# Influence of Mix Proportions on Rheological Properties, Air Content of Wet Shotcrete--A Case Study
## 3 Results and Discussion
### Influence of Various Factors on Rheology and Air Content of Fresh Wet Shotcrete
#### 3.1.2 Effect of Sand Ratio  
Figure 6 shows that with the increase of SR, the slump decreases gradually, while the air content increases. Within the range of 50-60% of SR, the relative yield stress increases, and the relative viscosity changes slightly with a downward trend. Some researchers [51; 52; 53] have reported the same result that increasing strontium increases the yield stress but decreases the plastic viscosity. [PERSON] et al. [54] also found that natural fine aggregate has little effect on plastic viscosity.  
The analysis shows that the texture of sand is smoother than that of gravel when \"ball effect\" is considered. The increase of SR reduces the content of gravel, and more  
Figure 5: Shootability compression between low slump and large slump.  
Figure 6: Effect of sand ratio on slump, air content, relative yield stress and viscosity.  
sand as a kind of ball can fill the gap between coarse aggregates, resulting in the release of cement paste embedded in the voids and the decrease of inter particle friction. However, if the sand volume fraction is further increased, the total surface area of solid particles will increase, the consumption of cement paste will increase, the yield stress and friction between particles will increase, which will reduce the fluidity of fresh wet shotcrete. The author [55] divides the influence of SR on the fluidity of concrete into two parts: the smaller volume of SR has a negative effect on the fluidity of concrete, because there is no mortar filling the gap in the coarse aggregates; the higher volume SR also reduces the fluidity, because the high specific surface area of fine aggregate consumes too much free paste that lubricates the aggregate particles and the inner wall of the pipe. Therefore, there is a critical SR value, and the positive and negative effects of free paste or ball bearing are balanced. Considering the shootability, the round sand particles are unfavorable to the accumulation thickness. The greater the sand content, the greater the density of shotcrete and the higher the compressive strength.  
According to Table 3, SR60 is the best value to obtain both good pumping performance and good shooting performance in the range of SR50 to SR70. Of course, due to the small number of experimental samples in this study, further research is needed to find the accurate critical point. The positive and negative effects of SR on liquidity of wet shotcrete are shown in Figure 7.  
In terms of air content, adding more fine aggregate will increase the total surface area of solid particles in fresh wet shotcrete, which provides more opportunities for liquid-solid interaction, thus bringing more bubbles [56; 57]. It was noted that the mixture labeled \"SR70\" was mixed with 0.3% water-reducing agent to ensure good pumping effect. Therefore, when the content of SR is 60-70%, the yield stress and viscosity are reduced by further increasing SR. In addition, the change of SR affects the mix proportion and bulk density. [PERSON] et al. [58] pointed out that the volume fraction and bulk density of aggregate have obvious influence on the rheological properties of concrete. The bulk density is related to the type, gradation, and size distribution of aggregates. Relevant studies on these aggregate parameters are given in [51; 58; 59].  
According to the linear fitting results in Figure 6, the fitting effect between SR and slump or air content is good, R\({}^{2}\) is greater than 0.9. However, the fitting of rheological parameters is poor, the relative yield stress R\({}^{2}\) = 0.479, the relative viscosity R\({}^{2}\) = 0.7741, which may be due to the introduction of superplasticizer in SR70.