Study On Mechanical Properties And Damage Degradation Of Concrete With Soft Cut Pile Head Under Cyclic Loading And Unloading

Because of its good plasticity and economic practicability,concrete is widely used in China’s construction engineering,road and bridge,water conservancy engineering and underground engineering and other fields.However,in the actual service process,the concrete structure has been subjected to all kinds of loads and adverse environment for a long time,and the internal damage keeps accumulating,which leads to its mechanical properties and durability greatly reduced,causing a huge hidden danger to the safety of the engineering structure.And concrete as a heterogeneous material,its macroscopic mechanical properties will inevitably be affected by the internal meso-structure.,therefore,in addition to the study of mechanical properties of concrete material from the macroscopic level,from the perspective of mesoscopic study of concrete material under the action of load internal damage cumulative process of development,gradually,between the two will be able to more accurate study of concrete damage degradation mechanism of the materials under complex stress conditions,and ensure the safety and stability of engineering structures.The paper takes S270 project of Guangfo River Expressway as the engineering background,and the pile head concrete after the construction of the new technology is sampled and the macroscopic mechanical test and CT mesoscopic mechanical test are carried out.The mechanical properties of soft cut pile head concrete are studied by combining the experimental and theoretical methods.The main research results of this paper are as follows:(1)A new soft cutting technology is proposed to innovate the traditional technology of pile head breaking,which is made into standard samples through core drilling.Conventional triaxial tests and cyclic loading and unloading tests with constant stress lower limit were carried out to obtain the stress-strain curves under different confining pressure strengths.In the conventional triaxial test,the curve is concave on the whole,and the curve is approximately linear at the initial loading stage.In the cyclic loading and unloading tests,with the increase of the number of cycles,the curve presents the phenomenon of "sparse--dense--sparse",and produces a hysteretic loop with the same change law in the direction of strain development.The failure modes of the samples under conventional triaxial and loading and unloading are relatively consistent.When the confining pressure is not applied,the samples are split failure,and when the confining pressure is applied,the samples are single bevel shear failure.And the compressive strength of pile head concrete is proportional to the increase of sampling depth.(2)In the cyclic loading and unloading test with constant stress limit,the elastic modulus of the sample increases first and then decreases with the increase of the upper limit of stress.The elastic modulus of the sample degrades with the increase of the number of cycles in each cycle.In the process of loading and unloading,the Poisson’s ratio generally increases with the increase of the upper limit of stress,and the curve rises in a step shape.The variation rule of Poisson’s ratio in each cycle is in the form of "decrease--stable--increase",and it has obvious buffering phenomenon in the last failure stage.(3)Under cyclic loading and unloading,the irreversible strain of the sample increases exponentially.Every time to uninstall generate irreversible strain as a damage parameter,calculates the cumulative damage parameters and absolute damage parameters,analyze its after the discharge under the action of pile head concrete specimens in the development of irreversible deformation is divided into three stages,namely,initial stage,stable stage,rapid growth stage,the corresponding samples in the process of loading consolidation stage,elastic stage and plastic yielding phase.(4)The failure process of pile head concrete specimen under conventional triaxial loading was observed by using CT scanning technology,and the CT scanning images under different load intensities were obtained.By comparing the scanning tomography images at different stages,it is found that pore initiation and compaction occur simultaneously in the whole loading process.By means of gray mean statistical analysis,the internal heterogeneity of the sample itself was reflected.It was found that the generation and distribution of internal damage under load were random,and the internal structure heterogeneity gradually increased.Based on damage mechanics and the definition of damage variables in CT images,the extended evolution process of internal damage of concrete specimens with soft shear pile head under triaxial compression was analyzed with gray value as the damage variable,and the damage evolution equation and damage constitutive equation were proposed.Based on high-precision CT images,3D reconstruction and visual analysis of concrete samples are realized,which can more intuitively observe and analyze the development and distribution of internal damage of samples.It is found that most of the internal defects of samples in the whole loading process are under the aperture of 0.5mm.