Experimental And Theoretical Study On Static Mechanical Performance Of RPC-NC Composite Beam

As a new kind of cement-based composite material,reactive powder concrete(RPC)has excellent mechanical properties and durability which are incomparabale by normal concrete(NC).RPC has caused deep attention from many researchers since its birth.However,use of expensive componential materials,such as silica fume,quartz sand and steel fibres,leads to a relatively high cost.Moreover,factory prefabrication is the most reasonable way at present to ultilize RPC,because special curing condition,such as steam curing,is essential in RPC manufacture procedure.Those deficiencies limit the promotion of RPC to engineering application in large span structures which need relatively more concrete,especially in large span bridge since the use of integral casting technique for total cross section of large span RPC beam will lead to extra hoisting construction cost.Therefore,in this paper the static load mechanical properties of RPC-NC composite beam,in which tensile zone is placed by prefabricated gravel RPC beam and other parts are cast in place by NC,is studied through test and theoretical method to provide reference to its possible engineering application.The main work and conclusion in this paper is as follows:(1)Twelve sets of double-sided shear specimens of RPC-NC composite interface were tested on static shearing scheme.Affecting factors includes strength grade of NC and interface pressure.Test results indicate that the failure mode of the interface is brittle,the separated prisms of RPC and NC remain intact.The increment of NC tensile strength can to some extent causes the increment of ultimate shearing capacity of those specimens without interface pressure,while it can effiently increase specimens ultimate shearing capacity in linear relationship when interface pressure is applied.Based on the considered factors in existing calculation formula,in this paper a simplified calculation model of ultimate shearing capacity for RPC-NC composite interface is introduced and the expression of ultimate shearing capacity is achieved based on regression analysis of measured data.(2)To study the flexural behavior of RPC-NC composite beam,twelve specimens were fabricated and tested on simply supported static loading scheme.Affecting factors includes height of RPC,strength grade of NC and ratio of longitudinal reinforcement.Test results show that the shape of load-deflection curve of RPC-NC composite beam is similar to that of NC beam and can be devided into three stages,namely the linear stage,the crack development stage and the destruction stage.The split points of these three stages are cracking of concrete and yielding of longitudinal tendons.The linear stage of load-deflection curves before cracking possesses the percentage from 30.6%to 58.3%of ultimate load while the reduction of ultimate load causes the increment of this proportion.It proves that RPC-NC composite beam has good crack resistance performance and stiffness storage.The cracking location of RPC-NC composite beam goes from the NC tensile edge to the bottom of prefabricated RPC part when the height of RPC is increased.The ratio between final deflection and the deflection at tendon yielding is from 3.05 to 4.68,thus the ductility requirement needed by general flexural member is satisfied.The measured strain data accord with plane section assumption in the main.(3)Analysis of cracking moment and flexural rigidity was conducted based on measured data of RPC-NC composite beam specimens.With the calculation model in which tensile ductility of RPC and NC is both considered,theoretical results of cracking moment have good agreement with the measured ones if tensile ducility coefficients of RPC and NC take 3.12 and 2.51 respectively.Formula of plastic influence coefficient for tensile zone of RPC-NC composite beam is given and its simplified expression is achieved on condition that RPC height is higher than 30%of total beam height.Rigidity-moment curves are numerically studied based on the tensile constitutive model of RPC with tension stiffening effect,results indicate that the bending moment,at which rigidity of composite beam begins to decrease,is from 1/3.1 to 1/2.2 of the ultimate moment,while the value is from 1/6.5 to 1/4.3 for integral concreting NC beam.Short-term rigidity of RPC-NC composite beam is analyzed based on effective inertial moment method and stiffness analysis method,then simplified expression is given and good agreement with measured data is met.Average crack interval of RPC-NC composite beam can be calculated with the formula in "GB50010-2010 Code for design of concrete structures",but the effective tensile area for concrete is recommended in this paper to be replaced by the area of prefabricated RPC part.Crack width correction coefficient is introduced to modify the maximum crack width formula in "JTG D62-2004 Code for design of highway reinforced concrete and prestressed concrete bridges and culverts",and according to regression analysis of measured data,0.40 is the most suitable value with conservativeness.(4)Considering the contribution of RPC tension to the ultimate bending capacity,simplified calculation formula for T-shaped RPC-NC composite beam is derived,and the tensile influence coefficient of equivalent rectangular stress for RPC is recommended to take 0.26.The critical reinforcement ratios of RPC-NC composite beam are studied theoretically.Results indicate that secondary load effect to the maximum reinforcement ratio can be ignored.The minimum reinforcement ratio of composite beam is higher than that of integral concreting NC beam with same section configuration,and can be even higher with the increment of RPC height.Based on fixed angle soften truss model(FASTM)and finite element method,the influence of RPC tensile strength,NC tensile strength,longitudinal reinforcement rato,stirrup ratio,shear span ratio and RPC height to the ultimate shearing capacity of RPC-NC composite beam is studied,and then simplified calculation formula is reccmmended.(5)Extended finite element method(XFEM)with cohesive force between crack faces and its implementation in the open source software OpenSees are introduced,then numerical study on crack behavior of RPC-NC composite beam on static bending is conducted.Results show that the increment of RPC height causes the reduction of crack width on bottom edge of NC and RPC at the same load level.The load,at which slope of load-deflection curve begins to decline,increases with the increment of RPC height,this theoretically proves that the increment of RPC height can efficiently increase the crack resistance performance and the overall rigidity of RPC-NC composite beam.