| Most of Sichuan-Tibet Railway is built in plateau area.The average number of freezing-thawing cycles of concrete in plateau area is higher than that in plain area due to the high average altitude,low air pressure,frequent positive and negative temperature alternation and large temperature difference between day and night.Therefore,concrete is required to have good frost resistance.The freezing resistance of concrete will be effectively improved by adding air-entraining agent,but the effect of air-entraining agent will be affected under low air pressure environmental conditions,so its entraining effect is obviously weakened after the use of conventional air-entrainment agent in the project,resulting in the decline of concrete frost resistance,that is,the use effect of air-entraining agent is not good in the plateau low pressure area.Therefore,in order to meet the demand of air entrainment in plateau area,enhance the entraining effect of air-entraining agent under low pressure environment and improve the frost resistance of concrete,a new type of high performance air-entraining agent was developed by mixing the conventional air-entraining agent sodium dodecyl sulfate(SDS)solution with the self-made modifier F solution in this paper,that is,the self-assembled complex air-entraining agent for plateau railway concrete.Then the following aspects of research were carried out:Firstly,the relative mechanism and performance mechanism of high performance air-entraining agent under normal pressure was studied.It was found that compared with the sodium dodecyl sulfate(SDS)solution without modifier F,the surface tension of the high performance air-entraining agent was significantly decreased,the strength of bubble liquid film was significantly increased,the absolute value of Zeta potential was significantly increased,and the foaming performance and foam stability were significantly improved.Moreover,with the increase of modifier F concentration,the surface tension and bubble liquid film strength of high-performance air-entrainment agent solution were decreased first and then increased,the absolute value of Zeta potential was increased,the foaming performance was increased first and then decreased,and the foam stability was increased.When the modifier F concentration was 0.2wt%,the maximum surface tension of high performance air-entraining agent solution was 16.8 ± 0.2 m N/m,the maximum bubble diameter was 149.82 ± 3.20 mm,and the maximum foaming ability was 80 ± 2 ml.When the modifier F concentration was 0.4wt%,the absolute value of Zeta potential of high performance air-entraining agent solution was reached the maximum value 24.23 ± 0.97 m V,the maximum foam stability was 89.7 ± 1.2%.Then the concrete with high performance air-entraining agent solution was tested.It was found that with the increase of modifier F concentration,the air content of fresh concrete was significantly increased,the 7-day compressive strength of hardened concrete was decreased and the air void spacing factor of hardened concrete was significantly decreased.When the modifier F concentration was0.2wt%,the air content of fresh concrete was reached the maximum 4.4%,the 7-day compressive strength of hardened concrete was decreased to 17.6 ± 0.2 MPa,the air void spacing factor of hardened concrete was reached the lowest value 144 ± 1 μm,that is,the pore structure was reached the optimal value.It was expected to achieve better frost resistance.Secondly,the relative mechanism and performance mechanism of high performance air-entraining agent under low pressure environment was studied.It was found that the surface tension and foaming performance of the high performance air-entraining agent solution were decreased,and the bubble film strength and foam stability were improved under low pressure compared with that under normal pressure.In addition,with the increase of modifier F concentration,the above properties were all showed a trend of increasing first and then decreasing under low pressure environment.When the modifier F concentration was 0.2wt%,the maximum surface tension of the high performance air-entraining agent solution was 16.9 ± 0.1m N/m,the maximum bubble diameter was 154.42 ± 1.19 mm,the maximum foaming ability was 70 ± 1 ml and the maximum bubble stability was 90 ± 5%.Then the mortar and concrete with high performance air-entraining agent solution were tested under low air pressure.It was found that with the increase of modifier F concentration,the air content of fresh mortar and concrete were significantly increased,and the air void spacing factor of hardened mortar was significantly decreased.At the same time,the frost resistance of hardened mortar and hardened concrete mixed with high performance air-entraining agent solution were significantly enhanced.When the modifier F concentration was 0.2wt%,the best air entraining effect of high performance air-entraining agent was achieved under low air pressure.The air content of fresh mortar was reached the maximum of 4.1%,the air content of fresh concrete was reached the maximum of 4.2%,and the air void spacing factor of hardened concrete was reached the lowest value of 151 ± 5 μm.The strength loss rate of hardened mortar and hardened concrete mixed with high performance air-entraining agent solution were both reached the lowest after 50 freeze-thaw cycles,which were 8.06 ± 0.35% and 1.18 ± 0.02%,respectively.It was showed that the high performance air-entrainment agent can be still achieved good frost resistance under low pressure environment.Finally,in order to develop antifreeze concrete for Sichuan-Tibet railway,the performance mechanism of mixing high performance air-entraining agent and polycarboxylate superplasticizer was studied.It was found that SPF0 mixed with SDS and polycarboxylate superplasticizer was reached the highest surface tension,the weakest surface activity,the lowest liquid film strength of bubbles,and the worst foaming and stabilizing performance.After adding modifier F with a concentration of 0.2wt%,the properties of the mixed solution SPF2 were significantly improved,and the lowest surface tension was 26.5 ± 0.2 m N/m,the maximum bubble diameter was 130.31 ± 1.01 mm,and the maximum foaming ability was 60 ± 2 ml.The maximum foam stability rate was 84.3 ±0.4%.At the same time,the performance of SP4% solution with 4% air content mixed with SDS was better than that of SPF0,but weaker than that of SPF2.Then the concrete for plateau railway with high performance air-entraining agent and polycarboxylate superplasticizer was tested.It was found that the air content and slump of fresh concrete mixed with SDS and polycarboxylate superplasticizer SPF0 were the lowest,both slump loss over time and air content loss over time were the largest,the 7-day compressive strength and the air void spacing factor of hardened concrete were the highest.After adding modifier F with a concentration of 0.2wt%,the performance of fresh concrete mixed with mixed solution SPF2 was significantly improved with air content reaching the maximum 4.5% and slump reaching the maximum 230 mm,both slump loss over time and air content loss over time were reduced to the minimum.The 7-day compressive strength of hardened concrete was decreased to 25.5 ± 0.1 MPa,and the air void spacing factor was reached the lowest value 127 ± 1 μm.At the same time,the performance of SP4% concrete with 4% air content mixed with SDS was between the two,better than SPF0 but worse than SPF2. |
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