Air Pore Structure Regulation And Durability Of Concrete Under Low Air Pressure On Plateau

The workability and the frost resistance of concrete can be enhanced due to the proper amount of air bubbles with suitable characteristics entrained by air entraining agents.For simplicity and convenience,the air content of fresh concrete is taken as the design parameter in the design specifications of concrete mix ratio at home and abroad.However,the empirical formula is generally based on a normal atmospheric pressure environment.Due to the special low air pressure environment of the plateau,the formation and stability of bubbles in concrete would be affected.Therefore,the characteristic parameters of air pores of hardened concrete cannot be predicted.Conventional air entraining methods may not be able to effectively improve the workability and frost resistance of concrete on plateau.In this paper,relevant tests were carried out in laboratory and field respectively.The foam stability of air entrainment agent solution and the time-varying performance of air content and density of cement mortar under different air pressure were studied.The development and collapse mechanism of concrete bubbles in low pressure environment was proved.The air pore structure,mechanical properties and durability of concrete under different atmospheric pressure were analyzed,and the influence mechanism of pore evolution on concrete properties under low atmospheric pressure was clarified.Based on the synergistic effect of nano-particle and air entrainment agent,the optimization method of pore structure stability of cement-based materials was designed,the method of bubble coalescence and collapse grade was proposed,the mechanism of nano-particle and air entrainment agent synergistic bubble stabilization was revealed,and the properties of cement-based materials with nano-particle and air entrainment agent were evaluated.Furthermore,the applicability of the method of bubble stabilization in concrete was verified in Lhasa,Tibet.The design of long-life concrete for plateau environment was realized by combining foaming,stabilizing,pore filling and pore interface optimization.The main conclusions are as follows:（1）The application of multi-parameter coupling environmental test system simulated the low air pressure environment,which realized the whole preparation and test process of entrainment agent solution and mortar sample in the stable air pressure environment.The air pressure environment had little influence on the air entrainment ability of air entrainment agent in cement mortar,and the influence degree was related to the type of air entrainment agent.The variation of volume of cement filtrate foam under different air pressure can approximate the influence degree of air pressure environment on the air entrainment agent performance.In low pressure environment,the stability of entrained air bubbles in cement-based material deteriorated obviously,and the larger the air content was,the more unstable the bubbles inside the system were,and the air pore content of hardened mortar decreased significantly.（2）Compared with the mortar with cationic and ionic entrainment agent,the fluidity of the mortar with anionic and non-ionic entrainment agent was more stable under low air pressure.At the same time,increasing the amount of air entrainment agent can also reduce the fluidity variation degree of fresh cement mortar.The air pore content of hardened concrete formed and cured in low pressure environment decreased obviously.When concrete specimens were formed and tested at 65 k Pa,the hardened air content was average 25.5%lower;the maximum pore diameter was higher;the proportion of pore less than 300μm was lower;the compressive strength of concrete at 28 days was 6.18%～13.58%lower;the flexural strength of concrete at28 days was 8.20%～15.00%lower;the durability index of concrete was 3.7%～44.0%lower than that at 95 k Pa.It’s proved that it was more difficult to stabilize bubbles under low pressure,which led to abnormal pore structure and deterioration of concrete performance.（3）Aiming at the problem of bubble instability under low atmospheric pressure,a nano-particle and entrainment agent co-stabilizing bubble design method was developed for cement-based materials.It’s found that the adsorption of NS particles on the bubble shell improved the strength of the bubble shell and the stability of the fine bubble,while the adsorption of nano-silica（NS）particles on the bubble increased the surface roughness of the bubble,resulting in the increase of the friction resistance of the bubble movement in the slurry.Thus,the migration and aggregation trend of fine bubbles were reduced and the stability of fine bubbles was improved.The crystal symmetry of C₃S on the bubble shell adsorbed by NS decreased,making its chemical properties more active.More C-S-H gel and Ca（OH）₂ can be produced by rapid hydration of high activity C₃S.The hydration products formed the cross-linked network.The strength of bubble shell was further improved,and the coalescence of small bubbles was prevented.（4）NS adsorbed on the air-liquid interface affected the whole evolution process of air pore in air-entraining mortar.In the curing process,NS still reacted with Ca（OH）₂ in cement mortar continuously at the interface of air pores,densifying pore wall hydration products,strengthening pore wall structure,filling pore pores,and improving the strength and durability of air entraining mortar.Considering the properties test results of air entraining mortar with different NS dosage,the recommended dosage was 0.5%.（5）Based on the synergistic effect of NS and air entrainment agent,a design method of long-life concrete suitable for low-pressure environment on plateau was proposed.Field application in plateau showed that compared with conventional concrete,the proportion of fine air pores in air entrained concrete mixed with NS was significantly increased,and the average pore diameter can be reduced by 43.3%.By strengthening the air pore wall structure and increasing the specific surface area of air pores,the maximum flexural performance of concrete can be increased by 26.3%,the maximum number of freezing-thawing cycles can be increased by 100%.The structure safety and service life of plateau concrete were improved.