Mechanism Study On The Effect Of Vibration And Vacuum Mixing On Pore Structure And Performance Of Concrete

Concrete is a porous composite material composed of solid,liquid and gas phases,and the development and changes of each phase have an essential impact on the performance of concrete.The various pores contained in the gas phase of concrete are important components of the concrete structure.Under the action of external loads,temperature,humidity,and corrosion,the volume changes,such as expansion and contraction of concrete,are more caused by pores.The gas phase in concrete mainly comes from the air voids generated during the mixing process and the pores generated during the hardening process.The latter is associated with the material composition,such as the water-cement ratio,while the former is related to the mixing method and process.The latter can be improved by adjusting the mix ratio,adding admixtures and mineral admixtures,while the former is mainly the residual air during the mixing process,which is the most challenging component to disperse uniformly in the concrete.Therefore,it is of great theoretical significance and engineering practical value to study the difference between concrete pore structures prepared with various mixing methods,including ordinary mixing,vibration mixing,vertical shaft mixing,horizontal shaft mixing,and vacuum mixing,as well as the impact on concrete performance.The paper analyzes the mixture’s force and state evolution and its gas phase change in the four stages of the mixing process.In the first and second stages,the gas inside the mixture is mainly discharged,and the air content continues to decrease.The gas in the third and fourth stages changes from volume to structure and size.Therefore,the improvement of pore structure by vibration mixing and vacuum mixing occurs in the second half of the mixing process.Moreover,according to different sources,the gas inside the mixture during the mixing process can be divided into the gas contained in the mixture itself,the gas brought in during the mixing process,and the gas introduced by adding an air-entraining agent.The analysis of the mixing process shows that the gas contained in the mixture itself is the primary source of the air content in the fresh concrete.Based on the digital image process,the paper studies the influence of different mixing methods,including ordinary mixing and vibration mixing,horizontal shaft mixing and vertical shaft mixing,and vacuum mixing on the concrete pore structure.The results show that compared with normal mixing,vibration mixing can significantly improve the pore structure of concrete.The air content and slump of fresh concrete are increased,the volume of permeable pore space of hardened concrete is lower,and the number of pores is more significant.The average pore size and the pore characteristic distance are smaller.Since the mixing shaft,mixing arm and blade of the horizontal shaft vibration mixer are all vibration sources,the vibration energy is larger than that of the vertical shaft mixing at the same vibration intensity,and the vibration mixing effect is more significant.When similar vibration power consumption per unit volume of concrete,it is found that the air content,average pore size and the number of pores of horizontal shaft mixing are higher than those of vertical shaft mixing.The difference in air content between the horizontal and vertical shaft mixing,as well as the boundary conditions and the movement of the mixing device,also confirm that the air brought in during the mixing process is not the primary source of the gas phase in the fresh concrete but the air contained in the concrete itself.Vibration mixing can refine and homogenize the pore structure,thereby improving the various performance of concrete.When vibration mixing is used,the compressive strength of concrete is improved,and the dispersion coefficient is reduced.Vibration mixing can improve the freeze-thaw resistance of concrete by reducing the size of pores,increasing the number of pores,and reducing the characteristic distance.Vibration mixing can also enhance the antipermeability of concrete by reducing the volume of permeable pore space and increasing the density of concrete.Comparing the performance of different concretes,it is found that the movement trajectory of the mixture is more complicated using vertical shaft mixing,so the compressive strength is slightly higher than that of horizontal shaft mixing.Because the pore size is the same,but the number of pores is more,the freeze-thaw resistance of vertical shaft mixing is better than that of horizontal shaft mixing.The volume of permeable pore space is smaller,the number of pores is smaller,and the density is higher,so the anti-permeability of horizontal shaft mixing is better than that of vertical shaft mixing.The paper also conducted an experimental study on the effect of vacuum mixing on the pore structure and performance of concrete.The results show that the vacuum degree significantly impacts concrete’s air content and compressive strength more than the filling rate and vacuum action time.When using vacuum mixing with a vacuum degree of-0.09 MPa,the pore structure of C80 concrete changes significantly,the air content decreases by about 80%,the numbers and average size of pores are smaller,and the microstructure is more compact.Moreover,the pore size distribution has also significantly changed.Between 0.05 and 0.2 mm,the number of different pores in vacuum mixing is more significant than that of non-vacuum mixing,while between 0.2 and 1 mm,the number of various pores in non-vacuum mixing concrete is much larger than that of vacuum mixing.In addition,by comparing the difference of concrete compressive strength between horizontal shaft and vertical shaft,it is also found that the effect of horizontal shaft vacuum mixing is better than that of vertical shaft vacuum mixing.