Design and Key Performance of Eco-nano Ultra-high Strength and Ultra-high Ductility Cementitious Composites

Author:Lei Dong Yi

Supervisor:guo li ping viktor mechtcherine


Degree Year:2019





In order to achieve the "The Belt and Road" strategy and national security strategy,some buildings with an ultra-high compressive strength(greater than 150 MPa)are necessary.In addition,subjecting to the tensile stress caused by high-intensity earthquakes,the shooting of precision weapons,high-intensity explosion shocks,and high blow-up pressure,the structural design for high safety,high energy consumption and high durability requires that the concrete components within the buildings need to have the special function of "stretching without brittle failure".Therefore,it is urgent to design and develop a concrete with an ultra-high compressive strength and an ultra-high ductility,this is a frontier topic in the concrete field.Although the compressive strength of ultra high performance concrete(UHPC)can exceed 150 MPa and has a high bending toughness,UHPC still exhibits the single-crack strain softening failure under an uniaxial tensile stress,its ultimate tensile strain is hard to exceed 0.5%.The high ductility cementitious composites(ECC)have an ultimate tensile strain of 3% to 7%,but its compressive strength is still low(mostly below 100 MPa).The UHPC and ECC are unable to meet the performance requirements for both ultra-high compressive strength,ultr-high ductility(multi-cracking)and the pseudo strain hardening characteristics.Here,after modification for silica fume with aminosilane,the silica fume nanoparticles with a significantly improved dispersion were used as the reinforcing component of the mortar matrix.Based on the theoretical analysis for the mesh size formed by the randomly distributed fibers in matrix,the principle on particle size selection for fine aggregates was proposed.Then,based on the particle dense packing theory,and drawing on the design theory of ECC,the polyethylene(PE)fiber with a high strength and a high elastic modulus was used as the energy-consuming component,thereby,an ultra-high strength and ultra-high ductility cementitious composites(UHS-UHDCC)with a compressive strength exceeding 160 MPa and a tensile ductility more than 6% was developed,it meets the requirements of the aforementioned buildings.The mechanical and deformation properties of UHS-UHDCC were studied.The connection between the microscopic and macroscopic properties was analyzed.The mechanism on the pseudo strain hardening behavior of UHS-UHDCC was revealed and its micromechanical design theory was established.The shrinkage performance and the key durability such as frost resistance,carbonation resistance,impermeability,fatigue were studied.The aminosilane molecules were grafted onto the silica fume particles by the covalent grafting reaction.When the dosage of aminosilane was 4×10-5 mol/g(by the ratio of molar mass of aminosilane to mass of silica fume),the surface potential of silica fume rised from-21 m V to +1m V,the adsorption amount towards the superplasticizer molecules rised from 4mg/g to 8mg/g,thus the dispersion of silica fume was significantly improved.The rheology of the modified silica fume-cement slurry was better than that of the unmodified silica fume-cement slurry.The modified silica fume not only accelerated the acceleration period of cement hydration,but also increased the cumulative heat.At the hydration age of 28 d,the compressive strength of the modified silica fume-cement sample was 114.05% of that of unmodified silica fume-cement sample,the flexural strength was 115.41% of that of unmodified silica fume-cement sample.When the dosage of aminosilane was 8×10-5 mol/g,the adsorption amount towards aminosilane molecules reached the saturation,at this time,the dispersion of silica fume and the mechanical properties of silica fume-cement sample were optimal.Based on the theoretical analysis for the mesh size formed by the randomly distributed fibers in matrix,the river sand with the size less than 500μm and the average size of 240μm was as the aggregate.The modified silica fume was used as the mineral admixture.The modified Andreasen equation was used as the objective function to optimize the particle gradation.Based on the particle dense packing theory,and drawing on the theory designing ECC,the UHS-UHDCCs with a high particle packing degree were designed,the deformation and mechanical properties of UHS-UHDCC with different mix proportions were studied.The results calculated from the least squares method and the gray correlation method all showed that when the silica fume dosage was 40%,the dry matrix mixture had the highest particle packing degree.With the increasing of silica fume dosage or the decreasing of water-binder ratio,the compressive strength and the flexural strength of UHS-UHDCC firstly increased and then decreased.When the water-binder ratio was 0.17,the compressive strength was the highest,when the water-binder ratio was 0.19,the flexural strength was the largest.Moreover,when the water-binder ratio was 0.19,the compressive strength,the uniaxial tensile ductility,the average crack of UHS-UHDCC was 163 MPa,more than 6%,85μm,respectively,and it showed a high residual flexural strength,high residual compressive strength,good compressive deformation capacity and high compression toughness.The connection between microscopic properties of fiber,matrix,fiber-matrix interface and the macroscopic tensile properties was analyzed.The strain hardening behavior(PSH)design framework of UHS-UHDCC was proposed.Within a certain range,increasing the plastic viscosity of slurry can improve the fiber dispersion,however,once the viscosity was too high,the dispersion reduced.Compared to the fluorescence analysis,the backscattering analysis greatly improved the accuracy of the fiber dispersion test,and its experiment procedure was simpler and the result was more accurate.With the increasing of silica fume dosage or the decreasing of water-binder ratio,the porosity of UHS-UHDCC reduced and the pore diameter refined,the nanocompressive elastic modulus increased in fiber-matrix Interface transition zone(ITZ)and the difference of elastic modulus between ITZ and matrix got smaller and smaller.Within a certain range,with the increasing of the fiber dispersion degree,the tensile ductility increased.The realization of saturated multi-crack strain hardening behavior needs to comprehensively tailor the fiber dispersion degree and the interfacial bonding strength.Based on the theories of micromechanics,fracture mechanics and statistics,analyzing the relationship between the single fiber pullout stress and the displacement,the relationship between the single-crack stress and the opening width,and the synergistic effect between the fiber,matrix and the fiber-matrix interface on tensile strain hardening properties,thus,the micromechanical design theory on UHS-UHDCC was proposed.The debonding behavior of hydrophobic PE fiber was defined as "physical debonding".For UHS-UHDCC,the slip hardening coefficient(β)increased with the increasing of the fiber embedment angle.Introducing the "slip hardening increasing coefficient-ω" corrected the traditional understanding that the ECC model regarded β as a fixed constant.Compared with the ECC model,the fiber bridging stress-crack opening(σ-δ)curve calculated from the UHS-UHDCC model more closely matched with the test result,and the strength and energy criterias of ECC theory were verified.The shrinkage performance and the key durability such as frost resistance,carbonation resistance,impermeability,fatigue were studied.Increasing the freeze-thaw cycles,the ultimate tensile stress of UHS-UHDCC decreased and its ductility increased.Until 400 freeze-thaw cycles,UHS-UHDCC still did not appear mass loss,its frost resistance was excellent.The effect of freeze-thaw cycle on tensile performance(29)the effect on flexural performance(29)the effect on compressive performance.The drying shrinkage strain increased rapidly in the early 20 days of shrinkage age,then the increasing rate decreased.The drying shrinkage of UHS-UHDCC was much smaller than that of ECC.The RCM result was similar with the electric flux result,that is,Cl-had a very low migration rate in UHS-UHDCC,UH-UHDCC exhibited an excellent chloride-penetration resistance.The carbonization of 28 d had no significant influence on the deformation and mechanical performances of UHS-UHDCC.When the fatigue stress was 0.66 times of the ultimate stress,after 2×106 sinusoidal stretching or sinusoidal bending loading cycles,the tensile strain of UHS-UHDCC was 1.18%,the mid-span deflection was 1.68 mm,they accounted for a very small proportion in own total deformation.