Experimental Study On Durability Of Fair-Faced Self-Compacting Concrete In Cold Area

Fair-faced self-compacting concrete can be compacted by its own weight without vibration or less vibration,and does not need to do any decoration,so it can be used as a green building material in the field of engineering construction.Combined with the field conditions of cold saline soil corrosion area in northwest China,through the test of fair-water self-compacting concrete mixture and formed concrete,the basic performance indexes such as filling,gap passage,segregation resistance,apparent properties,mechanical properties and micro-pore structure were studied.The durability tests under freeze-thaw cycle,sulfate corrosion and the coupling action of freeze-thaw cycle and sulfate corrosion were carried out.By analyzing the changes of macroscopic indexes such as quality,dynamic elastic modulus and compressive strength of fair-faced self-compacting concrete at different test ages,the effects of water-binder ratio,fly ash,mineral powder and silica fume on macroscopic indexes were explored.The following research conclusions are obtained:(1)through the basic performance tests of fair-faced self-compacting concrete with different mix ratios,it is found that increasing the content of fly ash will increase the slump,slump extension,J-ring expansion and segregation rate of fair-faced self-compacting concrete,while the above indexes will decrease with the increase of mineral powder content;the expansion time will be reduced with the increase of fly ash content,while the expansion time will be increased with the increase of mineral powder content.With the addition of fly ash,mineral powder and silica fume,the filling,clearance passage and segregation resistance of concrete are higher than those of single and double admixture.(2)based on the analysis of the apparent quality of the concrete after demoulding,it is found that the new and old formwork and the type of release agent have great influence on the apparent performance of fair-faced self-compacting concrete.in order to ensure the surface effect of fair-faced self-compacting concrete,new formwork should be used as far as possible or newer formwork should be treated to the new formwork standard.The release agent should be water-based release agent,and the diluted water-based release agent should be sprayed on the surface of the form.after demoulding,the surface is smooth,no color difference and less air bubbles,so as to reduce the surface defects of concrete.(3)through mechanical property test and microstructure analysis,with the increase of mineral powder content,the total porosity increases,while the total proportion of harmless holes and less harmful holes increases and decreases;the addition of 6% silica fume can increase the total proportion of harmless holes and less harmful holes,and plays a significant role in improving the early compressive strength of concrete.(4)through the test of clear water self-compacting concrete specimens under freezethaw cycle,it is found that the water-binder ratio has a great influence on frost resistance.The higher the water-binder ratio,the greater the mass loss in the later stage of freeze-thaw cycle,and the smaller the relative dynamic elastic modulus and corrosion resistance coefficient of compressive strength.The influence of different amount of fly ash on frost resistance is different,and the frost resistance is the best when the content of fly ash is 20%.When the content of mineral powder is 10%,the frost resistance is not obvious,and when the content of mineral powder is 20%,the frost resistance is improved.6% silica fume can significantly improve its frost resistance.Under the action of sulfate corrosion and the coupling action of freeze-thaw and sulfuric acid corrosion,the effects of water-binder ratio,different amount of fly ash,different content of mineral powder and 6% silica ash on mass loss rate,relative dynamic elastic modulus and corrosion resistance coefficient of compressive strength are basically consistent with those under a single freeze-thaw cycle.the difference lies in the time to reach the peak and the degree of damage deterioration in the later stage of the test.(5)based on the analysis of damage mechanics theory and experimental data,the damage evolution model of fair-water self-compacting concrete under the action of sulfate corrosion and the coupling action of freeze-thaw cycle and sulfate corrosion is established.the service life of concrete under the coupling action of freeze-thaw cycle and sulfate corrosion is predicted.