Effect Of Low Atmospheric Pressure On Air Entraining Effectiveness And Pore Structure Of Concrete

Qinghai-Tibet Plateau is characterized by high altitude,thin air and intense radiation,which make diurnal temperature differences.The frequency of positive-negative temperature cycling in Qinghai-Tibet Plateau is much higher than that in other regions with the same latitude.Therefore,concrete materials in the plateau are more prone to destroyed by freeze-thaw attacks.It is well known that the application of air-entraining agents is the main measure to improve the frost resistance of concrete.Although there have been several projects reported that the air entrainment was difficult in the plateau,the research on the influence of low atmospheric pressure environment on the air-entraining effectiveness of concrete in the plateau is still insufficient.Based on such background,this dissertation project investigated the effect of low air pressure mixing on bubble properties of air-entraining agent solution,pore structure(such as porosity,pore diameter distribution)of hardened cement paste,and air-void characteristics(such as air content,air-void spacing factors)of cement mortar by simulating low air pressures.The effect of low air pressure environment on the initial foam height and the variation of which with time was investigated by bubble shaking experiments.Results showed that the reduction of air pressure had negative effect s on the foaming capacity and bubble stability of most air-entraining agent solutions.The initial foam height of each group of air-entraining agent solution at low pressure is lower than that at standard air pressure by 6.6%.By simulating low air pressures,pore structure and hydration level of hardened cement paste with different air-entraining agents were investigated.The air-entraining agents included a commercially available triterpenoid saponin based air-entraining agent called SJ-2 and two new air-entraining agents respectively called FC-1 and FC-2.Pore structure analysis found that both the porosity and the most probable pore radius increased as the air pressure decreased,porosity increased by 9%-46%.Namely,the pore structure was adversely affected.This regularity was more obvious in early ages.Moreover,cement hydration was not found to be affected by air pressure in chemically combined water tests on h ardened cement paste.By simulating low air pressures,air-void characteristics and compressive strength of cement mortar respectively with SJ-2,FC-1 and FC-2 were investigated.Results showed that with the reduction of air pressure,cement mortar with ei ther SJ-2 or FC-2 had slightly higher air contents,unchanged air-void sizes,and slightly lower air-void spacing factors,indicating a slight improvement in frost resistance.For cement mortar with FC-1,although the air content decreased as air pressure decreased,either air-void sizes or spacing factors were almost unchanged,indicating no significant influence on its frost resistance.In addition,the compressive strength of cement mortar made in different air pressures decreased linearly with the air content increase.It verified that mixing and setting in low air pressure would not bring negative effects to the strength development of cement-based materials.Consequently,mixing and setting in low air pressure would not cause adverse effects on the pore structure of concrete with SJ-2.Compared with SJ-2,both FC-1 and FC-2 have superiorities in the reduction of air-void sizes and spacing-factors,however,further researches and improvements are necessary before they are used as air-entraining agents in concrete industry.Low air pressure could not cause air entraining difficult in concrete.