Research On The Self-sensing Properties Of GFRP-reinforced Smart Concrete Beam Components

Self-sensing concrete fabricated with conductive fillers integrates structural and sensing properties,providing a new method for structural health monitoring.However,most of the studies on the sensing performance of self-sensing concrete focus on the axial compressive and tensile conditions,while studies on the self-sensing performance when subjected to flexural loading are relatively few.In addition,when the sensing performance of self-sensing concrete beam components/structures is studied,the object of study is usually unreinforced concrete beams,which is difficult to meet the actual needs of the engineer.While the configuration of ordinary reinforcement will contribute to interfering the conductive network due to its conductivity within self-sensing concrete.Furthermore,most of the self-sensing concrete applications are in the form of embedded sensors,which exists the problem of strain coordination between the sensors themselves and the surrounding substrate.Based on this,in this paper,carbon nanotubenanocarbon black(CNT-NCB)composite fillers with good dispersion and both longrange and short-range conductive effects,and super-fine stainless wires(SSWs)with excellent conductive properties and corrosion resistance are used as conductive functional fillers to fabricate smart self-sensing concrete,and all(in the bulk form)or surface(in the coating form)smart concrete and non-conductive glass fiber reinforced polymer(GFRP)reinforcement is used as reinforcement to fabricate smart concrete beam components,and then the mechanical and electrical properties as well as the sensing properties under monotonic and cyclic flexural loadings are investigated.The main research contents and conclusions are as follows:(1)GFRP reinforced smart concrete beam components with CNT-NCB composite fillers in the bulk and coating forms were prepared,and their mechanical properties under four-point bending load were analyzed,and the DC and AC conductive properties of the beams were investigated during the curing age.The results showed that the destruction modes of concerete beams in the bulk and coating forms are shearcompression damage.The bearing capacity of concrete beams in the bulk form with CNT-NCB composite fillers decreases with the increase of contents,and the bearing capacity of beams in the coating form is higher than that of beams in the bulk form.The incorporation of CNT/NCB composite fillers can impart excellent conductive performance to concrete beams,and the DC and AC resistivities of beams in the bulk/coating forms range from 33 to 76 Ω·cm/18 to 85 Ω·cm and 34 to 57 Ω·cm/19 to85 Ω·cm,respectively.The DC and AC resistivities of beams vary less between different contents of the added fillers.The DC resistivity of the beams hardly changed with the growth of the curing age after doping CNT-NCB composite filler,indicating that a stable conductive network has been formed internally.(2)The self-sensing properties of GFRP reinforced smart concrete beam components with CNT-NCB composite fillers under monotonic and cyclic flexural loadings were investigated.The results showed that there is a clear correspondence between FCR in the tension and compression zones of the concrete beam in the bulk and coating forms and the damage process under monotonic flexural loading to failure.When microcracks appear,the resistance in the tension zone of beams in the bulk and coating forms is the first to increase significantly,and the fractional change in resistivity(FCR)in the tension zone of the pure bending section of the beam in the coating form with CNT-NCB composite filler doping of 1.8 vol.%,2.0 vol.% and 2.2 vol.% can reach60.6%,55.9% and 19.8%,respectively.The FCR of bending and shear sections of beam in the coating form all exhibited a similar variation pattern with the development of oblique cracks: the FCR curve changed gently at the beginning of the crack generation,then gradually increased with the crack development,and finally increased sharply when the beam components were damaged.Under cyclic bending loading,the FCR of both beams in the bulk and coating forms exhibited good synchronization and stability with the variation of mid-span deflection/strain.The beam in the bulk form incorporated with 2.0 vol.% CNT-NCB composite filler has the best self-sensing ability with FCR amplitude up to 14.2%.The FCR amplitude of the beam in the coating form with 1.8vol.% CNT-NCB composite fillers can reach 299.4% and 77.8% in the tension zone of bending-shear and pure bending sections,respectively.In addition,the FCR showed a progressive increase in cyclic loading at different amplitudes,and there were obvious residual values of resistance changes,indicating that irreversible damage such as cracks were generated inside the beam.With the increase of cyclic loading amplitude,the FCR amplitude in the bending-shear section and the tensile zone of the pure bending section of both beams in the bulk and coating forms increased significantly,indicating that the cracks significantly destroy the conductive network formed by the CNT-NCB composite filler.(3)GFRP reinforced smart concrete beam components with SSWs in the bulk and coating forms were prepared,and their mechanical properties under four-point bending load were analyzed,and the DC and AC conductive properties of the beams were investigated during the curing age.The results showed that the destruction modes of concerete beams in the bulk and coating forms are shear-compression damage.The bearing capacity of beams in the bulk form doped with 0.4 vol.% SSWs could be increased by up to 31.9%.The DC resistivity of beams in the bulk form with 0.4 vol.%SSWs was the lowest,ranging from 4.9 to 7 Ω·cm,while the DC resistivity of the coated beams with 0.2 vol.%～ 0.4 vol.% SSWs ranged from 2 to 59 Ω·cm.The 28-day AC resistivities of beams in the bulk and coating forms ranged from 3 to 55 Ω·cm and 3 to64 Ω·cm,respectively.In addition,the DC resistivities of the beams hardly changed with the growth of the curing age,indicating that the SSWs were overlapped to each other to form a stable conductive network.(4)The self-sensing properties of GFRP reinforced smart concrete beam components with SSWs under monotonic and cyclic flexural loadings were investigated.The results show that the FCR in the tension zone of the purely bending section of beam in the bulk form with 0.2 vol.%,0.3 vol.%,and 0.4 vol.% SSWs increases from 0.3%to 14.7%,from 1.4% to 31.4%,and from 0.1% to 15.9%,respectively,when microcracks appear under monotonic flexural loading to failure.The variation of FCR in the tension zone is more significant than that in the compression zone.The FCR of beam in the bulk form with 0.2 vol.% SSWs corresponds best to the load variation in the tension and compression zones.The FCR signal-to-noise ratio of beam in the coating form become relatively low after reaching the peak load.Under cyclic flexural loading,the beam in the bulk form with 0.3 vol.% SSWs has better sensing stability and higher damage sensing capability,and beam in the coating form with 0.3 vol.%SSWs has the best resistivity variation regularity.In addition,beams in the bulk and coating forms show significant residual values of FCR during different amplitudes of cyclic flexural loading,which show irreversibility,indicating that damage such as cracks were generated inside the beam components.(5)The FCR response(slope change,progressive upward trend,residual value)of concrete beams in the bulk and coating forms fabricated by adding CNT-NCB composite filler well corresponds to the damage process of the beam components,indicating that the internal damage of the beam components could be monitored in realtime by self-sensing the relative change in the resistance of the concrete.The concrete beams in the bulk form prepared with SSWs have better sensing performance than the concrete beams in the coating form,and the low FCR signal-to-noise ratio is caused by the easy debonding of SSWs which leads to the constant change of SSWs-cement matrix contact.The concrete beams with CNT-NCB composite filler showed higher FCR signal-to-noise ratio and better sensing stability in comparison with SSWs-doped concrete beams.