Mechanical Properties And Damage Mechanism Of Manufactured Sand Concrete Under Condition Of Freeze-Thaw Cycle

The excessive excavation of natural sand leads to the lack of the resource reserve and damages the ecological environment.What is more,it is difficult to repair the damage of the ecological environment.At the same time,the development of economic construction demands a large number of building sand;this makes manufactured sand the best alternative of natural sand.However,manufactured sand is quite different from natural sand in composition,grain shape,grain composition and fineness modulus,etc.Concrete also faces the effect of a variety of environmental factors and their coupling effect.So it is necessary to further study the mechanical properties and durability of the concrete with manufactured sand,to ensure the reliability of the manufactured sand concrete in engineering application.The paper mainly studies mechanical properties of manufactured sand concrete under the coupled effect of salt solution erosion and freeze-thaw cycles.The paper focuses on the following aspects:（1）Properties test on the raw materials were carried out.Based on design and adjusting,the mix proportions of manufactured sand concrete were determined with different strength grade（C30,C40,and C50）,and then specimens were made for following experiments.（2）Experiment of freeze-thaw cycle on manufactured sand concrete with grades of C30,C40,and C50 were carried out respectively in pure water,5 wt%Na₂SO₄ solution,5 wt%Na Cl solution,and mixture of 5 wt%Na₂SO₄ solution and 5 wt%Na Cl solution.Three parameters were studied:the rate of mass loss,the relative dynamic modulus of elasticity and compressive strength.The law of them changing along with the freeze-thaw cycles were analysed.The results show that the mass,dynamic elastic modulus and compressive strength of different strength grades decline different degrees in different salt solution,and the presence of salt exacerbated the surface spalling of manufactured sand concrete during freeze-thaw cycle,but slowed the decreasing of relative dynamic elastic modulus.（3）After 20,40,60 and 80 cycles of freezing and thawing,the ascent portion stress-strain curves of C40 manufactured sand concrete were obtained,and the changes of peak stress and peak strain were analyzed.The results show that with the number of freeze-thaw cycles increasing,the static compressive elastic modulus of manufactured sand concrete after freeze-thaw cycles showed a trend of accelerated decline,and the peak stress decreased significantly,meanwhile,the peak strain had an increasing trend.（4）With the aid of CT technology,scan imageries were obtained after freeze-thaw cycle test.The internal cracks and pores were observed,and the development of cracks and pores were analysed.The results show that obvious cracks occurred in the concrete after the freezing and thawing,but the expansion rate of cracks is different under different salt environment.