Residual performance of fire-exposed textile reinforced concrete : experimental investigations and modeling techniques

• Κύριος συγγραφέας: Καψάλης, Παναγιώτης
• Άλλοι συγγραφείς: Kapsalis, Panagiotis
• Γλώσσα: English
• Έκδοση: 2022
• Θέματα: Textile reinforced mortars; Textile reinforced concrete; Fire; High temperature; TRM; Modeling; Ινοπλέγματα σε ανόργανη μήτρα (ΙΑΜ); Πυρκαγιά; Υψηλή θερμοκρασία

Textile reinforced concrete (TRC) is a promising composite material with enormous potential in structural applications because it offers the possibility to construct lightweight but strong and sustainable elements. However, despite the reasonably good heat resistance of the inorganic matrices and the well-established knowledge on the high-temperature performance of the commonly used fibrous reinforcements, their application in TRC elements with very small thicknesses makes their effectiveness questionable. The concrete cover protecting the textile reinforcement is an order of magnitude smaller than in the case of steel-reinforced concrete elements. Furthermore, the experimental investigations that have been conducted so far are limited; hence, several knowledge gaps can be identified in the current state-of-the-art. Finally, among the published relevant studies there are large inconsistencies in the testing methods and conditions; thus, it is difficult to draw reliable conclusions about the thermomechanical behavior of the material. This research investigates the residual performance of TRC after exposure to fire via an extensive experimental campaign which provided data on the loadbearing resposne (tensile and flexural) of six TRC compositions, on a wide range of temperatures (20 °C to 700 °C). The six compositions differed by the reinforcing material (carbon or combination of glass and carbon), the sizing of the reinforcement (uncoated, polymer coated or impregnated) and the amount of reinforcement. The novelties of this campaign lie in (i) the quantification of the effect of the textile sizing and the protective concrete cover, (ii) the investigation of the performance of hybrid reinforcement, and (iii) the heat treatment of the specimens with actual fire tests (with durations varying between 7 and 37 minutes). The test results were modeled by analytical and numerical approaches. Existing analytical approaches were used as a tool to describe the behavior of fire exposed TRC in tension, while numerical simulations were developed to model its flexural performance. The novelty of the numerical model lies in the application of a mechanical layer-wise model in combination with a heat transfer model that determines the exposure temperature at every position of a fire-exposed specimen. The first part of this thesis introduces the need for research on this topic and provides the necessary theoretical background. This thesis starts with Chapter 1 that gives a more elaborate introduction to the research scope and the problem statement, as well as a clear statement of the objectives and the methodology to address them. Chapter 2 gives an introduction to structural fire engineering and the fire performance of cementitious materials, in order to acquaint the reader with some fundamental aspects that are used in this thesis. Chapter 3 introduces the reader to the mechanical performance of TRC at ambient conditions. Then a state-of-the-art review of the performance of TRC at increased temperature is provided. The second part of the thesis is devoted to the experimental campaign that was conducted for this doctoral research, aiming for the characterization of various TRC compositions after fire exposure. Chapter 4 gives an overview of the campaign. The geometrical and mechanical properties of the utilized materials, the testing techniques, and the tested compositions are described. Chapter 5 presents the first explorative fire tests. The results yielded valuable qualitative insights that served to set up the strategy of the main experimental campaign and justify specific choices made concerning material choices and test temperatures. Chapter 6 presents the results obtained from the fire tests. The observations on the response of the specimens to the fire exposure (after cooling down) and the temperature recordings are presented. Chapter 7 presents the results obtained from the tensile and flexural tests on TRC specimens (fire exposed and non-exposed). The residual mechanical capacity is defined for each tested composition at every exposure temperature or exposure duration. The evolution of cracking patterns and failure modes with increasing temperature are also discussed. Comparisons among the tested compositions trigger the discussion on the effect of the considered parameters: fiber material, fiber volume fraction, concrete cover and textile coating. Emphasis is given on the effect of the coating as it is identified as the most important parameter. The third part of the thesis concerns the modeling of the results. Chapter 8 covers the analytical approaches. It starts with the first approach, which is an assessment of the applicability of the Aveston-Cooper-Kelly analytical model to predict the stress-strain law of fire exposed TRC. The validity of the research assumptions is evaluated by comparing the analytical results to the experimental ones. Next, the second modeling approach is presented, which aims for the detailed definition of a degradation law for the mechanical properties of every tested composition by fitting the “Bisby” model to the experimental results of this research. It is shown tha tthese degradation laws can be used as input to expand the applicabilityof the Aveston-Cooper-Kelly model to fire exposed TRC elements. Chapter 9 presents the numerical modeling techniques that were adopted in this research. It starts by presenting the investigations on modeling the heat transfer properties of TRC at fire exposure and the presentation of the results (temperature profiles at flexural specimens). Next, it presents the assumptions and the methodology of numerically simulating the flexure tests. The temperature profile that was determined in the previous step and the material tensile properties that were determined by the experimental campaign, are used as input to the numerical model. The results of the simulation of the flexure tests are compared to the experimental results; hence, the validity of the adopted modeling technique is assessed, highlighting its strengths and limitations. Finally, the thesis ends with Chapter 10, which gives the conclusions of the conducted research, highlighting its main contributions. This chapter also gives suggestions for future research. The thesis concludes that the most decisive parameter for the residual capacity of TRC after exposure to fire is the surface treatment of the reinforcement; uncoated carbon fiber textiles being the best solution (owing to the superiority of carbon fibers at elevated temperature compared to other materials). Polymer coatings lead to faster reduction of the mechanical properties as temperature increases (due to the deterioration of the textile-to-matrix interface as the coating burns) while impregnation with epoxy resins inhibits an increased risk of thermal spalling due to the evaporation of the resin. Based on the provided knowledge, compositions with hybrid reinforcement can also present a favorable behavior with a proper design. This research also shows that the temperature profile of a fire exposed TRC element can be determined with a conduction heat transfer model and, consequently, the flexural performance of the element can be modeled with a layer-wise technique (assigning the predicted temperature to each layer). This modeling technique can be used to simulate correctly the flexural capacity of a heated element. Based on the flexure tests and their numerical simulation it is concluded that the mortar plays a key role too on the residual flexural capacity of fire exposed TRC elements; hence, its contribution should be modeled with accuracy. Finally, this research showed that the applicability of the Aveston-Cooper-Kelly theory can be applied to TRC elements at high temperatures as long as a representative degradation law is adopted. Such a law can be obtained by fitting analytical expressions to experimental results on fire exposed TRC elements tested in tension.