Insights into Alkali-Silica Reaction and Delayed Ettringite Formation through Advanced Characterization Techniques - Mehdi Rashidi

Where: 
Mason Building, Room 2119
When: 
Friday, December 8, 2017 - 09:00

 

Abstract:

Alkali-silica reaction (ASR) and delayed ettringite formation (DEF) are expansive reactions, which can damage concrete structures. However, for both ASR and DEF, the relationships between constituent materials, microscale damage propagation, and bulk expansion is not well understood. To address these knowledge gaps, this study quantifies ASR and DEF-induced damage at the microscale by nonlinear impact resonance acoustic spectroscopy (NIRAS) and when augmented with data from other standard and advanced materials characterization approaches, provides a basis for the new understanding of the factors influencing the extent and rate of damage by these reactions.This dissertation makes three main contributions. First, the influence of ASR gel composition on its structure and the potential for expansion is explored through the characterization of lab-produced samples by small-angle neutron scattering, 1H nuclear magnetic resonance relaxometry, and rheological measurements. Relying upon that improved understanding of the effects of gel composition along with an understand physics of nonlinear acoustic measurements, in the second part, a hypothesis is presented for interpreting the relationship between measured expansion and temporal material nonlinearity. In the third part, a similar approach is used to explore the relationship between compositional and environmental factors, and microscale damage and expansion derived from DEF.Results show that ASR samples with low silicate content are mainly made of monomeric silicate anions. These samples exhibit low water-binding ability and low viscosity. However, as the silicate content increases, the formation of dense mass fractal agglomerates is favored. The agglomerates cause a substantial increase in water-binding ability and the viscosity of the ASR products. For ASR- and DEF-affected mortars, despite the increase in expansion with exposure period, their temporal acoustic nonlinearity shows an increasing and later a decreasing trend. It is hypothesized that the increase in the nonlinearity of ASR-affected samples despite their low expansion level (especially in the accelerated mortar bar test) is mainly due to the dissolution of aggregates at the aggregate-paste interface and the transport of low viscosity gel to the surrounding porosities. However, as reaction develops and the expansivity and viscosity of gel increases, the two mechanisms of increasing in nonlinearity due to the creation of microcracks and the decrease due to the mechanisms such as increased gel pressure compete, and nonlinearity shows variation. At later ages, as the dominating mechanism is the filling of weak interfaces with reaction products, nonlinearity decreases. For the DEF-affected mortars, similar interpretation is used, except the reaction product (i.e., gel in ASR) is ettringite, and the effect of the dissolution of aggregates is most likely minimal. Overall, this study uses an interdisciplinary approach to improve the understanding of the degradation mechanisms during ASR and DEF.

Advisors:

Dr. Kimberly Kurtis and Dr. Laurence Jacobs

Committee:

Dr. Jin-Yeon Kim, Dr. Yuanzhi Tang (EAS), and Dr. Reza Zoughi (ECE-MS&T)