# 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
#### 3.1.4 Effect of Air-Entraining Agent  
Figure 10 shows that with the increase of air-entraining agent content, the air content slump increase, while the relative yield stress and viscosity decrease. The results show that adding more air-entraining agent in fresh wet shotcrete will produce a lot of bubbles, which plays a role of \"ball effect\". These entrained bubbles play a key role in lubricating aggregate and further affect the consistency of fresh wet shotcrete. The fluidity of wet shotcrete is improved, the slump is increased, and the yield stress and viscosity are reduced. Similarly, [PERSON] et al. [35] has been pointed out that the torque viscosity decreases significantly with the increase of air content. However, in an earlier study [61], the author showed that there was a critical air content of 5%, before which both flow resistance and torque viscosity decreased; after this point, the two rheological parameters tended to be stable. In our study, when 0.02% air-entraining agent is mixed, the air content reaches 13.5%, far exceeding the critical value of 5%. The corresponding criterion in reference [61] is invalid. This difference can be explained by two factors: one is the application of different types of test equipment or the type of fresh concrete (ordinary concrete with aggregate size greater than 30 mm and shotcrete with maximum particle size of 10 mm); the other can be found in [58]. The author [58] proposed that since bubbles are attracted to the cement particles to form a bubble bridge, which increases the bond strength between cement particles (similar to flocculated particles), the entrained bubbles may lead to an increase in yield stress. Once the bubble bridge breaks under strong shear, some of the retained paste is released, apparently acting as a lubricant, as shown in Figure 11. We can conclude that bubbles have two functions: binding and lubrication. Both are related to shear strength and aggregate parameters. Therefore, some reports [62; 63] show that the yield stress increases or remains stable with increasing air content.  
Figure 10: Effect of air-entraining agent on slump, air content, relative yield stress, and viscosity.  
In our study, adding air-entraining agent can improve the pumping performance of concrete in Table 3. Therefore, we infer that in this measurement process, the lubrication effect of bubbles is significantly greater than that of adhesion, with the range of AE0.02%-AE0.06%. Due to the spraying process of fresh wet shotcrete, bubbles will break and disappear, this kind of fresh concrete has better shotcreting property with good cohesion. Therefore, the air-entraining agent can improve the pumpability and sprayability in a certain range, which is worthy of recommendation in the application of wet concrete.  
According to the linear fitting results in Figure 10, the air-entraining agent has a good fitting effect on the rheological parameters, with the relative yield stress R\({}^{2}\) = 0.9657 and the relative viscosity R\({}^{2}\) = 0.988. Air-entraining agent has the worst fitting effect on air content, which may be the cause of instability of fresh concrete with more bubbles. The prediction results of total rheological properties and air content are ideal.