Research On Compressive Behavior Of FRP Confined Recycled Concrete With Recycled Clay Brick Aggregates(RCBAs) Incorporating Nanomaterials

With the fast development of civilization and construction in our country for 30years,the emerge of construction and demolition waste（CDW）which has led to inferior influences on the society,economy and environment,the excessive abuse of natural resources and the restriction between human and nature,are becoming the urgent core problem to civilization.The pile of CDW has aggravated the land occupation and resources shortage,leading to environmental pollution,inferior effect on urban beauty,demands for both high technologies of CDW process and huge cost.It has severely influenced the coordinated development of society,economic,human and nature.During the past ten years,the recycled concrete has been proposed to solve the reusing issues of CDW,while the actual application of recycled concrete has been restrained due to the drawbacks of recycled aggregates,i.e.,inferior mechanical and physical properties,e.g.,lower strength,obvious shrinkage and creep,easy-cracking of recycled concrete.Fiber reinforced polymers,shortly called as FRP,present high strength and quality ratio and superior cost-performance ratio,well corrosion-resisting and easy-shaping characteristics.During the recent twenty years,a lot of scholars have focused on the research of FRP applied in civil engineering and acquired satisfying outcomes.Nanomaterials act as one innovative material and become cost-appropriate due to the development of science and technology,some scholars suggested to use nanomaterials in concrete structures to improve the properties of concrete and trigger novel workability.In this study,FRP and nanomaterials were combined to improve the properties of recycled aggregate concrete（RAC）with recycled clay brick aggregates（RCBA）.The research includes:（1）By testing the crushing indexes and moisture content of recycled clay brick aggregate（RCBA）,and conducting the pre-experiments of the mix ratio of recycled aggregate concrete with recycled clay brick aggregate（RAC-RCBA）,the performance parameters and mix ratios of RAC-RCBA with certain strength were acquired.The compressive and split-tensile tests of RAC-RCBA were conducted.（2）Three types of nanomaterials（i.e.,nanoSiO₂,nanoCaCO₃,multi-wall carbon nanotube）were selected to be used in the improvement of RAC-RCBA.Firstly,the microscope observation,compressive and tensile tests of mortar incorporating with nanomaterial were conducted to test the influence of surfactant and ultrasonication on the dispersion of nanomaterials.The slump test,compressive and split-tensile tests were conducted on the RAC-RCBA incorporating with nanomaterials,the variables including dosage and type of nanomaterials,surfactant and ultrasonication,two types of two-stage mixing approach of concrete（TSMA）,replacement ratio of RCBA were tested to investigate the influence on the workability,compressive and split-tensile performance of RAC-RCBA.Finally,the tested split tensile strength values were compared to the calculated values based on standards of five countries.（3）The axial compressive experiments were conducted on 57 FRP confined RAC-RCBA cylindrical specimens to investigate the influence of replacement ratio of RCBA,thickness of FRP tube,type of FRP,size effect on the axial behavior of FRP-RAC-RCBA.The tensile test of FRP laminate and axial compressive tets of concrete filled in FRP tube（CFFT）were conducted firsly,and the confinement system of concrete filled in FRP and steel wire mesh tube（CFFST）was proposed to improve the brittleness of FRP.The failure modes,axials stress-strain behavior,strength ratio,ductility,dilation rate and Poisson ratio of specimens with different variables were analyzed.The size effect was evaluated by comparing the values of confinement ratio,strength ratio and strain enhancement ratio of specimens with three kinds of size.（4）The axial compressive experiments of FRP confined RAC-RCBA incorporating with nanomaterial cylindrical specimens were conducted.The variables included the replacement ratio of RCBA,thickness of FRP tube,type and dosage of nanomaterials,dispersion method of nanomaterials,mixing method of fresh concrete,etc.The strength ratio,ductility indexes,axial stress-strain behavior,lateral strain-axial strain behavior,dilation rate,axial strain distribution of FRP-NPRAC-RCBA were discussed.The comparison between FRP-RAC-RCBA and FRP-NPRAC-RCBA was conducted by comparing the values of strength,strength ratio and strain enhancement ratio.（5）The confinement mechanism of FRP used in concrete confinement were summarized.The existing confinement models were collected and classified in the dissertation,the CFRP confined concrete and GFRP confined concrete specimens under axial compression from lots of literatures were collected to build the FRP confined concrete database.The statistical software SPSS was used in the accuracy evaluation of confinement models.The JC method realized by MATLAB software was used to discuss the application range and reliability of confinement models.Based on the evaluation of confinement models,the model proposed by Lam and Teng was applied in the modeling of stress-strain behavior of specimens FRP-RAC-RCBA and FRP-NPRAC-RCBA.（6）The ABAQUS software was used in finite element（FE）analysis of axial compressive behavior of FRP-RAC-RCBA and FRP-NPRAC-RCBA.Based on the axial compressive FE model of FRP-RAC-RCBA and FRP-NPRAC-RCBA,the eccentric axial compressive,cyclic axial compressive and quasi-static cyclic lateral loading models were simulated.