# Influence of Mix Proportions on Rheological Properties, Air Content of Wet Shotcrete--A Case Study
## 3 Results and Discussion
### Comparison of the Effects of Fly Ash, Silica Fume on Rheological Properties and Air Content  
To understand deeply the effects of silica fume or fly ash on the properties of fresh wet shotcrete, the comparison between these two and other admixtures were analyzed.  
According to the comparative analysis of Figure 21, it can be concluded that:  
Figure 21: _Cont._In the aspect of relative yield stress, the effect of silica fume (SF) on the yield stress of wet shotcrete is similar to that of low water-binder ratio (WC), medium sand ratio (SR) and fiber (FI), almost exceeding the average value of 0.745 Nm. Fly ash (FA) and air-entraining agent (AE) have the same effect, showing a downward trend. It can be seen from the data in Table 3 that the high yield stress is beneficial to the shoot property, and the addition of silica fume or a small amount of fly ash can produce a better build-up thickness.  
In the aspect of relative plastic viscosity, the influence degree of fly ash on plastic viscosity is similar to that of air-entraining agent, which is lower than the average value of 1.31 Ns. The similar effect can be produced by low dosage of water-reducing agent, high water-binder ratio, or high sand ratio. When the dosage of silica fume is 5-15%, the plastic viscosity is little affected and fluctuates around the average value. For the flow of fresh wet shotcrete in the pipeline, if a lubricating layer is formed on the inner wall of the pipe, the higher viscosity will improve the anti-segregation and anti-bleeding ability of the wet shotcrete containing silica fume, avoid blocking and increase the build-up thickness. However, for shear flow, lower viscosity is beneficial to the pumping performance of fresh wet shotcrete. Therefore, adding fly ash is beneficial to pumping.  
In terms of slump: the effect of adding silica fume and fiber in fresh wet shotcrete is similar, and the slump of both is lower than the average value of 7.4 cm. The effect of adding fly ash on slump is similar to that of low air-entraining agent or low water-reducing agent. According to the test of pumpability and shootability, high slump is beneficial to pumping. Fly ash is better than silica fume in pumping process, but the shootability is opposite. It should be noted that the change trend of slump is opposite to that of relative yield stress, which is also reported in [29; 52].  
In terms of air content, the air content of wet shotcrete mixed with silica fume is lower than the average value of 6.2%, which is similar to that of fiber, low water-binder ratio, or high sand ratio. However, fly ash can produce similar effect of air-entraining agent or water-reducing agent at high air content. If the assumption that spraying behavior will  
Figure 21: Comparative analysis of influence between fly ash, silica fume, and other factors.  
reduce air content is true, fresh wet shotcrete with high air content is beneficial to pumping and spraying. Therefore, it seems that adding silica fume is not conducive to the air content of wet shotcrete, while fly ash with relatively high air content is better.  
Compressive strength: the influence of fly ash content on compressive strength is moderate, slightly higher than the average value. Considering the pumping and shotcreting performance of fly ash, fly ash is undoubtedly the common admixture of wet shotcrete. From the rheological parameters and air content, although the fluidity of concrete mixed with silica fume is low, it has great advantages in strength and sprayability. In some special projects that need to add silica fume into shotcrete, the pumpability can be improved by adding water-reducing agent or air-entraining agent.