Effect Of Sand Gradation On Mechanical Properties Of Big Recycled Aggregate-filled Concrete

As a new type of concrete,big recycled aggregate-filled concrete has many advantages in mass concrete engineering,such as reducing heat generation,fast construction speed,reducing project cost,which is beneficial to the recycling and utilization of construction waste.To realize circular economy,to protect ecological environment,and to build a resource-saving and environment-friendly society.On the basis of studying the material composition of recycled aggregate,the working performance of self compacting concrete,the mechanical properties and durability of recycled concrete,studying the influence of sand gradation on the compressive strength of recycled self compacting concrete and the microcosmic characteristics of new and old concrete.In this paper,four kinds of recombined graded sand with small voids are selected by the formula uniform design method,studying the correlation laws between the flexural strength and compressive strength of self compacting mortar and the binding area of graded sand.The filling compactness of self compacting concrete in wasted concrete accumulations is analyzed comprehensively by means of elastometer and ultrasonic pulse,and the compressive failure test of big recycled aggregate-filled concrete specimen is carried out.Finally,using SEM to analyze the microcosmic characteristics of the transition zone of bond surface under the condition of different grade of sand distribution with different strength grades.The main conclusions are as follows:1.When the volume ratio of fine: medium: coarse size sand is 0.293: 0.118: 0.589,the combination sand is maximum density,the porosity is 5.21% lower than ordinary medium sand,the compressive strength of mortar can be increased by 10.1%,and the flexural strength can be increased by 3.28%.In the gradation range of medium sand,the content of coarse sand increases relatively,the specific surface area decreases and the compressive strength increases.the content of fine sand is increased relatively,the specific surface area increases and the flexural strength increases.2.When it's the maximum density of graded sand,the strength of big recycled aggregate-filled concrete specimen with low grade strength is increased by 14.68%.With the increase of concrete strength grade,the influence of graded sand on strength is weakened.There are two kinds of failure modes: one is similar to the failure formof ordinary concrete,the bottom edge of the specimen diffuses 45° to the center,the other is 60° angle failure with the compression surface.3.Under the condition of maximum density gradation,the pores at the bonding surface of old and new concrete are relatively reduced.Compared with the interface transition zone between mortar and coarse aggregate in self compacting concrete,some new characteristics are produced: The porosity of the transition zone decreases,the morphology of CH crystal is changed by space limitation,the transition zone and cement slurry matrix are more homogenized.