## 6 Conclusions  
A simple computational modelling approach for studying the capsular clustering effects on the fracture mechanism of encapsulation-based self-healing concrete is developed in this paper. Two-dimensional specimens of a three-phase composite consist of a concrete matrix, a microcapsule shell, and an interface between them. The eXtended Finite Element Method (XFEM) is used to model the concrete and the capsule parts, while the interface between them is modelled by the cohesive surface technique. The microcapsule circumferential contact length (Lcs), which represents the interface contact surface between the microcapsule and the concrete, is used to define the degree of microcapsule clustering. The microcapsule size is defined by the core/shell ratio; the following conclusions are made:  
* The microcapsule circumferential contact length (Lcs) between the microcapsule shell and the concrete matrix has a significant role in governing the load-carrying capacity of the specimen. The load-carrying capacity of the sample with the microcapsule core/shell ratio 1:1 and Lcs 100% is reduced by 4.6% in the case of Lcs 25%. While the load-carrying capacity of the sample with the microcapsule core/shell ratio 15:1 and Lcs 100% is reduced by 1.1% in the case of Lcs 25%. This shows the higher the Lcs (i.e., the lower the degree of microcapsule clustering), the higher the load-carrying capacity, and vice versa. * The Lcs also affects the crack pattern; however, it is not recommended that the microcapsule core/shell ratio be 1:1, which has the largest shell thickness but the smallest volume of the healing agent. Consequently, the debonding of the microcapsule from the concrete will happen regardless of the ratio of Lcs. Otherwise, when the Lcs is lower than 25% of the microcapsule circumference, it will result in a greater possibility for the debonding of the microcapsule from the concrete. * The microcapsule size has an impact on the load-carrying capacity of SHC specimens. The load-carrying capacity for the sample with Lcs 100% and the microcapsule core/shell ratio 1:1 is reduced by 3.8% in the case of the microcapsule core/shell ratio 15:1. While the load-carrying capacity for the sample with Lcs 25% and the microcapsule core/shell ratio 1:1 is reduced by 0.28% in the case of the microcapsule core/shell ratio of 15:1. It is obvious that the lower the core/shell ratio (larger shell thickness) of the microcapsule, the higher the load-carrying capacity of the specimen, and vice versa. * The microcapsule core/shell ratio has also a significant effect on the crack pattern as it determines whether the microcapsule will fracture, or debond from, the concrete. Figure 17: Crack pattern of microcapsule ratio 10:1 with different Lcs ratios: (**a**) Lcs 100%, (**b**) Lcs 75%, (**c**) Lcs 50%, and (**d**) Lcs 25%. Figure 18: Crack pattern of microcapsule ratio 15:1 with different Lcs ratios: (**a**) Lcs 100%, (**b**) Lcs 75%, (**c**) Lcs 50%, and (**d**) Lcs 25%. The greater the core/shell ratio (smaller shell thickness), the greater the likelihood of microcapsules being fractured. * It can be concluded from studying the effects of the microcapsule circumferential contact length and the microcapsule core/shell ratio on the load-carrying capacity that the microcapsule clustering has a higher effect than the microcapsule size on the load-carrying capacity of the SHC samples. * It is important not only to take care of microcapsules size, but also the clustering effects during the mixing process of the SHC, to minimize its effects on the contact surface between the microcapsule and the concrete to ensure the breakage of the microcapsules and the subsequent release of the healing agent. As a possible future perspective of this research, the proposed computational framework for modeling the fracture mechanism in encapsulation-based SHC could be used to investigate the minimum microcapsule interdistance that should be considered in the design issue to obtain an effective fracture healing mechanism.