Impact of matrix modification on concrete fracture: Insights from mechanical testing and in-situ high speed imaging

The fracture performance of concrete is influenced by both the internal architecture and binder composition. While there have been considerable studies focusing on the role of binder modification, the role of aggregates alone has not received significant attention. At the same time, the widespread adoption of Supplementary Cementitious Materials (SCMs) introduces complexities not just in the microstructure but also on the mechanical performance. This raises questions about the continued validity of the existing empirical relations that are widely used in structural design. Hence, this study presents a two-pronged investigation combining aggregate packing optimization and binder variation in concrete aimed at addressing these two intertwined challenges. Firstly, the impact of optimizing aggregate packing on fracture behavior of concrete, while keeping the binder phase constant has been evaluated. Eight unique concrete mixes were developed with systematically varied packing efficiencies (86% to 94%) using a continuous packing model. The fracture response was evaluated at 3 and 7 days of hydration using the Two-Parameter Fracture Model (TPFM) in combination with an in-situ cross-sectional high-speed Digital Image Correlation (DIC). We find that improving the aggregate packing efficiency from 86% to 94%, with a constant binder ratio, increased fracture energy (GF) by 66.7% and 72.5% at 3 and 7 days of hydration, respectively. Similarly, the critical stress intensity factor (KIC) and Critical Crack Tip Opening Displacement (CTODc) increased by 80.1% and 155.5% (at 3 days), and by 57.7% and 61.6% (at 7 days), respectively. These findings highlight the pivotal role of optimizing granular skeleton in enhancing fracture resistance, independent of binder chemistry. Next, the effect of incorporating four SCMs (fly ash, slag, silica fume, and metakaolin) at multiple replacement levels on the validity of the empirical Modulus of Rupture (MOR) versus f’c relationship is explored. Flexural and compressive tests were conducted on binary concrete mixes incorporating SCMs across multiple replacement levels at 1, 3, 7, and 28 days. The results reveal that the relation between MOR and f’c dramatically changes based on the SCM used in the matrix. Specifically, fly ash and slag mixes consistently exhibited steeper trends with fr = 5.13 (f’c)0.58, and fr = 12.25 (f’c)0.47, respectively compared to the empirical relation of fr = 7.5 (f’c)0.5 proposed in ACI 318-19, while silica fume and metakaolin yielded more scattered behavior. The power law exponents, slope, and intercept exhibited distinct variation when a regression model was fitted onto the experimental data. We observe that fly ash and slag systems aligned reasonably well with the modified formulation proposed, yielding an R2 value of 0.87, and 0.82, respectively. In contrast, the R2 value obtained with the proposed formulation for silica fume, and metakaolin was 0.47, and 0.63, respectively. These findings underscore the limitations of the current empirical relationship in representing the flexural performance of the binary mixes incorporating silica fume and metakaolin as SCMs. Moreover, with the recent advancements in Limestone Calcined Clay Cement (LC3) mixes, relying heavily on metakaolin, a more comprehensive understanding of the mechanical response is required for a widespread adoption. Hence, a unified empirical model may not be suitable for all types of emerging binders and a recalibration of the prediction model is recommended as the next step.

Name thesis:degree_name

M.S.

Level thesis:degree_level

Thesis

Discipline thesis:degree_discipline

Civil Engineering

Grantor

University of Illinois Urbana-Champaign

Year dc:date

2025

Author dc:creator

• Kanel, Nischal

Contributors dc:contributor

• Garg, Nishant

• Concrete Fracture
• Aggregate Packing
• Supplementary Cementitious Materials
• Mechanical Strength
• Fracture Process Zone

Statement dc:rights

• © 2025 Nischal Kanel

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/129785

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F129785