# Numerical and Experimental Investigation of Recycled Brick Coarse Aggregate Concrete
## 4 Conclusions  
In order to study the mechanical and microscopic properties of RBCA concrete, the slump, apparent density, and compressive strength of RBCA concrete with different contents were tested in this paper. Combined with numerical analysis, the damage mechanism of the finite element model was studied and the content of RBCA and the thickness of the interface transition zone were analyzed, resulting in the following conclusions:  
1. Replacement of NCA with RBCA reduces the weight of concrete. Our experimental results show that when the RBCA content in concrete was 25%, 50%, 75%, and 100%, the apparent density of concrete was 3.95%, 7.52%, 10.46%, and 15.55% lower than that of ordinary concrete. In addition, replacing NCA with RBCA reduces the fluidity of concrete. The slump decreases significantly with increasing RBCA replacement rate, although this effect decreases significantly when the replacement rate is less than 50%.
2. Compared with ordinary concrete, the reduction rate of the compressive strength of 25%, 50%, 75%, and 100% RBCA concrete were 5.7%, 14%, 20.6%, and 28.6%, respectively, the peak strain of the concrete increased by 2.5%, 7.5%, 10.5%, and 15.0%, respectively, and the elastic modulus decreased by 10.4%, 18.9%, 27.4%, and 38.5%, respectively. Focusing on the compressive strength of RBCA concrete and taking into account the workability of fresh concrete and the principle of improving the utilization rate of RBCA, the best replacement rate is 25%.
3. The failure process of recycled concrete was predicted by numerical simulation. When the replacement rate of RBCA is 100%, 1-2 oblique continuous failure cracks form on the section. When the replacement rate of RBCA is 0-50%, the internal cracks are relatively discrete. The stress state of RBCA concrete under peak strain was obtained by numerical analysis. Tensile stress and shear stress concentration exist in the interface transition zone and the edge of the aggregate. The concentrated tensile and shear stress area coincides with the damage and failure area, failing the model requirements.
4. The compressive strength, peak strain, elastic modulus, and ratio of ultimate strain to peak strain were studied for different RBCA substitution rates. The compressive strength, tensile strength, and initial elastic modulus decreased with increased RCA content, while the peak strain increased slightly.
5. Based on the numerical model, the effects of the thickness of the interface transition zone and the replacement rate of recycled aggregate on the compressive strength of recycled concrete were studied. The substitution rate of RBCA and the thickness of the interfacial transition zone negatively correlate with compressive strength and positively correlate with elastic modulus and peak stress.  
Conceptualization, Y.J. and L.W.; methodology, D.W.; software, D.W.; validation, Y.J., L.W. and D.W.; formal analysis, D.W.; investigation, L.W.; resources, Y.J.; data curation, Y.J.; writing--original draft preparation, D.W.; writing--review and editing, Y.J.; visualization, D.W.; supervision, Y.J.; project administration, Y.J.; funding acquisition, Y.J. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Fundamental Research Funds for the Central Universities (No. 2572022 BJ03) and the Heilongjiang Province Studying Abroad Student (Startup Class) Scholarships.  
Not applicable.  
All data generated or analyzed during this study are included in this published article.  
The authors would like to acknowledge support from the Fundamental Research Funds for the Central Universities (No. 2572022 BJ03) and the Heilongjiang Province Studying Abroad Student (Startup Class) Scholarships.  
The authors declare no conflict of interest.