PhD Oral Exam - Olusola Olajide, Civil Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Alkali silica reaction (ASR) is a widely known deterioration mechanism in concrete; sufficient moisture and temperature are crucial for initiating and sustaining the reaction. The influence of these factors on ASR-induced expansion has been previously studied. However, little is known about their influence on ASR-induced damage. This work aims to apply a multi-level assessment protocol combining microscopic and mechanical properties tools to evaluate the impact of a wide range of moisture conditions and temperatures on ASR-induced expansion and deterioration. Concrete specimens were manufactured in the laboratory incorporating aggregates displaying different levels of reactivity (i.e., moderate, high, and very-high) and containing two alkali loadings (i.e., 3.82kg/m³ and 5.25kg/m³ Na₂Oeq by mass of cement). The specimens were conditioned at five relativity humidities (i.e., 100%, 90%, 82%, 75%, and 62%) and three temperatures (i.e., 21°C, 38°C, and 60°C) and monitored for internal and external moisture, and ASR expansion over time. A time-based assessment was conducted, and upon reaching pre-defined ages (i.e., 3, 6, and 12 months), the Damage Rating Index (DRI), Stiffness Damage Test (SDT), Direct Shear, and compressive strength tests were conducted to appraise ASR-induced deterioration in the specimens. The results show that moisture inside concrete is typically around 90% RH from batching, which enables rapid ASR-induced expansion. However, lower external moisture reduces internal moisture and may induce shrinkage cracks that influence the overall damage pattern. Moreover, at elevated temperatures, ASR-induced cracks exhibit greater density but shorter length and narrower width compared to lower temperatures despite similar expansion levels. These conditions influence the microscopic and mechanical properties of ASR-affected concrete, with the DRI and SDT outcomes presenting a strong correlation to expansion. Through the multi-level assessment protocol, this study establishes that the moisture threshold required to trigger ASR is temperature- and aggregate-dependent. While the 80% RH has been widely used in the past, the findings of this study suggest that a lower range (62-75% RH) might be required for high temperatures and reactivity of the aggregates. A novel damage classification table that accounts for a wider range of exposure conditions is thus proposed, offering a more comprehensive tool for the condition assessment of ASR-affected concrete.