Date of Award

Summer 8-20-2025

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Civil Engineering

First Committee Advisor

Roberto A. Lopez-Anido

Second Committee Member

Sunil Bhandari

Third Committee Member

William G. Davids

Abstract

The increasing demand for corrosion-resistant alternatives to steel reinforcement in concrete structures has catalyzed the development of fiber-reinforced polymer rebars. While thermoset-based (TS) glass fiber reinforced polymer (GFRP) rebars have been widely adopted and standardized, thermoplastic (TP) GFRP rebars with potential for field bendability and recyclability remains relatively underexplored. This research investigates the viability of economical TP composite rebars, glass fiber with polybutylene terephthalate (PBT), polyamide 12 (PA12), and polypropylene (PP) polymers with a focus on their physical properties, mechanical performance, durability, thermal stability, compliance with ACI and AASHTO design codes and specifications and the corresponding ASTM referenced standards D7597 and D8505.

TP composite bars were manufactured using a Continuous Forming Machine developed at the University of Maine's Advanced Structures and Composites Center. Their baseline characterization included cross sectional area, density, fiber content, void content, thermal transition, tensile properties, transverse shear strength and apparent shear strength. Among the thermoplastic systems, GF + PBT composite bars demonstrated the most promising performance, meeting the ASTM standards and performing comparably to thermoset rebars. The results indicated that while the fiber system was robust, processing improvements are needed in matrix consolidation to enhance overall performance.

Durability assessment involved immersion of bars in alkaline environments (pH 12.6–13.0) at different temperatures (room, 60 °C and 80 °C) to simulate short to long-term exposure conditions. Mechanical strength retention was assessed after 30 and 60-days conditioning. Thermoset rebars showed minimal to no degradation, maintaining structural integrity, while thermoplastic bar studied, GF+PBT exhibited significant loss of mechanical properties and degradation of matrix system, correlating with higher moisture uptake and weaker alkaline resistance. These results underscore the need for improved chemical resistance of matrix system and lower void content in thermoplastic composite bars manufacturing to enhance long-term durability in aggressive environments.

To evaluate thermomechanical performance, short beam shear tests were conducted at elevated temperatures. The semi-crystalline nature of thermoplastic matrices allowed them to retain apparent shear capacity even at elevated temperatures, unlike thermoset composites, which exhibited significant loss in strength at high temperatures. These findings highlight the need for performance-based qualification criteria over Tg based limits specified in ASTM standard for composite bar qualification.

A customized anchorage system was developed for tensile testing of thermoplastic composite bars following ASTM D7205, using reusable aluminum clamping fixtures. The use of aluminum clamps proved remarkably effective, for thermoplastic rebars with higher matrix content, enabling valid failure modes in gage region. Conical anchor reshaping, proper surface preparation, and torque-controlled longitudinal confinement were critical in avoiding slippage and stress concentrations at the grips.

Overall, the study concludes that among thermoplastic rebars studied, those made from GF+PBT, showed significant promise as viable alternatives to thermoset rebars in structural applications. However, further optimization in processing, matrix formulation for improved chemical resistance and surface treatment is needed. Future research should explore optimizing the bar constituents and improved manufacturing with surface profiles, then evaluating baseline properties and performance and further comprehensive durability testing on different sizes of bar to fully characterize performance and guide standardization efforts.

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