Experimental Study On Construction Technology And Vertical Bearing Capacity Of Precast Pier Foundation Filled With Demolished Concrete Lumps And Fresh Concrete

China is an important stage of urbanization development at present.A large number of construction waste will be generated in a series of projects such as large-scale old city reconstruction,demolition and temporary violation,and so on.It has become a trend to accelerate the resource utilization of construction waste.Combined with the favorable geological conditions of Nanchang city,this paper made a preliminary exploration,and poured large-size demolished concrete lumps and fresh concrete into pier foundation,which was used as a new type of building foundation form in the engineering.Based on this paper carried out an experimental study on the construction technology and vertical bearing capacity of the precast pier foundation filled with demolished concrete lumps and fresh concrete,providing guidance and theoretical basis for the practical application of the precast pier foundation filled with demolished concrete lumps and fresh concrete.The main work contents and conclusions are summarized as follows:(1)Through 13 groups of 52 cylindrical specimens with dimensions of(?)250mm×500mm,considering the gradation of demolished concrete lumps and the fresh concrete layered pouring of "wire mesh" according to the height setting,the experimental study was carried out.The uniaxial compression and flexural strength of the piece were tested in two batches.Through the conclusions of the test and analysis,the construction technology recommendations for guiding the construction of the precast pier foundation filled with demolished concrete and fresh concrete are formulated.The results of the experimental research show that.?Considering the gradation of demolished concrete lumps and the fresh concrete layered pouring of "wire mesh" according to the height setting,the compressive strength and flexural strength of the compound concrete made of demolished concrete lumps and fresh concrete are improved as a whole;?When the gradation of demolished concrete lumps is R3(40?60mm:60?80mm=6:4),the compressive strength and flexural strength values of the test piece are the largest.The compressive strength of the specimens H1-R3 is increased by 24.57%coMPared with H1-R1,and the flexural strength of the specimens H3-R3 is 14.28%higher than that of H3-R1;?The compound concrete made of demolished concrete lumps and fresh concrete specimens are considered to be layered and poured on each layer.By adding the "steel wire mesh" method,the average compressive strength and average flexural strength of the compound concrete can be improved,and the increase ranges are 5.38%?7.89%and 0.68%?3.42%,respectively;?The peak strain of the compound concrete specimens between 1800×10-6 and 2200×10-6,and the stress is 0.5fc0 that fc0 refers to the peak stress,the test piece is in the elastic stage.When the stress is between 0.7fc0 and 0.9fc0,the test piece is in the stage of crack development;?By using single variable analysis,the mean value,mean variance and coefficient of variation of the test data were taken to analyze and conclude that the compressive strength and flexural strength of the specimens were the best when the height of the layered pouring was H2 and the gradation of the demolished concrete lumps was R3.At the same time,it was shown that the construction process had the best effect under this condition.(2)Through the development of two precast pier foundations filled with demolished concrete lumps and fresh concrete(dimensions:0.8m×2.5m,lm×2.5m)and two ordinary concrete cast-in-place pier foundations(dimensions:0.8m×2.5m,1m×2.5 m)Vertical bearing capacity coMParison test,load-settlement curve characteristics of the precast pier foundation filled with demolished concrete lumps and fresh concrete,the load transfer law of pier-soil system,the earth pressure distribution of the pier bottom foundation,the failure mode and vertical bearing capacity formula calculation of the pier foundation are analyzed.The analysis and research have provided a theoretical basis for the application of the pier foundation in engineering.The results of the experimental research show that:?The Q-s curve of precast pier foundation filled with demolished concrete lumps and fresh concrete is slow deformation when loaded,which is consistent with that of the ordinary concrete cast-in-place pier foundations.The characteristic value of vertical bearing capacity of each pier foundation was determined by Q-s curve.It is found that the precast pier foundation filled with demolished concrete lumps and fresh concrete is 25%?30%lower than the ordinary concrete cast-in-place pier foundation;?For pier foundations of the same diameter,the precast pier foundations has a higher rebound rate than the ordinary concrete cast-in-place pier foundation after vertical unloading;??The theory of load transfer in pier-soil system was established,and the accuracy of the theory was verified by experimental analysis;?For the precast pier foundation is the same as the ordinary concrete cast-in-place pier foundation,the ground pressure of the pier bottom foundation is parabolic distribution that the ends are small and the middle is large;?In the sandy soil of the test site,the failure form of precast pier foundation filled with demolished concrete lumps and fresh concrete is local shear failure,while that of the ordinary concrete cast-in-place pier foundation is piercing-type shear failure;?It is analyzed that the existing calculation formula is the most reasonable for calculating the vertical compressive ultimate bearing capacity of the precast pier foundation filled with demolished concrete lumps and the ordinary concrete cast-in-place pier foundation.