Investigation On Preparation,properties,and Application Of Expansive UHPC

The main girder of hybrid cable–stayed bridge is composed of steel girder and concrete girder longitudinally.It has significant advantages in long-span bridges,including excellent mechanical performance,large span capacity and high cost performance.As a connection of the steel and concrete girders,the steel–concrete joint bears the large axial loads transmitted from the main span.In addition,the stiffness and strength of steel girder and concrete girder are quite different,leading to a complex stress condition in the steel-concrete joint and a stress concentration in the concrete.Therefore,the steel–concrete joint is the key part in the steel–concrete hybrid cable–stayed bridge.The steel-concrete joint with lattice is widely used in the steel–concrete hybrid cable-stayed bridge.But its requirements for workability,strength and shrinkage of concrete filled in lattice are quite high due to the complicated internal structure.Due to low strength,low toughness and high shrinkage of ordinary concrete,the steel-concrete joint poured with ordinary concrete has caused many problems,such as disengaging at steel/concrete interface,concrete cracking,and insufficient fatigue strength of the joint.It has negative impact on the bearing capacity of the structure,which is the problem to be solved for the steel-concrete joint with lattice.Using UHPC(Ultra-High-Performance Concrete)as the grout of steel–concrete joint may improve the pouring quality of concrete,bearing capacity and fatigue resistance of steel-concrete joint effectively due to the high strength and toughness of UHPC.However,high dosages of cementitious materials and low water-to-binder ratio lead to high shrinkage and heat of UHPC.As a result,the steel-concrete joint poured with UHPC would cause many problems,such as early age concrete cracking owing to temperature stress,disengaging at steel/concrete interface,late age concrete shrinkage cracking and decline in fatigue resistance,which restrict the application of UHPC in steel-concrete joint.Low heat expansive UHPC is expected to solve the key problems of UHPC in the application of steel-concrete joint.Based on the Wuxue Yangtze River Highway Bridge and the financial support from the technology project of Hubei Provincial Department of Transportation(project number: 2017-538-2-14),the material composition,preparation theory and principle,expansion mechanism,shrinkage and creep characteristics,hydration heat and engineering application of low heat expansive UHPC were studied.It was expected to solve the shortage of steel-concrete joint with ordinary concrete and provide a reference and basis for the engineering application of UHPC in the steel-concrete joint of steel–concrete hybrid cable-stayed bridge.The main contents and conclusions of this research are presented as follows:(1)Preparation technology of expansive UHPC.Based on the basic theory of expansive concrete and the characteristics of low water-to-binder ratio and high early strength of UHPC,the basic preparation conditions in the preparation of expansive UHPC were proposed;Calcium oxide expansive agent(CaO-EA),super absorbent polymer(SAP)and shrinkage reducing agent(SRA)were selected as the shrinkage compensating materials of UHPC.Firstly,determine the reasonable dosage of CaO-EA in UHPC by studying the effect of CaO-EA with different dosage on the properties of UHPC.Secondly,owing to the promoting effect of SAP on hydration of CaO-EA and the inhibiting effect of SRA on shrinkage,determine the reasonable dosage of SAP and SRA in UHPC by studying the effects of different dosage of SAP and SRA on the properties of UHPC with CaO-EA.The preparation technology of expansive UHPC is developed.When CaO-EA,SAP and SRA were used together,expansive UHPC with can be prepared,the expansion at 180 d was 68.2με.(2)Preparation mechanism of expansive UHPC.Experimental studies were carried out on the micro-characteristics of expansive UHPC,using X-ray diffraction analysis,thermal analysis,calcium oxide content analysis,internal relative humidity analysis and porosity analysis.The effects of CaO-EA,SAP and SRA on hydration products,the relationship between the reaction degree of CaO-EA and volume expansion,the effect of internal relative humidity on early age volume expansion,and the relationship between porosity and strength were investigated.The preparation mechanism and method of expansive UHPC were studied.(3)Shrinkage and creep properties of expansive UHPC.The mechanical properties,shrinkage properties,and creep properties of expansive UHPC and ordinary UHPC were experimentally studied for 1080 d.The development trend of long-term properties of UHPC,the influence of shrinkage compensation materials on long-term properties of UHPC and the applicability of different specification formulas on shrinkage and creep of UHPC in indoor environment were clarified.The suitable specification formula for predicting shrinkage and creep of ordinary UHPC was determined.A suitable model for predicting the shrinkage and creep of expansive UHPC was proposed.DBJ43/T325-2017 model can predict the shrinkage and creep of ordinary UHPC precisely at indoor environment after standard curing for 7 days.Considering the shrinkage and creep influence coefficient of shrinkage compensating composites,the modified DBJ43/T325-2017 model can also predict the shrinkage and creep of expansive UHPC precisely at indoor environment after standard curing for 7days(4)The material hydration heat of expansive UHPC is analyzed theoretically,the calculation model of hydration heat of expansive UHPC was established.The hydration heat of expansive UHPC with different fly ash content and temperature was tested.The calculation model of material hydration heat is verified and modified based on the test results of material hydration heat,and the mix proportion of low heat expansive UHPC is determined.The hydration heat of mass structures with low heat expansive UHPC was tested respectively,test results showed that the maximum temperature of ordinary and low heat expansive UHPC were 27.2 and 15.5 ℃respectively.The hydration heat calculation model was used in finite element analysis to verify the applicability of the calculation model in the finite element analysis of hydration heat.The effect of component size was also analyzed.(5)Application of low heat expansive UHPC in steel-concrete joint.Based on the steel-concrete joint filled with low heat expansive UHPC in Wuxue Yangtze River Highway Bridge,the mechanical properties and hydration heat of low heat expansive UHPC was tested to evaluate its application of low heat expansive UHPC in practical engineering.The finite element analysis model was established by using Midas civil,a special finite element analysis program for bridge structure.The structural stress of steel-concrete joint with different pouring materials(C55 concrete,ordinary UHPC and expansive UHPC)in construction stage and operation stage was analyzed.And the influence of shrinkage and creep characteristics of concrete on the stress of steel-concrete joint was discussed.The effect of creep on the longitudinal stress of steel-concrete joint was about 10% of the shrinkage.Therefore,the shrinkage characteristic of pouring material is more critical than the creep characteristic.It indicated that expansive UHPC is suitable for steel-concrete joint.