Analysis of Self-Healing Strategies in Smart Concrete using Fuzzy Analytic Hierarchy Process

: Despite the fact that the concrete is prone to fracture formation, it is a material that is widely utilized in the construction industry as it is affordable and easily available. Therefore, during the past two decades, there has been a surge in interest in self-healing materials, especially those that have the ability to repair themselves in environmentally friendly and sustainable concrete materials. However, because each research institute uses its own testing methods to determine how effective a treatment is, choosing the most efficient method is difficult. Self-healing concrete, also known as SHC, has the ability to heal and reduces the need to locate and repair internal damage, such as cracks, without the need for intervention from outside the structure. This helps in reducing the costs, increase durability and reduces reinforcement corrosion and concrete deterioration. This study provides a comprehensive overview of SHCs, taking into account the strategies, and effectiveness of self-healing. Based on the application, multiple self-healing concrete strategies are identified through Literature. Using Multi-Criteria Decision Making Tool (MCDM), Fuzzy Analytic Hierarchy Process (FAHP), it is found that out of five factors selected, Ease of Availability Factor got maximum preference and Performance Time Factor got least preference. Among four strategies identified, Chemical based Self-Healing has maximum weight followed by Bacteria based Self-Healing. Natural Self-Healing is least preferred. Besides, limits and future possibilities of SHC, as well as the area of interest research points for future examinations, are additionally effectively featured.


Introduction
Concrete is the second-most-used substance in the world after water. Due to its tremendous compressive strength, Concrete is the most commonly used building material. Concrete's sensitivity to fracture formation due to its low tensile strength is one of its disadvantages. For that reason, concrete is mostly combined with steel reinforcement to carry the tensile loads. These rebars limit the crack's width, but they are typically not made to entirely stop cracks from forming. As liquids and gases may enter the matrix along cracks and cause damage, cracks put concrete buildings' longevity in jeopardy. As a result, cracks could enlarge and the reinforcement might become exposed to the outside environment. The structure may completely collapse once the reinforcement begins to erode. This requires concrete crack examination, maintenance, and repair. However, when fissures are hidden or difficult to reach, crack healing becomes challenging [1]. These challenges paved a path in development of Smart Concrete. Due to its natural autogenous healing abilities, concrete has a well-established and well-known ability to self-heal [1]. Fissures may eventually heal as a result of the continuing hydration of clinker minerals or the carbonation of calcium hydroxide [Ca(OH)2]. On the other hand, autogenous healing is limited to small cracks and only works when water is available, making it challenging to fully control or predict its accuracy. However, concrete can be modified to contain a method for sealing that stimulates bacterial growth. In the 1990s of the previous century, initiatives to construct autonomous SHC started [2]. At Delft University of Technology in the Netherlands, microbiology professor Henk Jonkers created self-healing concrete (SHC) in 2006 as a new type of concrete [3]. After years of research, he found the Perfect-Self Healing Agent, named Bacillus. SHC, which is referred to as concrete's capacity to repair cracks automatically or autonomously, has a unique mechanism [4]. By secreting some sort of substance, SHC mimics the body's natural ability to repair itself of wounds [7,8]. SHC is made by mixing particular components (such as fibres or capsules containing mending solutions) into the concrete mix [9], where, when cracks occur, the fibres or capsules shatter and the liquid inside them spreads right away to fill the crack. Due to the poor tensile strength of the concrete structures, concrete cracks occur often [10,11]. The concrete's endurance over time is decreased by these created fissures since they leakage of hazardous gases and liquids [1]. Microcracks in concrete can cause it to degrade, but attacks brought on by gases and liquids seeping into the concrete system can also injure steel reinforcement bars [2]. Therefore, it is essential that the fractures are addressed soon to prevent them from expanding. Concrete constructions' ability to self-heal cracks could increase sustainability, increase the longevity of the structure, and make it more ecologically friendly [7].

Fuzzy Analytic Hierarchy Process (FAHP)
The Saaty created Analytic Hierarchy Process (AHP) which is a simple analytical hierarchy process that incorporates fuzzy theory [17]. Basic AHP has been enhanced by utilising fuzzy logic since it does not incorporate vagueness for subjective judgements. Through the linguistic variables, which are represented by triangular numbers in FAHP, pair-wise comparisons of both criteria and alternatives are carried out [14]. Based on the provided criteria and alternatives, it assists in making decisions that are more effective, adaptable, and realistic by applying the FAHP technique [15]. In order to determine the best self-healing strategy, the FAHP approach is identified and chosen. The following are the fundamental actions involved in applying for an FAHP: i. Breaking the problem into manageable issues using a hierarchy of goals at the top, standards or evaluation criteria at the next level, and potential solutions for achieving the goals at the bottom. ii. Each branch is then further subdivided into appropriate level of detail (At the end, the iteration process transforms the unstructured problem into a manageable challenge organised both vertically and horizontally under the form of a hierarchy of weighted criteria). iii. Listing the characteristics of the alternatives alongside their corresponding values in a matrix. iv. By transforming the verbal scale into a fuzzy triangular scale of 1 to 9, each attribute from the questionnaire survey is given a numerical weighting.

Methodology
The steps involved in Methodology are shown with the help of a flowchart:

Fig.1. Flowchart of Adopted Methodology
The secondary information gathered from literature search of books, research papers, professional articles, etc. allows one to fully comprehend the Smart Concrete scenario. Four Self-Healing Strategies are identified using the internet and market research. The strategy alternatives selected are obtained through primary data collection gathered from information obtained from discussions with key informants. A questionnaire survey has been conducted where four strategies have been considered for the present study: NB: Non-beneficial attribute; B: Beneficial attribute; S: Strategy, S1: Natural Self-Healing; S2: Mineral Admixtures; S3: Chemical-Based Self-Healing; S4: Bacteria-Based Self-Healing. A total of 102 responses are recorded out of which 55 are academicians, 27 are industry experts and 20 are research scholars. Order of preferences has been set through a questionnaire survey based on the following five criteria: i. Ease of Availability Factor ii. Cost Factor iii. Performance Time Factor iv. Durability Factor v. Performance Factor Ranking of the technologies is carried out using A Multi-criteria Decision Making (MCDM) tool which enables the user to select the appropriate choice of technology in different contexts. Fuzzy Analytical Hierarchy (FAHP) process is the MCDM tool primarily used for the analysis. Using the results of FAHP, the most effective Self-Healing Strategy is selected for the research based on all the factors considered.

Conduction of Questionnaire Survey to determine the relative importance of
Criterions.  C5 Performance Healing Capacity.

Attribute Comparison
A 5-point scale is used to establish and compare attributes for AHP comparison, with 1 denoting "VERY LOW," 2 denoting "LOW," 3 denoting "MEDIUM," 4 denoting "HIGH," and 5 denoting "VERY HIGH." The values are then categorized as attributes that are beneficial or not beneficial. Beneficial attributes are those the higher upsides of which are desired, whereas non-beneficial attributes are those the lower values of which are preferred. The values are given in Table 3.

Criteria Weightage
A questionnaire survey that was sent to experts in the same field was used to assign the weights during the judgments. To represent the decisions went with by the choice producers, Table 4 shows the network used to work out the loads of the primary rules. With the assistance of the Saaty scale, the matrix values are transformed into fuzzy numbers, which is what fuzzification means-converting linguistic terms into membership functions. In order to calculate the weightages, additional steps include locating the reciprocal values and geometric methods.

Ranking of Criteria
Priority values have emerged following a comparison of the attributes at each level, indicating that the Ease of Availability Factor is given most preference, as the associated materials for self-healing strategies are required to be easily available. Performance (Healing Capacity) is preference after Ease of Availability followed by Cost Factor .The next preferred factor is Durability of healed crack. Performance Time is given the least preference among all.

Ranking of Strategies
Based on the factors considered, Chemical based Self-Healing is most preferred strategy, followed by Bacteria based Self-healing. Natural Self-Healing is least preferred among all the strategies.

CONCLUSION
Self-Healing Concrete finds diverse application in Civil Engineering structures. Depending on the application, multiple Self-Healing Concrete Strategies are identified through Literature. A Multi Criteria Decision Making Tool, Fuzzy Analytic Hierarchy Process is used to identify the most suitable Self-Healing Strategy. Following observations are concluded from the study: i. Out of five factors selected for study, Ease of Availability Factor got maximum preference.
ii. Performance Time Factor got least preference among all.
iii. Among four strategies identified, Chemical based Self-Healing has maximum weight followed by Bacteria based Self-Healing.
iv. Natural Self-Healing is least preferred among all the Strategies.
Much of Research work is required in the field of Smart Self-Healing Concrete.