## 6 Effect of Particle Size Ratio  
In engineering practice, aggregates are usually divided into coarse aggregate and fine aggregate according to particle size. It is generally believed that the coarse aggregate plays the role of skeleton building in mixtures, while the fine aggregate mainly plays the role of filling air voids. Technical requirements for coarse aggregate and fine aggregate are accordingly put forward respectively. However, there are many different criteria for the classification of coarse and fine aggregates. 4.75 mm or 2.36 mm are often used as the boundary for the classification of coarse and fine aggregates, but this method does not consider the influence of the maximum particle size. In the Bailey method, 0.22 times of nominal maximum particle size is taken as the boundary for coarse and fine aggregates. Some researchers also suggest that the 0.25 times should be selected. Up to now, there is no consensus on the classification criteria of coarse aggregates and fine aggregates.  
Compare the \(I_{pf}\) and \(V_{a}\) of 16 VS 4.75, 16 VS 2.36 and 16 VS 1.18 in Figures 5 and 6, it is discovered that distributions of \(I_{pf}\) and \(V_{a}\) corresponding to different size combinations are significantly separated, indicating that the size ratio has a significant effect on the packing characteristics of the mixture.  
It can be observed from Figure 5 that, for the combination of 16 VS 4.75, when \(P_{bv}\)\({}^{s}\) is larger than 5%, \(I_{pf}\) of 4.75 mm particles in crushed stone mixtures are all above 0.5, while that of gravel is higher and above 0.6. For the combination of 16 VS 2.36 where \(I_{pf}\) of 2.36 mm is 0.148 times of 16 mm, when \(P_{bv}\)\({}^{s}\) is larger than 5%, the \(I_{pf}\) of 2.36 mm in crushed stone and gravel mixture are above 0.3 and increases fast with the increase of \(P_{bv}\)\({}^{s}\). Even for the combination of 16 VS 1.18 where the ratio of small particle size to large particle size is reduced to 0.074, the \(I_{pf}\) of 1.18 mm particles falls below 0.1 only when \(P_{bv}\)\({}^{s}\) is very small. \(I_{pf}\) also increases fast with the increase of \(P_{bv}\)\({}^{s}\). When \(P_{bv}\)\({}^{s}\) is larger than 20%, \(I_{pf}\) increases above 0.3.  
Compared with 4.75 mm, 2.36 mm or 0.22 times of the nominal maximum particle size used in the traditional classification of coarse and fine aggregates, it seems that the functions of particles in a packed blend cannot be simply distinguished by these standards. The function of particles cannot be clearly distinguished into skeleton building and air voids filling. In most cases, particles have both of these functions in different degrees in the mixture. The ideal air voids filling or the skeleton building function in the aggregate blend is rare. The change of \(I_{pf}\) with the proportion of smaller size particles indicates that the function of particles is closely related to the specific composition of different size particles, and cannot be simply divided according to the relationship between particle sizes. In the analysis of the packing function of particles, the proportion of particles should be considered.  
Figure 8: \(\Delta_{V_{a}}^{CS-G}\) for the dry-roded test.  
In addition, Figure 6 shows that the \(V_{a}\) of the same aggregate mixture decreases significantly with the increase of size ratio.