| The 21 st century will be the century in which human beings will exploit the undergroundspace. The development of river-crossing tunnel, sea-crossing tunnel and urban subway hasbeen greatly accelerated. In their transportation tunnels, quit a few tunnels will be shieldtunnels which are constructed by shield method. Reinforced concrete segment is commonlya precast member used for shield tunnel engineering. Reinforced concrete segment is themain body of structure, waterproof and impermeability in river-crossing or sea-crossingshield tunnels. Furthermore, it is long-term exposed to corrosive environments of deleteriousmedium, such as chloride ion and sulfate ion. Hence, it is required that the performance ofreinforced concrete segment should be quite well, especially in durability.Funded by the Hi-tech Research and Development Program of China (863 Program) No.2005AA332010 named Material & Structure Design of High Impermeablity LongService-life Large Dimension Segment of Shield tunnel and its Engineering Application,based on the first tunnel to cross the Yangtze River named Wuhan Yangtze River TunnelEngineering, and surrounded the durability problem of reinforced concrete segment, thedesign, preparation and properties of functionally gradient concrete segment (abbr. FGCS)are presented in the paper.Based on the study of material and structure design, durability,shrinkage, interface property, preparation process and microstructure of FGCS, the relationamong material, structure, property and process of FGCS is discovered. And the keytechnologies of material and structure design, preparation process, performance detection,durability assessment and engineering applying of FGCS are established. The main researchwork and compliments are listed as follow.1. The design theoretical system of FGCS is proposed. The design concept(performance-cost ratio of the whole service-life cycle)and design theories (functional andstructural integration, long service-life and interface strengthening) of FGCS are put forward.In addition, the design principles (strength, durability, volume stability, multi-function andpreparation process) and design method of FGCS are established. Based on the abovementioned design theoretical system, the theoretical foundation for functionally gradientdesign of concrete used in underground engineerings is provided.2. The functional and structural integrated design of FGCS is accomplished. The designthought of gradient function is introduced to the structure design of reinforced concretesegment. Two design schemes of FGCS are proposed, one is the design scheme of HPC highimpermeability cover, the other is the design scheme of MIF high impermeability cover,which have high compact and waterproof outer-layer, high impermeability and corrosion-resistance cover, high strength and performance structural-layer andfire-precaution and blast-resistance inner-layer.3. Meso-interfacial transition zone-free cement-based materials (abbr. MIF) is developed.To cancel coarse aggregate and fine aggregate of traditional cement-based materials, tointroduce super-fine sand and high active supplementary cementitious materials, and toblend some ingredients modified property such as shrinkage reducing ingredient,anti-cracking ingredient, hydrophobic ingredient, etc, MIF is developed, in which interfacialtransition zone between aggregate and cement paste is reduced or eliminated. MIF is usedfor concrete cover in underground engineering structure. Meanwhile, design theory of MIF isput forward, mechanical property, impermeability, sulfate attack resistance and shrinkage ofMIF is investigated, and microstructure of MIF is analyzed through SEM-EDXA, XRD,TG-DTG, MIP and microhardness.①The microhardness of interfacial transition zone in MIF is obviously increased in 10 to 30μm distance apart from the surface of aggregate, and its value exceeds 395 MPa, while that ofordinary concrete is only 150 to 250 MPa. The main hydration product of interfacial transitionzone in MIF is CSH gel, but few is Ca(OH)2 crystal. Besides, the orientation of Ca(OH)2 crystalisn't very distinct. In comparison with ordinary concrete whose thickness of interfacial transitionzone is 60 to 100μm, the thickness of interfacial transition zone of MIF is lower than 30μm.Penetration paths to corrosive medium are effectively interdicted in MIF.②MIF has several key technical indexes: compressive strength is more than 60 MPa at theage of 28 days, chloride diffusion coefficient is lower than 0.8×10-13m2/s, Conductive chargefor 6 hours is lower than 300 coulombs, impermeability grade can be raised up to S40.4. Interface property and microstructure of FGCS is Systematically investigated. Interfacemechanical property, transport property, shrinkage of FGCS is investigated by means ofsplitting tensile test, natural diffusion method, accelerating diffusion method and ANSYSsimulation. Meanwhile, Interface microstructure of FGCS, such as interface hydrationproduct and its distribution, pore structure and its characteristic, and interface bond status,etc, is analyzed through SEM-EDXA, MIP and microhardness. In addition, interfacestructure model is discussed.①When two functional layers of functionally gradient concrete were cast, respectively,interface bond strength between two functional layers was increased by 10% to 35% bymeans of imprinting process as-compared to the control without imprinting process.Imprinting process can result in the effect of interface strengthening, and resolve the problemof interface bond strength decreasing.②In comparison with the single concrete used for high strength structural-layer, chloridediffusion coefficient of functionally gradient concrete which is made up of between concreteused for high impermeability cover and concrete used for high strength structural-layer, isobviously decreased. The apparent chloride diffusion coefficient was increased by 25% to 50% by means of natural diffusion method, the chloride diffusion coefficient was decreased by one totwo orders of magnitude by means of rapid chloride diffusivity test (NEL), and the conductivecharge for 6 hours was lower than 400 coulombs by means of the rapid chloride permeability testmethod as designated in ASTM C1202. Thus, the impermeability of functionally gradientconcrete is obviously improved, especially, the ability to resist chloride ion penetration.③To take the FGCS with design scheme of MIF high impermeability cover as anexample, the maximum interface tensile stress due to shrinkage in interface bond zone wascalculated by ANSYS software (finite element analysis tool), and calculation value was lessthan test value of interface splitting tensile strength. Interface tensile stress due to shrinkagein interfacial bond zone didn't result in cracking of FGCS, and high impermeability cover ofFGCS didn't peel off. Therefore, The compatibility of interface volume deformation was quitwell, and the sliding deformation of interface layers would not generate.④Microstructure and pore structure of functionally gradient concrete are obviouslyimproved. In comparison with interfacial bond zone of functionally gradient concrete withdesign scheme of HPC high impermeability cover, the microhardness of functionallygradient concrete with design scheme of MIF high impermeability cover was higher, and thepore whose radius is over 25 nm was much less. Furthermore, there are more CSH gel andless Ca(OH)2 crystal. Besides, the orientation of Ca(OH)2 crystal is more poor.⑤According to durability and materials component gradual transition of functionallygradient concrete, interface structure model base on durability chang and interface structuremodel base on materials component change were put forward. According to thecharacteristic of interfacial transition zone between aggregate and cement paste incement-based materials, such as the thickness of interfacial transition zone and the content ofCa(OH)2 crystal and its orientation, structure models of interfacial transition zone of MIFand ordinary concrete were also put forward.5. The key technologies of production and engineering applying of FGCS are provided.The key preparation technologies (preparation process, imprinting cover board, steam-curingsystem and quality control), performance detection technology and durability assessmentmethod of FGCS are established. FGCS is produced, whose chloride diffusion coefficient is4.9×10-13m2/s. According to the multi-component diffusion equation, a service life of 280years for FGCS is predicted. Furthermore, FGCS and MIF were successfully applied toWuhan Yangtze River Tunnel Engineering.
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