Static And Fatigue Behavior Of Rc Flexural Members Strengthened With BFRP Grids

To extend the service life of the degenerated reinforced concrete members,strengthening techniques such as increasing section area,externally bonded steel plate,external prestressing or externally bonded fiber-reinforced polymer(FRP)are commonly used to strengthen these members.However,the above-mentioned strengthening techniques have some problems such as complex process or poor durability,which affect the construction efficiency and structural operation safety.In view of this,scholars have developed a new strengthening technology,namely externally bonded FRP grids,which is convenient in construction and excellent in durability.Nevertheless,up to now,studies on this strengthening technology are still limited,so that the corresponding application specifications or design guidelines can not be formulated.In addition,most of the existing studies mainly focuses on the expensive carbon fiber-reinforced polymer(CFRP)grids.If the CFRP grids are selected as strengthening material in practical application,the strengthening cost will be much higher than other strengthening techniques,which undoubtedly limits the popularization and application of the externally bonded FRP grids strengthening technology.Therefore,in this paper,the basalt fiber-reinforced polymer(BFRP)grids are selected as the strengthening material due to the reasonable material costs.The effectiveness of the externally bonded BFRP grids is systematically studied from three aspects: bond behavior of the interface between BFRP grids and concrete,static behavior of the strengthened members and fatigue behavior of the strengthened members.It is hoped that this study can provide reference for the formulation of subsequent application specifications or design guidelines.Main research contents of this study are as follow:(1)The bond behavior between the FRP grids and concrete was investigated according to the single-shear test.The variable parameters included the FRP grid widths,FRP grid types,bonding material types and FRP product types.The experimental results showed that the width ratio of the FRP grids to concrete block,the stiffness of the FRP grids and the types of bonding materials had great influence on the bond behavior of the FRP grid-concrete interface.Compared with the FRP sheet-concrete interface,the stress transfer mechanism of the FRP grid-concrete interface was more complicated,but this interface exhibited a higher interfacial fracture energy(1.95-4.09 times of the FRP sheet-concrete interface).Compared with epoxy resin,when polymer cement mortar was used as the bonding material,the interfacial stress between the FRP grids and concrete would decrease more quickly after reaching the maximum value,that was,the interfacial brittleness was greater.In addition,based on the regression of test data,models for the width factor,effective bond length,bond-slip relationship and bond strength of the FRP grid-concrete interface were proposed.(2)The flexural behavior of reinforced concrete beams strengthened with BFRP grids was investigated.The effects of FRP grid type,amount of FRP grid and types of bonding materials on the flexural behavior of the strengthened beams were discussed.In order to further improve the flexural behavior of the strengthened beams and enhance the strength utilization efficiency of the BFRP grids,this study developed a simple anchorage,and proposed a method of multiple prestress release.The test results showed that BFRP grids can effectively improve the flexural stiffness and load capacity of the test beams.When the prestressed BFRP grids were applied,the cracking load,yield load and flexural stiffness of the test beams were further improved.Meanwhile,with the increase of the prestress release times,the failure mode of the prestressed beams can be changed from “concrete cover separation” to “concrete crushing” or “BFRP grids rupture”.In addition,based on the plane section principle,the calculation formula of the load capacity of the beams strengthened with BFRP grids was derived.(3)The flexural behavior of reinforced concrete slabs strengthened with BFRP grids was studied.First,the strengthening effects of three strengthening technologies,including externally bonded BFRP sheets,embedded BFRP bars and externally bonded BFRP grids,were compared.The results showed that the BFRP grids were more effective than the BFRP sheets and bars at enhancing the flexural stiffness and load capacity of test slabs when each BFRP product possessed the same axial stiffness.At yield and ultimate loads,the economic indices of the BFRP bar strengthened slab were 0.44 and 0.82 times of those of the BFRP sheet strengthened slab,respectively.However,the economic indices of the BFRP grids strengthened slab were 1.68 and1.60 times of those of the BFRP sheet strengthened slab,respectively.Therefore,the economic advantage of the externally bonded BFRP grids strengthening technology was more prominent.Then,the flexural behavior of the slabs strengthened with prestressed and non-prestressed BFRP grids were compared.The results showed that the application of the prestress improved the strength utilization efficiency of the BFRP grids(from 52.6% to 84.5%-96.4%),and further improved the load behavior and serviceability of the strengthened slabs before yielding occurred.(4)The shear behavior of reinforced concrete beams strengthened with BFRP grids was studied.The variable parameters included the shear span ratios,types of strengthening materials,types of bonding materials and arrangement of BFRP grids.The results showed that the contribution of the BFRP grids to shear capacity was almost independent of the shear span ratio.Compared with the epoxy resin,the polymer cement mortar performed better in improving initial stiffness of the test beams,but the diagonal crack development and stiffness degradation of these beams were faster.The fracture energy of the interface between BFRP sheet and concrete was low.Even if additional anchorage measures were adopted,the BFRP sheet was prone to debond.However the interface between the BFRP grids and concrete could maintain reliable bonding during the whole loading process.In addition,compared with the parallel BFRP grids,the diagonal BFRP grids displayed better in restricting the development of cracks and restraining degradation of the flexural stiffness.(5)The fatigue behavior of reinforced concrete beams strengthened with BFRP grids was investigated.The variable parameters included the load amplitude,prestress and prestress release times.The results showed that the fatigue life of the beams strengthened with non-prestressed BFRP grids was improved 57%-117%.However,the fatigue life of the unstrengthened beams and non-prestressed BFRP grid strengthened beams decreased with the increase of the load amplitude.Nevertheless,all of these beams failed in the fatigue fracture of the steel bars.For the non-prestressed BFRP grid strengthened beams,the failure mode of the strengthened layer was different when the load amplitude was changed.For the prestressed BFRP grid strengthened beams,the fatigue life of the strengthened beam was not improved when the prestress just released once.However,the fatigue life of the strengthened beam would be significantly improved when the prestress was released twice.In addition,based on the regression of test data,the fatigue life prediction model of the BFRP grid strengthened beams was developed.