# 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
#### 3.1.5 Effect of Water Reducer  
Figure 12 shows that the relative yield stress, viscosity, and air content decrease with the increase of water reducer dosage from 0.3% to 1.2%, while the slump is opposite. Under the same water-binder ratio, the fluidity of fresh concrete is greatly improved when the quality of water-reducing agent is mixed. When 1.2% water-reducing agent is added, the slump value reaches 15.1 cm, which is much higher than that of wet shotcrete with other admixtures. There was no bleeding or separation during tests. [PERSON] et al. [58] indicated that the spatial resistance effect of water-reducing agent is one of the reasons for this phenomenon. After adding water reducer, the yield stress and viscosity of fresh concrete decrease significantly. As shown in Figure 13, after adding water reducer, the flocculability of cement particles decreases because water reducer makes cement particles bringing same charge. In addition, the efficiency of water reducer also depends on the adsorption of water-reducing agent molecules to cement particles and the repulsion force produced by adsorption molecules. [PERSON] et al. [64] found that the addition of water-reducing agent increases the effective layer thickness of cement particles covered by paste. The increase of the bridging distance between particles reduces the maximum attraction between particles. [PERSON] et al. [65] concluded that with the increase of water-reducing agent dosage, the average surface spacing increases, and the colloidal interaction and flocculation degree between particles are reduced, thus reducing the rheological parameters of fresh wet shotcrete.  
Figure 11: Schematic of the effect of air bubble on flowability of WET SHOTCRETE.  
When WR exceeds 0.9%, the relative yield stress decreases slowly, the plastic viscosity increases inversely. It might be explained that there is a critical saturation point for the efficiency of superplasticizer. Beyond this point, more superplasticizers will increase the viscosity of the liquid. Compared with other admixtures (except air-entraining agent), the air content of wet shotcrete with water-reducing agent is relatively high, with an average of 12.25%. The analysis shows that the water reducer contains a certain amount of surfactant and thus introduces some bubbles. If the superplasticizer is continued to be added, excessive water reducer will increase the viscosity of the liquid and hinder the generation of bubbles, which will lead to the decrease of air content.  
According to the fitting results in Figure 12, the slope of the linear equation represents the rate of increase or decrease. The decrease of relative yield stress is greater than that of relative plastic viscosity. The results show that superplasticizer has good fitting effect on slump and relative yield stress, R\({}^{2}\) = 0.9592 and R\({}^{2}\) = 0.9176, respectively, which can be used to predict the changes of these two properties.  
Table 3 shows that adding more superplasticizers can reduce the stacking thickness. It is pointed out that although water-reducing agent is almost used in modern pouring concrete due to its economic and technical benefits, the application of water-reducing agent is not conducive to improving the shootability of concrete. It is suggested that when premixing water-reducing agent in front of pump, additives such as viscosity agent or accelerator should be added at the same time to restore or improve its shooting performance.  
Figure 12: Effect of water reducer on slump, air content, relative yield stress, and viscosity.  
Figure 13: Mechanism of water reducer affecting cement paste.