[CCoE Notice] Dissertation Defense: Cementitious Materials with Ultra-High Damage Tolerance for Hazards Mitigation

[Grayson, Audrey A]

Ph.D. Dissertation Defense
Prakash Bhat

Cementitious Materials with Ultra-High Damage Tolerance for Hazards Mitigation

Committee members:
Dr. Mo Li (chair of the committee), Dr. Yi-Lung Mo, Dr. Kaspar J. Willam,
Dr. Gangbing Song, Dr. Jose H. Vazquez
Location: N61 Engineering Building 1
Date: December 3, 2015 Time: 9:30 a.m.


Abstract

Concrete structures, including civil infrastructure and energy infrastructure, are facing safety concerns under extreme loading events, such as earthquakes, elevated temperature, and impact loading. The low fracture resistance of concrete often results in brittle failure modes such as concrete spalling, fragmentation, loss of bond with reinforcing steel, and catastrophic structural failure. In order to improve the resistance of concrete structures to natural and manmade hazards, a material-based approach is proposed, which will fundamentally transform concrete from a brittle material to a damage-tolerant material exhibiting pseudo strain-hardening behavior and extraordinary ductility. This is achieved through a multi-scale method, which links micromechanics-based material design theory spanning nano-scale to composite material-scale, to material processing and experimental characterization, and finally to the development of new constitutive models and finite element analysis of structural behavior.

In this dissertation, we first improve and expand the theoretical framework for designing ductile strain-hardening cementitious materials, taking into account of the random distribution of flaws and fibers. Analytical modeling coupled with experimental measurements of micromechanical and statistical parameters are interwoven at different scales of this framework. Based on the theoretical framework, two categories of new fiber reinforced cementitious composite materials are successfully designed and processed: one is strain-hardening cementitious composites (SHC) featuring optimized tensile ductility, minimized width of microcracks, fracture energy two orders higher than regular concrete, and large energy dissipation capacity under cyclic loading; the other is ultra-light-weight SHCs featuring a low density 40-60% less than regular concrete, meanwhile maintaining a large tensile ductility more than 500 times that of normal concrete and compressive strength higher than 40 MPa. Both categories of SHCs exhibit ultra-high damage tolerance under static and cyclic loading, as well as elevated temperature effect. The ultra-light-weight SHC in particular allows great potential for mitigating the impacts of earthquakes on structures by simultaneously reducing seismic demand on structures while increasing structural seismic resistance. It also offers significantly improved resistance to elevated temperatures.

The new SHC materials are experimentally characterized to understand their behavior under various loading and environmental conditions. New constitutive models are developed. These constitutive models bridge the gap between material behavior and structural performance, and are incorporated into a finite element analysis program to fundamentally elucidate how material damage tolerance and lightweightness can be translated into improved seismic performance of structures.
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