Characteristics Of Air Bubbles Over The Life Cycle Under Low Air Pressure And Improvement Of The Performance Of Air Entrained Concrete

The high-altitude plateau area accounts for about a quarter of China's land area,and the construction in the plateau area is rapidly developing.However,it was found the conventional air-entraining agent?AEA?in high-altitude plateau applications had met series of problems:the air content of fresh concrete is low,the fluidity of fresh concrete is insufficient,and the freeze-thaw resistance of hardened concrete is decreased when compared with the concrete in the plain area.This indicates that the effectiveness of AEA is affected by the low air pressure environment of the plateau.A new test method measuring the characterization of bubble structure under low air pressure is established to ascertain how the plateau environment affects AEA's effectiveness.The data describing the effect of low air pressure on AEA performance is obtained in both the laboratory simulation and the actual plateau construction.The characteristics of the AEA bubbles in its life cycle under low air pressure are studied and compared with that under normal air pressure.The characteristics of the AEA bubble under low air pressure are clarified.Based on these results,a new type of AEA is developed to improve plateau concrete's performance.The following test results are obtained:?1?Equipment testing the structure of AEA bubble under simulated low-air-pressure plateau environment have been developed,which ensures that both the stirring process and performance testing on the solution or cement paste of AEA are carried out under low air pressure,avoiding the error caused by air pressure change during sample transportation.The characteristic parameters of the AEA bubble under the pressure of20 kPa?100 kPa,such as the foam volume V_s,bubble diameter in AEA solution d_s,and bubble diameter in cement paste d_c were obtained.The results showed that when compared with 100 kPa,the foam volume V_s of 6 kinds of AEAs decreased by 9.4%on average,the d_s-5min and the d_c of saponin AEA increased by 4%and 18%respectively under 60 kPa.The deterioration of the air-entraining effect and the decrease of air content under low air pressure on the plateau are closely related to bubble structure change.?2?Comparison tests on the performances of 6 kinds of commercial AEAs were conducted in Beijing?50 m,100 kPa?and Lhasa?4200 m,60 kPa?.Test results showed that the air content of the air-entrained concrete is related to the type and molecular structure of the AEA:Air content of concrete mixed with saponin,rosin,and cationic gemini surfactants under 60 kPa is less than 10%lower than that under 100 kPa,while that of concrete mixed with alkylbenzene sulfonate,polyether,and anionic gemini surfactants decreased by more than 15%under 60 kPa.The study also found that when compared with the concrete under 100 kPa,the pore diameter of hardened concrete under 60 kPa increased by 21%,the bubble spacing factor increased by 45%,the chloride diffusion coefficient increased by 80%,and the relative dynamic elastic modulus of concrete after 200 freeze-thaw cycles decreased by 11.6%on average.These results indicate that both the pore structure of air-entrained concrete deteriorated and the concrete's durability decreased under 60 kPa low air pressure.?3?It is proposed that the AEA bubble's life cycle can be divided into three stages—generation,mixing-separation,and decaying stage.The bubble characteristics of AEA solution in each stage are described as following:?1?In the bubble generation stage,a large number of bubbles are produced in the AEA solution by stirring,V_s increases rapidly to the maximum value V_s-max within 20 s.It is observed that the V_s-max-max under low air pressure is 5%lower when compared with that under normal air pressure.?2?In the mixing-separation stage,the small bubbles suspend in the solution.The whole solution becomes a mixed system of water and bubbles.V_s can maintain a constant value for a short period,and then some bubbles rise rapidly because of the buoyancy,the gas and the liquid separated,then V_s decreases rapidly.Under low air pressure,the bubble rises faster from the solution:the duration of this stage is 30 s under low air pressure,33%less than the 45 s duration under normal air pressure.?3?At the bubble decay stage,the bubble ripens,drains,and merges,and the foam volume V_s decreases gradually with time.The bubble stability under low air pressure is worse than normal air pressure.This stage's duration is 8 h under low air pressure,which is 33%less than the 12 h duration under normal air pressure.?4?The main reasons for the development and change of bubbles in the life cycle under low air pressure are analyzed from the points of compression/diffusion of gas and nucleation/rise of the bubbles.The gas volume compression proportion under 60 kPa is reduced by 6%than under normal air pressure,which is one reason for the decrease in entrained gas volume.The surface tension of the AEA solution under 60 kPa increases by 4%compared with that under 100 kPa.Under low air pressure,the bubble diameter increases,which leads to the faster escape of the bubble from the solution and the decrease in the volume of gas retained in the dispersion medium.The gas solubility under 60 kPa is 40%lower than that under 100 kPa.The volume of dissolved soluble gas decreases,the supersaturated dissolved gas enters the bubble,and the bubble growth rate increases by 104%,which means the bubble's decaying is accelerated.?5?The parameters that significantly affect the air-entraining capacity under low air pressure in each stage of the life cycle are proposed—volumetric compressibility conversion coefficient k_Z,gas retention correction coefficient k_B,and bubble stability conversion coefficient k_S.A prediction model for air content of concrete under low air pressure is established based on these parameters.After engineering verification,the difference between the calculated value and the test value is less than the error value0.5%specified in the standard.?6?To improve the bubble stability conversion coefficient k_S,maleic rosin-based gemini AEA,including MRE and MRP,are designed and synthesized specifically for low air pressure.The ternary phenanthrene ring group was designed to increase the molecular structure rigidity,and the gemini linking group was introduced to reduce the electrostatic repulsion between the hydrophilic groups.Compared with rosin AEA,the strength of the bubble liquid film of the MRP is 11%higher,the molecular arrangement density on the bubble liquid film is 50%higher,and the bubble stability is improved.Under low air pressure,when compared with air-entrained concrete with rosin,the air content reduction rate of air-entrained concrete with MRP is 5%lower,and the increase rate of bubble spacing factor under low air pressure is 87%lower,which effectively improves the workability and freeze-thaw resistance of plateau concrete.