Ingestigation On The Mechanical Behavior Of Damaged Reinforced Concrete Beams Repaired With Ultra-High Performance Cement Materials

The large scale infrastructure construction has entered into a stable phase in China,and some of the structures have been used for decades.Most of these constructions were made of concrete which is likely to suffer damage after years of service.On the other hand,the functionally obsolete buildings are routinely demolished,and building new constructions is quite difficult due to the planning,environmental and financial limitation.The restoration of existing buildings is the most appropriate solution.The appearance of Ultra High Performance Concrete(UHPC)provides a good support for the repair and reinforcement of concrete structures.A large number of studies have shown that UHPC reinforced the existing concrete can improce the mechanical properties and durability singnificantly.Considering the materials constitutent and interface performance,this paper studies the performance of existing concret structures reinforced by UHPC and the corresponding materials from UHPC material hydration mechanism.Taking UHPC reinforced beam as the research object,the comprehensive performance of reinforced beam in relation to various parameters is obtained,and the design method of reinforced beam is proposed.It is expected to provide a data support and theoretical basis for the application of UHPC in existing concrete repairing and reinforcement.The work firstly focused on improving the performance of UHPC by replacing silica fume by granulate ground blast furnace slag and determing the hydration mechanism of so prepared cementitious materials.Then the shear performance of the interface between UHPC and existing concrete,the flexural behavior of reinforced concrete beams was evaluated.Based on the experimental study and finite element simulation,the calculation method of normal section flexural capacity of reinforced concrete beams by UHPC was proposed.Considering the need for rapid reinforcement of damaged reinforced concrete structures in some condition,a kind of fiber reinforced magnesium phosphate cement(FRMPC)was proposed for the rapid repair of cracked concrete beams.Combined with the calculation method of flexural bearing capacity,the mechanical performance of the rapid reinforced beam was analyzed and the reinforced design method is then proposed.The main findings in this work can be summarized as follows:(1)The preparation and performance of UHPC made with slag and silica fume.The Linear Packing Model(LPM)model was used to calculate the theoretical bulk density of the UHPC binder.The calculation results indicated that the addition of slag and silica fume could increases the bulk density,which was consistent with the test results of wet bulk density.The model confirmed that the combination of slag and silica fume is able to improve the performance of UHPC.The slag has little effect on the water film thickness,while the silica fume will directly affect the average water film thickness.Using different supplementary cementitious mateirals to realize the synergistic improvement and regulation of the UHPC performance via the nucleation effect and pozzolanic effect.Slag and silica fume particles with high bulk density provides a large number of nucleation sites for the formation of hydration products promoting the hydration degree of binders.(2)Investigation on the shear properties of the interface between UHPC and the existing concrete.The key to the joint of the UHPC reinforcement layer and the original reinforced concrete(NC)beam is a reliable interface bonding.Therefore,the thesis firstly uses the method of direct shearing(pull-out test)to study the shear resistance of the interface between UHPC and NC.The failure modes and shear properties of the specimens(load-slip curve,interfacial shear strength,etc.)were determined.According to the experimental failure mode,the limit value of the minimum length-diameter ratio of the planting reinforcement was proposed,which provides a theoretical basis for the interface design of the existing concrete beam reinforced by UHPC and the subsequent finite element simulation analysis.(3)The flexural strength behavior of concrete beams reinforced by UHPC.Through the four-point bending test,the effects of interface treatment method(interface chiseling and reinforcement),shear-span ratio,UHPC layer reinforcement ratio on the failure mode,bearing capacity,stiffness,interface slip and crack development and distribution of UHPC-reinforced reinforced concrete beams were studied.The research results show that: when the UHPC layer does not contain steel reinforcement,the specimen would lose its contribution to the reinforced concrete beam due to the complete rupture of the UHPC reinforcement layer.For the specimen reinforced with UHPC,the steel reinforcement can effectively restrain the development of UHPC macro cracks.The bearing capacity and initial stiffness of the specimen increased significantly with the increase of reinforcement ratio and shear-span ratio,the specimen changed from typical bending failure to bending-shear failure mode.The interface treatments of chisel hair and planting reinforcement can effectively ensure the reliable connection between the UHPC reinforcement layer and the existing concrete beam.(4)Numerical analysis and calculation method of bearing capacity of UHPC reinforced concrete beams under flexural stress.Based on the flexural test of UHPC reinforced concrete beams,a refined finite element model considering the initial damage of reinforced concrete beams and different interface connection properties was proposed.The influence of parameters involving reinforcement ratio,UHPC thickness and reinforcements spacing on the flexural properties of UHPC-reinforced concrete beams was evaluated.Based on the damage evolution and stress distribution of each component,the flexural failure mechanism of UHPC reinforced concrete beams was revealed.According to the experimental test and numerical simulation results,a calculation method for the flexural bearing capacity of UHPC-reinforced concrete beams was proposed successfully.(5)Mechanical properties of rapid reinforced concrete beams by magnesium Phosphate Cement(MPC).MPC is characterized by rapid setting,high early strength,well volume stability,high bonding strength with traditional concrete substrates,and excellent reinforcement properties,making it very suitable for rapid reparing of concrete structures.After expoloring the mechanical properties of fiber reinforced magnesium phosphate cement(FRMPC),the shear performance of the FRMPC-NC interface was investigated through push-out tests.According to the failure mode,it was determined that the anchorage length-diameter ratio of planting bars of FRMPC reinforced concrete beams should be no less than 5.The flexural strength of FRMPC reinforced concrete beams at early age was evaluated through four-point bending loading test.