# 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
#### 3.1.7 Effect of Silica Fume  
Figure 16 shows that the addition of silica fume reduces the fluidity of fresh wet shotcrete, reduces the slump, and increases the relative yield stress and viscosity. In terms of filling effect and ball bearing effect, the average particle size of silica fume (about 0.1-0.3 \(\upmu\)m) is smaller than that of cement (30-60 \(\upmu\)m). [PERSON] et al. [58] pointed out that the particles below 0.1 \(\upmu\)m account for more than 80% in silica fume, and the particle geometry is circular. These conditions provide the possibility of filling silicon powder particles between larger particle gaps as ball bearings. In this normal case, the free paste effect enhanced by particle filling effect will improve the fluidity of fresh wet shotcrete. This mechanism is similar to that of fly ash. However, due to the high silica content (85-96%), silica fume has high chemical activity. It participates in the hydration process of cement and increases the consistency of cement and the friction between particles. After adding silica fume, the fluidity of fresh wet shotcrete decreases because the effect of free paste is less than the effect of flocculation. In this study, the relative yield stress and viscosity increased after adding 5% silica fume. This result is similar to previous work [35], which reported that the flow resistance of fresh concrete increases with silica fume content greater than 7%.  
Considering that silica fume reduces the fluidity of wet shotcrete, superplasticizer was added into the mixture to ensure good pumping performance. It can be seen from Figure 16 that the effect of water-reducing agent on wet shotcrete with silica fume is not obvious, because after adding water reducer, the slump cannot reach a large slump, and  
Figure 16: Effect of silica fume on slump, air content, relative yield stress, and viscosity.  
the slump value is less than 5 cm. It has been reported in the previous literature [35; 38; 66] that the high specific surface area of silica fume adsorbs water-reducing agent molecules and reduces the water-reducing effect. Air content decreases with the increase of silica fume content. This may be caused by the decrease of water because the high fineness and high chemical activity of silica fume increase the demand for water. Under the same water content, adding more silica fume consumes too much water and reduces the chance of introducing bubbles into fresh shotcrete.  
As shown in Table 3, the increase of yield stress and viscosity is beneficial to build-up thickness. Due to the additional flocculation of silica fume, the bond strength of concrete mixture on surface sprayed is enhanced, and the segregation and bleeding in the pumping process are reduced. [PERSON] [31] gave a similar result, i.e., when 10-15% silica fume is added to wet shotcrete, silica fume has a positive effect on both sprayability and pumpability. It can be seen from the fitting results in Figure 16 that the silica fume has a good fitting effect on the air content and rheological parameters.  
We try to illustrate the influence of various factors on shotcrete performance through pictures, such as Figures 4, 5 and 7, etc. It should be noted that only three or four factor levels are selected in this study. Figures 3, 6, 8, 10, 12, 14 and 16 only adopt linear fitting method, but in fact, exponential fitting equation may be more suitable for the change trend of some influencing factors, such as w/c ration. Because the test data of each factor is relatively small and the fitting variance is small, all the fitting patterns in those figures are only used as a reference. In future studies, we will increase the number of test samples.