## 5 Self-Healing Mechanisms and Design Strategies  
It is well-known that human skin and tissues can repair themselves through the replacement of the damaged areas based on the nutrient uptake. Similarly, the essential products serve as nutrients in the cement-based materials with a self-repair capability, enabling these materials in repairing the damage or deterioration (Figure 3). Many studies have been performed in recent years to discover new methods for achieving an effective self-repair material alongside the durability of the cement-based materials. Figure 4 illustrates an overview of such methods. It is customary to discuss the self-healing mechanisms shown by varieties of materials, including plastics, polymers, paints, coatings, metals, alloys, ceramics, and concretes. These materials possessing their own self-healing traits are useful for the design strategies of the concrete mixes. The primary self-healing processes are the: (i) release of healing agent; (ii) reversibility of the cross-links; and (iii) miscellaneous technologies. Liquid-active self-healing agents, such as monomers, dyes, catalysts and hardeners enclosing the micro-capsules, hollow fibers, or channels, are usually added into the polymer system as reservoirs during their production. In the presence of cracks, these reservoirs are broken, thereby pouring the reactive agents into the cracks via the capillary forces; thereupon, they solidify due to the existence of pre-disposed catalysts, effectively healing the cracks. The cracks propagation is considered the main driving stimuli in the healing mechanism. Conversely, the stress needed from the cracks for relieving remains the main limitation of this automatic mechanism, which happens in the absence of any physical or exterior  
Figure 4: Various strategies for cement-based materials self-healing. Figure 3: Healing process of micro-cracks in concrete cover because of unhydrated cement nuclei-assisted continued hydration [12]. intervention. Various concepts, such as the embedment of (i) microcapsules, (ii) hollow fibers, and (iii) micro-vascular systems involved in designing the self-healing materials are explained hereunder. Microencapsulation is a process of attaching micron-sized particles of solids, droplets of liquids, or gases in an inert shell, which in turn isolates and protects them from the external environments [71; 72]. The end product of the microencapsulation process is termed as microcapsules. Generally, these microcapsules are spherical or irregular in shape with varying sizes in the range of few nanometers to micrometers. Various self-healing polymeric composites have been designed by incorporating different healing agents or catalysts enclosing microcapsules. It has been suggested [73] that the implementation of microcapsules-embedded healing agents in the polyester network matrix could yield excellent self-healing efficiency, however, not successful enough to produce self-healing components useful for practical purposes. [PERSON] et al. [74] first demonstrated the possibility of getting practical self-repairing material, wherein the self-healing capabilities were achieved by embedding encapsulated healing agents into the polymer matrix containing dispersed catalysts. Figure 5 depicts the self-healing strategy via the microcapsules embedment. There are many advantages to the microcapsule-based self-healing approach, but the uncertainty about how to achieve a complete and/or multiple healing agent limits their applications [76]. Multiple healing is only feasible when excess healing agent is available in the matrix after the first healing occurs. Thus, to achieve multiple healing in the composite materials, another type of reservoir that might be able to deliver larger amount of liquid healing agent was developed. However, such an approach has not received wide popularity due to limited success. Various composite systems formulated using the filled glass fibers have limitations.