PhD Defense by Rebecca Nylen
Ph.D. Thesis Defense AnnouncementResidual Capacity of Concrete Subjected to Impulsive Loads by Rebecca Nylen Advisor(s): Dr. Lauren Stewart (CEE) Committee Members:
Dr. Kimberly Kurtis (CEE), Dr. Laurence Jacobs (CEE), Dr. Min Zhou (ME), Dr. James Cox (SNL)
Date & Time: July 15th, 2020 at 2:00pm
This research aims to achieve a better understanding of the behavior and residual capacity of concrete materials subjected to impulsive loading conditions such as blast and impact. Previous research efforts investigating the residual capacity of concrete subjected to these loading conditions have primarily focused on the response of full-scale structural components and frames. Existing test methods to investigate the strain rate dependent properties of concrete typically apply impulsive loads in such a magnitude that the concrete is fully damaged and has no residual capacity following the test. As such, little is known about how varying levels of damage imparted by impulsive loading affects the residual strength and stiffness of plain concrete. This information is vital to ensure that the damage formulations used in concrete constitutive models are accurately representing the material properties of concrete in the range of damaged states that are relevant to structural-scale residual capacity studies. To address this issue, a dynamic compressive testing system and experimental protocol capable of damaging plain concrete cylinders under intermediate strain rates is developed. Following the impulsive loading, the residual capacity of the damaged specimen is assessed with a standard uniaxial compression test. The impulsive loading is achieved with the use of a high speed actuator, which accelerates a flyer mass to a desired velocity before the mass impacts the test fixture. The testing system is capable of producing impulsive loads in a controlled and repeatable manner to subject the specimen to single or repeated impacts.
Using the new experimental method, an experimental study was conducted on plain concrete cylinders to evaluate the residual strength and stiffness of concrete subjected to impulsive loads of varying intensity. The experiments were able to capture a clear decrease in the residual strength and stiffness for increasing levels of impulsive load. Most notably, the normalized loss of stiffness is much greater than the loss of strength at lower levels of damage. This differs from the behavior of concrete as it sustains damage through quasi-static mechanical loads, where the loss of strength versus stiffness is demonstrated to be nearly proportional. So, this trend in the impulsively damaged concrete is likely due to damage mechanisms that only are influential or present during impulsive loading. The existence of this trend is previously unknown and provides novel insight on the behavior of impulsively damaged concrete. Finally, a numerical study was conducted to evaluate the suitability of concrete constitutive models for predicting the residual capacity of concrete subjected to impulsive loads. Three models commonly used for blast and impact applications were examined: the Karagozian and Case model, the Holmquist-Johnson-Cook concrete model, and the Johnson-Holmquist ceramic model. The concrete models show a more rapid loss of strength before the material experiences any significant reduction in stiffness, which does not match the trend observed in the experiments. This points to a potential shortcoming with the damage formulation in these models, as the experimentally observed loss of stiffness is not captured. To improve the damage formulations, more experimental data is needed to fully characterize the relationship between the loss of modulus and loss of strength for impulsively damaged concrete and identify the damage mechanisms responsible.