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Self-Healing Concrete
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Structural and Fire Engineering.
2019 (English)Licentiate thesis, comprehensive summary (Other academic)Alternative title
Självläkande Betong (Swedish)
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 [en]
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: urn:nbn:se:ltu:diva-76527ISBN: 978-91-7790-490-8 (print)ISBN: 978-91-7790-491-5 (electronic)OAI: oai:DiVA.org:ltu-76527DiVA, id: diva2:1366001
Presentation
2019-12-12, C305, University of Technology, Luleå, 08:00 (English)
Opponent
Supervisors
Funder
Svenska Byggbranschens Utvecklingsfond (SBUF)Swedish Transport AdministrationAvailable from: 2019-10-28 Created: 2019-10-28 Last updated: 2019-11-27Bibliographically 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: 2019-10-28Bibliographically 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?
Show others...
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)
Note

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

Available from: 2019-10-25 Created: 2019-10-25 Last updated: 2019-11-20Bibliographically 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)31783574 (PubMedID)
Note

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

Available from: 2019-10-28 Created: 2019-10-28 Last updated: 2019-12-09Bibliographically approved

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