| To solve these problems of high energy consumption of cement,low utilization rate of fly ash,shortage of freshwater and river-sand resources,an original concrete material,high-volume fly ash self-compacting concrete(HVFA-SCC)mixed with seawater and sea-sand,is researched and developed for offshore engineering.Also,to solve the problem of the decline of durability and the shorten of service life caused by the corrosion of steel bars,a glass fiber reinforced polymer(GFRP)bar,which has the advantages of light weight,high strength and corrosion resistance,is encouraged to replace steel bars in concrete structures.Therefore,HVFA-SCC mixed with seawater and sea-sand and GFRP bars are used for offshore engineering structures,which can not only achieve sustainable construction,but also improve the durability and service life of these structures.However,the fracture characteristics,crack propagation process and failure mechanism of the seawater sea-sand HVFA-SCC structures reinforced with and without GFRP bars have not been clearly elucidated.In view of this,this paper conducts research on the fracture behavior of seawater sea-sand HVFA-SCC beams reinforced with and without GFRP bars by using experimental,numerical and analytical methods.The main research contents and conclusions are as following:(1)The mix proportion design of seawater sea-sand HVFA-SCC is carried out,and the microscopic and mechanical properties of seawater sea-sand HVFA-SCC are tested.The test results show that the use of seawater and sea-sand instead of freshwater and river-sand to mix SCC can increase its compactness,improve its early compressive strength and split tensile strength.Seawater sea-sand HVFA-SCC(50% replacement ratio of fly ash)has good workability and mechanical properties,which is feasible to be used in actual engineering projects.The fracture test of seawater sea-sand HVFA-SCC three-point bending beams is carried out,and the effect of these parameters,replacement ratio of fly ash,water-sand type and initial crack-depth ratio,on fracture characteristics of seawater sea-sand HVFA-SCC is analyzed.The results show that the variation of fly ash proportion(no greater than 50%)has little effect on fracture characteristics of SCC three-point bending beams.SCC mixed with seawater and sea-sand instead of freshwater and river-sand can improve its fracture energy,fracture toughness and crack propagation resistance.Since a large number of crystalline particles fill the pores in SCC mixed with seawater and sea-sand,and increase the structural density and cohesion.The initiation and unstable fracture toughness of seawater sea-sand HVFA-SCC are not affected by the initial crack-depth ratio,which can be regarded as material parameters.(2)The bilinear tensile softening constitutive model parameters are determined based on the inverse analysis of the nonlinear hinge model by using the P-CMOD curves of the SCC three-point bending beams.The results show that the bilinear tensile softening curve of SCC is almost unaffected when the replacement ratio of fly ash does not exceed 50%.The stress of the bilinear tensile softening curve of seawater sea-sand SCC is greater than that of freshwater river-sand SCC with a same crack width,which caused by the denser microstructure of seawater sea-sand SCC.A size-independent bilinear tensile softening constitutive model is determined for seawater sea-sand HVFA-SCC based on the relationship between bilinear tensile softening curve parameters and the initial crack-depth ratio.The numerical models of three-point bending beams are established based on the extended finite element method,which aims to verify the validity of the determined bilinear tensile softening constitutive model of SCC.In addition,the influence of the parameters of the bilinear tensile softening constitutive model on the fracture characteristics of the seawater sea-sand HVFA-SCC three-point bending beam is discussed.(3)The fracture test of GFRP reinforced seawater sea-sand HVFA-SCC beams is carried out,and the effects of these parameters,including reinforcement type,initial crack-depth ratio,reinforcement diameter,thickness of the protective layer and the concrete type,on its fracture behavior are analyzed.Based on the evolution law of fracture characteristics,P-CMOD curve,concrete strain and bar strain of reinforced concrete beam,a four-stage fracture failure model is proposed,namely the linear elastic stage,the micro-crack propagation stage,the macro-crack propagation stage and the unstability failure stage.In addition,the damage state in the fracture process of the reinforced three-point bending beams is identified by using the smart aggregate transducers based on the time reverse method,and the proposed four-stage fracture failure model was verified.The micro-cracking and macro-cracking fracture toughness are introduced to describe the crack propagation state of FRP reinforced concrete structures under normal service conditions,and corresponding crack propagation criteria and calculation methods for fracture control parameters are proposed.It is noted that the micro-cracking and macro-cracking fracture toughness can be regarded as a constant when the initial crack-depth ratio is not more than 0.4,which can be used as the material parameters of seawater sea-sand HVFA-SCC reinforced by GFRP bar.(4)The numerical models of reinforced concrete beams are established by adopting concrete damaged plasticity model and cohesive elements.The cohesive elements are inserted into the interface between the reinforcing bar and the concrete to consider the bond-slip behavior.The results obtained from the numerical model are compared with that of obtained from the experiment,verifying the validity of the established numerical model.Based on the established numerical model,the evolution law of the principal stress and strain of the concrete at the ligament and interface is analyzed,the crack propagation and interface damage of reinforced concrete beams are discussed,and the four-stage fracture failure model of the reinforced concrete beams is verified.The parameter analysis is carried out based on the established numerical model of reinforced concrete beams.It is found that the damage degree of concrete at the interface of the GFRP reinforced three-point bending beam is weakened by increasing the axial stiffness of the reinforcement,increasing the ratio of the tensile softening curve,decreasing the initial stiffness of the bond-slip curve.(5)The governing equations for the fracture process of the GFRP reinforced seawater seasand HVFA-SCC three-point bending beam are established by considering the cohesive stress between the crack surfaces according to the bilinear tensile softening curve.Also,based on the three-linear and the double ? bond-slip constitutive model,respectively,combined with the governing equations,the analytical methods for the fracture process of the GFRP reinforced seawater sea-sand HVFA-SCC three-point bending beam are proposed.The results show that the proposed analytical methods can effectively reflect the fracture process of the GFRP reinforced seawater sea-sand HVFA-SCC three-point bending beam.Based on the analytical method of the three-linear bond-slip constitutive model,the influence of different parameters on the fracture characteristics of the GFRP reinforced seawater sea-sand HVFA-SCC threepoint bending beam is discussed.It demonstrates that the relative crack length corresponding to the maximum FPZ length is increased with the increase of the initial crack-depth ratio,changed little with the increase of the beam depth and thickness of the protective layer,and decreased with the increase of the diameter of the reinforcement. |
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