| In order to compensate the structural defects of recycled coarse aggregate(RA),nano silica(NS)and fly ash(FA)were added into cement mortar to modify recycled coarse aggregate concrete(RAC).In this study,228 standard cubes and 57 standard cylinders were tested to systematically study the basic mechanical properties of nano silica and fly ash modified recycled concrete(NFRAC).(1)Specimens with basic strength of C40,water binder ratio of 0.47,different NS(0%,1%,2%,3%),FA(0%,15%,20%)and RA(0%,50%,100%)replacement ratio were tested in this study.The slump of fresh concrete shows that NS has more adverse effect on the workability of recycled concrete than natural concrete.(2)The cube compressive test and cube splitting tensile test with curing age of 7d and 28 d were carried out to study the influence of fly ash and nano silica on the strength of recycled concrete.Compared with NAC control group,the strength of natural concrete with 10% FA and 3% NS increased by 46.6% in 7d and 29.0% in 28 d.(3)The cylinder uniaxial compression test was carried out,and the stress-strain curves were collected to explore the effects of replacement ratio of RA,FA and NS on the peak stress and strain,elastic modulus,Poisson’s ratio,ductility coefficient and energy consumption coefficient of NFRAC.(4)Compared with the existing concrete uniaxial compression constitutive relationship,the mathematical model of recycled concrete constitutive relationship with the highest fitting degree is selected,and the uniaxial failure criterion and constitutive relationship suitable for this study are proposed,which provides theoretical basis for engineering application.(5)The microstructure of interfacial transition zone(ITZ)of natural concrete and recycled concrete mixed with NS and FA was observed by tungsten filament scanning electron microscope(SEM),and the structure development law and physical properties of composite modified recycled aggregate were analyzed from the micro level. |
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