Cracking Methods for Testing of Self-Healing Concrete: An Experimental Approach

Abstract

With the advent of new sustainable construction materials, self-healing concrete has been used and tested in the last decade, raising the question of the efficacy of said mechanisms to prevent water permeation after crack formation. Thus, new novel mechanical methodologies have been introduced to induce controlled cracks in concrete specimens to improve the standardisation and effectiveness of permeability tests. This research explores those new mechanical techniques to create consistent and reproducible crack patterns, crucial for assessing the efficacy of self-healing mechanisms in concrete. This study systematically evaluates how different crack configurations influence the self-healing ability of the material. Findings from this research are expected to aid in refining testing protocols and to contribute significantly to the field of material science within civil engineering by demonstrating the potential of self-healing concrete to revolutionise building practices.

1. Introduction

This manuscript revolves around the creation of new cracking methods to test self-healing concrete. Although more complex methods already exist, these new smaller scale and easy-to-reproduce samples are more accessible to recreate for testing and, therefore, for quality control of said self-healing concrete.

An introduction to self-healing concrete is provided, with work already developed by the authors, followed by observing the existing literature regarding self-healing concrete and subsequent gaps using a bibliometric network mapping tool.

It is followed by a showcase of already documented methods of cracking concrete, with the most varied results, and then permeability methods and calculations used by several authors to determine liquid or gas permeability under different conditions.

The methodology showcases the bacterial self-healing mixture and is used in the following new cracking methods proposed by the authors. The three novel cracking methods proposed are the nylon strings, wedge, and plate methods, which are explained in detail, with the permeability setup used.

The discussion presents the outcomes and effectiveness of the cracking methods proposed in the water permeability test for the self-healing concrete mixture while comparing them with a fourth and less efficient cracking method.

1.1. Self-Healing Concrete

Self-healing concrete (SHC) represents a transformative approach in civil engineering aimed at enhancing the longevity and durability of concrete structures. Traditional concrete, while robust, often succumbs to cracking due to environmental stressors and load-bearing demands, leading to structural vulnerabilities. SHC addresses this inherent weakness by incorporating mechanisms that allow concrete to repair cracks autonomously, potentially reducing maintenance costs and extending the infrastructure’s lifespan.

There are two primary methodologies employed in SHC: autogenous and autonomous healing. Autogenous healing occurs naturally when unhydrated cement particles within the concrete react with water and carbon dioxide to form new hydrates and carbonates that bridge cracks. This method is generally adequate for small cracks, typically up to 0.2 mm wide. However, its efficiency decreases as crack width increases, limiting its application under broader structural conditions.

In contrast, autonomous healing involves embedding healing agents directly into the concrete. These agents can be chemical, mineral, or biological. When a crack forms and exposes the agents to water, they are activated—chemically or biologically—to precipitate healing products that fill the cracks. For instance, bacterial SHC utilises specific bacterial strains that precipitate calcium carbonate, effectively plugging the cracks and restoring structural integrity. This method extends the applicability of SHC to wider cracks and provides a controlled healing process, enhancing the reliability of the repair.

The structural engineering performance of SHC is critically linked to its ability to maintain or restore the mechanical properties of concrete following damage. Studies have shown that both autogenous and autonomous SHC can significantly improve the durability of concrete by sealing cracks that would otherwise lead to rapid deterioration through water ingress and corrosion reinforcement. Moreover, incorporating SHC technologies helps maintain the concrete’s initial strength over time and can contribute to the structure’s overall resilience to environmental challenges.

Compared with non-healed samples, experimental assessments in SHC research often focus on evaluating mechanical properties such as tensile strength, compressive strength, and flexural responses of healed samples. These studies highlight the capacity of SHC to recover significant percentages of its original strength post-healing, with variations depending on the healing method used and the concrete composition.

In sum, the development of SHC technologies, mainly through the innovative use of bacterial agents in autonomous healing systems, promises to revolutionise the approach to concrete durability and sustainability. By effectively bridging the gap between material science and structural engineering, SHC offers a proactive solution to the age-old problem of concrete cracking, paving the way for more resilient and maintenance-efficient infrastructures. This aligns with the growing global emphasis on sustainable construction practices and the urgent need to enhance the lifecycle of civil engineering projects [1].

Recent research has underscored the potential of bacterial self-healing concrete to autonomously mend cracks ranging from 0.3 mm to 1.5 mm in width. This healing process is mediated by bacterial agents embedded within the concrete matrix that produce calcium carbonate precipitates, effectively filling and sealing the cracks [2]. Sealing such a range of crack widths augments the durability of concrete structures by shielding the internal reinforcements from environmental harm and substantially extending their operational lifespan.

Nevertheless, integrating bacterial self-healing agents and supplementary materials such as fly ash and ground granulated blast-furnace slag (GGBS) introduces particular challenges. Observations from the studies indicate an initial decline in crucial mechanical properties of the concrete, like compressive and flexural strength, during the early phases of curing relative to traditional concrete mixes [3]. Although these mechanical properties generally improve over time and may surpass those of conventional concrete, the early-stage reductions could pose significant challenges in construction contexts that require high initial strength.

Moreover, while the results concerning crack healing are promising, the effectiveness of these self-healing mechanisms in preventing water ingress—vital for maintaining structural integrity—has yet to be fully established. Water permeability tests are critical as they offer definitive evidence of the concrete’s ability to impede water penetration through the healed cracks. These tests are essential to determine whether the cracks have been merely closed visually or if there has been a genuine restoration of the concrete’s structural integrity against water and other corrosive elements. Therefore, conducting comprehensive water permeability tests on bacterial self-healing concrete is crucial to confirm the practical applicability of this innovative technology in real-world settings. Such empirical validation would support self-healing concrete’s long-term durability and functionality and promote its broader adoption in the construction industry.

1.2. The Literature Network

Many associated keywords demonstrate clustered research-themed connections when focusing on the latest research regarding bacterial self-healing concretes. The following bibliometric network, provided by VOSviewer [4], demonstrates related keywords into four significant clusters identified by colour, as seen in the following figure.

This bibliometric mapping is valuable for identifying the core themes, the relationships between them, and potential gaps or emerging trends in concrete research. It offers a visual and quantitative overview that can guide future research directions, inform practitioners, and support the development of innovative concrete technologies.

Figure 1 is a network visualisation from a bibliometric analysis, a method used to quantitatively assess the literature within a specific field—in this case, concrete research. This visualisation maps out the interconnectedness of keywords from related articles, revealing the field’s major themes and research areas.

In an overview, this image presents an overall view of the research landscape, showing the interconnectedness between various keywords within the field of concrete research. Colours of significant clusters indicate different thematic concentrations, such as durability, cracking, reinforcement, and self-healing concrete.

The green cluster is centred around self-healing concrete, focusing on innovative materials that can repair themselves through biological or chemical processes. Keywords like “bacteria”, “calcite”, and “biomineralisation” suggest an emphasis on bio-based solutions for self-repairing concrete, reflecting a trend towards sustainable and innovative materials in construction.

The red cluster concentrates on the durability aspects and mechanical properties of concrete, using keywords like “compressive strength”, “chlorine compounds”, and “durability”. It highlights the interest in enhancing concrete through chemical additives and understanding the factors contributing to its structural integrity and longevity.

When the blue cluster is observed, it focuses on the structural integrity of concrete, particularly concerning crack formation and the resulting permeability issues. Terms like “cracks”, “permeability”, and “concrete construction” are prominent, suggesting research into how cracks affect concrete’s performance and durability.

The yellow cluster delves into reinforced concrete, examining the use of fibres and other reinforcing materials to improve concrete’s performance. Keywords such as “fibre-reinforced concrete”, “fibres”, and “tensile strength” indicate studies on different types of reinforcement and their effects on the mechanical properties of concrete under various environmental conditions.

2. Self-Healing Concrete Testing

2.1. Cracking Methods

Initial documented approaches to crack creation are described [5], where standardised cracks are created by inserting 0.3 mm copper sheets on the top of freshly cast concrete, allowing the creation of grooves 10 to 20 mm deep after 24 h when demoulding in a 160 × 160 × 70 mm slab specimen. Alternatively, a splitting tensile test is used to force a longitudinal crack in cylinders bound by a layer of glass fibres reinforced with epoxy resin. Using this approach, instead of a natural crack, there would be a smooth interface in large specimens, making permeability tests difficult and not corresponding to reality.

Lee et al. proposed a method [6] where the cylinder is submitted to a splitting tensile test until it cracks into two halves. Silicon sheets are placed between these halves and finally clamped together, thus allowing a variable crack thickness depending on the clamp tightness. If performed by itself, the splitting tensile test will produce randomly oriented cracks, adding to the tall cylinder samples, making water permeability hard to perform with a high volume of uncentered cracks.

The most common methods of crack induction, seen in Lahman et al. [7], stem from mechanical test procedures to assess concrete capacity. A tensile splitting test performed on cylinders shows that predictable cracking occurs from tensile forces, albeit the crushed sections in the compression contact area will compromise observation with multiple crack widths. Using a four-point bending test on a beam, as used in flexural tests, the specimens are usually submitted to a rotation of the crack interface, crushing the compressive region and increasing the crack width. When compressive tests are performed in cubic specimens, the cracks usually appear due to excessive loading, creating many widths and deteriorating the specimen. Another approach is created when performing direct tensile tests, where the formed crack pattern is parallel but needs a custom and expensive setup. In contrast, water permeability tests are more challenging to perform in large quantities.

Akatsu et al. [8] introduce a novel method for controlled cracking of industrial ceramic waste, including concrete, using a steam pressure cracking (SPC) agent. This non-explosive chemical agent offers a safe and controlled alternative to traditional methods like mechanical cutting or explosive crushing by expanding within the concrete. The research involves experiments with cylindrical concrete specimens, demonstrating that cracking control is more effective in water than in air. The SPC agent ignites within a hole in the concrete, leading to controlled cracking. Notably, the method differs from explosive techniques by producing fewer larger pieces of concrete, which are more suitable for recycling processes. Guide holes are used to direct the cracking path, with the SPC agent’s mild reaction allowing for precise control over crack initiation and propagation. The study also concludes that the SPC method is an efficient controlled approach for processing sizeable industrial concrete waste, significantly contributing to waste management and recycling practices. Although a method of cracking concrete, the imprecision of unpredictable randomised crack patterns makes it hard to reproduce and standardise this method to test water permeability and, subsequently, the effectiveness of crack healing of self-healing concrete.

Wiggenhauser et al. [9] present a method to create controlled cracks in concrete using expanding mortar—“Betonamit”—placed into blind holes aligned in a line, combined with strategically placed reinforcement. The reinforcement’s position and alignment control the crack direction and depth. A layer of additional parallel reinforcement limits the depth of the crack to the specimen’s surface. The expanding mortar is carefully applied, and the pressure builds up to create the crack, which becomes visible within 24–48 h. The crack’s depth and other properties are meticulously controlled and reproducible, making this method valuable for developing and validating non-destructive testing methods for crack evaluation in concrete. Although this method has some limitations, the time-dependent reaction, the chemical insertion by drilling and random cracking creation, and the costs make this option less desirable for research in smaller samples.

Trussel et al. [10] investigate water transport in wet sprayed concrete, both standard and with an ethylene-vinyl acetate (EVA) copolymer, focusing on capillary suction and crack permeation. Cracks are induced using a tensile splitting technique, which applies compressive force to create a central crack, ensuring precise control over crack width. Crack measurements are accurately made using digital image correlation (DIC) and confirmed microscopically. For permeability testing, the concrete discs are exposed to water on one side, and the flow through the crack is measured to calculate the permeation flow rate coefficient. This coefficient quantifies the concrete’s permeability, highlighting the role of crack width and the presence of EVA copolymer in influencing water transport. The research finds that wider cracks increase water permeability while EVA reduces capillary suction and permeation rates. The study provides critical insights into the impact of cracking and material composition on the water transport properties of concrete, making this a complex and costly process that is hard to replicate for statistical acuity.

2.2. Water Permeability

Regarding permeability testing methods, some authors propose the use of small concrete disks of 100 mm in diameter by 50 mm in height [5,6], as seen in Figure 2, where the specimen is wedged between two chambers and two rubber seals, allowing a constant water overhead to form on the top chamber while collecting what passes through the specimen’s crack in the bottom chamber.

The following study by Hou et al. [11] emphasises the importance of autogenous healing or adding healing agents into cementitious materials. It reviews and analyses empirical formulas linking the quantified geometry of a single crack (such as crack width, tortuosity, and surface roughness) to its transmissivity. The relationship between crack geometry and sealing ratio is established and discussed, with recommendations provided for the quantitative evaluation of the self-healing capacity of cracked concrete.

The water permeability test (WPT) measures the water transmissivity of cracked or healed concrete. In this test, a laminar flow passes through water-saturated and interconnected crack voids under a specific pressure gradient. The flow rate is measured once a steady state is reached, allowing for the calculation of water transmissivity changes over time. This information calculates the sealing ratio (SR) of the specimens. Two types of devices are generally used for WPT: falling head tests and constant head tests. The falling head test measures the drop in a water reservoir pipe to calculate the permeability of macrocracks, while the constant head test records the weight of leaked water to assess the transmissive capacity of cracks.

A uniform pressure gradient and a no-slip boundary condition are applied for the transmissivity of a single natural crack. The velocity distribution of an incompressible Newtonian viscous laminar flow in a smooth parallel-plate crack is described using a parabola-shaped velocity function along the x-axis, dependent only on the z-axis [11,12].

$$u\left(z\right)=\frac{1}{2\mu }\cdot \frac{dp}{dx}\cdot \left(z^2-H^2\right)$$

(1)

where $u\left(z\right)$ represents the velocity along the z-axis, $\mu$ is the dynamic viscosity of the fluid, $\frac{dp}{dx}$ is the pressure gradient, and $z$ and $H$ are coordinates along the z-axis, representing the positions of the upper and lower surfaces of the crack, respectively.

The total fluid volume through an ideal crack is calculated using the following relationships.

• The Cubic Law Formula [11,13]:

$$Q=-\frac{w\cdot b^3}{12\mu }\cdot \frac{dp}{dx}$$

(2)

where $Q$ stands for the total volume of fluid through a crack, w and b are the length and width of the crack, respectively, $\mu$ is the dynamic viscosity of the fluid, and $\frac{dp}{dx}$ is the pressure gradient.

• Darcy’s Law Formula [11,14,15]:

$$Q=-\frac{\kappa \cdot w\cdot b_H}{\mu }\cdot \frac{dp}{dx}$$

(3)

where similarly to the cubic formula, $Q$, $\mu ,$ and $\frac{dp}{dx}$ have the meaning mentioned above, $\kappa$ represents the permeability, $b_H$ the hydraulic crack width, and $w$ the length of the crack, while the transmissivity formula is determined by the following:

$$T=\frac{b_H^3}{12}$$

(4)

The sealing ratio (SR) [11,16] is a crucial metric in quantifying the changes in crack transport capacity. It is expressed as a function of the water volumetric flux of cracked and healed concrete and the transmissivities of the same. The SR formula incorporates the hydraulic width of cracked and healed concrete to determine the effectiveness of the self-healing process.

$$SR=\frac{Q_{cracked}-Q_{healed}}{Q_{cracked}}=1-\frac{b_H^{heale{d}^3}}{b_H^{cracke{d}^3}}$$

(5)

where $b_H^{healed}$ and $b_H^{cracked}$ are the hydraulic widths of the healed and cracked concrete, respectively.

Similarly, Lee et al. [6] focus on developing and evaluating self-healing concrete technologies and acknowledge that a single test or parameter cannot sufficiently identify self-healing performance due to various influencing factors. Thus, the paper emphasises the need for standardised test methods to verify and compare the performance of self-healing materials. The research involves producing self-healing mortars based on inorganic admixtures designed to seal 0.3 mm cracks with a healing index of 90%. These mortars are used to validate the water permeability test and establish protocols for evaluating self-healing performance. The paper proposes an equivalent crack width, estimated from the water flow rate, as a rational evaluation index and introduces a self-healing performance chart to comprehensively display the healing performance in cement-based materials. The permeability methods used in the study involve two primary approaches depending on the outflow amount: a constant water head permeability test for large outflows and a variable water head permeability test for smaller outflows. The study selects a constant water head permeability test, which is more suitable for evaluating self-healing performance in cracked concrete specimens. The water flow rate of this method is expected to vary with time, decreasing as the cracks fill with healing products. A constant water head of 300 mm is maintained during the tests to ensure uniform test conditions and minimise the influence of head loss. The test measures the water from the equipment for 7 min after stabilising the water head and flow. The water flow rate is then calculated in mL/(mm⋅min) units by dividing the amount of discharged water by the test duration and crack length.

The calculations in the study are based on Poiseuille’s Law. The proportional coefficients measured for the reduction factor of the cracks (ξ) range from 61.9 to 69.2 mL/(mm⋅min), corresponding to a ξ of 0.21 to 0.24. This range is similar to values obtained in other mortar and concrete mixtures studies, indicating the roughness and tortuosity of the cracks.

• Poiseuille’s Law Adaptation for Water Flow Rate [6,7]:

$$Q=\frac{\mathsf{\xi }\mathsf{\Delta }Pb}{12\mathsf{\eta }d}{w}^3$$

(6)

where $Q$ is the water flow rate through a crack, $\xi$ is the reduction factor reflecting crack toughness, $\mathsf{\Delta }P$ is the water head gradient between the inlet and outlet, $b$ is the length of the crack, $\eta$ is the absolute viscosity, $d$ is the flow path length of the crack, and $w$ is the crack width.

• Water Flow per Unit Length of a Crack:

$$q=\frac{Q}{b}=\frac{\mathsf{\xi }\mathsf{\Delta }P}{12\mathsf{\eta }d}{w}^3=\mathsf{\alpha }{w}^3$$

(7)

where $\alpha$ is a coefficient of proportionality relating water flow rate to the third power of crack width.

• Initial Crack Width Estimation:

$$w_0={\left(\frac{q_0}{\mathsf{\alpha }}\right)}^{\frac{1}{3}}$$

(8)

This equation is used to assess the initial crack width ($w_0$) using the measured initial water flow rate ($q_0$) and the coefficient $\alpha$. This is useful in cases where direct measurement of the crack width is difficult.

• Equivalent Crack Width at Time t:

$$w\left(t\right)={\left(\frac{q\left(t\right)}{\mathsf{\alpha }}\right)}^{\frac{1}{3}}$$

(9)

Furthermore, the self-healing index ($SHq$) is calculated using the formula:

• Self-Healing Index ($SHq$):

$$SHq=\left(1-\frac{q\left(t\right)}{q_0}\right)\times 100(\mathrm{\%})$$

(10)

where $SHq$ is the self-healing index based on the water flow rate, $q_0$ is the initial water flow rate measured just after the specimen is cracked without any healing effect, and $q\left(t\right)$ is the water flow rate at a given time. This calculation helps determine how much water tightness is recovered in the self-healing concrete.

Lauch, Desmettre, and Charron [17] present an in-depth study on the self-healing properties of concrete, utilising a sophisticated experimental setup to induce and monitor flexural cracks in fibre-reinforced concrete prisms. The setup’s precision allows for the controlled induction of cracks, ensuring reliability in subsequent water permeability measurements. Employing Darcy’s Law and a modified version of Poiseuille’s Law for crack flow, the study meticulously quantifies water flow through cracks while considering the unique characteristics of concrete, such as crack roughness and width variation.

Healing and closing ratios are central to the study, which provide a quantitative framework to assess the self-healing process. The healing ratio (HR) evaluates the reduction in water permeability over time, reflecting the healing process’s effectiveness at a micro level, possibly through sealing internal crack surfaces or mineral precipitation. The closing ratio (CR) complements this by offering a measure of the physical closure of cracks, indicating the restoration of the concrete’s structural integrity.

The research findings validate the experimental capability of the setup to accurately measure self-healing in concrete, with a notable correlation between crack characteristics, water permeability, and self-healing efficiency. This underscores the relevance of the theoretical models and applicability in understanding the fluid dynamics within cracked concrete structures. The quantitative measures provided by HR and CR offer significant insights, revealing the intrinsic factors influencing the self-healing process and highlighting the potential for enhancing concrete durability through self-healing technologies.

• Healing Ratio (HR):

$$HR\left(\mathrm{\%}\right)=\left(\frac{Kwi-Kwt}{Kwi}\right)\times 100$$

(11)

• Closing Ratio (CR):

$$CR\left(\mathrm{\%}\right)=\left(\frac{Ai-Af}{Ai}\right)\times 100$$

(12)

Lee et al. [18] report a detailed comparative analysis of water permeability and gas diffusion methods to evaluate crack width and self-healing performance in concrete. The study elaborates on the distinct flow characteristics of water and gas through concrete cracks, with water flow being influenced by crack width and viscosity and gas diffusion being governed by concentration gradients.

The study demonstrates that, while water permeability and gas diffusion tests effectively estimate crack width and assess self-healing performance, the physical properties of water and gas result in different flow behaviours and measurement outputs. This difference emphasises the need for precise methodologies in evaluating and enhancing the durability and functionality of concrete structures, considering the unique dynamics of fluid flow through cracks.

Through experimental methods, the work validates the effectiveness of the gas diffusion test as a reliable method for evaluating crack width and self-healing performance in concrete, presenting it as a complementary or alternative test to the traditional water permeability test. The findings contribute to the field by providing insights into the distinct behaviours of water and gases in cracked concrete, thus enabling a more accurate assessment of the material’s self-healing properties and overall durability.

In order to test permeability through cracked concrete, a controlled cracking method is needed to standardise and expedite the testing of internal healing and water-tightness capacity.

3. Materials and Methods

• Self-Healing Concrete Mixture

The following

Table 1. Bacterial self-healing mixture used and adapted from [2].

Material (kg/m3)
Sand (0/4 mm)	990
Gravel (4/10 mm)	878
Cement CEM I 52.5 R	380
Water	152
SHA—Basilisk	7.5

shows the design of the bacterial self-healing mixture used in all novel crack methods, as shown in the following bulleted points.

A maximum recommended quantity of self-healing agent is used for this specific mixture to observe the maximum healing capacity of this agent when used with an ordinary Portland cement (OPC) mixture design.

• Nylon Strings Method

In exploring controlled cracking techniques for concrete specimens, this study introduces a novel method using nylon strings to induce precise cracks in cubic and cylindrical concrete specimens. The methodology adheres to the BS EN 12390-1-2021 [19] standards for compressive and tensile concrete testing, ensuring a rigorous and standardised approach to specimen preparation and evaluation.

The experimental setup utilises nylon strings embedded in fresh self-healing concrete with diameters ranging from 0.3 mm to 1.0 mm. Specimens are moulded into cubic forms of 100 × 100 × 100 mm, supported by wooden or plastic strips measuring 10 × 10 × 140 mm, which serve to secure and tension the nylon strings within the moulds.

The methodology unfolds in several meticulously planned steps, as seen in Figure 3. Initially, the moulds are prepared by attaching nylon strings to the support strips, ensuring they are tightly secured and adequately tensioned to withstand the casting process. The nylon strings are pulled out from a demoulding hole in the bottom, where tension is created in these strings that allow the strips to remain in place. These strips are not glued onto the moulds since the cubic specimens need to come out of the mould with the strings embedded. The strip is meant to be relatively in the middle of the specimen, where the crack is to be positioned for permeability tests. The best results are obtained with four strings evenly distributed (around 20 mm apart). This setup is critical for embedding the strings within the concrete, allowing for controlled cracking positioning post-curing (a).

Following mould preparation, the fresh concrete mix is poured into the moulds, with particular care taken to maintain the tension of the nylon strings, ensuring their correct positioning (b). The specimens are then left to cure for 24 h, a period that varies depending on the specific mechanical properties of the concrete mix. Once the initial curing phase concludes, the strings are cut at the points of attachment to the strips yet remain embedded within the concrete, marking the commencement of the controlled cracking process (c and d).

Controlled cracks are introduced by applying gradual pressure with a steel rod aligned perpendicularly to the direction of the embedded strings, a technique that allows for precise manipulation of crack formation (e). After inducing the cracks, the specimens are carefully cleaned to remove debris, ensuring a clean interface for the self-healing agent to work effectively (f).

The study meticulously observes the bonding of the cracked interfaces, noting that the widths of the cracks range from that of the nylon string to less than 0.1 mm (g). This precision in crack width is crucial for evaluating the efficacy of the self-healing agent. Subsequently, the specimens undergo further curing and incubation, a phase during which the healing of the crack surfaces is closely monitored over time (h). This additional curing period is instrumental in assessing the long-term effectiveness of self-healing concrete, offering valuable insights into its potential applications in extending the durability and lifespan of concrete structures.

• Wedge Method

The wedge method creates indented wedges within 100 mm cubic concrete specimens to establish a smaller shear interface at a precise location. This technique offers an efficient reusable approach to specimen creation without needing external components to be embedded within the concrete. The unique concave wedge shape enables the placement of a 10 mm diameter metal rod into the indents on the opposing side faces of the specimen. Moreover, a manual compression jack facilitates a slow controlled increase in pressure to induce cracking.

The materials involved include Plexiglas (acrylic sheet) ranging from 2 to 3 mm thick and cut into 100 × 96 mm and 100 × 94 mm squares and 100 × 18 mm strips. Hot glue is utilised to assemble the strips and create the wedge. The concrete used is fresh and contains a self-healing agent. A 500 mm long worm hose with a clamp secures the split specimens alongside cubic moulds measuring 100 × 100 × 100 mm.

The methodology encompasses several steps, depicted in Figure 4. Initially, acrylic sheets are precisely cut and assembled with 18 mm strips at a 45-degree angle (a). This assembly is placed inside the cubic mould and treated with demoulding oil (b). Fresh concrete is poured, vibrated, and demoulded depending on the concrete mix after at least 24 h (c). Depending on its mix and necessary hydration time, the specimen is ready for cracking anywhere between 3 and 7 days; although, in this instance, cracking occurred after 28 days of curing (d). Incremental pressure is applied using a manual hydraulic compression jack and two rods inserted into the indents, with a clamped worm hose optionally used to manage the explosive tensile splitting (e). Post splitting, the cracked interface is cleared of debris until an airtight connection between both halves is achieved (f). Finally, the two halves are secured using the clamped worm hose (g), preparing the specimen for analysis and incubation (h).

• Plate Method

For this method, a 100 mm cubic mould is utilised alongside three plates, each measuring 130 mm in length, 7 mm in width, and having a thickness ranging from 0.3 to 1 mm. These plates are specifically designed to be bent to fit within the moulds. Wood or plastic strips measuring 10 × 10 × 140 mm and fresh concrete for the experiment are also used.

The procedure begins with the thin steel plates being precisely cut into lengths ranging from 100 to 125 mm. These plates are then glued or tied to the wooden or plastic strips, as Figure 5a depicts. After allowing adequate time for the adhesive to set, the strips are placed into the previously prepared concrete mould (b). Subsequently, fresh concrete, enhanced with a self-healing agent, is poured into the mould. It is crucial to ensure the plates are correctly aligned during this process. The newly formed specimen is then vibrated and left to air dry (c).

It is also worth mentioning that the metal plates are placed in the central area of the cube, where fresh concrete is poured on top. These do not carry tension, as the moulds are not being pierced through; instead, these are manually held in place while being cast and, after vibration, readjusted while fresh so they can harden in the correct position.

After at least 24 h, when the specimens should have achieved sufficient curing to allow for demoulding, they are carefully removed from the curing mould and placed into a curing tank. This step is crucial for achieving the desired compressive strength. Once the specimens reach the requisite strength, they are removed from the curing tank and again left to air dry (d).

The next phase involves the application of force, which is administered perpendicularly to the longitudinal alignment of the embedded steel strips. This force is gradually applied using a hydraulic jack equipped with steel rods (e). Following the splitting of the specimen, the surface interface is meticulously cleaned of any debris, and the plates are removed. This process facilitates the closure of the interface while ensuring the steel opening remains fully accessible (f).

Subsequently, the separate specimens are securely bound together using a worm hose clamp. This stage includes the initial observation, during which points of interest on the surface are identified and marked for continuous monitoring throughout the experiment (g). Finally, the specimen is prepared for both visual observations and permeability tests. It is important to note that the lateral openings must be sealed to ensure the accuracy of these tests (h).

• Water Permeability

To conduct the water permeability test, as seen in Figure 6, the following materials are required: a cubic specimen with a completely penetrated interface, a cylindrical PVC tube measuring 60 mm in diameter and 15 to 30 mm in height, a rubber membrane measuring 60 × 80 mm and 0.2 mm in thickness featuring a circular opening, a rubber disc with a diameter of 160 mm and a thickness of 10 mm equipped with an internal circular opening, a support structure to hold the specimen in place, a flask or jug for water collection, and a graduated cylinder for precise volume measurement. A flow regulator is adjusted to deliver water at 0.069 mL/s or 4.16 mL/min, ensuring a total water volume of 83.2 ml over 20 min. A hot glue gun and sticks seal any lateral cracks in the specimen.

The procedure begins with cleaning the pre-prepared cracked concrete samples to remove debris and water (a). The surface intended for permeation is selected, and any external parallel cracks are sealed using a hot glue gun (b). A thin rubber membrane is then affixed between the concrete surface and a tubular element to create a reservoir at the first water entry point (c). Once the specimen is prepared on the sides and top, it is placed within a hollow tripod holder, funnelling water directly into the middle of a flask or jug below (d). A hollow rubber ring is positioned between the holder and the specimen to prevent water from escaping the bottom surface, except for the middle section (e). The specimen is then positioned atop the holder to receive water at a predetermined rate, with any non-permeating excess water accumulating on top (f). The experiment is conducted over 20 min, with the flow of inlet water meticulously regulated.

To quantify the healing capacity of internal cracks, the formulation used is the sealing ratio (SR), where the maximum volume of water is known, as seen in the following formula:

$$SR=\frac{Q_{cracked}-Q_{healed}}{Q_{cracked}}$$

(13)

Having a total maximum volume of water that can permeate through the specimen crack in the 20 min of each experiment and subtracting the amount of water that permeates thoroughly through the specimen allows the observation of retained water due to the phenomena of surface tension, unhydrated cement particles, mineral and debris accumulation from the curing water, and the organic crystalline formation from the bacterial self-healing agent.

The water permeability method can only indicate an evolution of internal healing by determining the amount of water collected, or changes thereof, after a few days of curing. The paired use of other methods, like microscopy, is recommended to assess the surface crystalline formations from the healing agent and thus assess if the effect is due to autogenous or autonomic healing.

4. Discussion

• Nylon Strings Method

The “nylon strings method”, as detailed in the provided data tables and graphs, offers another approach for assessing the healing of concrete cracks, using nylon strings to measure or influence the healing process. This method involves recording the amount of water that passes through the samples over time, and the subsequent analysis involves calculating the percentage of healing.

In data

Table 2. Water volume obtained for each nylon strings method specimen at 0, 5, 13, and 25 days after crack.

Specimen	0 Days (mL)	5 Days (mL)	13 Days (mL)	25 Days (mL)
N1	83	66	61	61
N2	66	64	53	75
N3	43	72	72	65
N5	83	76	73	58
N6	51	71	71	64
N7	83	79	69	67
N8	47	73	63	68
N9	66	64	72	78
N10	83	70	64	62

, nine specimens are observed, with measurements taken at intervals at Day 0, Day 5, Day 13, and Day 25. The amount of water that passes through each specimen is recorded in millilitres, and these values decrease over time for most specimens, which suggests a reduction in crack permeability and, therefore, an indication of healing, as seen in Figure 7.

The statistical analyses in

Table 3. Crack sealing ratio according to reduction on water permeated and based on volume for nylon strings specimens at 0, 5, 13, and 25 days.

	% of Healing
Specimen	Day 0	Day 5	Day 13	Day 25
N1	0.00	20.48	26.51	26.51
N2	20.48	22.89	36.14	9.64
N3	48.19	13.25	13.25	21.69
N5	0.00	8.43	12.05	30.12
N6	38.55	14.46	14.46	22.89
N7	0.00	4.82	16.87	19.28
N8	43.37	12.05	24.10	18.07
N9	20.48	22.89	13.25	6.02
N10	0.00	15.66	22.89	25.30

and Figure 8 include the average amount of water passed, the maximum and minimum values, and the standard deviation, which provide insight into the variability and reliability of the data. Interestingly, the average percentage of healing initially decreases from 19.0% on Day 0 to 15.0% on Day 5, then fluctuates slightly higher to 19.9% by Day 13 and remains the same by Day 25. The average healing percentage does not show a significant increase over the 25 days, which could suggest a slower or less pronounced healing process compared with earlier methods. The standard deviation values, which are relatively high across all days, indicate considerable variability in the healing performance among the specimens.

In Figure 8, the bar graph portrays the average percentage of healing at each time interval, with error bars reflecting the standard deviation. The error bars are substantial, again indicating the specimen set’s variability. This variability could stem from the heterogeneity in crack sizes, differences in concrete composition, or variability in the distribution or activity of the healing agent.

The nylon strings method indicates some level of healing; however, the overall effectiveness seems to be less than what might be desired for a robust self-healing concrete application, based on the modest increase in healing percentage over time.

There seems to be an initial increase in healing on Day 0, which is reduced on the second reading on Day 5 and increases on the following days. This could indicate that, after cracking the specimens, the internal unreacted particles of the exposed cement react with the permeated moisture, retaining or slowing down the volume that would be measured in the process of autogenic healing.

After this initial healing, the joint effort of autogenic and autonomous healing seems to occur, as seen in the following Figure 9.

A linear trendline is observed when removing the initial sealing ratio at zero days, closely matching the bacterial healing activity.

By pairing with microscopy analysis, the nylon strings method can demonstrate the occurrence of healing, as observed in the following images in

Table 4. Surface microscopy analysis of some of the nylon strings specimens’ cracks and their evolution throughout 25 days of curing.

Specimen	Crack Width (mm)	Time in Days
		0	5	13	25
N8	0.5
N5	0.2
N7	0.2

.

From the table above, a calcium carbonate formation is observed, a limestone-like crystallisation on the surface at five days of crack healing, which, after a few days, seems to spall off the surface, indicating that there is no observable initial and instantaneous healing, only happening a few days after cracking. There is also an indication that, after the surface crystalline formation spalls (and considering the increase in the sealing ratio), the formation of a new calcium carbonate microstructure seems to proliferate inside the crack, thus slowing or sealing the passage of water.

The following Figure 10 shows an example of the mould preparation and the cracked specimen.

• Wedge Method

The effectiveness of the wedge method in quantifying the self-healing capabilities of concrete cracks treated with a bacterial agent can be discerned from the provided data, which present a nuanced picture of the healing process over 23 days, as seen in

Table 5. Water volume obtained for each wedge method specimen at 0, 3, 14, and 23 days after the crack.

Specimen	Day 0 (mL)	Day 3 (mL)	Day 7 (mL)	Day 14 (mL)	Day 23 (mL)
W1	74	49	71	54	45
W2	81	76	79	66	65
W3	68	58	63	57	57
W4	58	52	62	69	63
W5	77	74	71	67	61
W6	64	54	68	76	60

and Figure 11.

Initial observations from

Table 6. Crack sealing ratio according to reduction on water permeated and based on volume for wedge specimens at 0, 3, 7, 14, and 23 days.

	% of Healing
Sample	Day 0	Day 3	Day 7	Day 14	Day 23
W1	10.84	40.96	14.46	34.94	45.78
W2	2.41	8.43	4.82	20.48	21.69
W3	18.07	30.12	24.10	31.33	31.33
W4	30.12	37.35	25.30	16.87	24.10
W5	7.23	10.84	14.46	19.28	26.51
W6	22.89	34.94	18.07	8.43	27.71

and Figure 12 depict a somewhat irregular progression in the average percentage of healing, suggesting a non-linear healing process. This non-linear behaviour may reflect the complex dynamics of biological healing agents, where factors such as initial microbial colonisation, nutrient availability, and the physicochemical environment within the concrete matrix could influence the efficacy of crack healing over time.

A closer examination of Table 6 and the following bar graph reveals a marked increase in healing from Day 0 to Day 3, followed by a decrease on Day 7. A linear progression is observed from Day 7 to Days 14 and 23. These fluctuations imply that the healing process is not uniform and may be affected by the exposed internal unreacted cement particles in the early days of water permeability and curing. However, this does not exclude the possibility of changes in the kinetics of biological activity, including phases of rapid bacterial growth followed by periods of stabilisation or dormancy.

The wedge method’s statistical outputs, such as the average maximum and minimum percentages of healing, provide insight into the range of the healing agent’s performance. Some samples exhibit higher healing rates, which may suggest optimal conditions for the bacterial agent’s activity, while others with lower healing rates might indicate less-than-ideal conditions or less effective crack infiltration by the healing agent.

Removing the initial healing spike in the early days of crack permeability (Figure 13, day zero) removes the instant hydration of unreacted cement particles from the graph, leaving space for gradual healing consistent with the formation of bacterial calcium carbonate.

When paired with a microscopy analysis, as seen in the following

Table 7. Surface microscopy analysis of some of the wedge method specimens’ cracks and their evolution throughout 23 days of curing.

Specimen	Crack Width (mm)	Time in Days
		0	3	7	14	23
W3	0.3
W1	0.3
W5	0.7

, this occurrence is observed and corroborated:

When correlating the average healing spike at three days from Figure 12 and Figure 13, it is again observed that the bacterial healing process in the early days of crack healing seems to be non-existent due to the lack of crystal formations at the surface. However, it cannot be excluded that early bacterial healing might be in effect by capturing moisture from the unreacted cement particles deep inside the fracture, thus retaining water in its internal contact surface.

Given the observed variability, it is also essential to question the method’s reproducibility and the scale at which the healing is measured. The wedge method relies on the change in permeability as a proxy for internal crack healing; however, the healing at the macrolevel observed through permeability may not directly correlate with the microlevel biological and chemical processes within the cracks.

From the applied laboratory work of the method shown above, the following cluster of images, Figure 14, is observed:

This type of controlled cracking allows a linear crack formation between the reduced sectional area, thus controlling the risk of multiple parallel cracks that easily crumble and destroy the contact interface of the split specimen.

• Plate Method

A prototype and simplified method is used in the laboratory to assess these specimens, where, instead of three steel plates, only one is used, as seen in the following Figure 15:

The data collected from 10 cubic samples are observed in the following

Table 8. Volume of water collected through crack permeation, plate method.

Sample	Day 0 (mL)	Day 3 (mL)	Day 7 (mL)	Day 14 (mL)	Day 23 (mL)
S1	60	65	72	51	46
S2	48	63	58	46	41
S3	71	83	73	63	62
S4	45	67	68	62	58
S5	43	60	66	54	49
S6	58	82	70	61	59
S7	65	64	64	68	61
S8	54	54	53	65	65
S9	83	83	58	52	50
S10	64	60	61	53	50

and Figure 16, where a maximum of 83 mL of water is slowly poured in 20 min:

Table 9. Crack sealing ratio according to reduction on water permeated and based on volume for plate method specimens at 0, 3, 7, 14, and 23 days.

	% of Healing
Sample	Day 0	Day 3	Day 7	Day 14	Day 23
S1	27.71	21.69	13.25	38.55	44.58
S2	42.17	24.10	30.12	44.58	50.60
S3	14.46	0.00	12.05	24.10	25.30
S4	45.78	19.28	18.07	25.30	30.12
S5	48.19	27.71	20.48	34.94	40.96
S6	30.12	1.20	15.66	26.51	28.92
S7	21.69	22.89	22.89	18.07	26.51
S8	34.94	34.94	36.14	21.69	21.69
S9	0.00	0.00	30.12	37.35	39.76
S10	22.89	27.71	26.51	36.14	39.76

shows the sealing ratio of the cracks by applying the sealing ratio formula.

The data show that the plate method demonstrates a quantifiable metric for the self-healing efficiency of concrete samples treated with a bacterial self-healing agent. This method quantifies water flow through the cracks of the concrete, both in its initial cracked state and post-healing stages, to ascertain the extent of crack closure attributed to the self-healing process.

As per the data and Figure 17, the plate method reveals an overall positive trend in self-healing, with the percentage of healing increasing over the 23 days. The efficacy of the self-healing agent is particularly noticeable between the initial measurement Day 0 and Day 3, where a significant reduction in permeability is observed. The increment in healing from Day 14 to Day 23 is also substantial, indicating that the healing process is progressive and continues to evolve.

However, the healing agent’s efficacy appears inconsistent across samples, as reflected by the variability in healing percentages and the standard deviations provided. This suggests that, while the bacterial healing agent is effective, its performance may be sensitive to environmental conditions or variations in the concrete matrix, including factors such as pH, temperature, or competing microbial flora.

Notably, the healing process does not progress linearly, as indicated by the slight decrease in average healing percentage on Day 7. This non-linearity could be due to several factors, including re-cracking under stress, partial dislodgement of healing products, or an initial rapid consumption of the healing agent, followed by a slower regenerative phase.

The method’s effectiveness is supported by the calculated healing percentages using the sealing ratio (SR) formula, which reasonably assumes that the extent of healing is proportional to the reduction in fluid flow through the concrete. The results suggest that the bacterial agent contributes to crack closure and, consequently, a reduction in permeability.

In interpreting these findings, it is crucial to establish consistent and reproducible crack patterns in the concrete samples. The variation in initial crack widths and patterns could significantly affect the healing agent’s ability to seal the cracks, thereby effectively influencing the healing rates. Moreover, the environmental conditions under which the healing is allowed to progress, such as humidity and temperature, may affect the microbial activity and should be controlled or accounted for in future studies.

While the plate method provides valuable insights into the healing process, further research is necessary to minimise variability and optimise the conditions under which the bacterial self-healing agent operates most effectively. Longitudinal studies extending beyond 23 days may also offer additional insights into the long-term durability and the full potential of the self-healing process.

When analysing the sealing ratio that correlates with the internal crack sealing, an initial healing spike is observed at zero days, followed by a sharp reduction at three days and an incremental evolution on the following days. This process matches the cement particle hydration, as seen in the previous methods. The following Figure 18 is obtained by removing the initial cement hydration from the interaction surface and surface tension due to rugosity, thus promoting the slower autogenous healing process with added autonomic capacity.

Table 10. Surface microscopy analysis of some of the plate method specimens’ cracks and their evolution throughout 23 days of curing.

Specimen	Crack Width (mm)	Time in Days
		0	3	7	14	23
S10	0.4
S9	0.2
S2	2.0

shows the evolution of surface healing by microscopical observation, where samples of the plate method are observed. With crack widths ranging from 0.2 to 2.0 mm, the initial 0 to 3 days slow sealing progress is observed for samples S10 and S9, while at 7 days, calcium carbonate formations are observed in and around the cracked area, having seemingly better results in smaller crack width (S9 and S10), while creating a crystalline mass where the crack width is too broad, as seen in specimen S2.

The table above shows that at 0 days, there is no superficial crystalline formation between 3 and 7 days after crack curing, which also matches the initial healing spike’s initial assumption due to cement hydration.

• T Method

The T, or T-wedge, method is similar to the previously discussed wedge methods. The effectiveness of the T-wedge method in quantifying the healing of cracks in concrete is discernible from the tabulated data and corresponding graphical representation.

Table 11. The volume of water was collected through crack permeation using the T method.

Specimen	0 Days (mL)	5 Days (mL)	13 Days (mL)	25 Days (mL)
T1	40	71	58	63
T2	38	59	36	66
T3	43	64	65	62
T4	83	64	54	66
T5	67	65	61	61
T6	54	63	71	69

and Figure 19 provide a detailed quantitative analysis of water permeability through the cracks over time, offering a basis for calculating the percentage of healing at the following various intervals: Day 0, Day 5, Day 13, and Day 25.

The average maximum and minimum values provide insights into the range of the healing response among the specimens. A significant difference exists between the highest and lowest healing values, indicating that some specimens exhibit robust healing, while others may show less responsiveness to the bacterial healing agent, as seen in

Table 12. The crack sealing ratio is based on reducing water permeated and volume for T method specimens at 0, 5, 13, and 25 days.

	% of Healing
Specimen	Day 0	Day 5	Day 13	Day 25
T1	51.81	14.46	30.12	24.10
T2	54.22	28.92	56.63	20.48
T3	48.19	22.89	21.69	25.30
T4	0.00	22.89	34.94	20.48
T5	19.28	21.69	26.51	26.51
T6	34.94	24.10	14.46	16.87

and Figure 20.

When we transition to the graphical representation, the bar graph with error bars reflects the central tendency and variability in healing across the samples. The error bars associated with each mean percentage value are noticeably long, especially on Day 13, which aligns with the highest standard deviation observed in the tabulated data. The fluctuating pattern depicted in the graph points to a non-linear healing process, which may be characteristic of the natural variability inherent in biological healing processes and the influence of environmental factors.

From the tabulated data and graph above, an initial average healing of 34.7% on Day 0 is observed, which decreases to 22.5% by Day 5, which indicates the rapid initial hydration of unreacted cement particles, as seen in previous methods. However, the average percentage of healing experiences increases to 30.7% by Day 13 before settling at 22.3% by Day 25. The standard deviation at each time point suggests high variability among the samples, with the highest variation occurring on Day 0 and the lowest by Day 13.

The T-wedge method’s effectiveness is evident in its capacity to capture the dynamic nature of the healing process over time. The method’s reliance on the principle that healing can be equated to reduced water flow through the concrete’s cracks is both a strength and a limitation. While it offers a direct measure of crack sealing, it does not account for other factors, such as the healed area’s mechanical properties or the seal’s long-term durability.

Considering the variability observed, it becomes apparent that factors such as the size and location of the cracks, the distribution of the healing agent, the environmental conditions during the healing period, and the initial conditions of the concrete must be carefully controlled or accounted for to ensure the reliability of the T-wedge method.

By applying the same logic of the previous methods and removing the zero days of crack healing, which has a higher coefficient of variation, the following Figure 21 is obtained. At the same time, the lowest healing capacity is observed, with only 22.3% at 25 days.

Table 13. Surface microscopy analysis of some of the T method specimens’ cracks and their evolution throughout 25 days of curing.

Specimen	Crack Width (mm)	Time in Days
		0	5	13	25
T1	0.1
T1	1.0
T6	0.05

shows the evolution of healing capacity documented throughout the 25 days of crack healing, where calcium carbonate formation is again observed protruding through the cracks after five days.

In light of previous methods, a clear difference is seen in this method, where there is not an increasing and progressive evolution of the healing rate but rather a relative constant. This can be due to the imprecision of the method, precise difficulty in demoulding, and the creation of many parallel through cracks, which can lead to surface tension absorption of the permeated water, leading to significant variability of results in the time tested.

The T-wedge method demonstrates the potential for assessing the healing of cracks in concrete. Nevertheless, a more comprehensive understanding of the underlying biological and physical processes is required to enhance the method’s robustness and predictive power. Furthermore, long-term studies extending beyond 25 days would provide valuable insights into the sustainability and durability of the healing facilitated by the bacterial agent.

This method exhibits several challenges that undermine its effectiveness and sustainability. First, there is significant difficulty in demoulding, often resulting in the T-wedge breaking inside the specimen. This complication necessitates constantly recreating T-wedges, consuming considerable time and resources and questioning the method’s sustainability. Additionally, the produced cracks are not linear and are accompanied by parallel minor cracks across interfaces. These additional cracks increase the surface area at interfaces, where unreacted cement particles can absorb moisture, further complicating the analysis. Another critical issue is the sealing ratio, which remains unexpectedly constant between 5 and 25 days, contrary to the anticipated gradual increase. This constancy makes it challenging to distinguish between autogenic and autonomic healing processes in the concrete, thereby limiting the method’s diagnostic utility in evaluating the self-healing capabilities of concrete.

5. Conclusions

From the observed methods, the nylon strings method can show an average crack healing increase of almost 20% (with a coefficient of variation of 39.3%) in 23 days of testing while allowing the observation of the superficial crack sealing evolution. This method allows an easy-to-replicate assembly and fitting on the concrete moulds.

It is assumed that the initial healing at 0 days (15 min after cracking) and its high value of CV is due to the rehydration of the cement particles, retaining water in the interface, and thus, not an effect of the healing agent added to the concrete.

The wedge method shows the internal cement particle hydration can prolong for a few more days (3 days), albeit from Day 7 to Day 23, and incremental and progressive healing is captured, reaching a maximum of almost 30% (with a CV of 29.2%). This method is very easy to replicate since a pair of wedges can be easily inserted and removed from the moulds and samples and reused afterwards. Although it creates a less observable area, the captured healing capacity is increased.

The plate method has an initial healing spike observed at 0 and 3 days (with high values of CV), progressing steadily to almost 35% of average healing (with a CV of 27.4%), making this method one of the best in terms of healing capacity captured visually and through water permeability. This method also allows for the plates to be reused on other moulds.

A fourth method is also tested, with a T-shaped wedge, similar to Van Tittleboom et al. [5], but difficulties occur early with demoulding and removal of the wedges due to concrete expansion. Other than showing the initial healing spike at zero days, it only captured a maximum healing percentage of 22.3%. As the T-wedges used for the mould are also partially destroyed, the method is only used to compare the previous three methods and is not suggested as an efficient cracking method.

This investigation has substantially advanced the understanding of self-healing concrete, particularly by exploring mechanical methods for inducing controlled cracks in concrete specimens. By methodically comparing various techniques, such as the insertion of copper sheets and the application of tensile splitting tests, this research highlights the critical role of crack standardisation in assessing the self-healing efficacy of concrete. The results indicate that precise control over crack formation can significantly enhance the predictability and effectiveness of self-healing processes, thereby improving the durability of concrete structures.

The implications of this research are profound for both material science and the construction industry. Standardising and controlling crack formation benefits the laboratory testing environment and enhances real-world applications. An improved understanding of crack dynamics and their impact on self-healing processes can lead to the development of more durable construction materials. This advancement could reduce maintenance costs and extend the lifespan of concrete infrastructures, supporting more sustainable construction practices by decreasing the need for repairs and thus reducing raw material consumption and environmental impact over time.

Looking ahead, several research avenues appear particularly promising. Optimising these mechanical cracking methods to suit different environmental conditions and construction needs could maximise their efficiency and applicability. Furthermore, integrating self-healing concrete with emerging technologies such as smart sensors and the Internet of Things (IoT) could provide comprehensive solutions that enable real-time monitoring and maintenance of structural health, paving the way for smarter and more responsive infrastructures. Additionally, longitudinal studies that monitor the performance of self-healing concrete over extended periods and under real conditions are essential to validate laboratory findings and assess the practical viability of these technologies in the field.

In conclusion, the methodologies developed and insights gained from this study could revolutionise the use of concrete in construction by promoting the broader application of self-healing materials. These materials improve the durability and sustainability of structures and offer significant economic benefits by reducing maintenance costs. As the construction industry moves towards greater sustainability and resilience, the ongoing development of self-healing concrete represents a critical step forward in our ability to construct economically viable and environmentally responsible infrastructures. This research bridges significant gaps in our understanding of self-healing concrete and sets a foundation for future innovations in this exciting area of material science.