## 5 Application of External Blending Method in Recycled Coarse Aggregate Concrete
The co-mingling method involves pouring liquid and powdered admixtures into the mixer together with concrete materials, or mixing liquid admixtures with water, then mixing them with other materials. Concrete's performance can be effectively enhanced by all of these techniques. The external admixture of 20 mesh MRP with 4% admixture is the benchmark group of RCAC. In order to compare the three ways of testing the compressive and flexural strengths of concrete, the first admixture method and the identical admixture method were employed as the control group. Figure 9 displays the outcomes of the experiment. Figure 6 presents the statistical analysis of the impact of the three procedures on the flexural and compressive strengths of concrete. In Figure 9, the compressive strength of the concrete will go through three stages over time: initial strength, rising strength, and stabilized strength. There is not much difference between the stages of the three admixtures; 0-10 days belongs to the initial strength stage. The strength of concrete grows faster in this stage, mainly due to the hydration reaction of cement. The growing strength stage occurs from 10 to 20 days. The rapid strength growth rate of the concrete gradually slows down, mainly due to the continuation of the hydration reaction of the cement and the gradual reduction of the pore structure within the concrete. The stabilized strength stage occurs from 20 to 35 days. The growth rate of the concrete almost stops at this stage, and the concrete reaches its final strength. Compared to the internal and coadmixing methods, the external method produces the concrete with the highest final strength, mainly due to the fact that the external method allows the admixture to be better dispersed in the concrete. This dispersion improves the homogeneity of the concrete and allows all parts  
Figure 9: Three methods for determining the compressive strength and flexural strength of concrete. of the concrete to realize their full strength potential. In Figure 9b, the three methods of admixture have little effect on the cracking strength of concrete, which is due to the fact that the amount and manner of admixture of the substances did not change the basic structure and properties of the concrete. A quick freeze-thaw cycle test was used in the experiment to evaluate how the three inclusion techniques affected the longevity and frost resistance of the concrete. Figure 10 displays the outcomes of the experiment. In Figure 10a, the three admixture methods did have different effects on the mass loss of concrete. Among them, the concrete produced by the external admixture method exhibited the least mass loss. For the concrete produced by the coadmixture method, the mass loss was higher during 0-140 freeze-thaw cycles (FTC). And after 140 FTC, the most significant mass loss was observed for the concrete produced by the first-mix method. These differences can be attributed to the effect of the admixture method on the internal structure and compactness of the concrete. The external mixing method minimizes the quality loss due to better dispersion of the admixtures, resulting in a more homogeneous and dense internal structure of the concrete, which is more effective in resisting the damage caused by FTC. In the early freeze-thaw cycle, the homogeneous mixing method may result in higher quality loss due to the uneven mixing of the admixtures with the concrete raw materials. The first-mixing method may have a certain degree of admixture aggregation during the production process, which makes the concrete susceptible to damage in these aggregated areas after several FTC, resulting in the greatest loss of quality. As the number of FTC increases, the trend of the elastic modulus change of the concrete made using the other two admixture procedures(i.e., internal admixture and coadmixture) does not differ significantly, as shown in Figure 10b.