## 1 Introduction  
Rock-filled concrete (RFC) technology has been developed based on self-compacting concrete (SCC), which is a mass concrete construction technique. Its construction process includes stacking large-size stones (particle size > 30 cm) in the specified formwork, and then pouring SCC. When pouring SCC, its high fluidity is used to fill the gaps between stones without vibration or rolling process. Compared with ordinary concrete, RFC has the advantages of high construction efficiency, low hydration heat, energy conservation, and environmental protection [1; 2; 3]. Due to its excellent performance, RFC has been put into practical engineering application since 2005, and by the end of 2021, there have been 126 RFC dams completed or under construction in China, including 115 gravity dams and 11 arch dams [4]. In a study of RFC material properties, [PERSON] et al. [1] first proposed RFC construction technology, and tested the flow performance of SCC in the rockfill and the compressive strength of RFC. [PERSON] et al. [5] tested the compactness, compressive strength, tensile strength, and permeability of RFC, indicating that RFC meets the requirements of hydraulic concrete. [PERSON] et al. [6] established the RFC model containing rock, SCC, and interface transition zone, and tested the elastic modulus of RFC by compression simulation experiments, which put forward the prediction formula of the elastic modulus of RFC in the hardening process based on the Dual Eigenstrain method. [PERSON] et al. [7] conducted compression creep and shrinkage tests on RFC and SCC, revealing that the creep and shrinkage strains of RFC were lower than those of SCC; additionally, they found that rock aggregates had a significant impact on the creep behavior of RFC. [PERSON] et al. [8] tested the mechanical properties of RFC specimens by coring the borehole of the dam, which showed that the average compressive strength of RFC was slightly greater than the SCC strength, but much less than the rock strength. [PERSON] et al. [9] conducted uniaxial compression tests on 39 RFC cube cutting specimens; these cutting specimens were taken from a large volume test block cast in situ using the same material, process, and environment as the construction site. The results of the test showed that the compressive strength of the cutting specimens ranged from 20.7 Mpa to 51.1 Mpa, with a standard deviation of 6.3 Mpa. The strength distribution was quite scattered, which could be attributed to the differences in the size of the aggregate in the specimen, the randomness of the aggregate shape, the randomness of the spatial distribution of the aggregate, and the different proportions of the aggregate in different specimens. The three RFC cutting specimens from the construction site in Figure 1 showed the highly random shapes and spatial distribution of the aggregates. However, it is important to note that the impact of specimen size on the results has not been fully considered in previous studies on the mechanical properties of RFC. Additionally, in practical projects, the aggregate size in RFC is often randomly distributed between 30 cm and 100 cm, and in recent years, some projects have even used aggregate sizes exceeding 100 cm. It is unclear whether the effect of aggregate size on the mechanical properties of RFC is significant or not. Therefore, it is crucial to investigate the impact of both specimen size and aggregate size on the compressive strength of RFC. The strength of quasi-brittle materials, such as rock and concrete, exhibits a size effect, where the strength decreases in a regular manner as the specimen size increases. For concrete, [PERSON] et al. [10] conducted an experiment on lightweight concrete specimens and compared their compressive strength with that of common concrete under different specimen sizes; the results indicated that the size effect of lightweight concrete was more pronounced, and the researchers proposed a prediction formula for the size effect, which takes into account the unit weight of concrete.