Research And Application Of Lightweight Ultra-High Performance Concrete With Low Shrinkage

Ultra high performance concrete（UHPC）with excellent mechanical strength can be used to improve the bearing capacity of building components,and reduce the size of components and the amount of reinforcement.But the UHPC was limited to popularize and apply in the fields of long-span bridge and super-high building structure due to its large shrinkage,steam curing and high apparent density.Aiming at the above problems,this paper based on the Nation Natural Science Foundation of China for“Preparation and microstructure formation mechanism of UHPC with light weight and low shrink”（51878003）and the Major Special Project of technological innovation of Hubei Provincial for“Development of lightweight UHPC and its application in prefabricated bridge engineering”（2018AAA001）,using the NMR,DSI,SEM-EDS、3D XRM and others modern testing technologies to study the lightweight and low shrink ultra high performance concrete（LUHPC）.In the paper,the influences of composition of cementitious materials,physical properties of lightweight aggregate,content of steel fiber and water-binder ratio on the working performance,mechanical properties,volume stability and durability of the LUHPC were studied,the high strength lightweight aggregate with high water absorption,good water storage and release abilities were developed,the formation mechanism of internal curing effect of pre-wetting spherical lightweight aggregate on the microstructure of the LUHPC were investigated,and the LUHPC with lightweight,low shrinkage,ultra-high strength and high durability has been prepared and applied in engineering.The main innovative achievements of this paper were as follows:（1）The collaborative design and preparation method of LUHPC with low density,low shrinkage,ultra-high strength and high durability was proposed.The LUHPC with apparent density≤2000 kg/m³,28 d compressive strength≥100 MPa,28d flexural strength≥15 MPa,28 d tensile strength≥7.5 MPa,90 d shrinkage≤300×10^-6,56 d electric flux≤100 C and mass loss rate after 500 freeze-thaw cycles≤1%has been prepared.The synergistic mechanism of the internal curing effect of pre-wetting spherical lightweight aggregate with high fine connected porosity and high water absorption and the compensating shrinkage of expansive agent to reduce shrinkage of the LUHPC was researched.The formation mechanism of the density arch shell structure around the lightweight aggregate with the higher strength than the matrix,similarly to tunnel lining role and the reduction compressive stress of lightweight aggregate when the concrete was under pressure was illustrated,and the preparation of the LUHPC above C100 strength grade by using the lightweight aggregate（the cylinder compressive strength was 6-7 MPa）with lower strength and the fiber crack resistance were revealed.（2）The preparation technology of lightweight aggregate with high micro connected porosity,high water absorption and medium-high strength applying into the LUHPC was developed,and the lightweight aggregate with the water absorption rate≥15%,apparent density≤1.45g·cm^-3and cylinder pressure strength≥6.5MPa has been prepared.The influence mechanism of the composition and microstructure of the lightweight aggregate on its performances was proved.The main mineral compositions of the lightweight aggregate were mullite and calcium feldspar.Among,the fine granular calcium feldspar grains composed the aggregate skeleton structure could induce the transgranular fracture occurring,the short columnar high-strength mullite grains could hinder crack propagation,which made the light aggregate with high water absorption and mechanical strength.The Ca²⁺,Mg²⁺and other ions increased the viscosity of high temperature liquid phases during the firing process,it hindered the migration,discharge and mutual aggregation of the pores to endow the fine connected pores at 0.01-100nm size range in lightweight aggregate was more than 50%of the total porosity.As a result,the lightweight aggregate had high water absorption rate,stable water storage during LUHPC mixing process and good internal curing effect to reduce shrinkage during the hardening process.（3）The internal curing effect of the pre-wetting spherical lightweight aggregate with high micro connected porosity and water absorption and the formation mechanism of the dense high-strength arched shell structure around the lightweight aggregate are revealed.The pre-wetting spherical lightweight aggregate with high micro connected porosity and water absorption had stable water storage capacity in the capillary action during the mixing stage of low water-binder ratio cementing slurry,and consistently and slowly release the absorbed water into the surrounding slurry by the coupling effect of osmotic pressure and humidity difference during the hydration hardening stage.It slowed the decrease trend of internal humidity in the cement based materias,and increase the degree of hydration,the polymerization degree and average molecular chain length of the hydrated product C-S-H gel of the cementing materials around the lightweight aggregate.Furthermore,the arch shell with higher mechanical properties than that of the cement matrix was formed around the lightweight aggregate.It dispersed compressive stress of lightweight aggregate when the concrete was under pressure,and which was beneficial to improve the volume stability,mechanical properties and the ability of resisting harmful ions erosion of the LUHPC.（4）LUHPC has excellent durability,and its performance indexes such as freezing and thawing resistance,carbonization resistance and chloride ion penetration resistance was close to the ordinary UHPC under the same strength grade.LUHPC used for steel bridge deck pavement could reduce the static load of the bridge,and effectively solve the problems of cracking,sliding and enveloping for the pavement layer of steel bridge deck.The research results have been applied into the steel bridge deck pavement engineering and obtained good economic benefits,which has the value of further research and application.