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Stimulated autogenous self-healing of mechanically and thermally cracked cementitious materials
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.ORCID iD: 0000-0001-8039-692x
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: urn:nbn:se:ltu:diva-94762ISBN: 978-91-8048-227-1 (print)ISBN: 978-91-8048-228-8 (electronic)OAI: oai:DiVA.org:ltu-94762DiVA, id: diva2:1716868
Public defence
2023-02-21, C 305, Luleå tekniska universitet, Luleå, 10:00 (English)
Opponent
Supervisors
Funder
Svenska Byggbranschens Utvecklingsfond (SBUF)Swedish Transport AdministrationAvailable from: 2022-12-07 Created: 2022-12-06 Last updated: 2024-02-01Bibliographically approved
List of papers
1. Autogenous Self-Healing: A Better Solution for Concrete
Open this publication in new window or tab >>Autogenous Self-Healing: A Better Solution for Concrete
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
National Category
Other Materials Engineering
Research subject
Building Materials
Identifiers
urn:nbn:se:ltu:diva-75206 (URN)10.1061/(ASCE)MT.1943-5533.0002764 (DOI)000475694700023 ()2-s2.0-85067520596 (Scopus ID)
Note

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

Available from: 2019-07-03 Created: 2019-07-03 Last updated: 2022-12-06Bibliographically approved
2. Does a High Amount of Unhydrated Portland Cement Ensure an Effective Autogenous Self-Healing of Mortar?
Open this publication in new window or tab >>Does a High Amount of Unhydrated Portland Cement Ensure an Effective Autogenous Self-Healing of Mortar?
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2019 (English)In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 12, no 20, article id 3298Article in journal (Refereed) Published
Abstract [en]

It is commonly accepted that the autogenous self-healing of concrete is mainly controlled by the hydration of Portland cement and its extent depends on the availability of anhydrous particles. High-performance (HPCs) and ultra-high performance concretes (UHPCs) incorporating very high amounts of cement and having a low water-to-cement ratio reach the hydration degree of only 70–50%. Consequently, the presence of a large amount of unhydrated cement should result in excellent autogenous self-healing. The main aim of this study was to examine whether this commonly accepted hypothesis was correct. The study included tests performed on UHPC and mortars with a low water-to-cement ratio and high cement content. Additionally, aging effects were verified on 12-month-old UHPC samples. Analysis was conducted on the crack surfaces and inside of the cracks. The results strongly indicated that the formation of a dense microstructure and rapidly hydrating, freshly exposed anhydrous cement particles could significantly limit or even hinder the self-healing process. The availability of anhydrous cement appeared not to guarantee development of a highly effective healing process.

Place, publisher, year, edition, pages
MDPI, 2019
Keywords
continued hydration, ultra-high performance concrete, cracking, microstructure, calcite
National Category
Other Materials Engineering
Research subject
Building Materials
Identifiers
urn:nbn:se:ltu:diva-76510 (URN)10.3390/ma12203298 (DOI)000498402100021 ()31614436 (PubMedID)2-s2.0-85074298266 (Scopus ID)
Note

Validerad;2019;Nivå 2;2019-10-28 (johcin)

Available from: 2019-10-25 Created: 2019-10-25 Last updated: 2022-12-06Bibliographically approved
3. The effect of exposure on the autogenous self-healing of Ordinary Portland cement mortars
Open this publication in new window or tab >>The effect of exposure on the autogenous self-healing of Ordinary Portland cement mortars
2019 (English)In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 12, no 23, article id 3926Article in journal (Refereed) Published
Abstract [en]

Exposure conditions are critical for the autogenous self-healing process of Portland cement based binder matrixes. However, there is still a significant lack of fundamental knowledge related to this factor. The aim of this paper was to investigate and understand the effects of various potentially applicable curing solutions on the efficiency of the crack closure occurring both superficially and internally. Four groups of exposures were tested, including exposure with different water immersion regimes, variable temperatures, application of chemical admixtures, and use of solutions containing micro particles. The self-healing process was evaluated externally, at the surface of the crack, and internally, at different crack depths with the use of optical and scanning electron microscopes (SEM). The phase identification was done with an energy dispersive spectrometer combined with the SEM. The results showed very limited self-healing in all pure water-based exposures, despite the application of different cycles, temperatures, and water volumes. The addition of a phosphate-based retarding admixture demonstrated the highest crack closure, both internally and externally. The highest strength recovery and a very good crack closure ratio was achieved in water exposure containing micro silica particles. The main phase observed on the surface was calcium carbonate, and internally, calcium silicate hydrate, calcium carbonate, and calcium phosphate compounds. Phosphate ions were found to contribute to the filling of the crack, most likely by preventing the formation of a dense shell composed of hydration phases on the exposed areas by crack unhydrated cement grains as well as by the additional precipitation of calcium and phosphate-based compounds. The micro sized silica particles presumably served as nucleation sites for the self-healing products growth. Changes in the chemical composition of the self-healing material were observed with a distance from the surface of the specimen.

Place, publisher, year, edition, pages
MDPI, 2019
Keywords
autogenous self-healing, cementitious materials, cracking, exposure, microstructure, calcium phosphate
National Category
Infrastructure Engineering
Research subject
Building Materials
Identifiers
urn:nbn:se:ltu:diva-76526 (URN)10.3390/ma12233926 (DOI)000510178700122 ()31783574 (PubMedID)2-s2.0-85075858886 (Scopus ID)
Note

Validerad;2019;Nivå 2;2019-12-09 (johcin)

Available from: 2019-10-28 Created: 2019-10-28 Last updated: 2022-12-06Bibliographically approved
4. Improved self-healing of mortars with partial cement replacement
Open this publication in new window or tab >>Improved self-healing of mortars with partial cement replacement
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2020 (English)Conference paper, Poster (with or without abstract) (Refereed)
Abstract [en]

Making the European Union’s economy sustainable is the “European Green Deal” strategy announced by The European Commission. One of the major aims is becoming climate-neutral by 2050. Since global cement production accounts for approximately 8% of anthropogenic carbon dioxide emissions, the development of concrete with waste by-products as alternative binders and efficient self-healing properties would be a significant milestone towards the circular economy. The self-healing efficiency of cementitious composites with alternative binders requires further improvement as there is still insufficient information on this topic. The latest results for cement mortars showed the promotion of crack closure, both internally and externally, when the healing medium is a mixture of phosphate-based retarding admixture and water. The current study verifies whether satisfactory healing may also be achieved for cementitious composites with 20%cw slag and fly ash replacement subjected to phosphate-based exposure. The efficiency of the proposed solution is compared with other types of environmental conditions such as deionized or lime water immersion. The self-healing process is quantitatively assessed after 4 weeks of healing based on the crack closure and flexural strength regain. All exposure conditions applied resulted in efficient external crack closure; however, the phosphate-based retarding admixture showed the most impressive internal filling of the crack (Figure 1). Based on the Scanning Electron Microscope (SEM) with Energy Dispersive Spectroscopy (EDS) analysis, the majority of the self-healing products were identified as calcium carbonate crystals. Calcium phosphate compound and calcium silicate hydrate (C-S-H) were visible inside the crack in case of retarding admixture exposure contributing also to a limited flexural strength recovery.

National Category
Other Civil Engineering
Research subject
Building Materials
Identifiers
urn:nbn:se:ltu:diva-94755 (URN)
Conference
74th RILEM Annual Week and 40th Cement and Concrete Science Conference, Sheffield, England [Online], August 31 - September 4, 2020
Funder
Swedish Transport AdministrationSvenska Byggbranschens Utvecklingsfond (SBUF)
Note

Funder: Skanska

Available from: 2022-12-06 Created: 2022-12-06 Last updated: 2024-02-06Bibliographically approved
5. Autogenous self-healing of low embodied energy cementitious materials: Effect of multi-component binder and crack geometry
Open this publication in new window or tab >>Autogenous self-healing of low embodied energy cementitious materials: Effect of multi-component binder and crack geometry
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2023 (English)In: Construction and Building Materials, ISSN 0950-0618, E-ISSN 1879-0526, Vol. 376, article id 130994Article in journal (Refereed) Published
Abstract [en]

Concrete's ability to auto-repair the cracks reduces the need for maintenance and repair. Autogenous self-healing is an intrinsic property of concrete highly dependent on the binder composition. The urgent necessity to decrease CO2 emissions of concrete by replacing cement with “greener” materials provides challenges and opportunities for self-healing cementitious materials. This research aims to verify the self-healing behavior of environmentally friendly multi-component binders. An experimental study is conducted to test the effect of binder composition-related parameters (e.g., phase composition, porosity) and crack geometry on the self-healing efficiency of the “green” mortars. Cementitious materials with 50 wt.%cement replacement with limestone powder blended with fly ash, blast furnace slag, and silica fume are investigated. Sorptivity change, compressive strength regains, and crack closure after self-healing are used to quantify the self-healing efficiency. Quantitative analysis and correlations between chemical composition/microstructural features, geometrical crack characteristics, and self-healing measures are investigated. The results indicate that “green” binder composition affects the self-healing mechanism leading to different levels of performance recovery. Some SCMs-limestone binder formulations enable a better self-healing efficiency than pure OPC or OPC/limestone cementitious materials, presumably due to a synergistic effect between the limestone and the mineral additions. Correlation analysis indicated that geometrical complexity characterized by fractal dimension and tortuosity of the crack does not affect the external crack closure, whereas the fractal dimension and maximum crack width are correlated with the internal crack healing.

Place, publisher, year, edition, pages
Elsevier, 2023
Keywords
Cracking, Microstructure, Mortar, Autogenous self-healing, Low embodied energy, Fractal dimension
National Category
Other Civil Engineering Other Materials Engineering
Research subject
Building Materials
Identifiers
urn:nbn:se:ltu:diva-94753 (URN)10.1016/j.conbuildmat.2023.130994 (DOI)2-s2.0-85150247385 (Scopus ID)
Funder
Swedish Transport AdministrationSvenska Byggbranschens Utvecklingsfond (SBUF)
Note

Validerad;2023;Nivå 2;2023-04-12 (hanlid);

Funder: Skanska AB, Sweden;

This article has previously appeared as a manuscript in a thesis

Available from: 2022-12-06 Created: 2022-12-06 Last updated: 2023-05-08Bibliographically approved
6. Interpretable Machine Learning for Prediction of Post-Fire Self-Healing of Concrete
Open this publication in new window or tab >>Interpretable Machine Learning for Prediction of Post-Fire Self-Healing of Concrete
Show others...
2023 (English)In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 16, no 3, article id 1273Article in journal (Refereed) Published
Abstract [en]

Developing accurate and interpretable models to forecast concrete’s self-healing behavior is of interest to material engineers, scientists, and civil engineering contractors. Machine learning (ML) and artificial intelligence are powerful tools that allow constructing high-precision predictions, yet often considered “black box” methods due to their complexity. Those approaches are commonly used for the modeling of mechanical properties of concrete with exceptional accuracy; however, there are few studies dealing with the application of ML for the self-healing of cementitious materials. This paper proposes a pioneering study on the utilization of ML for predicting post-fire self-healing of concrete. A large database is constructed based on the literature studies. Twelve input variables are analyzed: w/c, age of concrete, amount of cement, fine aggregate, coarse aggregate, peak loading temperature, duration of peak loading temperature, cooling regime, duration of cooling, curing regime, duration of curing, and specimen volume. The output of the model is the compressive strength recovery, being one of the self-healing efficiency indicators. Four ML methods are optimized and compared based on their performance error: Support Vector Machines (SVM), Regression Trees (RT), Artificial Neural Networks (ANN), and Ensemble of Regression Trees (ET). Monte Carlo analysis is conducted to verify the stability of the selected model. All ML approaches demonstrate satisfying precision, twice as good as linear regression. The ET model is found to be the most optimal with the highest prediction accuracy and sufficient robustness. Model interpretation is performed using Partial Dependence Plots and Individual Conditional Expectation Plots. Temperature, curing regime, and amounts of aggregates are identified as the most significant predictors.

Place, publisher, year, edition, pages
MDPI, 2023
Keywords
autogenous self-healing, cementitious materials, high temperature, artificial neural network, ensemble methods, mechanical properties, artificial intelligence
National Category
Other Civil Engineering
Research subject
Building Materials
Identifiers
urn:nbn:se:ltu:diva-94752 (URN)10.3390/ma16031273 (DOI)2-s2.0-85147941374 (Scopus ID)
Funder
Swedish Transport AdministrationSvenska Byggbranschens Utvecklingsfond (SBUF)
Note

Validerad;2023;Nivå 2;2023-03-01 (joosat);

Funder: Skanska AB

Licens fulltext: CC BY License

This article has previously appeared as a manuscript in a thesis.

Available from: 2022-12-06 Created: 2022-12-06 Last updated: 2023-05-08Bibliographically approved
7. Is “smart” concrete capable of self-repair after a fire?
Open this publication in new window or tab >>Is “smart” concrete capable of self-repair after a fire?
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Other Civil Engineering
Identifiers
urn:nbn:se:ltu:diva-94750 (URN)
Available from: 2022-12-06 Created: 2022-12-06 Last updated: 2022-12-06
8. Self-healing of thermally cracked cement paste subjected to different post-fire curing
Open this publication in new window or tab >>Self-healing of thermally cracked cement paste subjected to different post-fire curing
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Other Civil Engineering
Identifiers
urn:nbn:se:ltu:diva-94751 (URN)
Available from: 2022-12-06 Created: 2022-12-06 Last updated: 2022-12-06

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