Axial Compression Performance And Theoretical Model Study Of CFRP-Steel Double Tube Confined Gangue Aggregate Concrete

Coal gangue is the main solid waste produced in the process of coal mine production.The application of coal gangue as the coarse aggregate of concrete to the infrastructure construction under the mine and the mining area can not only solve the problem of solid waste treatment,but also realize the local use of building materials.However,gangue aggregate concrete has low strength and stiffness,high brittleness,which limits its practical application in engineering to a great extent.After confined by CFRP which has the characteristics of lightweight,high strength and anti-erosion,the strength and ductility of gangue aggregate concrete can be improved effectively and its durability can be also improved under long-term environmental action.But the axial stiffness of gangue aggregate concrete is not significantly improved by CFRP confinement.Especially when gangue aggregate concrete is used in the support system under the mine,the section area of the support column is generally large,so the section form of FC-GAC will cause the diameter thickness ratio of CFRP tube to be larger,and then reduce the confinement effect.In view of this,the cross section form of CFRP-steel Double Tube Confined Gangue Aggregate Concrete（DTC-GAC）is proposed.The embedded steel tube can not only effectively improve the confinement effect of gangue aggregate concrete,but also improve the axial stiffness of the members,while premature buckling and surface erosion of steel tube can be avoided.The mechanical properties of FC-GAC are the basis for the study of DTC-GAC.Therefore,the mechanical behavior and stress-strain constitutive model of FC-GAC under monotone axial compression are firstly studied in this dissertation.On this basis,the mechanical properties and constitutive models of DTC-GAC under monotonic and cyclic axial compression were systematically studied.The main research contents and results are as follows:（1）The mechanical properties and theoretical research of CFRP-confined gangue aggregate concrete（FC-GAC）under monotonic axial compressionThe monotonic axial compression test of CFRP-confined gangue aggregate concrete and nature aggregate concrete column was carried out,considering the strength of plain concrete,the type of concrete aggregate and the number of CFRP layers.The test results show that the failure mode of gangue aggregate concrete after confined was changed from the shear crushing of coarse aggregate to the simultaneous failure of coarse aggregate and interface,and its strength and ductility had been greatly improved,and the stress-strain curve appears obvious strengthening section and presents a double-line ascending line.At the same mix proportion,the strength and elastic modulus of gangue aggregate concrete are greatly reduced compared with nature aggregate concrete,but the confined gangue aggregate concrete shows a higher improvement rate of strength and ductility,and is more inclined to volume compression.This volume compression behavior delays the crack development of gangue aggregate concrete and further strengthens its mechanical properties.The theoretical model of FC-GAC was systematically studied.Firstly,combined with the test data,the existing classical ultimate strength and strain models were evaluated.All the predicted values of the models underestimate the test values,and the ultimate strength and strain models of FC-GAC based on confinement strength ratio and confinement stiffness ratio were proposed.By analyzing the expansion behavior of unconfined gangue aggregate concrete and the confinement benefit of CFRP,an axial strain-hoop strain relationship for its volume compression behavior was established combined with the model of Teng.The analysis-oriented and design-oriented models of FC-GAC were proposed by combining the existing stress-strain basic formula.（2）The mechanical properties and theoretical research of CFRP-steel double tube confined gangue aggregate concrete（DTC-GAC）under monotonic axial compressionThe monotonic axial compression test of DTC-GAC was carried out,with the strength of plain concrete,the number of CFRP layers and the relative diameter ratio（outer diameter of steel tube/inner diameter of CFRP tube）as the main test parameters.The experimental results show that the bearing capacity and axial stiffness were further improved compared with FC-GAC,while ductility was improved to some extent but not significantly.The axial force-strain curve shows a typical double rising line,and the increase of the strengthening section of the curve was more obvious with the increase of the relative diameter ratio.The local buckling of the built-in steel tube was effectively restrained,which not only provided axial stress,but also provided considerable confinement stress for the internal gangue aggregate concrete.The synergistic effect between CFRP,steel tube and gangue aggregate concrete of DTC-GAC had an obvious gain effect on the improvement of bearing capacity,and this gain effect became more significant with the increase of relative diameter ratio.Mises yield criterion and Prandtl flow rule were used to determine the bi-axial stress state of steel tube.Based on the analysis model of FC-GAC,the stress-strain calculation method of gangue aggregate concrete was given,and the analysis oriented theoretical model of DTC-GAC was proposed combined with plastic increment theory.Based on the design model of FC-GAC in this dissertation,the ultimate strength and strain model of DTC-GAC was established considering the relative diameter ratio of steel tube,and the calculation formula of stress-strain design model was proposed.（3）The mechanical properties and theoretical research of CFRP-steel double tube confined gangue aggregate concrete（DTC-GAC）under cyclic axial compressionThe cyclic axial compression test of DTC-GAC was carried out,considering the strength of plain concrete,the number of CFRP layers,the relative diameter ratio and the cyclic loading system as the main test parameters.The experimental results show that the ultimate bearing capacity and ductility under cyclic axial compression were almost the same as those under monotone axial compression,but the hoop strain was slightly higher than that under monotone axial compression,and the envelope lines of axial force-axial strain and axial strain-hoop strain can be predicted by using corresponding curves under monotone axial compression.Under cyclic axial compression,the plastic strain accumulation of steel tube and gangue aggregate concrete was quite different,which led to the dissimilarity of unloading/reloading rate between them,and the coal gangue concrete still bear certain compressive stress at the end of unloading path.In addition,the degradation of DTC-GAC was mainly caused by the stress damage of gangue aggregate concrete,and the loading history has an effect on the accumulation of plastic strain and the degradation of bearing capacity.Based on the comprehensive discussion and analysis of the existing cyclic axial compression models,the quantitative calculation method of plastic strain and loading capacity degradation of DTC-GAC was clarified,and the prediction model of the whole section under cyclic axial compression was proposed.The axial force-strain envelope was predicted by using the design model under monotonic axial compression,the unloading/reloading curve was predicted by using the improved Lam-Teng model,with the slopeE_{u n,0}was improved considering the effect of steel tube.There are 85 figures,26 tables and 224 references in this dissertation.