Study On Uniaxial Properties Of Ultra High Performance Concrete And Bonding Properties With High Strength Rebar

Ultra high performance concrete(UHPC)is usually defined as cement-based cementitious material with compressive strength exceeding 150 MPa.UHPC has excellent impermeability,tension and compression performance,and is the optimal solution for the design of high-strength and durable structures.At present,UHPC has been widely used in many architectural structures such as bridges,sidewalks,and houses.By carefully controlling the size and distribution of its constituent particles and combining high-strength discontinuous steel fibers,UHPC achieves ultra-high packing density and improves material properties such as compressive strength,ductility and durability.However,the cost of UHPC is relatively high,and at the same time there is a lack of more comprehensive experimental research,and its further application is limited.With the increasing demand for UHPC in modern buildings,the in-depth study of its tensile and compression properties is becoming more and more important.At the same time,in actual engineering practice,UHPC components mostly appear in the form of reinforcement,so the bond-slip performance analysis between UHPC and steel bars(especially high-strength steel bars)has also become the premise of UHPC’s behavior analysis at the component level.In order to find a relatively simple test method to capture all the stress-strain curves during the tensile and compression process,some uniaxial stress tests study were carried on the UHPC based on a self-designed test device.The uniaxial pull-out experiment is used to study the bonding properties between UHPC and high-strength steel bars.The main main conclusions and research process are listed blow.(1)The results of experimental investigations on the compression and tensile response of UHPC are presented.Conventional test methods for ordinary concrete are unreliable for measuring the post-cracking behavior of UHPC in tension and compression.The experimental tests include 36 cylinders compressive tests and 36 dog-bone shaped tension tests to evaluate the stress-strain relationship of the material under compression and tension.The rigid auxiliary component and the deformation test frame were adopted in the compressive tests to obtain the decline curve successfully.Based on self-designed uniaxial tensile test equipment,a series of UHPC uniaxial tensile tests were completed.(2)The increase of steel fiber content has little effect on the peak strain and elastic modulus of UHPC tensile and compressive stress.However,there is a significant improvement in the form of failure and the ductility after cracking,which is the decisive factor for the transformation of UHPC from brittleness to ductility.2% of the steel fiber content is proved to be able to make the failure form of UHPC into ductile failure.The value of the water-to-binder ratio affects the strength of the UHPC concrete matrix and the modulus of elasticity.Tests have shown that a water-to-binder ratio of 0.18 is the most suitable choice without reducing the strength of UHPC.(3)In this paper,24 central pull-out tests were conducted in order to characterize the bond behaviour of UHPC and high-strength steel bar reinforcement.The tests were conducted using steel bar reinforcement with nominal diameters of 16 mm,18 mm,and20 mm.Other experimental parameters include four embedment Lengths(3d,4d,5d and6d)and two steel bar reinforcement grades(HRB600 and HRB500).The results of experimental investigations on the bond-slip curve are presented.(4)In the specimens with a short embedment length,the specimens showed the highest ultimate bond stress,whose failure mode was due to the interfacial rupture between the high strength steel rebar and the UHPC block.In these situations,a very small crack could be observed on the surface.In the other specimens with higher embedded lengths,the rupture of the high strength steel rebar limited the bond stress of the specimens.The ultimate bond strength decreases with increasing embedding length and reinforcing bar diameter.The use of high-strength steel bars can significantly improve their bonding strength.(5)The constitutive relationship originally used to describe the bond-slip of ordinary concrete is no longer applicable.Based on the experimental load-displacement at the free end responses,a new bond-slip relationship with is proposed.Due to the excessively high bond strength,the effect of strain penetration that were originally present only in bars with long embedment length were also considered in the new proposed bond-slip relationship.The bond stress distribution of different location was obtained by innerembedded strain gauge,the position function which reflects this change is deduced.Combined with product of average bond stress-silp constitutive equation and positon function of UHPC,a more accurate description of bond-slip constitutive relation was presented.