| Author: | Xu, Lingyu |
| Title: | High-strength engineered cementitious composites incorporating artificial geopolymer fine aggregates |
| Advisors: | Dai, Jian-guo (CEE) |
| Degree: | Ph.D. |
| Year: | 2022 |
| Subject: | Cement composites Composite materials Waste products as building materials Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | xxvi, 271 pages : color illustrations |
| Language: | English |
| Abstract: | In the past three decades, Engineered Cementitious Composites (ECC) have emerged as one of the milestones of concrete technology because ECC are featured with strain-hardening and multiple cracking characteristics. ECC overcome the quasi-brittle problems of conventional concrete materials and can achieve very high tensile ductility (typically 3~10%). ECC also facilitate the excellent crack-arresting and self-healing abilities, which can significantly enhance the structural durability and safety. Very recently, two major trends have been seen in the development of ECC materials: (1) development of ECC with combined high/ultra-high strength and superior ductility (e.g., through the use of high-modulus and high-strength synthetic fibers); (2) use of more environmentally friendly matrix materials for improved sustainability (e.g., through the utilization of industrial by-products and wastes as raw materials). This dissertation aims to develop a sustainable high-strength ECC material by replacing the conventional fine silica sand (FSS) with artificial geopolymer aggregates (GPA), which are manufactured through alkali activation of industrial by-products such as fly ash and slag. The GPA are an effective one-stone two-bird solution which eases the environmental burden caused by exploitation of natural aggregates, and in the meantime, reduces the number of industrial/urban/agricultural wastes and by-products in landfills. In this dissertation, GPA are innovatively used to tailor the ductility and crack width of high-strength ECC with improved sustainability. The dissertation starts from a feasibility study of high-strength GPA-ECC, followed by in-depth investigations into its short-term and long-term mechanical performances, performance-based design through the hybrid use of GPA and FSS, and crack control mechanisms under the micromechanical design principle. The research work conducted has been distributed into five chapters (Chapters 3-7) besides an introduction (Chapter 1), literature review (Chapter 2) and summary part (Chapter 8). Detailed contents of each chapter are briefly summarized as follows: Chapter 3 presents a feasibility study on high-strength GPA-ECC production. The mechanical performances of three types of ECC, i.e., fine silica sand ECC (FSS-ECC), cement-bonded aggregate ECC (CBA-ECC) and GPA-ECC, were compared. Based on the comparison results, it was concluded that GPA-ECC exhibited the best strain-hardening performance. Preliminary discussions on the overall mechanical performance, fiber failure modes, and crack width distributions of GPA-ECC under direct tension were carried out. Chapter 4 provides a multi-scale investigation to understand the microstructures and tensile properties of GPA-ECC using the GPA-to-binder ratio as the key variable. The mechanical properties including the compressive strength, full-range tensile stress-strain curves, and crack patterns of GPA-ECC were investigated in details. In order to understand the mechanism behind the macroscopic mechanical properties, the reaction kinetics of the matrix and the interfacial properties between GPA and matrix were analyzed based on the calorimetry test, Back Scattered Electron (BSE) with Energy Dispersive Spectroscopy (EDS), and microhardness test, as compared to those of counterpart matrix using FSS. The "additional flaw effect" of GPA on the performance of GPA-ECC was proposed and demonstrated based on X-ray computed tomography (XCT) analysis of the flaw size distribution. Chapter 5 presents an extensive study on the long-term mechanical performance of GPA-ECC after accelerated aging, and the obtained long-term properties were compared with the short-term data obtained in Chapter 4. FSS-ECC specimens were also prepared for comparison. The microhardness values of cementitious matrix, GPA, and GPA-to-matrix interface were tested to elaborate the compressive strength evolution mechanisms of GPA-ECC. Particular attention was paid to the aggregate-to-matrix interface transition zone (ITZ) in order to achieve an in-depth understanding of the interaction between the cementitious matrix and two different types of aggregates (i.e., GPA and FSS). Both the short- and long-term tensile properties (strength, ductility, and crack resistance) of FSS-ECC and GPA-ECC were compared. The role of GPA as additional flaws on maintaining the stable long-term tensile ductility of GPA-ECC was clarified. Finally, a cost analysis was conducted to demonstrate the commercial potential of GPA-ECC. Chapter 6 presents how to strategically tailor the tensile strain-hardening behavior of high-strength ECC through the hybrid use of GPA and FSS. With such hybridization, the GPA (i.e., relatively low strength and stiffness) can be strategically utilized in the performance-based design of ECC, while retaining the sustainability benefit of GPA. A comprehensive multiple-scale experimental study was conducted on the performance of the ECC through mechanical tests, BSE analysis, digital image correlation (DIC) investigation and XCT analysis. A Weibull-based stochastic modeling approach was adopted to present the effective and ineffective flaw distributions and explain their influences on the designed performance of the produced ECC. Chapter 7 developed ultra-high-strength GPA-ECC with different aggregate sizes that influence the crack width distribution and material ductility. The calorimetry test was carried out to understand the reaction kinetics of GPA with different particle sizes in the ECC matrix. The nanoindentation test was used to identify the hardness distribution of the GPA-to-matrix interfacial region. Both the compressive and tensile properties of GPA-ECC were examined, and single-crack tests were conducted to evaluate the pseudo strain-hardening (PSH) characteristics of GPA-ECC. Finally, the over-saturated cracking behavior of GPA-ECC caused by the adoption of very fine GPA (< 0.60 mm) was explained by micromechanics and theoretical crack analysis. |
| Rights: | All rights reserved |
| Access: | open access |
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