Effect of matrix self-healing on the bond-slip behavior of micro steel fibers in ultra-high-performance concrete
Abstract: This study investigates the bond-slip behavior of micro steel fibers embedded into an Ultra-High-Performance Concrete (UHPC) matrix as affected by the self-healing of the same matrix in different exposure conditions. The UHPC matrix contains a crystalline admixture as a promoter of the autogenous self-healing specially added to enhance the durability in the cracked state. For the aforesaid purpose, some samples were partially pre-damaged with controlled preload (fiber pre-slip at different levels) and subjected to one-month exposure in 3.5% NaCl aqueous solution and in tap water to study the fiber corrosion, if any, and the effects of self-healing; after that, they were subjected to a pull-out test, to be compared with the behavior of analogous non-pre-slipped samples undergoing the same curing history. Moreover, some samples were cured in the chloride solution, intended to simulate a marine environment, to study the effect of marine curing on the pull-out behavior of steel fiber. The steel fiber corrosion and self-healing products attached to the surface of the steel fiber were analyzed via Scanning Electron Microscopy (SEM), and Energy -Dispersive Spectroscopy (EDS). The results indicate that the newly healed particles formed on the highly damaged fiber-matrix interface significantly enhance the friction phase of the bond-slip behavior and result in a significant residual capacity compared to non-pre-slipped specimens. On the other hand, the self-healing effect in specimens subjected to low damage pre-slip contributed more to the chemical adhesion region of the bond-slip behavior. Owning to the dense microstructure of the matrix, curing in 3.5% NaCl aqueous solution was not found to significantly affect the pull-out resistance as compared to the samples cured in tap water.
Reference of this article: Al-Obaidi, S., He, S., Schlangen, E. et al. Effect of matrix self-healing on the bond-slip behavior of micro steel fibers in ultra-high-performance concrete. Mater Struct 56, 161 (2023).
Affiliations:
Salam Al-Obaidi and Liberato Ferrara: Department of Civil and Environmental Engineering, Politecnico Di Milano, Piazza Leonardo DaVinci 32, 20133 Milan, Italy
Salam Al-Obaidi: Roads and Transportations Engineering Department, University of Al-Qadisiyah, Diwaniyah 58001, Iraq
Salam Al-Obaidi, Shan He and Erik Schlangen: Department of Civil Engineering and Geoscience, Delft University of Technology, 2628, CN, Delft, The Netherlands
Microstructural characterization of crack-healing enabled by bacteria-embedded polylactic acid (PLA) capsules
Abstract: The current study investigates short-term and long-term crack-healing behaviour of mortars embedded with bacteria-based poly-lactic acid (PLA) capsules under both ideal and realistic environmental conditions. Two sets of specimens were prepared and subjected to different healing regimes, with the first set kept in a mist room for varying short durations (i.e., 1 week, 2 weeks, 3 weeks and 8 weeks) and the second set placed in an unsheltered outdoor environment for a long-term healing process (i.e., 1 year). Alteration of microstructure because of self-healing was characterized by backscattered electron (BSE) imaging and energy dispersive X-ray spectroscopy (EDS) via crack cross-sections. Results show that visible crack healing enabled by bacteria began after 2 weeks in a humid environment. The healing products initially precipitated at crack mouths and gradually moved deeper into cracks, with the precipitated calcium carbonate crystals growing larger over time. After 8 weeks, healing products can be found even a few millimetres deep inside cracks. Observations of crack healing in a realistic environment revealed significant differences compared to healing under controlled conditions. While no healing products can be found at crack mouths, a substantial healing process was observed throughout the entire crack depth. It is likely that the environmental actions such as rainfall and/or freeze and thaw cycles may have worn away the healing products at crack mouths and thus led to a deeper ingress of oxygen into cracks, which promoted the activation of healing agents and associated calcium carbonate precipitation deep inside a crack.
Reference of this article: Shan He, Zhi Wan, Yu Chen, Henk M. Jonkers, Erik Schlangen, Microstructural characterization of crack-healing enabled by bacteria-embedded polylactic acid (PLA) capsules, Cement and Concrete Composites, Volume 143, 2023, 105271, ISSN 0958-9465
Affiliations:
Shan He, Zhi Wan, Yu Chen, Henk M. Jonkers and Erik Schlangen: Microlab, Faculty of Civil Engineering and Geosciences, Delft University of Technology, Delft, the Netherlands
Direct ink writing of vascularized self-healing cementitious composites
Abstract: Direct ink writing of cementitious materials can be an alternative way for creating vascular self-healing concrete by intentionally incorporating hollow channels in the cementitious matrix. In this study, a 3D-printable fibre reinforced mortar was first developed. Three groups of specimens were fabricated using direct ink writing, where the two top and bottom printing layers were printed with different printing directions. The macrostructure of the hardened specimens was studied using CT scanning. Four-point bending tests were carried out to investigate the initial flexural strength and the strength recovery after healing with injected epoxy resin. Furthermore, water permeability test was used to evaluate the healing potential of the samples. The results from CT scanning show that printing direction influences the actual volumes of hollow channels and the volume of small pores which are a consequence of the deposition process. The hollow channels of all samples were squeezed by the upper layers during the printing process, and the longitudinally printed samples were the most affected. When printing direction changes from longitudinal to transverse, the initial flexural strength decreases. Similarly, the average permeability of the cracked samples increases when the printing direction changes from longitudinal to transverse. Although the healing effectiveness regarding flexural strength is remarkable for all specimens, it was only possible to perform a single healing process as hollow channels were then blocked by the epoxy resin. The rough surface of the hollow channels is inferred to make it difficult to extract the epoxy resin out of the specimens.
Reference of this article: Zhi Wan, Yading Xu, Shan He, Yu Chen, Jinbao Xie, Branko Šavija, Direct ink writing of vascularized self-healing cementitious composites, Cement and Concrete Composites, Volume 144, 2023, 105295, ISSN 0958-9465,
Affiliations:
Zhi Wan, Yading Xu, Shan He, Yu Chen, Jinbao Xie and Branko Šavija: Microlab, Faculty of Civil Engineering and Geosciences, Delft University of Technology, Delft, the Netherlands
An enhanced lattice beam element model for the numerical simulation of rate-dependent self-healing in cementitious materials
Abstract: This paper describes the development of a discrete lattice model for simulating structures formed from self-healing cementitious materials. In particular, a new approach is presented for simulating time dependent mechanical healing in lattice elements. The proposed formulation is designed to simulate the transient damage and healing behaviour of structures under a range of loading conditions. In addition, multiple and overlapping damage and healing events are considered. An illustrative example demonstrates the effects of varying the healing agent curing parameters on the computed mechanical response. The model is successfully validated using published experimental data from two series of tests on structural elements with an embedded autonomic self-healing system. The meso-scale model gives detailed information on the size and disposition of cracking and healing zones throughout an analysis time history. The model also provides an accurate means of determining the volume of healing agent required to achieve healing for all locations within a structural element. The importance of the information provided by the model for the design of self-healing cementitious material elements is highlighted.
Reference of this article: Sina Sayadi, Ze Chang, Shan He, Erik Schlangen, Iulia C. Mihai, Anthony Jefferson, An enhanced lattice beam element model for the numerical simulation of rate-dependent self-healing in cementitious materials, Engineering Fracture Mechanics, Volume 292, 2023, 109632, ISSN 0013-7944,
Affiliations:
Sina Sayadi, Iulia C. Mihai and Anthony Jefferson: School of Engineering, Cardiff University, Cardiff CF24 3AA, UK
Ze Chang, Shan He and Erik Schlangen: Faculty of Civil Engineering and Geosciences, TUDelft, Delft 2628 CN, the Netherlands
Self-Waterproofing Performance of Repair Mortars With Inorganic Healing Agents
Abstract: In Europe, about 55% of concrete bridges are about 50 years old and require non-structural rapid repair strategies to reinstate the aesthetic and durability performances. Existing strategies focus primarily on superficial restoration that continues to demonstrate premature deterioration due to inevitable micro-crack formations that further propagate to macro-cracks leading to the ingress of moisture along with harmful ions. In this study, the benefits of self-healing technology to control moisture ingress at the microscale were investigated. For this, tailored microcapsule with inorganic healing agent, specifically, commercially available water-repellent agent (SIKAGARD 705L) was added to mortar with two types of commonly used binders namely CEMI 52.5N and CEMI 52.5R. The compatibility assessment in terms of capsule integration, fresh and hardened properties was done. The baseline healing efficiency of the mortars without any healing additions was obtained to understand the autogenous healing capacity of the reference mortars. Subsequently, the reference mortar mixes were compared with mixes containing varying fractions of microcapsules (3, 5, and 10%) for autonomous healing efficiency with capillary absorption as the main durability function. The healing efficiency was further investigated for two different crack mouth widths (350 μm); representative of non-structural residual crack widths. In mortars with microcapsules, a maximum reduction of sorptivity coefficients up to 82% and 78% with CEMI 52.5N and CEMI 52.5R mortars, respectively, for specimens cracked after 7 days of curing was observed. Subsequently, a synergetic effect of autogenous healing action and autonomous water-repellent action for durability recovery was identified and proved useful for repair mortar applications. The healing agent investigated, capsule content, and healing environment considered in the current study lay a foundation for further optimisation to improve the performance and to suit different applications.
Reference of this article: Self-Waterproofing Performance of Repair Mortars With Inorganic Healing Agents Padmapriya Arul Kumar, Sripriya Rengaraju and Abir Al-Tabbaa MATEC Web Conf., 378 (2023) 03001
Affiliations:
Padmapriya Arul Kumar, Sripriya Rengaraju and Abir Al-Tabbaa : Department of Engineering, University of Cambridge, United Kingdom
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