Experimental Study On Physical And Mechanical Properties And Acoustic Characteristics Of Nano-SiO₂-Mechanical Sand Concrete Under Freeze-thaw Cycles

As natural sand is restricted to be mined and not renewable,it makes the application of mechanism sand in engineering construction more and more widely.At the same time,the construction of cold areas in China is increasing,and there is a certain demand for the anti-freezing property of concrete.Therefore,in this thesis,based on the research status of domestic and foreign mechanism sand concrete,nano-materials and ultrasonic detection technology,we investigate the incorporation of nano-SiO₂into mechanism sand concrete through corresponding physical tests.To analyze the influence law of the mechanism sand concrete on the frost resistance of concrete under different nanometer SiO₂admixture.Meanwhile,there are few studies on the ultrasonic wave propagation characteristics under freeze-thaw cycles of concrete.In this thesis,the ultrasonic wave propagation characteristics under freeze-thaw cycles of concrete are studied on the basis of this study,and the effect of admixture of nano-SiO₂into machine-sand concrete on the frost resistance of concrete and the change law of acoustic parameters under freeze-thaw cycles are investigated,and the correlation between the number of acoustic-cycles and acoustic-stress of concrete is analyzed.The main research works done in this thesis are as follows:（1）Determine the raw material mechanism sand stone powder content of 10%,and formulate the corresponding cement mortar and mechanism sand concrete according to different stone powder contents（5%,7.5%,10%,12.5%and 15%）,and derive the stone powder dosage of 10%according to the compressive strength.Formulation of nano-SiO₂machine-sand concrete yielded a significant increase in its water consumption.Therefore,the dosage of the corresponding water reducing agent was determined without changing the water-cement ratio.Thereafter,the strength of the nano-SiO₂machine-sand concrete was tested at different ages and it was concluded that the nano-SiO₂increased the strength of the machine-sand concrete by a large amount in the early stage and by a small amount in the later stage.（2）Freeze-thaw cycles were carried out on the concrete with different nano-SiO₂admixtures,and the physical and mechanical properties of the concrete after freeze-thaw were analyzed at different admixtures.With the increase of the number of freeze-thaw cycles,the mass loss rate of the concrete with different nano-SiO₂admixtures increased,then decreased,and then increased again;the decrease of the relative dynamic modulus was slow in the early stage,and the decrease of the relative dynamic modulus was faster in the later stage;the compressive strength of the concrete specimens with different nano-SiO₂admixtures decreased with the increase of the number of freeze-thaw cycles.The mass loss rate,relative modulus of elasticity and strength of concrete with nanometer SiO₂admixture are smaller than those of concrete without admixture.The frost resistance of the concrete with nanosized SiO₂at 1%is better.（3）Microscopically,its composition and microstructure were analyzed by XRD and SEM.Nano-SiO₂greatly reduces the pore space of concrete,while the admixture of nano-SiO₂in concrete has basically no effect on the composition of machine-made sand concrete,but more on its generation products and CH crystals.（4）The ultrasonic time domain parameters（ultrasonic wave velocity,first wave amplitude and waveform）,frequency domain parameters（principal frequency amplitude,principal frequency and spectral area）and wavelet transformed frequency domain parameters（weighted principal frequency amplitude,weighted principal frequency and weighted spectral area）of nanosized SiO₂machine-sand concrete were analyzed.The variation law of each parameter under load for unfrozen-thawed and frozen-thawed concrete was analyzed,as well as the variation law of each parameter under different number of freeze-thaw cycles.It is found that each acoustic parameter differs from the stress and the number of freeze-thaw cycles.