Study On The Frost Resistance Characteristics Of Ⅲ Fly Ash Replacing Fine Aggregate Concrete

The massive accumulation of Ⅲ-grade fly ash and increasingly scarcity sand mining resources,which has caused great pressure to the ecological environment.Expanding the application range of Ⅲgrade fly ash and improving its utilization rate,reducing the amount of sand mining are urgent;At the same time,freeze-thaw damage is one of the main reasons for damage of concrete structure,The concrete structure in northern and southern parts of China is generally damaged by different degree,which causes many safety hidden trouble and huge economic loss.In this paper,according to the regional characteristics of Ningxia region,the frost resistance and mechanical law of Ⅲ-grade fly ash replacing fine aggregate concrete and the damage law after freezing will be studied,It will provide technical support for studying the frost resistance of fly ash concrete,and provides an effective use of the large quantity of fly ash in Ningdong area.Based on the study of C30 concrete and the substitution of Ⅱ-grade fly ash for fine aggregate concrete,the optimum substitution rate of fly ash was determined by using principal component analysis and statistic method.The mechanical properties of the concrete with the replacement rate of 0,15%,25%and 35%were tested in 7d,28d and 56d years by replacing the Ⅲ-grade fly ash with the aggregate,which is mainly compressive strength,strength and.The results show that Ⅲ-grade fly ash as fine aggregate can improve compressive strength of concrete,has early strength effect,7d compressive strength is above 45MPa;28d flexural strength is above 5MPa,compressive strength can be deduced accurately by combineing flexural strength and after-flexural compressive strength.Freeze-thaw cycle durability test using "fast freezing method" to study the quality loss,relative dynamic modulus,compressive strength,flexural strength and the intensity of healing changes of concrete after freezing.The results show that with the increase of the freezing and thawing cycles,the relative dynamic modulus,mass,compressive strength and flexural resistance of Ⅲ-grade fly ash fine aggregate concrete specimens are gradually reduced;At the same substitution rate,the greater the ratio of water to cement,the higher the concrete mass loss and the relative modulus of elasticity decrease;At the same freezing-thawing cycle times,the greater replacement rate,the larger relative modulus of decay,but the quality loss rate is similar;The flexural strength is more sensitive to the freezing-thawing cycle times than the compressive strength;By introducing the concept of relative strength,it is better to investigate the relationship between intensity attenuation and freezing-thawing cycle times.the after-flexural compressive strength of secondary 28d curing after the freeze-thaw is basically unchanged;The strength advantage of fly ash concrete is no longer obvious at the later stage.At the same time,the 1.1m large volume concrete cube was constructed on site to measure the strength change of the freeze-thaw concrete during the natural environment for 3 years.During the period,the hydration heat change of mass concrete and the relationship between the core intensity and the rebound intensity was monitored.The attenuation model of mechanical properties of natural and the indoor freeze-thaw circulating concrete are compared.The results show that at the early stage fly ash of large volume fly ash concrete does not participate in hydration heat reaction,and the concrete crack resistance is good.The strength of the early core is changed with the depth,the latter is the same,the core strength is more similar to the actual strength of concrete than the rebound strength,the core strengthis 0.88 times the actual strength,there is a difference between the mechanical attenuation model of indoor freeze-thaw cycle and natural freeze-thaw cycle,and the actual project should pay attention to this phenomenon.