Study On Mechanical Properties Of UHPC-RC Composite Beams

Ultra-high Performance Concrete（UHPC,for short）,also known as Reactive Powder Concrete（RPC）,is a new type of cement-based composite material.Compared with ordinary concrete,it has superior material properties such as ultra-high strength,high toughness and high durability,which can meet the needs of building long-span structures such as bridges and buildings in harsh environment.It has become a new hotspot in the field of engineering material with bright prospects for application.With a higher price of UHPC than that of ordinary concrete,the full-section UHPC beam is plagued by low economic effectiveness,and the UHPC at the top plate cannot fully exerts its performance.To solve this problem,using combined cross-section of UHPC-RC will be a better option.However,as a new type of structure,the bending and shearing resistance of UHPC-RC composite beam should be bestowed great attention.Therefore,based on the basic mechanical tests of UHPC materials and the bending and shearing performance tests of UHPC-RC composite beam,the mechanical properties and calculation methods are discussed and studied in this paper.The main research contents and conclusions are as follows:（1）With axial compression,flexural performance,splitting tensile and elastic modulus tests carried out on UHPC cubes and prism specimens with specific ratios,the mechanical indexes such as UHPC cube strength,prism strength,flexural strength and relevant statistical parameters were obtained.And through the chi-square（?²）test of mathematical statistics,it is concluded that each mechanical index and relevant statistical parameters conform to the normal distribution.（2）Taking the UHPC-RC composite bridge with a span of 16m as a supporting project,a 1:1 full-scale model was designed and subjected to a flexural bearing capacity test to verify that the flexural bearing capacity of the UHPC-RC composite beam satisfies the design requirements of supporting project,the stress level and deformation performance in the elastic section are in good agreement with the design results and after entering the elastoplastic section,the bearing capacity still has more room to play.After the flexural bearing capacity test is completed,the ends of the two beams close to the support are each cut off by about 4.4m,and beam A and beam B with a 4m clear span are formed for the subsequent shear-resistant capacity test.The shear-resistant capacity of UHPC-RC composite beams was verified to exceed the design requirements.（3）Considering the dispersion of UHPC material,the geometric parameters of section and the dispersion of calculation errors,the uncertainty analysis of the structural resistance of prestressed UHPC beams is carried out to obtain the statistical parameters of structural resistance.Combined with the probability distribution,statistics of the effect parameters and the principle of the checkpoint method（JC method）,MATLAB failure probability analysis program has been compiled.Based on the reliability idea and according to the calculation results of the failure probability of UHPC components under axial compression,the UHPC material partial factors can be obtained.By summarizing the test results in the relevant references and in chapter 2 of this paper,the value of the coefficient of difference?₁ between the structural strength of the UHPC and the strength of the specimen,the cube strength,and the strength of the prism are given.The standard and design values of compressive and tensile strength of UHPC materials were determined.（4）Based on the assumption of a flat section,the calculation formula of the relative limit compression zone heightξ_b of the prestressed reinforced UHPC member without obvious yield point is derived.（5）Through the comparison and analysis of the calculation results and test values of the existing UHPC beam ultimate bearing capacity algorithm,it is proposed to use a more accurate deformation coordination method to calculate the flexural bearing capacity of the normal section of the prestressed UHPC-RC composite beam.Considering that the actual engineering design calculations at home and abroad often refer to formulas in the French UHPC design guide AFGC-2016,and that has yielded good engineering results,this article recommends that formulas in the French UHPC design guide AFGC-2016 should be taken into consideration when calculating the shear-resistant capacity of prestressed UHPC-RC composite beams.Meanwhile,some adjustments were made in these formulas in this paper and the comparison between calculation results and the test results was made.The results show that the calculated bearing capacity is basically consistent with the test value,which has certain applicability to the test beam.