## 5 Application of External Blending Method in Recycled Coarse Aggregate Concrete  
The main objective of the external admixture method is to increase the volume of coarse aggregate in concrete without changing the concrete mix ratio in order to achieve the effective utilization of RCA in concrete [20; 21]. The basic principle of the external admixture method is to disperse the rubber powder in the matrix in the form of individual particles, rather than in the form of a dispersed phase, so as to improve its compatibility with the matrix and filling properties [22; 23]. The application of external admixture in RCAC has several advantages. By using an external mixing method, concrete's volume stability can be increased. It can also prevent shrinkage and creep, increase the volume of coarse aggregate in the concrete, and enhance its volume stability. Concrete can have its mechanical qualities enhanced by the addition of an external admixture. This will increase the RCAC's compressive strength, flexural strength, and other mechanical characteristics, bringing it closer to the technical specifications [24; 25; 26]. Second, the external admixture approach can enhance the durability characteristics of RCAC, such as its resistance to frost and impermeability, and increase its ability to adjust to variations in the surrounding environment. Finally, costs can be reduced by the external admixture method, which does not require changes in the concrete mix ratio and, therefore, does not increase the cost of new materials. It can also reduce the cost of waste disposal [27; 28]. In summary, the use of the external admixture method in RCAC can enhance concrete performance while lowering costs and reducing environmental pollution. It can also help to realize resource utilization of waste. The schematic diagram of the application process of external admixture method in RCAC is shown in Figure 8.  
The raw materials used in the experimental process included cement, RCA, crushed stone, and external admixtures. A 42.5 grade ordinary silicate cement was used. RCA was taken from a construction and demolition material treatment plant and made after cleaning, crushing and grading, and its performance indexes conformed to the relevant standards [29]. Medium sand with a fineness modulus of 2.6 and continuous graded gravel  
Figure 8: The schematic diagram of the application process of the external mixing method in recycled coarse aggregate concrete. ranging from 5 to 25 mm were used. Silica fume, slag powder, and experimentally prepared MRP were also used, and their performance indexes met the relevant standards. A high-efficiency water-reducing agent with a 25% water reduction rate, specifically polycarboxylic acid high-efficiency water-reducing agent, was utilized. The experimental method contains four main steps. First, it is necessary to design the mixing ratio, replacing an amount of sand with an equal amount of silica fume, and designing the test using external mixing method, with the water-cement ratio of 0.35, the dosage of high-efficiency water reducing agent of 1.5%, the dosage of silica fume of 16%, and the sand rate of 34%. After that, the concrete specimens were made in accordance with the design mix ratio and cured under typical curing conditions for 35 days. The test of compressive strength, which is the third phase, was conducted according to the Standard for Test Methods of Mechanical Properties of Ordinary Concrete. Each specimen's compressive strength value was noted. Lastly, test data analysis was carried out to investigate the impact of the external admixture method on the compressive strength of RCAC. In addition to the external admixture method, the first-mix method and the co-mix method are also commonly used in concrete. The first-mixing method involves mixing the powdered admixture with cement before adding aggregate and water to the mix.