## 5 Conclusions  
In this study, the GPGPU-accelerated 3D FDEM has been developed with the introduction of the 1D rebar model to analyze the demolition process of the RC structure in detail. The fracturing of the concrete is modeled on the framework of the 3D FDEM and can reproduce the fracture process of the concrete by modeling continuum behavior, crack initiation and propagation and contact between discrete elements. The reinforcement of the RC structure is also modeled using the 1D rebar model, which includes two-node REs and two-node RJEs. As the constitutive model, linear elastic and elastic-perfect plastic models were applied to describe stress statements in the RE and RJE induced by its deformation. The 3D FDEM parameter was firstly determined based on our previous study [33] to properly simulate the fracture process of the plain concrete by simulating fundamental mechanical tests, i.e., UCS and BTS test. On the other hand, RE was modelled as 1D linear elastic material while RJE was modeled as 1D elastic-plastic material. The three-point bending test for the RC beam was simulated to verify the proposed 3D FDEM. Different element sizes in the FDEM mesh for the concrete model were constructed and investigated to consider the size effect of element size on the fracture behavior. The mid-span loading displacement curves and resultant fracture pattern for the RC beam were also discussed based on the results of the three-point load testing for the reinforced and plain concrete beams. In addition, the blasting demolition of the small-scale RC structure, composed of the three stories as a \(1\times 1\) bay structure, was also simulated to verify the applicability of the proposed FDEM for simulating the progressive collapse of the RC structure. The following conclusions can be taken from this study:  
1. The fracture process of the concrete part in RC structure is highly dependent on the FDEM element size and, depending on the selected element size, exhibits different mechanical behavior when subjected to three-point loading. The RC beam model with an average element size of 100 mm showed a splitting mode whose direction of main fracture was parallel to the three-point loading direction, which is not observed in reality. On the other hand, with the decrease of the element size (50 mm, 25 mm  
Figure 17: Comparison of the x-axis displacement in experiment and numerical simulation (This figure has been reproduced from [7] under the terms and conditions of the CC BY-NC license). and 12.5 mm in this study), the obtained crack propagation pattern became more and more realistic compared with the experiment. Therefore, the optimal size of the element size for concrete must be determined by carefully checking the effect of the element size on the fracture process of concrete in the RC structure. Once the proper element size is determined, the rebar model incorporated in this paper was found to be less sensitive with respect to the element size. In addition, by comparing the cases with and without the reinforcement, brittle fracturing occurred in the former case, while the latter case showed the clear effect of reinforcement characterized by its yielding. The loading-displacement curves and resultant fracture pattern for the RC beam obtained by FDEM were compared with the experimental results, and these exhibit a high degree of agreement. It may be concluded that the developed 3D FDEM can be used to simulate the collapse process of the RC structure effectively. 2. The FDEM simulation of collapse process in the small-scale RC structure including the beams, slabs, and columns for blasting demolition was successful in terms of the reinforcing effect of each component of the RC structure. When compared to the history of displacement at a specific point in the RC structure, the numerical simulation results agreed well with experimental observation to some extent. However, the differences between the FDEM simulation and the experiment were also noticeable after the falling RC structure dynamically contacts with the ground.