Effects of elevated temperature exposure on cement-based composite materials

Concrete structures are considered to have higher degree of thermal and fire resistance than steel structures due to their non-combustible nature and low thermal conductivity. Despite concrete structures having high degree of thermal resistance, there is loss in mechanical properties and durability when they are exposed to extremely high temperatures. This research aims to improve the understanding of complex mechanisms that drive thermal degradation of cement-based composite materials. The majority of this dissertation focuses on understanding the degradation that occurs at temperatures higher than 300 °C as this temperature was reported to be critical based on previous research. Morphological, mechanical, and chemo-mechanical characterizations of cement paste samples are conducted to study changes in micro and nanostruture due to the decomposition of hydration products that could finally result in the global loss of mechanical properties after exposure to high temperatures. Specifically, relatively new advances in nanotechnology are adopted in this research, such as atomic force microscopy (AFM), static and dynamic nanoindentation, and coupled nanoindentation/scanning electron microscopy with energy dispersive x-ray analysis (SEM-EDS). Thermal degradation of calcium silicate hydrate (C-S-H) is assessed based on AFM image analysis and a statistical analysis of static and dynamic nanoindentation. Furthermore, the application of combined nanoindentation/SEM-EDS successfully overcomes the difficulty of identifying phases with statistical deconvolution based solely on mechanical data. From a combination of experimental results and existing literature, new degradation processes driving the loss of mechanical properties of cement pastes are proposed. The development of microcracks at the interface between unhydrated cement particles and paste matrix, a change in C-S-H nanostructure, and shrinkage of C-S-H are considered as dominant factors that cause the thermal degradation of cement pastes. This research also suggests a mitigation strategy of thermal and chemical degradation in cement-based composite materials. A detailed, experimental characterization is performed to study the effects of incorporating small dosages of nanosilica on the degradation of cement pastes exposed to various heating and cooling regimes and accelerated carbonation. Carbonation proves to be the critical cause of damage in cement paste after high temperature exposure. It is confirmed that increased thermal and chemical stability of hydration products against high temperature exposure and carbonation by incorporating nanosilica.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Civil Engineering

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2015

Author dc:creator

• Lim, Seungmin

Contributors dc:contributor

• Mondal, Paramita
• Struble, Leslie J.
• Popovics, John S.
• Jasiuk, Iwona

• Cement paste
• High temperature exposure
• Nanoindentation
• Nanosilica
• Carbonation

Statement dc:rights

• Copyright 2015 Seungmin Lim

Language dc:language

en

Handle dc:identifier

http://hdl.handle.net/2142/78451

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/78451