CONCRETE OF SELF-CURE: DEFINITION, MECHANISM, AND APPLICATION IN DIFFERENT TYPES OF STRUCTURES
Concrete is one of the most commonly used building materials. However, it is one of the main producers of carbon dioxide (CO2) that directly contributes to destroying our environment. Not to mention that huge costs are being spent every year to maintain concrete constructions. Cracks of various sizes are formed in all concrete constructions that must be sealed manually shortening the useful life of a particular construction. On the other hand, self-healing concrete (SHC) is a revolutionary construction material that has the solution to all these problems and is definitely the building material of the near future. Therefore, we must understand its properties and mechanisms and foresee how it affects the architectural designs of the times to come, what elements are needed to create buildings and useful and aesthetic constructions.
Keywords: self-healing, concrete, construction material, intelligent material, cracks, mechanism, repair, design, architecture.
Introduction
The concrete word comes from the Latin word "concretus" which means condensed and hardened. The first use of cement goes back to twelve million years ago, while the first use of construction material similar to concrete goes back to 6500 BC. However, it was not formed as concrete until later during the Roman Empire.
As revolutionary as it was and still is, modern concrete (based on lime) has a short lifespan caused by the formation of cracks that shorten the longevity of a particular construction. Many researchers have been trying to improve the concrete to obtain a better longevity among many other things. This is how the concept of self-healing becomes concrete. There are two main areas of research when it comes to developing this type of concrete; the natural form of hydrates to seal cracks with time, and the artificial way of sealing cracks that requires a man-made intervention. The main purpose of this work is to increase the durability of concrete, which will have a great positive impact on both the environment and the economy.
On the other hand, it could also improve architectural designs by forcing new design methods and, therefore, changing the shape of internal spaces to serve many functions and provide flexibility.
Definition of self-healing
A self-healing material is described as a material that is capable of repairing itself to its original state. The self-healing concrete (SHC) concept that occurs over time (autogenous) has been observed for more than 20 years. It can be seen in many ancient structures that have remained standing for long periods of time even though they have limited maintenance. This observation concludes that the cracks are cured when the moisture interacts with the unhydrated cement clinker in the crack. However, in current constructions, cement is lowered as a result of modern construction methods. Therefore, the amount of non-hydrated cement available is lower and, therefore, the natural healing effect is reduced.
The main phases of the natural healing capacity are the inflammation and hydration of cement pastes; followed by the precipitation of calcium carbonate (CaCO3) and, finally, the obstruction of the flow paths as a result of the deposition of impurities in the water or the movement of some concrete drills that break off during the entire cracking process . Many factors are considered in the natural way of healing, such as; Temperature, degree of damage, freezing and thawing cycles, the age of the concrete and the condition of the mortar.
As for the artificial way of repairing cracks in concrete, the man-made self-healing process was first invented in 1994. The main method and first approach was to use a healing agent (adhesive) that is encapsulated inside of a microcapsule, once a crack is formed, causes the microcapsules to break, releasing the healing agent and, therefore, 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, researchers from the University of Cardiff, the University of Cambridge, the University of Bath and researchers tackled more effective mechanisms, and the Korea Construction Institute. In this article, two of the main approaches, which look promising and distinguished, will be briefly addressed together with the advantages and disadvantages of using this type of concrete, which will soon be used inevitably throughout the world.
Main approaches and their mechanics
There are many approaches to create smart concrete and improve its properties while reducing the cost of general material use. Many of these approaches were dedicated to creating SHC; Two of the main approaches have proven to be efficient and easy to use.
Healing process based on bacteria
Also known as Bio-Concrete; This type 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 such as calcium lactate (Ca (C3H5O2) 2) and (ii) bacteria planted in microcapsules (or simply added to the mixture) that will then germinate , Once the water reaches the crack. As soon as the bacteria germinate, they produce limestone (CaCo3) caused by the multiplication of bacteria. Dr. Richard Cooper of Bath's Department of Biology and Biochemistry says that adding bacteria to the concrete adds a double-layer shield to prevent corrosion in the steel. Not to mention that it uses oxygen present, which would benefit the steel corrosion process.
The bacteria that are applied in this type of concrete are spore-forming bacteria and resistant to alkalis. The bacteria of this group are the most suitable because they form spores and can live for more than 200 years in dry conditions . Therefore, the use of bacteria as a healing mechanism is one of the best mechanisms to produce this type of concrete due to its sustainable organic properties.
Form memory polymers
New intelligent materials (SMP) that are able to return to their state of initiative by changing their form when applying a stimulus. This mechanism employs autogenic and autonomic principles. It uses a man-made system to increase natural autogenic healing and seal cracks in concrete. This type of polymers are semicrystalline polymers that have a predefined shape memorized in their structure that then helps the polymers to return to their original state.
When a crack occurs, the system is activated, therefore, the shape memory polymer within the crack is activated through heating, which can be in the form of direct heat, or an electric current. As soon as it is activated, the effect of shape memory or shrinkage occurs, and due to the restricted nature of the tendon, a pulling force is generated, whereby the crack closes on itself. After that, the autogenous cure begins.
Factors that affect the use of self-healing concrete
There are many factors that intervene with the use of this type of concrete. As you notice; It is not yet used in all new constructions, as it is still in development. Recently, concrete-based self-healing bacteria has been successfully tested on a large scale at the University of Bath in the United Kingdom. However, the cost of use is not yet determined, since it is difficult to predict a total cost. Cost efficiency is one of the most important factors and will determine if the material will have a limited use restricted to difficult places to repair and important constructions such as bridges and roads.
In addition to cost, long-term efficiency is one of the most important factors along with the size of the cracks formed that should not exceed 150 millimeters deep to establish an ideal result.
All in all, some factors that will definitely determine if SHC will be used as a concrete replacement are; The economic factor, long-term efficiency, potential suppliers and safety factors.
Application in Architectural Designs and Structures.
Since the use of SHC looks promising, we must understand how that will affect future architectural designs. It is difficult to make a general forecast, since the function and the size of the construction play a very important role in determining if this type of concrete could be adequate and, therefore, it will be analyzed separately.
Application in small and medium buildings (residential and public)
The size and function of a building usually determine the approximate lifespan desired for this particular construction. Small buildings are usually residential and are located in suburbs, towns or villages. And like most buildings, concrete is one of the main building materials used, especially for foundations (slabs or columns), since small residential buildings rarely change their function, it is practical to want to increase their useful life and, Therefore, use SHC.
Medium-sized buildings use more concrete than any other building size, unlike skyscrapers that use more steel and smaller buildings that use more stone or wood. However, medium-sized residential and public buildings appear to be eligible for the use of SHC, and especially in public buildings as life expectancy increases, designs must be flexible and easy to change the function of the interior space so that be efficient To use this type of concrete. Therefore, instead of demolition, it will be reshaped when the service maintained within the building is no longer needed in a particular area, which in turn has a positive effect on reducing CO2 emissions by avoiding building.
Application in large buildings and roads (residential and public)
SHC is particularly suitable for bridges and all road constructions, as they often experience small cracks due to heavy loads and need constant maintenance. The use of this type of concrete will significantly reduce the cost of maintenance and increase safety, therefore, its use is recommended due to its many benefits.
All large buildings will definitely benefit from the use of this type of concrete just as the infrastructure will be improved by providing safety and durability.
