**1. Introduction**

The concept of self-healing concrete came from the principle of the self-healing properties of the skin, a form of natural defense mechanism. Nature plays an active role in this process by the development of clots to seal the break. This is the first process of skin healing. Self-healing technology is a novel branch of engineering aimed at the protection of concrete infrastructure from developing minor and major cracks. In a bid to improve the strength and durability of concrete

which, is one of the most pervasive material in the world in terms of infrastructural construction, self-healing technology was adopted. The use of concrete has been adopted in the design and construction of major infrastructure for national growth. Globally, concrete is widely used for the construction of structural and pavement elements [1]. The first usage of concrete in the world was in the Roman Empire, for the construction of the Pantheon, which is a very great structure and still in a functional state till date [2]. Concrete microstructure consists of a multiphase nanostructured material in the composite form which ages over time. The structural strength of concrete to a large extent depends on the micro- and nanoscale structural properties of the constituent element.

Despite the uniqueness of concrete infrastructures using these innovative materials, they are still prone to cracks. The research of [3] as reported in [4] revealed that concrete crack is a result of shrinkage, weather action, thermal stresses, and so on. Using self-healing technology, the strength and durability of concrete can be improved using biotechnological method by adopting the calcite precipitation principle. Self-healing technology seems to be very effective if the crack size is not more than 0.8 mm at the early age. However, the research of [4] revealed that hydro-gel encapsulation, vascular systems, and capsules are also good methods of self-healing concrete structures. Recent research focuses on the use of biotechnology and nanomaterial and the use of autogenous principle in self-healing technology which is espoused in this review.

## **1.1 Self-healing technology**

The concept of self-healing was birthed some few decades due to the crack induced in some water retaining structures [5]. One of the major causes of concrete structural failure is the crack that can occur both in the plastic and hardened states [6–9]. The effect of crack may not be pronounced at the early stage, but it affects the mechanical strength at the late age which involves a lot of money for repair. The research of [7] showed that the active treatment of cracks seems to be an effective method as compared with the passive method of crack treatment.

The main concept was to make sure that this concrete structure affected by crack regained its mechanical strength by the hydration of the cement particles present in it [10, 11]. The concept of autogenic healing was used in this approach. According to [11], autogenous healing is a procedure where materials self-heal by nature. The same author avowed that this self-healing may be due to the formation of the carbonate or the hydroxide of carbon (calcium carbonate and calcium hydroxide). Additionally, the sedimentation of particles and swelling of the cement matrix in the concrete proved to be likely causative factors [12]. Asserted the problem of sedimentation and swelling can be averted and corrected using the self-healing capacity of the material composition of concrete.

Self-healing is an example of the active process of crack treatment. This method can operate independently in different conditions regardless of the crack position. The design of materials with healing properties is now gaining acceptance in concrete technology due to its numerous advantages.

#### **1.2 Sustainable materials used in self-healing concrete**

Sustainable structures provide environmentally friendly infrastructure, add long-term value to facilities, and improve the structural stability of structures. In concrete technology, different materials have been used in self-healing technology through three main strategies as shown in **Table 1**.


## *1.2.1 Autogenous healing*

This process of healing occurs when the continuity of two sides of cracks is restored without any external repair [25]. The same author avowed that water passing through concrete dissolve the calcium present in the cement mortar of concrete. The passage of water oftentimes is through the presence of cracks either in the hardened or plastic state. The calcium is transported in the insoluble form in the voids which eventually seal the crack without any external approach. The cracks did not only heal, but the mechanical properties were also restored. Additionally, the healed concrete becomes impermeable to water, thereby improving the mechanical strength. The principle of sealing cracks with calcium carbonate crystals from carbon dioxide in the surrounding soil, air, or water is the autogenous healing process. This reaction with the free calcium oxide and calcium hydroxide from the hydration of tricalcium silicate of the cement helps in crack healing also. However the main product that fills the void is the calcium carbonate [25].

Furthermore, the research of [26] showed that calcium carbonate is a versatile material that can be used in crack healing for the filling of voids and improved porosity. The research of [13, 14] showed that the presence of unhydrated cement in the concrete composition can affect autogenous healing. Additionally, the presence of water and humidity are also critical factors. The improvement of this approach of crack treatment depends on the water-cement ratio used in the concrete design. The lower the water-cement ratio, the better the autogenous healing process. Moreover, the success of this approach depends on the diameter of the crack induced in the concrete structure. The research of [18] showed that only cracks ranging from 0.1 to 0.3 can be filled using this approach.

### *1.2.2 Encapsulation of polymeric material*

This process involves coating of the hydrophobic nanoparticles with an additional polymer layer. This process involves the foaming of the healing agent in the presence of moisture. It also involves the use of fibers in concrete. Encapsulation also uses capsules that can survive in concrete matrix. The addition of this capsule must not interfere with rheology and mechanical properties of the concrete both in the plastic and hardened states [27]; this factor according to the research of [19] as stated in [28] makes this method difficult. The research of [19, 28] stated that encapsulation involves the use of liquid, gas, or fine solid particles incorporating synthetic polymer in concrete technology. The research of [19] stated that to provide


**Table 1.** *Self-healing methods.* protection to the constituents of the healing agent, the healing process begins when the capsule is opened to crack and the applied load breaks the capsule which invariably opens the healing agent [4]. This method can be categorized into the following:


The materials used in this method are as shown in **Table 2**.

The drawback of this approach is the tendency to repeat itself over time, and this invariably leads to repeated healing. Moreover, the moisture content required is high to make the healing process effective. Research of [42–44] showed that insufficient capillary action could render the method ineffective. The cost of production is another shortcoming of adopting this method.
