Seismic retrofitting with alkali-activated textile reinforced mortar (AAM-TRM)
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Date
2024-04-16
Authors
Azdejkovic, Lazar
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Abstract
The present dissertation covers the potential use of alkali-activated materials (AAM) in seismic retrofitting of the existing structures within the textile reinforced mortar (TRM) strengthening system. For the purpose of the presented research an AAM mortar mix was developed, based on the raw and waste materials which were available in Greece. Two AAM mortar mix designs were employed as TRM matrix materials: the first, interim mix based on metakaolin and ladle furnace slag was tested in seismic retrofitting of unreinforced masonry walls (URM) made of natural stones; the final AAM mortar mix was the ternary blend of metakaolin, fly ash and ladle furnace slag which was studied through seismic retrofitting of RC elements and URM walls made of perforated fired-clay bricks.
A total of three AAM-based TRM strengthening configurations were studied on three different levels: at a microscopic scale with the focus on fiber-matrix interface; at the level of TRM characterization conducted with standard tensile and pull-out bond tests; and at cyclic tests of mid- or large-scale structural elements retrofitted with the previously developed and characterized TRM configurations. The main objective of the present material is to provide firm evidence that AAMs (geopolymers) combined with various textile meshes represent an effective and durable alternative to the traditional OPC-based TRM applicable in seismic retrofitting. Additionally, this study aims to cover the strengthening with TRM of particular RC and URM structural elements, for which no or very little data was identified in the available literature: RC short columns, three-leaf URM masonry walls made of natural stones and single-leaf beam-type URM walls made of perforated (hollow) fired-clay bricks.
In the first large-scale experimental campaign, the effectiveness of TRM confining jackets based of uncoated carbon-fiber textiles applied at shear-deficient (short) RC columns was studied. A set of cyclic loading tests on RC columns with rectangular cross-section retrofitted with AAM- and OPC-based TRM jackets were performed aiming primarily to study the behavior of RC short columns confined with TRM jackets, secondly to showcase the applicability of AAMs in seismic retrofitting of RC elements and finally, to compare the two materials involved. The data obtained experimentally was applied in adapting and formulating the existing analytical models in order to reach a reliable procedure for predicting the response of TRM-confined RC short columns which sustained failure initiated by the TRM rupture, after yielding of the flexural reinforcement.
In the second major experimental campaign of this thesis, seismic retrofitting of three-leaf URM walls made of natural stones with TRM consisting of coated basalt-fiber textile and AAM mortar was studied in three different testing configurations: slender walls subjected to in-plane and out-of-plane cyclic loading, and squat walls subjected only to in-plane cyclic loading. The effectiveness of the applied AAM-based TRM strengthening scheme was evaluated by comparing the responses of retrofitted walls to that of the unretrofitted ones. Moreover, the proposed retrofitting configuration was compared to the configuration taken from the literature, based on natural hydraulic lime (NHL) mortar and flax-fiber textile, which was applied at the walls of the same geometry and tested by following the identical procedure.
The final experimental campaign was designed to investigate the effectiveness of AAM-based TRM applied at beam-like, single-leaf URM walls made of perforated (hollow) clay bricks. Moreover, a combined energy and seismic retrofitting of beam-type walls with AAM- or OPC-based mortars, alkali-resistant (AR) glass-fiber textile and extruded polystyrene (XPS) thermal insulating boards was studied. All specimens were subjected to in- and out-of-plane four-point bending cyclic tests with a main objective to explore the relative effectiveness of seismic, and combined energy and seismic retrofitting with AAM-based TRM, compare it with the respective commercial OPC-based solutions, and finally provide preliminary data about the use of the alternative binders with lower environmental impact compared to the traditional cements.
Preliminary tests have shown that AAMs have a good adhesion with textile mesh materials. Good tensile response and great adhesion to concrete substrates of AAM-based TRM composites reinforced with uncoated carbon-fiber mesh were confirmed with uniaxial tensile tests, pull-out bond tests and large-scale RC column tests. Additional confinement achieved with the application of AAM-based TRM jackets improved significantly the cyclic response of shear-deficient RC columns: increased energy dissipation, lateral load, and deformation capacity; changed the failure from brittle shear-governed to shear-flexure governed, which occurred after extended yielding of the flexural reinforcement and was controlled by the TRM jacket failure. Finally, the response of TRM-retrofitted RC short columns was approximated analytically with a bilinear model.
The cyclic tests of URM walls made with natural stones or fired-clay bricks showed that the application of AAM-based TRM overlays improved the responses in terms of load, deformation and energy dissipation capacity. Moreover, the experimental tests on beam-type URM walls subjected to integrated retrofitting showed very good adhesion between AAM and XPS panels. However, it was observed in cyclic tests of URM walls that the exploitation of basalt- and AR glass-fiber textiles was rather poor. Therefore, an extensive study on durability of AAM-based TRM made with coated basalt- and glass-fiber textile reinforcements was carried out. Detrimental effect on basalt- and AR glass-fibers caused by the high alkalinity of AAM matrix materials was confirmed mechanically and chemically at the fiber-matrix interface.
The dissertation is divided into eight chapters. Chapter 1 serves as an introduction to the topic of the study, provides some fundamental information on AAM technology, outlines the current state-of-the-art, and describes in detail the subject of the dissertation as well as the research gap that it tries to cover and its novelty. Chapter 2 covers the development of AAM mortar mixes suitable for applications in seismic retrofitting with TRM. The characterization of TRM composites comprising the selected AAM-based mortar mixes and carbon-fiber textile is included in Chapter 3. Next, Chapter 4 covers the effectiveness of AAM-based TRM in seismic retrofitting of existing RC short column elements. Analytical modeling of the observed responses of the specimens subjected to cyclic loading is contained in this chapter as well. Then, Chapter 5 and Chapter 6 deal with seismic retrofitting of URM walls with AAM-TRM: natural stone walls with basalt-fiber, AAM-based TRM (Chapter 5) and beam-type fired-clay brick walls with AR-glass fiber, AAM-based TRM. Chapter 6 covers combined seismic and energy retrofitting of beam-type URM walls, too. The durability study of AAM-based TRM consisting of basalt- and glass-fiber textiles is given in Chapter 7. Finally, in Chapter 8 the thesis summary is given, the most important conclusions are drawn and recommendations for future work are provided.
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Keywords
Geopolymers, Textile reinforced mortars, Reinforced concrete columns, Alternative cementitious materials, Unreinforced masonry walls, Strengthening of existing structures