Subject Area
Mechanical Engineering
Abstract
Concrete stands as the most extensively used construction material in both military and civil structural applications, attributed to its excellent durability and high impact resistance. Over the years, high-strength concrete (HSC) has evolved to withstand higher compressive strengths, making it a viable substitute for conventional concrete (CC) in many applications. Hence, HSC is commonly used for structures and structural elements that are exposed to extreme loading conditions such as hurricanes, wave impacts, ship-bridge collisions, explosions, earthquake loads, etc. The mechanical response of concrete subjected to dynamic loading conditions differs dramatically from its response under quasi-static conditions due to its inherent strain rate sensitivity. Furthermore, external loading leads to the initiation and accumulation of damage inside the concrete structure elements at microscopic and macroscopic levels. The accumulated damage results in the degradation of the material's physical and mechanical properties. Continuum damage mechanics express the transition of a material from an intact to a damaged state via a single scalar damage variable, D, which ranges from 0 for the intact/undamaged to 1 for the completely damaged/fractured material. Consequently, continuum damage models have been extensively utilized to predict the mechanical response and damage evolution of concrete under dynamic loading conditions. The validation of these models requires an accurate characterization of the mechanical properties of concrete materials under a wide range of strain rates. Nonetheless, the current dataset on the mechanical properties of HSC under dynamic loading circumstances has three primary gaps that are compromising the accuracy of the finite element (FE) simulation results.
These gaps can be summarized as follows: (a) Residual mechanical properties: Concrete experiences a degradation of its residual mechanical properties when exposed to stress levels higher than the onset of plastic strain. However, the majority of strength-based plasticity damage models implemented in numerical simulations tend to overestimate the loss of residual strength at the early stages of damage without accurately capturing the corresponding reduction in stiffness. Accordingly, the reported FE simulations do not replicate the actual behavior of concrete as reported from laboratory experiments. The inaccuracy in modeling the behavior of concrete, particularly under dynamic compressive loading conditions, is due to the scant dataset that accurately quantifies the concrete's residual mechanical properties under such conditions. Therefore, most of the damage models that have been developed to predict the dynamic damage evolution of concrete materials are not based on the actual physical mechanisms that drive this behavior but rather are fitted to a limited set of experimental data. (b) Constitutive damage parameters and crack morphology: The primary damage mechanism in concrete is the development and propagation of macrocrack networks, which impact the residual mechanical properties of the bulk material. Therefore, it is necessary to extend the current framework of damage models to establish a correlation between the measurements of damage-induced cracks and their impact on the residual mechanical properties as manifested by the constitutive damage parameters. Nevertheless, the majority of studies concerning this matter have predominantly focused on ultra-high-performance concrete (UHPC) and conventional concrete (CC), with HSC receiving comparatively less attention. (c) Dynamic tensile properties: The urgent need for dynamic tensile properties of concrete materials to accurately calibrate constitutive material models is met with significant challenges in developing an experimental technique for dynamic direct tensile testing under valid testing conditions. These challenges primarily stem from two factors: firstly, the necessity to design an appropriate clamping technique to secure the specimen without any slippage during the test, and secondly, the inherent characteristics of these materials such as high heterogeneity, pronounced brittleness, and asymmetrical mechanical responses in compression and tension. To date, there is no universally accepted experimental technique to accurately quantify the dynamic tensile response of concrete materials under direct tension. The aim of the present study is to fill the research gaps identified earlier.
The residual mechanical properties of HSC subjected to dynamic compressive loading were quantified using a modified Kolsky compression bar coupled with a momentum trapping technique. This technique was used to generate a precisely controlled pre/post-peak intermittent loading to introduce distinctive states of damage inside HSC specimens without complete fragmentation. Subsequently, the recovered specimens were subjected to a second monotonic loading using the Kolsky compression bar to evaluate their residual mechanical properties (e.g., residual stiffness and residual strength).
The X-ray micro-CT scanning technique was utilized to quantify the relationships between the damage-induced crack networks and the associated degradation of residual mechanical properties in HSC. A micro-CT scanner was used to examine the HSC specimens before and after loading with the Kolsky compression bar. Hence, the damage-induced crack measurements, such as crack volume and crack surface area, were extracted and correlated to the constitutive damage parameters. Moreover, a systematic analysis of the micro-CT scans was conducted to develop a better understanding of the inherent interaction between naturally preexisting voids inside the bulk material and the damage-induced crack network when subjected to dynamic compressive loading.
The present study also introduced an improved experimental technique for the testing of cementitious materials under dynamic direct tensile loading on a Kolsky tension bar. This technique involves using a pair of aluminum adapters attached to the bars, with the concrete specimen secured to these adapters by epoxy adhesive. The proposed technique presents challenges caused by the inertia forces from the mass of adapters as well as the overestimation of the specimen strain due to the deformation of adapters. These challenges were addressed by modifications to the conventional approach of data post-processing in Kolsky bar experiments. In addition, pulse shaping techniques were utilized to develop an optimal loading wave, thereby enabling the attainment of valid testing conditions. The proposed technique was evaluated through the testing of steel fiber reinforced concrete.
Degree Date
Spring 2024
Document Type
Dissertation
Degree Name
Ph.D.
Department
Mechanical Engineering
Advisor
Dr. Xu Nie
Number of Pages
156
Format
Creative Commons License
This work is licensed under a Creative Commons Attribution-Noncommercial 4.0 License
Recommended Citation
Abbas, Mohamed, "The Dynamic Response and Damage Evolution of High-Strength Concrete" (2024). Mechanical Engineering Research Theses and Dissertations. 58.
https://scholar.smu.edu/engineering_mechanical_etds/58
