application in Architectural Designs and Structures

Since the use of SHC seems promising, we must understand how that will affect the forthcoming architectural designs. A general prognosis is hard to make as the function and size of building plays a huge role to whether or not this kind of concrete might be suitable, and therefore, will be discussed separately.

Application in Small-Size and Medium-Sized Buildings (Residential and Public)

concrete architectural building smart

Size and function of a building usually determine the approximate life-span desired for this particular construction. Small-size buildings are usually residential and located either in the suburbs, towns or villages. And like most buildings, concrete is one of the main building materials used, especially for foundation (slabs or columns), as small residential buildings rarely change function it is practical to want to increase their life-span, and hence use SHC.

Medium-size buildings use more concrete than any other size of buildings, unlike like skyscrapers that use more steel and small-size buildings which use more stone or wood. Both residential and public middle-sized buildings seem to be eligible to the use of SHC, however, and especially in public buildings as the life-span increases, designs must be flexible and easy to change function of the inner space in order to be efficient to use of this kind of concrete. Therefore, instead of demolition there will be re-modeling when the service held within the building is no longer needed in a particular area, which in its turn has a positive effect in reducing the CO2 emission by avoiding construction.

Application in Large-Size Buildings and Roads (Residential and Public)

SHC is particularly adequate for bridges and all road constructions as they often experience small-sized cracks due to heavy loads and constantly need maintenance. The use of this kind of concrete will reduce the maintenance cost significantly and will increase safety, therefore, it's highly recommended to use due to its many benefits.

All large-size buildings will defiantly benefit from the use of this kind of concrete just as the infrastructure will be enhanced by providing safety and durability.

Discussion

There is still an ongoing research regarding SHC; many scientists are trying different approaches that ensure the same outcome which is closing cracks with minimum intervention while keeping cost at reasonable rates. SHC is much more effective than concrete; A brief comparison of some aspects is tackled below.

Safety: Since cracks in SHC are easy to close with no extra costs being added, the general safety of a particular construction is increased. However, that leaves a question regarding the resistant ability of concrete and whether crack closing would affect its strength. All research conducted so far show that the concrete gains about 25% of its original strength in the healed spot which more than the 15% gained back when the crack is sealed by current methods.

Cost: It is obvious that the initial cost of construction using SHC is higher, however, on the long-term, durable concrete is much more cost-efficient due to the low cost of maintenance, durability and the long life-span of the construction.

Durability: According to research and experimentation bacteria-based SHC is denser and more durable than concrete. [9, P. 14892]

Availability: As it is still under development, this kind of concrete is used on a limited scale and still not commercially wide-spread. Some main obstacles are cost and production.

Effects on Architecture & Design: By increasing the life-span of a construction, architects need to re-consider design standers. A long life-span impacts the design of any construction, as architects must take into consideration future prognosis of two main aspects: (i) the potential function within a particular building (potential technological needs, change of function, change of life style, etc.) (ii) The future function of urban space surrounding a certain building. Hence, architects' main task is to foresee the upcoming needs and the current ones to design and construct a useful, aesthetic and more importantly, highly-flexible buildings in order to be change function easily.

Environmental impact: Cement industry is one of the main two producers of carbon dioxide (CO2) emissions, which is directly harming our planet. Therefore, by using SHC the carbon dioxide emissions are reduced significantly.

Conclusion

As a conclusion, SHC appears to be much more efficient than usual concrete. It will definitely reshape how architects think and design. By comparison, we notice that it has more advantages than disadvantages and will transform concrete from an Eco-harming into an Eco-friendly material, as it reduces the CO2 emissions significantly. There are currently many undergoing studies using different approaches to produce SHC, however, the most promising approach is the bio-concrete which is bacteria-based due to its simplicity in comparison with other mechanisms. For the meanwhile, architects must develop new design methods; ones that allow flexibility in changing functions easily for instance by providing movable partitions which allow creating bigger or smaller spaces depending on the current needs.

References

1. Latin Dictionary// Oxford: Clarendon Press. 1879.

2. The History of Concrete // Dept. of Materials Science and Engineering, University of Illinois, Urbana-Champaign. [Electronic resource] URL: http://matse1.matse.illinois.edu/concrete/hist.html.

3. Gromicko N. The History of Concrete / N. Gromicko, K. Shepard // InterNACHI [Electronic resource] URL: https://www.nachi.org/history-of-concrete.htm#ixzz31V47Zuuj.

4. Christopher J. Experimental and Numerical Study of the Fracture and Self-healing of Cementitious Materials / Christopher Joseph // Cardiff University. 2008. p.19.

5. Micro-capsules and bacteria to be used in self-healing concrete // University of Baath [Electronic resource] URL: http://www.bath.ac.uk/news/2014/12/03/micro-capsules-and-bacteria-to-be-used-in-self-healing-concrete/. 2014.

6. Schlegel H. G. General Microbiology / Hans G. Schlegel, C. Zaborosch // Press Syndicate of the University of Cambridge. 1993. p.102.

7. Lendlein, A. Shape-memory polymers / A. Lendlein, S. Kelch, // Angew. Chem. Int. Ed. 2002. p. 2034.

8. Paine K. A Design and performance of bacteria-based self-healing Concrete / K. Paine, M. Alazhari, T. Sharma, R. Cooper, A. Heath // The 9th International Concrete Conference. 2016. p. 6-8.

9. Lakshmi.L, Durability and Self- Healing Behaviour of Bacterial Impregnated Concrete / L. Lakshmi, C.M. Meera, Eldhose Cheriyan // International Journal of Innovative Research in Science, Engineering and Technology. Vol. 5. 2016. p. 14892.

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