رسائل دكتوراة
English
2019
Experimental and Numerical Modeling of the Behavior of Ultra High-Performance Concrete Slabs Under Impact Loads
Shamshad Ahmad, محمد عمر خان
كلية تصاميم البيئة-جامعة الملك فهد للبترول والمعادن · السعودية
Ultra-High-Performance Concrete (UHPC) has recently evolved as a revolutionary structural material with potential of applications in special conditions. It possesses excellent mechanical and durability properties owing to its highly dense microstructure achieved through the proper selection of its ingredients (high amount of cement and ultra-fine mineral admixtures, fine quartz sand, steel fibers, and no coarse aggregate), optimum mixture design, special mixing and curing practices. The stress-strain behavior of UHPC is characterized by strain hardening in tension, accompanied by multiple cracking, which allows it to undergo high strain before failure resulting into its very high resistance against impact and crack propagation (i.e., fracture toughness). Furthermore, the capability of UHPC to absorb higher energy during impact loading due to greater fracture toughness makes it a promising smart material for the construction of the protective engineering structures subjected to impact and dynamic loadings for safety against explosion, tornado-generated projectiles and accidental loads. The UHPC can be used for construction of structural elements in nuclear power plants to offer resistance against high intensity accidental loads as well as to provide resistance against nuclear radiations, if the dry density of UHPC is increased to fall under the category of heavy-weight ultra-high-performance concrete (HWUHPC) by partially replacing the sand by a heavy-weight powder material.In this study, firstly, three mixtures of UHPC containing 10% hematite powder, by partially replacing sand with hematite powder having very high fineness (thirty times higher than cement) and density (two times heavier than sand), and three different dosages of fiber (2, 4 and 6%) were considered to examine the effect of fiber content on the mechanical properties. As expected, higher fiber content significantly improved the mechanical properties. An optimum dosage of fiber was taken as 6%, by mass of concrete, based on the test results showing variations of the mechanical properties with fiber content.After the completion of first phase, an attempt was made to first develop HWUHPC mixtures containing 0, 10, 20, 30, 40, and 50% of hematite powder (partially replacing the sand) and 6% fiber content (taken as an optimum dosage) and then to evaluate their performance in terms of mechanical properties and radiation shielding. The mechanical properties of the HWUHPC mixtures studied in the present work included compressive strength, modulus of elasticity, flexural strength, fracture toughness and resistance against impact loading. All the HWUHPC mixtures satisfied the flow requirements and their dry densities varied in the range of about 2600 kg/m3 to 2900 kg/m3 meeting the requirements of heavy-weight concrete mixtures. Although the HWUHPC mixtures had different dry densities, their mechanical properties did not vary significantly and showed no adverse effect of the replacement of sand by the hematite powder. This is due to the role of hematite powder in making the microstructure of the mixtures highly dense owing to its very high fineness and its non-interference with the hydration of the cementitious materials, as evident from the analysis of SEM, EDS and XRD test results obtained by testing the mixtures of HWUHPC.In order to study the resistance of the developed mixtures against impact loading, slab specimens (750 mm × 750 mm × 50 mm) were cast for all nine mixtures (1 conventional high-performance concrete with aggregates, 1 conventional UHPC, and 7 HWUHPCs). The slab specimens were tested under low-velocity impact loading using tailor-made drop-weight impact test set-up. The analysis of the experimental data indicates that the impact resistance of conventional high-performance concrete was far less as compared to UHPC and HWUHPC mixtures. Moreover, the impact resistance of the mixtures increased exponentially with increasing the fiber content. However, the impact resistance did not vary significantly with increase in hematite powder content and showed no adverse effect of the replacement of sand by the hematite powder. Utilizing the experimental data generated under the present study, the correlations between the applied impact energy and resulting deflection in slabs were obtained through regression analysis. Furthermore, finite element modeling (FEM) was performed in ABAQUS® using concrete damaged plasticity model, to simulate the behavior of slabs under low-velocity impact loading. The FEM results were found to be in good agreement with their corresponding experimentally generated results.The performance of the developed HWUHPC mixtures against nuclear radiation was evaluated by exposing the concrete specimens to gamma radiation. As expected, it was observed through the analysis of the experimental data that the radiation shielding increased with increase in the dry density of the HWUHPC mixtures. However, an important observation made through this study is worth noting. That is, at a same dry density, the radiation shielding of the HWUHPC mixtures was found to be around 40% higher as compared to that of the heavy-weight conventional concrete, indicating that the denseness of the microstructure also improves the radiation shielding besides the dry density of concrete, which has not been reported yet in context to the HWUHPC. Empirical correlations between the dry density of the HWUHPC and different indicators of the radiation shielding were obtained through regression analysis utilizing the experimental data.The outcomes of the present study would be very useful for optimal design of the mixtures of HWUHPC for meeting the structural as well as radiation shielding requirements of the special structures, for example, for structural members of a nuclear power plant where the structural members are susceptible to both, accidental loads and nuclear radiation.