| With the continuous development of the construction industry,a large amount of energy and natural resources are consumed while a large amount of construction solid waste(solid waste)is generated,which has an increasingly serious negative impact on the environment.The use of recycled aggregates obtained from secondary crushing of solid waste to configure recycled concrete is one of the methods to improve the utilization rate of construction solid waste.The colder winters in the northwest,with a large temperature difference between day and night,lead to the prevalence of frost damage to concrete.With the advancement of urbanization in China,building fires have also gradually become a major factor threatening the safety of human life and property.The study of mechanical properties of recycled concrete under frccze-thaw and high temperature environment is important for the long-term working performance,fire resistance assessment and post-disaster restoration of recycled concrete in cold regions.Due to the disadvantages of recycled concrete such as weak load bearing capacity,low high temperature resistance and poor frost resistance,the promotion of recycled concrete in actual projects is limited.Slurry-wrapping modified reinforced recycled aggregate and steel tube restrained recycled concrete can improve the load-bearing capacity of recycled concrete,which gradually becomes one of the hot spots of current research on recycled concrete.In this paper,a single-factor control method is used by combining two strengthening methods of slurry-wrapping modification and steel tube restraint.The section form,aggregate replacement rate,aggregate type,high temperature and number of freeze-thaw cycles are used as the influencing factors.The effects on the damage mode,ultimate strength,transverse deformation coefficient,initial stiffness and ductility coefficient of slurry-wrapping recycled concretefilled steel tube short columns at room temperature,high temperature and freeze-thaw environment were studied and investigated.The measured values obtained from the tests are compared with the calculated values from the codes of China,Europe,the United States and Japan.The expressions of stress-strain curves for the whole process of slurry-wrapping recycled concrete-filled steel tube short columns at room temperature were fitted,and the strength prediction equation of slurry-wrapping recycled concrete-filled steel tube short columns after high temperature was modified.It has important scientific value and practical significance for the promotion and application of slurry-wrapping recycled concrete-filled steel tube short columns.The main research results are as follows:(1)At room temperature,the ultimate strength of slurry-wrapping recycled concrete-filled steel tube short columns decreases and then increases with the increase of the aggregate replacement rate,and reaches the minimum and maximum ultimate strength at 25%and 100%aggregate replacement rate,respectively.The initial stiffness,residual bearing capacity and strength improvement coefficient of circular slurry-wrapping recycled concrete-filled steel tube short columns are higher than those of square slurry-wrapping recycled concrete-filled steel tube short columns.The strength improvement coefficient of slurry-wrapping recycled concrete-filled steel tube short columns was less affected by the aggregate replacement rate.Based on the comparative analysis between the measured values and the calculated values of the four national codes,the expressions of the full process stress-strain curves fitted according to the Chinese codes are in good agreement with the measured curves.(2)With the increase of high temperature,the average ultimate strength of slurry-wrapping recycled concrete-filled steel tube short columns showed a trend of rising and then decreasing,changing by+14.0%and-45.6%at 400℃ and 800℃,respectively.At 200℃-400℃,the ultrasonic wave velocity can reflect the change of internal pore space of concrete,while it is not consistent with the change of strength.With the increase of temperature,the transverse deformation coefficient and initial stiffness of slurry-wrapping recycled concrete-filled steel tube short columns showed a gradual decreasing trend,while the ductility coefficient showed a decreasing and then increasing trend due to the change of the confining factor.The measured values of slurry-wrapping recycled concrete-filled steel tube short columns after high temperature match with the calculated values of Chinese and European codes,and deviate from the calculated values of American and Japanese codes.The correction coefficient proposed by considering the effect of temperature change on material strength has good effect on the correction of strength prediction equation after high temperature.(3)With the increase of the number of freeze-thaw cycles,the ultimate strength and ultimate displacement of slurry-wrapping recycled concrete-filled steel tube short columns continued to decrease.Under 300 cycles,the average ultimate strength and ultimate displacement decreased by 9.9%and 13.0%,respectively.At different numbers of freeze-thaw cycles,the ultimate strength of ordinary concrete-filled steel tube short columns>slurry-wrapping recycled concrete-filled steel tube short columns>recycled concrete-filled steel tube short columns.The initial stiffness and ductility coefficient of slurry-wrapping recycled concrete-filled steel tube short columns gradually decreased with the increase of the number of freeze-thaw cycles.The effect of aggregate type on the ductility coefficient of slurry-wrapping recycled concrete-filled steel tube short columns is not obvious.The measured values of slurry-wrapping recycled concrete-filled steel tube short columns after freeze-thaw matched better with the calculated values of European code,and deviated more from the calculated values of Chinese,American and Japanese codes. |
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