| dc.description.abstract | Glass fiber reinforced polymer (GFRP) bars are becoming a reasonable alternative to steel bars in reinforced concrete members as a method to address issues of corrosion and electromagnetic interference. This growing interest in GFRP reinforcement results in an increase in the types of products and their quantities in the market. GFRP bars are typically produced in an automated pultrusion process; however, when curved or bent bars are to be manufactured non-standardized production processes are adopted, which differ among manufacturers. This results in variability of the products, which raises concerns regarding quality and durability of the bars. Thus, to use GFRP bars more efficiently two fundamental technological barriers need to be resolved: material property and durability uncertainties.
Quality control (QC) and assurance (QA) testing is a proper way to check the properties of bars before using them in structures. In this thesis, an attempt was made to study the influence of GFRP bars geometrical and mechanical characteristics variability on standardized quality control tests (tensile, shear, flexure, and cure ratio tests). Bars from two different companies with two different diameters and three different surface finishes were included in this study. Based on obtained results it was postulated that variability of the currently available GFRP bars has an influence on testing procedures. As for e.g., surface finishing affects anchorage length for the tensile test, or cost of the shear test can directly depend on a number of bar sizes that need to be tested. Tests, investigated in this research program, were found to be impractical and inconvenient in a rutile use. Thus. adjustment of testing procedures is needed to improve QA and QC testing.
Subsequently, possible correlations between the bar properties were analyzed. Based on the research outcome the tensile-flexure correlation was found to be a great asset for quality control testing of GFRP bars. The study was performed with recognition of the composite bimodular properties in different states of stress (tension and compression) and the results were compared with standardized flexural strength determination protocols. A Weibull “Weakest Link Model” was utilized in the tensile-flexure strength correlation. Based on performed analysis it was found that correlation between tensile-flexure strength does exist and the flexure test potentially can be used as a tensile strength prediction method for QA testing. The difference between the analytical value of the tensile strength, obtained by proposed methodology, and tensile strength measured directly from the test did not exceed five percent.
The second factor that partially impedes wider adaptation of GFRP bars is the absence of satisfactory life prediction models. Since GFRP bars are no longer used only as a longitudinal reinforcement, but also as stirrups in reinforced concrete structures, a model describing the deterioration of all properties is required. Thus, this research program is focused on the prediction capability of existing models to determine degradation of GFRP bar properties. An accelerated aging test (alkaline immersion) was introduced to study GFRP bars long-term performance. Specimens were kept in a highly alkaline solution (approximately 13 pH) under three temperatures: 50˚C, 60˚C, and 70˚C; for three different periods of time: 30, 90, and 150 days. Existing strength retention models were validated using the obtained data and durability of GFRP bar properties was investigated. Two from four introduced in this research program strength retention models were found to be proper estimations for the long-term behavior of GFRP bar properties. It was found that all properties of GFRP bars that directly depend on the bar cross-section area are characterized by a similar rate of degradation (tensile, shear strength). While the flexure strength degrades quicker. Also, smaller bar diameters are characterized by the higher speed of degradation than the bars with bigger diameters. In fact, degradation of GFRP bar properties depends on many factors, including the type of resin, fiber content or bar surface finishing. Thus, the durability of GFRP reinforcement is a complex problem that required further analysis. Discussion and comparison of results between different bar types are presented in this thesis. | en |
