Study On Compressive Mechanical Properties Of Coal Gangue Concrete Under Active And Passive Confinement

As a by-product of coal mining and washing,coal gangue accounts for more than 10%of the bulk solid waste discharged in China.The stock of coal gangue in China is about 7billion tons and shows a trend increasing year by year.The efficient and reasonable utilization of coal gangue is directly related to the green development of the national economy.It directly affects the realization of China’s goal of "carbon neutrality and carbon peak".Considering the similarity between the main components of coal gangue and natural sand aggregate,using coal gangue instead of natural sand aggregate to prepare green concrete with coal gangue has been widely concerned by the industry.Compared with traditional methods such as coal gangue direct accumulation,of coal gangue on the ground,treatment of coal mining subsidence are,and direct filling in the coal mine,the technology of preparing coal gangue concrete maximizes the additional output value of coal gangue,and provides a new idea for alleviating the current shortage of natural sand resources.Unlike from natural sand aggregate,coal gangue is a mixed material with complex sources and different compositions,and its significant heterogeneity leads to the poor mechanical properties of coal gangue concrete.In order to improve the application of coal gangue concrete in civil engineering,some new composite structures including steel tube coal gangue concrete structure and FRP tube coal gangue concrete structure have been developed from the perspective of changing the stress environment of coal gangue concrete（active constraint and passive constraint）.Based on previous research results,this subject focuses on the mechanical response mechanism of coal gangue concrete under a triaxial stress state,which is a key scientific issue.The effects of different replacement rates of a coarse aggregate of coal gangue（0%,25%,50%,100%）and different lateral confining pressures（including conventional triaxial and true triaxial）on mechanical properties of coal gangue concrete（including failure modes,stress-strain curves,volumetric strain-axial strain curves,etc.）were analyzed.A new composite structure is proposed,composed of Polyethylene terephthalate fiber reinforced polymer（PET FRP）and coal gangue concrete.For different FRP constraint stiffness（0mm,0.841 mm,1.682mm）and different coarse and fine aggregate of coal gangue,the volume replacement rates of a coarse aggregate of a coal gangue are 0%,25%,50%,100%;The mechanical properties of new composite structures were studied under the conditions of0%,12.5%,25%,50% volume replacement rates of gangue fine aggregate.The main research contents and conclusions of this topic are as follows:（1）Taking coal gangue aggregate of typical mines as the research object,it’s microscopic morphology,mineral composition and basic physical properties are tested,and the parameters of gangue bulk density,apparent density,water absorption and crushing index are obtained.Compared with natural sand aggregate,the results show that coal gangue aggregate meets the relevant requirements of concrete aggregate.（2）Seven groups of coal gangue concrete with different proportions were designed to study the change law of concrete’s basic physical and mechanical properties under different gangue aggregate replacement rates.The results show that with the increasing gangue aggregate replacement rate,the concrete’s uniaxial compressive strength and elastic modulus show a decreasing trend,and the corresponding peak strain increases.Compared with ordinary concrete,the uniaxial compressive strength and elastic modulus of coal gangue concrete decrease by 80% and 89% respectively,mainly due to the low self-strength of coal gangue aggregate.（3）In this study,56 standard cube specimens were prepared to study coal gangue concrete’s the active constraint mechanical response mechanism under conventional and true triaxial compression conditions under 7 groups of different lateral confining pressures.The results show that the failure modes of coal gangue concrete under triaxial compression mainly include columnar failure（confining pressure 3-3MPa）,oblique shear failure（confining pressure 3-6MPa,3-9MPa,6-9MPa）and extruded flow failure（confining pressure 6-6MPa,9-9MPa）.In addition,the compressive strength of coal gangue concrete can be increased more effectively by increasing the minimum and intermediate principal compressive stress at the same time compared with the single increase of intermediate principal compressive stress.With the increasing gangue aggregate replacement rate,the strength improvement efficiency of gangue concrete increases under the same lateral confining pressure.（4）Based on the triaxial compression test data of coal gangue concrete,the applicability of the existing theoretical model and failure criterion was evaluated.The results show that the existing models can not accurately predict the stress-strain behavior of coal gangue concrete under conventional triaxial compression.Based on the Popvics model,the stress-strain model suitable.for the characteristics of gangue concrete is obtained by modifying the model’s key parameters,including elastic modulus of concrete,peak stress and strain of confined concrete,strain corresponding to uniaxial compressive strength,and shape coefficient.On this basis,equivalent confining pressure is introduced to successfully predict the stress-strain relationship of coal gangue concrete under true triaxial compression.The Guo-Wang failure criterion and Ottosen failure criterion are verified,and the failure criterion parameters of gangue concrete with different replacement rates under true triaxial compression are obtained.（5）This study tested,21 unconfined concrete and 28 PET FRP confined concrete specimens.The effects of different gangue aggregate replacement rates and PET FRP constraint stiffness on the mechanical properties of passive confined concrete were studied.The reliability of the existing theoretical models to predict the axial compression characteristics of PET FRP confined coal gangue concrete was compared.The test results show that the strength and axial deformation capacity of coal gangue concrete constrained by PET FRP are significantly increased,and its failure mode is mainly the instantaneous failure of external PET FRP concentrated in the middle of the specimen after reaching the ultimate tensile strength.The existing theoretical models（especially the analytical models）can accurately predict the ultimate strength of PET FRP confined coal gangue concrete.In predicting ultimate axial strain,the analytical model is overestimated,while the design model is conservative.