Study On The Freeze-thaw Damage Characteristics Of Concrete Made Of Fly Ash Blended Sand Under Steam Curing

With the massive and rapid development of China’s infrastructure,sand is indispensable in the preparation of concrete,and the massive mining of river sand has led to its depletion and even the situation that there is no sand available in some areas.In order to solve the problem of no river sand available,nowadays many areas have started to use mechanism sand instead of river sand,mechanism sand instead of river sand in the future will be the inevitable trend.In this paper,we investigate the strength and frost durability of steam-cured concrete by varying the water-cement ratio and the amount of fly ash blended in steam-cured concrete,and analyse the effect of fly ash blended in steam-cured concrete with different water-cement ratios on the strength,porosity,dynamic modulus of elasticity,resistance to chloride ion penetration and frost durability of steam-cured concrete.The results of the study were applied to the preparation of concrete for cast-in-place box girders.The main components of the study and the results include the following.(1)A comparative study of the compressive strength,dynamic modulus of elasticity,pore structure,chloride ion permeability and frost resistance of the concrete with a 50°C steam curing programme was carried out to demonstrate the feasibility of this test programme for concrete with a 50°C steam curing programme.The comparative analysis revealed that as the steam curing temperature increased,the rate of cement hydration accelerated during the steam curing stage and the release strength of the machine-made sand concrete increased.However,the deterioration of the pore structure of the machine-made sand concrete increases under 60°C steam curing,which adversely affects the chloride ion penetration resistance and frost durability of the machine-made sand concrete,while the difference between 40°C steam curing and standard curing is not very large,which cannot play a good role in shortening the working period and mould turnover in the actual engineering application.(2)The strength growth of the steam-cured concrete with different water-cement ratios and the mechanical properties of the concrete at the end of the steam-curing period and the pore structure,dynamic modulus of elasticity and resistance to chloride ion penetration were investigated.The effect of fly ash on the properties of the concrete with different water-cement ratios was investigated.The compressive strength growth rate of the steam-cured concrete increased with the decrease of the water-cement ratio,and the strength growth rate of the concrete decreased after the incorporation of fly ash.The strength growth of the sandcrete at all ages after steam curing was relatively gentle,but the second growth occurred after 7 days of age and the greatest growth occurred at 28 d.The dynamic modulus of elasticity of the sandcrete increased with the decrease of the water-cement ratio,the pore structure was denser and the resistance to chloride ion penetration was better.The increase in fly ash admixture(≤30%)results in a greater increase in the dynamic modulus of elasticity of the concrete from 28 d to56d,optimises the pore structure and improves the resistance to chloride ion permeation.However,the strength,dynamic modulus of elasticity and pore structure of the concrete are affected to different degrees by the same specific weight of fly ash at different water-cement ratios.(3)A rapid freeze-thaw test was conducted to analyse and compare the macroscopic and microscopic parameters of the concrete with different water-cement ratios and the effect of fly ash blending on the frost resistance and durability of the concrete.It was found that the frost durability of concrete increased with the decrease of water-cement ratio in the rapid freezethaw test,and the incorporation of certain specific gravity of fly ash would improve the frost durability of machine-made sand concrete;the pore structure of concrete was analysed by nuclear magnetic analysis,and the changes of pore structure of different pore sizes under rapid freeze-thaw cycles were investigated.The results show that the incorporation of fly ash into the machine-made sand concrete will improve the frost durability of the machine-made sand concrete,but the optimal amount of fly ash for different water-cement ratios is not the same.In summary,the optimum fly ash blending rates of 0.29,0.32 and 0.35 water-cement ratios for steam-cured machine-made sand concrete at 50°C are: 20%-30%,20%-30% and 30%.(4)A grey correlation between macro and micro indicators was calculated and evaluated to rank the correlation between macro and micro indicators from highest to lowest.The three micro-indicators with the highest correlation with the macro-indicators of the sandcrete were total porosity,>200nm pore structure and 100-200 nm pore structure,from the highest to the lowest.The increase in total porosity and the increase in >200nm pore structure and 100-200 nm pore structure of the sandcrete under rapid freeze-thaw cycles had a significant effect on the deterioration of the frost durability of the sandcrete.(5)Prediction of the freeze-resistant service life of 0.29 and 0.32 sands concrete.Frost resistance service life of 0.29 and 0.32 water-cement ratio concrete was predicted based on the loss of relative dynamic elastic modulus in Changchun in the cold region and Yichang in the hot summer and cold winter regions.It is found that the frost service life of the sand concrete is increased by 1.24 to 1.32 times at the optimum amount of fly ash,and the frost service life of the sand concrete is much higher in Yichang than in Changchun.