Study On Dynamic And Static Mechanical Properties And Durability Of Nano Modified UHPC

With the rapid development of economic construction,bridges are facing many new challenges,such as the increase of traffic flow,the diversification of loads,the complexity of service environment,the extension of service life and so on,which have more and higher requirements for the use of engineering materials.Due to the characteristics of ultra-high strength,extremely low permeability,excellent durability and good toughness,Ultra-High Performance Concrete(UHPC)can not only meet the requirements of engineering for high strength,but also make the bridge structure better deal with harsh environments such as frost,chloride,acid rain and so on.With more and more applications of UHPC in engineering structures,how to further improve the mechanical properties and durability of UHPC is still the pursuit of researchers.At present,researchers have used nano silica and nano calcium carbonate to modify UHPC and achieved certain results.However,there is still a lack of relevant research,especially the effect of nano materials on the dynamic properties of UHPC.In this dissertation,nano materials were used to modify the matrix of UHPC.Through the combination of experimental research and theoretical analysis,the effects of nano materials and steel fibers on the working performance,splitting tensile strength,compressive strength,compressive stress-strain curve,dynamic axial compression impact performance,dynamic splitting impact performance and durability of UHPC under the coupling action of nano materials and steel fibers are studied by field emission electron microscope(SEM)The mechanism of UHPC modified by nano materials was analyzed by micro measurement techniques such as X-ray diffraction(XRD)and mercury injection test(MIP).In addition,nano modified UHPC is also used to strengthen the existing damaged concrete,which provides important test data for the engineering application of UHPC.The research on nano modified UHPC has important scientific significance for promoting the application of UHPC in engineering field.The main research contents and results of this dissertation are as follows:(1)By preparing 9 groups of UHPC with different ratios,the effects of nano materials and steel fibers on the working properties,compressive strength and microstructure of UHPC were analyzed.Combined with scanning electron microscope and X-ray diffraction(XRD),the strengthening mechanism of nano modified UHPC was explored.The results show that steel fiber can reduce the fluidity of UHPC and improve the strength of UHPC.Steel fiber can effectively bridge cracks,inhibit the development of cracks and improve the toughness of UHPC.Nano materials can enhance the transition zone between aggregate and matrix,make the matrix more dense and increase the strength.Excessive nano materials will lead to the increase of UHPC defects and the decrease of strength due to agglomeration.The addition of nano materials makes the early hydration degree of UHPC deeper and the early strength higher.(2)Uniaxial compression and splitting tensile tests were carried out on UHPC cylindrical specimens,the strengthening mechanism of nano material modified UHPC was revealed,and a constitutive model suitable for nano modified UHPC was proposed.The research shows that steel fiber can improve the tensile strength,compressive strength and toughness of UHPC,and the effect of end hook steel fiber is better than that of straight steel fiber.Nano materials can improve the microstructure and strength of UHPC matrix.Among them,2% of NC has the largest increase in compressive strength,and 0.3% of CNF has the best increase in tensile strength.Excessive nano material content will cause matrix defects and reduce the strength.Compared with the existing constitutive models of UHPC,the proposed model quantifies the effects of steel fibers and nano materials,which has not been reported at home and abroad.(3)The axial compression impact test of nano modified UHPC was carried out,and the effects of strain rate,steel fiber type,nano material type and nano material content on the dynamic compressive strength,axial compression impact failure mode,impact toughness,dynamic compression constitutive relationship and dynamic reinforcement coefficient of UHPC were analyzed.The results show that the dynamic compressive strength,dynamic toughness and CDIF of UHPC increase with the increase of strain rate;Steel fiber improves the dynamic compressive strength and impact toughness of UHPC and reduces the strain rate sensitivity of UHPC.The effect of end hook steel fiber is better than that of straight steel fiber;Proper addition of NS,NC and CNF can improve the axial compressive impact properties of UHPC,but excessive nano materials will have an adverse impact on the axial compressive impact properties of UHPC.Based on the experimental results,a UHPC dynamic enhancement factor model considering the coupling effect of nano materials and steel fibers is proposed,and its prediction effect is better than other existing models.(4)The splitting impact test of nano modified UHPC was carried out.The effects of strain rate,steel fiber type,nano material type and nano material content on the failure mode,time dynamic tensile stress curve,dynamic splitting strength and dynamic reinforcement coefficient of UHPC under Splitting impact load were analyzed.Based on the test results,the TDIF prediction formula suitable for UHPC was proposed.The results show that UHPC shows strain rate sensitivity,and the dynamic tensile strength and TDIF increase with the increase of strain rate;Steel fiber can effectively improve the dynamic splitting strength of UHPC and reduce the strain rate sensitivity of UHPC.The effect of end hook steel fiber is better than that of straight steel fiber;Proper addition of NS,NC and CNF can improve the splitting impact properties of UHPC,but excessive ns and NC will adversely affect the splitting impact properties of UHPC.(5)The effect of nano materials on the durability of UHPC was analyzed through water penetration test,chloride ion penetration test and rapid freeze-thaw test.Combined with mercury injection test(MIP),the durability mechanism of nano modified UHPC was explored.It is found that nano materials can optimize the internal pore structure of UHPC,reduce the harmful pore ratio,reduce the water permeability of UHPC,and improve the chloride ion permeability and freeze-thaw resistance.Among them,the improvement effect of NC and NS is better than that of CNF.(6)Through the combination of test and finite element,the stress of existing damaged concrete columns strengthened with UHPC is analyzed.The results show that the ultimate bearing capacity of reinforced concrete columns can be greatly improved by using UHPC wrapped reinforcement,and the reinforcement effect is good;The finite element model analysis results are consistent with the experimental results,which has a good prediction effect on the bearing capacity of existing damaged concrete columns strengthened by UHPC.