Document Type

Thesis - Open Access

Award Date

2025

Degree Name

Master of Science (MS)

Department / School

Civil and Environmental Engineering

First Advisor

Akram Jawdhari

Abstract

Fiber-reinforced cementitious matrix (FRCM) composites have gained increasing attention for the rehabilitation and strengthening of concrete and masonry structures due to their favorable mechanical properties, compatibility with existing substrates, and ease of application. The effectiveness of any strengthening procedure depends on the quality of the bond between the external reinforcement and the substrate, ensuring efficient stress transfer and enabling the system to reach its ultimate capacity without premature debonding. While research has focused on the bond performance of FRCM systems under pure shear loading and quasi-static, low strain rate conditions, the effects of mixed-mode loading and high strain rates representative of dynamic events such as earthquakes and vehicular impacts remain insufficiently explored. Studying these gaps is essential because current bond design equations are based on pure shear assumptions and neglect peel stresses caused by curvature in beams, slabs, and walls, which can lead to premature debonding. Additionally, short duration loads like impact and seismic events are common in practice but are not considered in existing bond models, which are typically developed under quasi-static conditions. In particular, the influence of peel angle (θ) and textile reinforcement ratio (ρ) on the bond behavior under mixed-mode conditions, as well as the effects of strain rate, bond length (Lb), and reinforcement ratio on bond performance at high loading rates, requires further investigation. This study addresses these gaps through two complementary experimental programs: (1) evaluating mixed-mode bond behavior by varying θ and ρ using modified shear tests, and (2) investigating the strain rate-dependent bond performance through single-lap shear tests incorporating varying displacement rates, bond lengths, and textile reinforcement ratios. The findings contribute to a better understanding of the FRCM-concrete bond behavior under peel stresses and high strain rate load events, characterization of bond properties, and development of design procedures accounting for the effects of these loading conditions. The first study evaluates the mixed-mode bond behavior between FRCM composites and concrete substrates. An experimental program comprising 36 specimens was conducted to investigate the effects of mode mixity by varying θ between the FRCM composite and concrete surface. The influence of θ, ranging from 0° to 36.9°, and ρ, varied at three values (2.61, 3.93, and 5.23%), was examined through modified shear tests utilizing a custom-designed loading apparatus. Test results indicated that the ultimate load (Pu) generally decreased with increasing θ, with the most significant reduction observed at low to moderate peel angles. For specimens with ρ = 3.93%, Pu decreased sharply by 88% as θ increased from 0° to 11.5°. While ρ exhibited no consistent influence on Pu, it notably affected the failure mechanisms—transitioning from FRCM–concrete interface debonding at lower ρ to textile–mortar interface debonding at higher ρ, attributed to the reduction in mechanical interlock with increased textile content. An analytical methodology integrating strain gauge data, digital image correlation (DIC), and test imagery was employed to develop a database comprising 66 Pu–θ data points. Two analytical models were proposed based on linear and non-linear regression analyses of the database. The average experimental-to-analytical Pu ratios were 0.78 and 1.14 for the linear and non-linear models, respectively, demonstrating the capacity of the regression-based formulations to capture the observed bond behavior trends. The next study investigated the bond characteristics of FRCM–concrete interfaces subjected to dynamic loading conditions. In this study, 36 single-lap shear specimens, each consisting of a concrete substrate bonded to a carbon-FRCM (C-FRCM) strip, were tested to examine the effects of strain rate, ρ, and Lb on bond behavior. The experimental matrix incorporated three displacement rates (0.20, 20, and 2000 mm/min), representing quasistatic to high-rate dynamic loading, alongside three ρ values (2.61, 3.93, and 5.23%) and four Lb values (75, 150, 225, and 300 mm). Results demonstrated that increasing the strain rate generally enhanced Pu across all Lb and ρ levels. For ρ values of 2.61, 3.93, and 5.23%, the maximum increases in Pu were 139.6%, 56%, and 105.9%, respectively, when the displacement rate increased from 0.2 to 2000 mm/min. Four primary failure modes were identified: debonding at the concrete–mortar and textile–mortar interfaces, textile slippage, and textile rupture. An effective bond length of 150 mm was established based on the Pu– Lb relationship and was found to be unaffected by strain rate. A multi-linear regression analysis, considering specimens failing by debonding, was conducted to derive an analytical expression for Pu incorporating strain rate, Lb, and ρ. The regression-based model achieved an average experimental-to-analytical Pu ratio of 0.97, outperforming two comparable models from literature, which yielded ratios of 1.15 and 0.93.

Library of Congress Subject Headings

Fibrous composites.
Cement composites.
Fiber-reinforced concrete.
Strains and stresses.
Peel angle.

Publisher

South Dakota State University

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Rights Statement

In Copyright