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Autogenous Self-Healing: A Better Solution for Concrete
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.ORCID iD: 0000-0001-8039-692X
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.ORCID iD: 0000-0001-7279-6528
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering. Concrete Specialist, Skanska AB, Göteborg.
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.ORCID iD: 0000-0001-6287-2240
2019 (English)In: Journal of materials in civil engineering, ISSN 0899-1561, E-ISSN 1943-5533, Vol. 31, no 9, article id 3119001Article in journal (Refereed) Published
Abstract [en]

Self-healing can be defined as the ability of a material to repair inner damage without any external intervention. In the case of concrete, the process can be autogenous, based on optimized mix composition, or autonomous, when using additionally incorporated capsules containing a healing agent and/or bacteria spores. The first process uses unhydrated cement particles as the healing material while the other utilizes a synthetic material or bacteria released into the crack from a broken capsule or activated through access of water and oxygen. The critical reviewing of both methods indicates that the autogenous self-healing is more efficient, more cost effective, safer, and easier to implement in full-scale applications. Nevertheless, a better understanding of the mechanism and factors affecting the effectiveness of the process is needed. The main weaknesses of the autonomous method were identified as loss of workability, worsened mechanical properties, low efficiency and low probability of the healing to occur, low survivability of the capsules and bacteria in harsh concrete environment, very high price, and lack of full-scale evaluation.

Place, publisher, year, edition, pages
American Society of Civil Engineers (ASCE), 2019. Vol. 31, no 9, article id 3119001
National Category
Other Materials Engineering
Research subject
Building Materials
Identifiers
URN: urn:nbn:se:ltu:diva-75206DOI: 10.1061/(ASCE)MT.1943-5533.0002764ISI: 000475694700023Scopus ID: 2-s2.0-85067520596OAI: oai:DiVA.org:ltu-75206DiVA, id: diva2:1334728
Note

Validerad;2019;Nivå 2;2019-07-03 (svasva)

Available from: 2019-07-03 Created: 2019-07-03 Last updated: 2022-12-06Bibliographically approved
In thesis
1. Self-Healing Concrete
Open this publication in new window or tab >>Self-Healing Concrete
2019 (English)Licentiate thesis, comprehensive summary (Other academic)
Alternative title[sv]
Självläkande Betong
Abstract [en]

Concrete is a brittle material prone to cracking due to its low tensile strength. Crack repairs are not only expensive and time-consuming but also increase the carbon footprint. Designing a novel concrete material possessing the ability to self-repair cracks would enhance its sustainability. Self-healing can be defined as a material’s ability to repair inner damage without any external intervention. In the case of concrete, the process can be autogenous, based on an optimized mix composition, or autonomous, when additional capsules containing some healing agent and/or bacteria spores are incorporated into the binder matrix. The first process uses unhydrated cement particles as the healing material while the other utilizes a synthetic material or bacteria precipitating calcite which are released into the crack from a broken capsule or activated by access to water and oxygen. The main disadvantages of the autonomous method are the loss of the fresh concrete workability, worsened mechanical properties, low efficiency, low survivability of the capsules and bacteria during mixing and the very high price. On the other hand, the autogenous self-healing was found to be more efficient, more cost effective, safer, and easier to implement in full-scale applications. Knowledge related to mechanisms and key factors controlling the autogenous self-healing is rather limited. Therefore, the aim of this research work was to better understand the autogenous self-healing process of concrete and to optimize the mix design and exposure conditions to maximize its efficiency. This licentiate thesis summarizes the main findings of the first 2.5 years of the PhD project. Several factors affecting autogenous self-healing were studied, including the amount of unhydrated cement, mix composition, age of material, self-healing duration and exposure conditions. The process was investigated both externally, at the surface, and deeper inside of the crack, by evaluating the crack closure and chemical composition of formed self-healing products. In addition, the flexural strength recovery was also studied. It was observed that a large amount of cement in the concrete mix does not ensure an efficient autogenous self-healing of cracks. A very dense and impermeable binder microstructure limited the transport of calcium and silicone ions to the crack and diminished the precipitation of the healing products. Addition fly ash increased the crack closure ratio close to the crack mouth, but its presence did not support the recovery of the flexural strength, presumably due to a very limited formation of load bearing phases inside the crack. Calcium carbonate was detected mainly at the crack mouth, whereas calcium silicate hydrate (C-S-H) and ettringite were found deeper inside the crack. The formation of C-S-H and ettringite presumably resulted in a regain of the flexural strength. On the other hand, calcite crystals formed close to the surface of the specimen controlled conditions inside the crack through its external closure. Healing exposure based on pure water appeared to be inefficient even despite the application of different temperature cycles and water volumes. The application of a phosphate-based retarding admixture in the curing water resulted in the highest self-healing efficiency. The admixture presumably inhibited the formation of a dense hydration shell on the surface of the unhydrated cement grains and promoted the precipitation of calcium phosphate compounds inside the crack. In addition, water mixed with microsilica particles caused a regain of the flexural strength through formation of C-S-H in the crack.

Place, publisher, year, edition, pages
Luleå tekniska universitet, 2019
Series
Licentiate thesis / Luleå University of Technology, ISSN 1402-1757
Keywords
cementitious materials, self-healing, exposure, fly ash, calcite, C-S-H, cracking
National Category
Engineering and Technology Other Materials Engineering
Research subject
Building Materials
Identifiers
urn:nbn:se:ltu:diva-76527 (URN)978-91-7790-490-8 (ISBN)978-91-7790-491-5 (ISBN)
Presentation
2019-12-12, C305, University of Technology, Luleå, 08:00 (English)
Opponent
Supervisors
Funder
Svenska Byggbranschens Utvecklingsfond (SBUF)Swedish Transport Administration
Available from: 2019-10-28 Created: 2019-10-28 Last updated: 2021-10-15Bibliographically approved
2. Stimulated autogenous self-healing of mechanically and thermally cracked cementitious materials
Open this publication in new window or tab >>Stimulated autogenous self-healing of mechanically and thermally cracked cementitious materials
2023 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

It is estimated that each year, approximately 8 billion cubic meters of concrete are produced worldwide, a vast number comparable to 1 m3 per person, making the construction industry a major contributor to overall global CO2 emissions. Throughout the manufacturing process of the most common cement binder, ordinary Portland cement (OPC), CO2 emissions reach 842 kg per ton of clinker produced. Besides production-related emissions, concrete is a brittle material prone to cracking, wherein the mechanical performance and durability of the material degrade. In addition, maintenance and repairs of concrete structures require material resources, adversely affecting the concrete's overall environmental impact.

At the same time, concrete is a very popular building material, primarily due to its low price, accessibility, and multifunctionality, enabling it to be used in most construction environments. Given its versatility and widespread use, decreasing its carbon footprint is essential. It can be achieved through different methods, such as partially replacing OPC with industrial by-products or activating waste materials, using low-carbon cement, or reusing and recycling. Another area of interest in achieving increased service life for concrete is developing and utilizing cementitious materials with self-healing properties.

Cementitious materials have an inherent ability to self-repair cracks up to widths of 150 μm. However, wider cracks can be healed by employing various "stimulators" to boost the self-healing process, such as adding specific types of fibers, crystalline admixtures, or particular exposure conditions. Partial healing can also be achieved in extreme conditions. For example, structures that sustained high-temperature damage can be partially healed by executing post-fire curing. The recovery mechanism involves rehydration and self-healing of high-temperature cracks. Several variables define the process efficiency, such as the curing conditions, binder type, loading temperature, and post-fire cooling. The goal of this Ph.D. research project was to investigate the physicochemical processes and mechanisms behind the autogenous self-healing of cementitious materials. Two types of damage were evaluated: mechanical Cracking and high-temperature damaged binders. Furthermore, identifying potentially novel stimulators for enhanced self-healing properties was one of the project objectives. The application of low-carbon cementitious materials was of primary interest.

A comprehensive exploratory and experimental program was devised and implemented to evaluate factors affecting autogenous self-healing, including the age of the material, exposure conditions, amount of unhydrated cement, and self-healing duration. Environmentally friendly binders were primarily used for the different mix compositions. Observations were made at the crack mouth and deep inside the crack by analyzing the crack closure and chemical composition of the newly formed self-healing products. In addition, the strength recovery and durability of the specimens were investigated. Quantitative analysis and correlations were examined between microstructural features, geometrical crack characteristics, and self-healing efficiency parameters. Physicochemical mechanisms for thermally and mechanically cracked cementitious materials were studied. Machine Learning techniques were used to predict the compressive strength recovery after high-temperature exposure numerically. Four algorithms were deployed and trained on a database of results collected from the literature review, and corresponding hyperparameters were tuned for optimized model results. Individual Conditional Expectation and Partial Dependency plots were used to visualize and interpret the results.

It was observed that high cement content in the concrete mix does not guarantee an efficient autogenous self-healing of cracks. A dense, impermeable binder microstructure constrained the transport of silicon and calcium ions to the crack and reduced the precipitation of the healing products. With the addition of fly ash, the crack closure ratio close to the crack mouth increased, but recovery of flexural strength was not supported, presumably due to the small number of loadbearing phases inside the crack. All SCM-limestone cementitious materials have shown superior self-healing efficiency compared to pure OPC or OPC/limestone binders, presumably due to a synergistic effect between the limestone and the mineral additions. The binder composition affected the self-healing mechanism, leading to varying levels of performance recovery. Calcium carbonate was detected mainly at the crack mouth, whereas ettringite and calcium silicate hydrate (C-S-H) were found deeper inside the crack. Flexural and compressive strength was regained, presumably because of C-S-H and ettringite formation.

On the other hand, after calcite crystals sealed the crack at the surface, the concentration of the ions inside the crack presumably increased, leading to better self-healing performance. Healing based on pure water exposure had limited efficiency despite applying various water volumes and temperature cycles. The highest crack closure was observed with the addition of a retarding admixture in the curing water. The admixture supposedly blocked the formation of a dense hydration shell on the surface of the unhydrated cement grains. Phosphorus and calcium were detected in the self-healing phases within the crack. Recovery of flexural strength by forming C-SH in the crack was recorded when using water mixed with micro silica particles.

Using lime water with a small dosage of carbon nanomaterials displayed marginally improved high-temperature crack closure and mechanical performance compared with ordinary cement paste and tap water curing. Two distinct processes were identified for the recovery process of a thermally cracked cementitious material, i.e., rehydration and self-healing of the cracks. Phase assemblage and the cement paste porosity were exposed to changes with increasing loading temperature. These changes were presumably partially reversed upon application of a water re-curing process after cooling, i.e., the unhydrated cement grains further hydrate, forming new hydrates, pores are filled with new hydration products, and existing phases react to form new ones, e.g., CaO reacted with water to form Ca(OH)2. It can be hypothesized that the mechanism of the crack healing is the same as in the mechanically cracked concrete, i.e., based on diffusion-dissolution-precipitation processes.The developed machine learning model interpretation indicated that strength recovery depends on the temperature range that caused the damage, re-curing conditions, and the amount of fine and coarse aggregate.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2023
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Other Civil Engineering
Research subject
Building Materials
Identifiers
urn:nbn:se:ltu:diva-94762 (URN)978-91-8048-227-1 (ISBN)978-91-8048-228-8 (ISBN)
Public defence
2023-02-21, C 305, Luleå tekniska universitet, Luleå, 10:00 (English)
Opponent
Supervisors
Funder
Svenska Byggbranschens Utvecklingsfond (SBUF)Swedish Transport Administration
Available from: 2022-12-07 Created: 2022-12-06 Last updated: 2024-06-05Bibliographically approved

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Rajczakowska, MagdalenaHabermehl-Cwirzen, KarinHedlund, HansCwirzen, Andrzej

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