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This research develops and validates computational tools for the design and analysis of structural components and systems constructed with Ultra-High Performance Concrete (UHPC). The modeling strategy utilizes the Lattice Discrete Particle Model (LDPM) to represent UHPC material and structural member response, and extends a structural-level triaxial continuum constitutive law to account for the addition of discrete fibers. The approach is robust, general, and could be utilized by other researchers to expand the computational capability and simulate the behavior of different composite materials. The work described herein identifies the model material parameters by conducting a complete material characterization for UHPC, with and without fiber reinforcement, describing its behavior in unconfined compression, uniaxial tension, and fracture toughness. It characterizes the effect of fiber orientations, fiber-matrix interaction, and resolves the issue of multi-axial stress states on fiber pullout. The capabilities of the computational models are demonstrated by comparing the material test data that were not used in the parameter identification phase to numerical simulations to validate the models' predictive capabilities. These models offer a mechanics-based shortcut to UHPC analysis that can strategically support ongoing development of material and structural design codes and standards. / Ph. D.

Yes / Considering the continuous development of sustainable development, energy saving, and emission reduction concepts, it is very important to reduce concrete's cement content in order to improve its environmental impact. Using reactive admixture to replace part of the cement in ultra-high performance concrete (UHPC) can effectively improve the overall performance of the concrete and reduce carbon dioxide emissions (CO2), which is an important aspect of environmental protection. Here, industrial waste residue (fly ash and slag), sea sand (SS), and manufactured sand (MS) were used to produce UHPC under standard curing condition, to reduce the material cost and make the it more environmentally friendly and sustainable. The effects of water-binder ratio, contents of cementitious materials, types of sands, and content of steel fibers on the mechanical performance of UHPC under standard curing were investigated experimentally. In addition, the effects of various factors on the depth under hydraulic pressure and electric flux of UHPC, mass loss, relative dynamic modulus of elasticity, flexural, and compressive strengths of UHPC specimens after freeze-thaw cycles were conducted to evaluate the impermeability, chloride, and freeze-thaw resistance of various UHPCs produced. The obtained experimental results show that the SS-UHPC and MS-UHPC prepared by standard curing exhibit high strength, excellent impermeability, and chloride resistance. The frost resistant grade of all groups of UHPCs prepared by standard curing are greater than F500 and had excellent freeze-thaw resistance, including those produced with local tap water or artificial seawater. The investigation presented in this paper could contribute to the production of new UHPCs of low cost and environmental-friendly and accelerate the application of UHPC in engineering structures.

This thesis presents the results of an effort to quantify the implications of using ultra-high performance concrete (UHPC) for shear walls in tall buildings considering structural efficiency and environmental sustainability. The Lattice Discrete Particle Model (LDPM) was used to simulate the response to failure of concrete shear walls without web steel bar reinforcement under lateral loading and constant axial compressive loading. The structural efficiency of UHPC with simulated compressive strength of f'c = 231 MPa was compared to that of a high-performance concrete (HPC) with f'c = 51.7 MPa simulated compressive strength. UHPC shear walls were found to have equal uncracked stiffness and superior post-cracking capacity at a thickness 58% of the HPC shear wall thickness, and at 59% of the HPC shear wall weight. Next, the environmental sustainability of UHPC with compressive strength f'c = 220-240 MPa was compared to that of an HPC with compressive strength f'c = 49 MPa with a life-cycle assessment (LCA) approach, using SimaPro sustainability software. At a thickness 58% of the HPC shear wall thickness, UHPC shear walls with 0% fiber by volume were found to have an environmental impact 6% to 10% worse than that of HPC shear walls, and UHPC shear walls with 2% fiber by volume were found to have an environmental impact 47% to 58% worse than that of HPC shear walls. The results detailed herein will allow for design guidelines to be developed which take advantage of UHPC response in shear. Additionally, this work may be implemented into topology optimization frameworks that incorporate the potential improvements in structural efficiency and sustainability through using UHPC. / Master of Science

Yes / Considering the continuous development of sustainable development, energy saving, and emission reduction concepts, it is very important to reduce concrete's cement content in order to improve its environmental impact. Using a reactive admixture to replace part of the cement in ultra-high-performance concrete (UHPC) can effectively improve the overall performance of the concrete and reduce carbon dioxide emissions, which is an important aspect of environmental protection. Here, industrial waste residue (fly ash and slag), sea sand (SS), and manufactured sand (MS) were used to produce UHPC under standard curing conditions to reduce the material cost and make it more environmentally friendly and sustainable. The effects of water-binder ratio, contents of cementitious materials, types of sands, and content of steel fibers on the mechanical performance of UHPC under standard curing were investigated experimentally. In addition, evaluations of the impermeability, chloride, and freeze-thaw resistance of various UHPCs produced were conducted by investigating the effects of various factors on the depth under hydraulic pressure and electric flux of UHPC, as well as the mass loss, relative dynamic modulus of elasticity, flexural strength, and compressive strength of UHPC specimens after freeze-thaw cycles. The obtained experimental results show that the SS-UHPC and MS-UHPC prepared by standard curing exhibit high strength, excellent impermeability, and chloride resistance. The frost-resistant grade of all groups of UHPCs prepared by standard curing was greater than F500 and had excellent freeze-thaw resistance, including those produced with local tap water or artificial seawater. The investigation presented in this paper could contribute to the production of new low-cost and environmentally friendly UHPCs and accelerate the application of UHPC in engineering structures.

Yes / Super-fine stainless wires (SSWs) with micron diameter and large specific surface area can simultaneously strengthen and toughen reactive powder concrete (RPC) at low volume fraction, so SSW reinforced RPC composites have potential for developing infrastructures bearing fatigue load or with aseismic requirements. In this paper, the uniaxial compressive fatigue characteristics of such composites under high stress levels were investigated, and the modification mechanisms of SSWs to RPC were revealed through failure state and microstructure analyses. The results showed that incorporating only 0.5 vol.% SSWs into RPC enables the fatigue life and energy dissipation capacity to increase by 252.0% and 262.3%, meanwhile, the fatigue limit strength of composites at the failure probability of 50% reaches up to 76.6% of static uniaxial compressive strength, due to the improvement effect on microstructure compactness, inhibiting effect on flaw initiation, and the ability to convert single main crack into radial multiple micro cracks centered on SSWs. Furthermore, the average maximum fatigue strain and residual strain of composites are improved by 73.7% and 87.2%, respectively, which can be ascribed to the bridging, debonding and being pulled-off effect of SSWs. It can be therefore concluded that the incorporation of SSWs endows RPC with excellent fatigue performance, thus further enlarging the application of composites. / The authors would like to thank the National Science Foundation of China (51908103 and 51978127), and the China Postdoctoral Science Foundation (2019M651116) for providing funding to carry out this investigation.

Yes / The development of ultra-high performance cementitious composite (UHPCC) represents a significant advancement in the field of concrete science and technology, but insufficient hydration and high autogenous shrinkage relatively increase the pores inside UHPCC, in turn, affecting the macro-performance of UHPCC. This paper, initially, optimized the pore structure of UHPCC using different types and dimensions of nanofillers. Subsequently, the pore structure characteristics of nano-modified UHPCC were investigated by the mercury intrusion porosimeter method and fractal theory. Finally, the fluid permeability of nano-modified UHPCC was estimated by applying the Katz-Thompson equation. Experimental results showed that all incorporated nanofillers can refine the pore structure of UHPCC, but nanofillers with different types and dimensions have various effects on the pore structure of UHPCC. Specifically, CNTs, especially the thin-short one, can significantly reduce the porosity of UHPCC, whereas nanoparticles, especially nano-SiO2, are more conducive to refine the pore size. Among all nanofillers, nano-SiO2 has the most obvious effect on pore structure, reducing the porosity, specific pore volume and most probable pore radius of UHPCC by 31.9%, 35.1% and 40.9%, respectively. Additionally, the pore size distribution of nano-modified UHPCC ranges from 10-1nm to 105nm, and the gel pores and fine capillary pores in the range of 3-50nm account for more than 70% of the total pore content, confirming nanofillers incorporation can effectively weaken pore connectivity and induce pore distribution to concentrate at nanoscale. Fractal results indicated the provision of nanofillers reduces the structural heterogeneity of gel pores and fine capillary pores, and induces homogenization and densification of UHPCC matrix, in turn, decreasing the UHPCC fluid permeability by 15.7%-79.2%. / National Science Foundation of China (51978127, 52178188 and 51908103), the China Postdoctoral Science Foundation (2022M720648 and 2022M710973) and the Fundamental Research Funds for the Central Universities (DUT21RC(3)039).