Academic literature on the topic 'Self-healing of cracks'

At present, the commonly used repair materials for concrete cracks mainly include epoxy systems and acrylic resins, which are all environmentally unfriendly materials, and the difference in drying shrinkage and thermal expansion often causes delamination or cracking between the original concrete matrix and the repair material. This study aimed to explore the feasibility of using microbial techniques to repair concrete cracks. The bacteria used were environmentally friendly Bacillus pasteurii. In particular, the use of lightweight aggregates as bacterial carriers in concrete can increase the chance of bacterial survival. Once the external environment meets the growth conditions of the bacteria, the vitality of the strain can be restored. Such a system can greatly improve the feasibility and success rate of bacterial mineralization in concrete. The test project included the microscopic testing of concrete crack repair, mainly to understand the crack repair effect of lightweight aggregate concrete with implanted bacterial strains, and an XRD test to confirm that the repair material was produced by the bacteria. The results show that the implanted bacterial strains can undergo Microbiologically Induced Calcium Carbonate Precipitation (MICP) and can effectively fill the cracks caused by external concrete forces by calcium carbonate deposition. According to the results on the crack profile and crack thickness, the calcium carbonate precipitate produced by the action of Bacillus pasteurii is formed by the interface between the aggregate and the cement paste, and it spreads over the entire fracture surface and then accumulates to a certain thickness to form a crack repairing effect. The analysis results of the XRD test also clearly confirm that the white crystal formed in the concrete crack is calcium carbonate. From the above test results, it is indeed feasible to use Bacillus pasteurii in the self-healing of concrete cracks.

Sustainable maintenance of existing steel-reinforced concrete structures becomes more important. Using non-reinforced sprayed mortar to maintain these structures often leads to cracks in this repair layer due to the alteration of crack widths in the ordinary structure. The water impermeability as well as the durability of the sprayed mortar will be reduced due to the described cracks. This presentation shows a solution for the described problem. The use of carbon yarns with a special inorganic coating as reinforcement in sprayed mortars leads to a self-healing of the arising cracks. Due to the inorganic coating applied on carbon yarns the excellent bond between mortar and yarn results in a fine distributed crack image with crack width below 0.1 mm. It is shown that these cracks heal themselves. Consequently we can provide a mainly mineral protection layer for existing steel reinforced concrete structures which is impermeably to water based solutions. The presentation focuses on the material development and characterization.

The autogenous healing potential of cement-based materials is affected by multiple factors, such as mix composition, crack width, pre-cracking age and external environmental conditions. In this study, the effects of curing conditions and supplementary cementitious materials (SCMs) on autogenous self-healing of early age cracks in cement mortar were investigated. Three curing conditions, i.e., standard curing, wet–dry cycles and incubated in water, and two SCMs, i.e., fly ash (FA) and blast furnace slag (BFS) with various contents (cement replacement ratio at 0%, 20%, and 40%) were examined. A single early age crack (pre-cracking age of 3 days) with a width of 200~300 μm was generated in cylindrical mortar specimens. Autogenous crack self-healing efficiency of mortar specimens was evaluated by performing a visual observation and a water permeability test. Moreover, microstructure analysis (XRD, SEM and TG/DTG) was utilized to characterize the healing products. The results indicated that the presence of water was essential for the autogenous self-healing of early age cracks in cement mortar. The efficiency of self-healing cracks was highest in specimens incubated in water. However, no significant self-healing occurred in specimens exposed to standard curing. For wet–dry cycles, a longer healing time was needed to obtain good self-healing compared to samples incubated in water. SCMs type and content significantly affected the autogenous self-healing ability of early age cracks. The self-healing efficiency of early age cracks decreased with increases in FA and BFS content. BFS mortars exhibited greater recovery in relation to water penetration resistance compared to the reference and FA mortars. Almost the same regain of water tightness and a lower crack-healing ratio after healing of 28 days in FA mortars were observed compared to the reference. The major healing product in the surface cracks of specimens with and without SCMs was micron-sized calcite crystals with a typical rhombohedral morphology.

Concrete is a popular building material all over the world, but because of different physiochemical processes, it is susceptible to crack development. One of the primary deterioration processes of reinforced concrete buildings is corrosion of steel bars within the concrete through these cracks. In this regard, a self-healing technique for crack repair would be the best solution to reduce the penetration of chloride ions inside concrete mass. In this study, a rapid chloride migration (RCM) test was conducted to determine the self-healing capacity of cracked mortar. With the help of the RCM test, the steady-state migration coefficient of cracked and uncracked specimens incorporating expansive and crystalline admixtures was calculated. Based on the rate of change of the chloride ion concentrations in the steady-state condition, the migration coefficient was calculated. Furthermore, bulk electrical conductivity tests were also conducted before and after the migration test to understand the self-healing behavior. It was evident from the test results that the self-healing of cracks was helpful to reduce the penetration of chloride ions and that it enhanced the ability of cracked mortar to restrict the chloride ingress. Using this test method, the self-healing capacity of the new self-healing technologies can be evaluated. The RCM test can be an acceptable technique to assess the self-healing ability of cement-based materials in a very short period, and the self-healing capacity can be characterized in terms of the decrease of chloride migration coefficients.

Cracks are caused by many factors. Shrinkage and external loading are the most common reason. It becomes a problem when the ingression of aggressive and harmful substance penetrates to the concrete gap. This problem reduces the durability of the structures. It is well known that self – healing of cracks significantly improves the durability of the concrete structure. This paper presents self-healing cracks of cement paste containing bentonite associated with ground granulated blast furnace slag. The self-healing properties were evaluated with four parameters: crack width on the surface, crack depth, tensile strength recovery, and flexural recovery. In combination with microscopic observation, a healing process over time is also performed. The results show that bentonite improves the healing properties, in terms of surface crack width and crack depth. On the other hand, GGBFS could also improve the healing process, in terms of crack depth, direst tensile recovery, and flexural stiffness recovery. Carbonation reaction is believed as the main mechanism, which contributes the self-healing process as well as the continuous hydration progress.

The self-healing efficiency of cementitious materials was improved by developing several strategies to provide and deliver the products (healing agents) needed for cracks to self-repair. This study evaluated the self-healing efficiency of microcapsules filled with calcium nitrate in reinforced and unreinforced concrete beams. The structural behavior and healing efficiency were evaluated by measuring and then comparing the initial stiffness, peak strength, and deformation with posthealing measurements. Furthermore, as part of this study, crack monitoring was conducted to evaluate crack healing over time. Then characterization analysis was carried out with energy dispersive X-ray spectroscopy to quantify the healing components in the cracked areas. Results showed that the air content in samples containing microcapsules was two times higher than that in the control samples. Furthermore, addition of microcapsules lowered the flexural strength of concrete beams compared with that of the control samples. A positive stiffness recovery was recorded for all groups, with and without microcapsules or steel. Control samples showed the lowest stiffness recovery; however, the use of steel with microcapsules presented a superior healing efficiency and improved stiffness recovery significantly by 38%. Results from image analysis showed that crack widths did not completely heal for the control samples, while using microcapsules allowed the cracked widths to heal more efficiently. The best observed performance was for the microcapsules–steel group, which yielded 100% healing of the cracks.

AbstractThe present study investigates the self healing behavior of both pristine and defected single layer graphene using a molecular dynamic simulation. Single layer graphene containing various defects such as preexisting vacancies and differently oriented pre-existing cracks were subjected to uniaxial tensile loading till fracture occurred. Once the load was relaxed, the graphene was found to undergo self healing. It was observed that this self healing behaviour of cracks holds irrespective of the nature of pre-existing defects in the graphene sheet. Cracks of any length were found to heal provided the critical crack opening distance lies within 0.3-0.5 nm for a pristine sheet and also for a sheet with pre-existing defects. Detailed bond length analysis of the graphene sheet was done to understand the mechanism of self healing of graphene. The paper also discusses the immense potential of the self healing phenomena of graphene in the field of graphene based sub-nano sensors for crack sensing.

ABSTRACTSelf-healing is one of the most desired material properties. Herein, we present the design and development of a new self-healing dental composite (SHDC) that can heal micro-cracks autonomously. The SHDC has two functional components in addition to contemporary dental composites: healing powder (HP) and healing liquid (HL) encapsulated in silica microcapsules. The autonomous healing is triggered by micro-cracks which fracture microcapsules in their propagation path and release the HL. As a consequence, the released HL dissolves and reacts with the HP, and then fill the micro-cracks with a cement-like new material. This 3-step crack-release-heal process prevents micro-cracks from causing restoration failure, thus improving the service life of dental restorative material. The mechanical performance of the SHDC prepared were evaluated in terms of elastic modulus and fracture toughness, which were in the upper level compared to commercial dental restorative materials, and the self-healing capability was confirmed through fracture toughness recovery test. In addition, the SHDCs were made with clinically-tested, biocompatible materials, which makes them readily applicable as medical devices.

Polymer based elements are frequently subject to high mechanical load. It is well known, that such components can spontaneously break although the mechanical stress has not reached the average maximum load. These fatigue fractures are caused by micro-cracks. A smart approach would be to implement a self-healing function that is able to heal a crack in an early stage and thus avoid crack propagation. Fraunhofer UMSICHT and the Plant Biomechanics Group Freiburg together with co-operation partners develop biomimetic self-healing elastomers having the capability to repair micro-cracks automatically without any intervention from outside.

This study proposed a method of applying coating on uncracked surfaces of test specimens in the electrical migration–diffusion test for the evaluation of the chloride penetration resistance of cracked cement-based composites. It was shown that, by applying the proposed method, the recovery of the chloride penetration resistance from self-healing of cracks can be evaluated more accurately because the application of surface coating reduces the test time and the error introduced by over-simplification. Based on observations of the self-healing-induced recovery of chloride penetration resistance, a phenomenological model for predicting the progress of crack self-healing in cement-based composites was suggested. This model is expected to evaluate the chloride penetration resistance more accurately in actual concrete structures with cracks.

Apesar dos avanços tecnológicos e do conhecimento disponível sobre o concreto e suas propriedades, muitas estruturas apresentam-se degradadas prematuramente. Assim, são necessárias frequentes intervenções para reparo havendo, em consequência, consumo de quantidades expressivas de materiais, com importantes impactos financeiros e ambientais. A preocupação crescente com o desempenho das estruturas de concreto tem incentivado vários segmentos da sociedade a buscarem soluções que assegurem sua durabilidade. Nesse sentido, muitas pesquisas acadêmicas vêm sendo desenvolvidas para melhor entender o comportamento do concreto frente a diferentes condições de uso, considerando-se suas propriedades mecânicas e de durabilidade. Esta pesquisa teve por objetivo analisar o desempenho de concretos frente à retração por secagem, após terem sido submetidos ao pré-carregamento. Foram utilizados os cimentos CP V ARI e CP IV, nas relações a/c 0,35, 0,50 e 0,70, e adotadas as idades de cura de 7 e 28 dias. Nas idades 1, 3 e 7 dias, os concretos foram submetidos a carregamentos de 25%, 50% e 75% da carga média de ruptura à tração na flexão (NBR 12142, ABNT, 2010). Realizou-se o ensaio para determinação da retração por secagem (ASTM C157/C157M-08, 2014e1), em concretos de referência e nos pré-carregados. Foram também realizadas análises complementares, como determinação da velocidade da onda ultrassônica (NBR 8802, ABNT, 2013), porosimetria por intrusão de mercúrio e microtomografia de raios X. Comparando-se o desempenho dos concretos pré-carregados ao dos concretos de referência, o cimento CP V ARI apresentou redução da retração média, exceto para a relação a/c 0,35 com cura de 7 dias. Nos concretos com cimento CP IV, a retração média diminuiu para a relação a/c 0,70, com ambas as idades de cura. O pré-carregamento provocou, em maior ou menor grau, a associação de dois diferentes efeitos. Por um lado, a compactação ocasionou a quebra dos compostos menos resistentes da matriz, havendo um efeito físico de redução dos vazios capilares, além da exposição de grãos anidros. Em paralelo, ocorreu a microfissuração do concreto, favorecendo, também, a exposição de grãos anidros. A cura por imersão possibilitou a continuidade da hidratação, com a formação de novos produtos resistentes na matriz, reduzindo as porosidades total e efetiva e, portanto, a taxa da retração do concreto. Além da densificação da matriz, a cura prolongada possibilitou a hidratação dos grãos anidros de cimento expostos nas paredes das microfissuras, favorecendo a sua autocicatrização (self-healing), o que contribuiu com a recuperação da capacidade resistente dos concretos, frente aos esforços da secagem. Assim, a associação destes fatores promoveu a redução da retração por secagem nos concretos estudados, principalmente nos mais porosos.
The growing concern with the performance of structures has encouraged several segments of society to find solutions to improve concrete durability. Several studies have been made to provide a better understanding of the mechanical properties and concrete durability in different conditions. This study assessed the drying shrinkage performance of preloaded concrete. Cement types CP V ARI, equivalent to Portland type III (ASTM C150/C150M-16e1) and CP IV, equivalent to Portland type IP (ASTM C595/C595M-16) were used with w/b = 0.35, 0.50 and 0.70 and curing ages of 7 and 28 days. At the ages of 1, 3 and 7 days, concrete mixes were subject to loads of 25%, 50% and 75% of their mean ultimate flexural strength (Brazilian Standard NBR 12142, ABNT, 2010). Drying shrinkage in reference and preloaded concrete was determined according to ASTM C157/C157M-14e1. Additional tests included ultrasound pulse velocity (NBR 8802, ABNT, 2013), mercury intrusion porosimetry (MIP) and X-ray microtomography. Preloaded CP V ARI cement concrete showed a drop in mean shrinkage when compared with the reference mix, except for the mix with w/b = 0.35 cured for 7 days. For CP IV cement, mean shrinkage dropped with w/b =0.70 for both curing ages. Early loading caused a compaction effect, promoting the weakest compounds of the matrix to be broken, filling the capillary voids. In this process some unhydrous grains probably were exposed. On the other hand, microcracking also occurred, lefting unhydrated grains exposed in the microcracks walls. Due to immersion curing, the availability of water promoted the hydration continuity. New resistant hydrates filled the large voids and reduced the total and effective porosities in the cement matrix. The hydration of anhydrous grains in the microcracks walls had led to self-healing, with partial resistance capacity recovery. The combined effect of these factors promoted the drying shrinkage reducing, mainly for the more porous concrets.

In den durchgeführten Versuchsreihen wurden die Wasserabsorption sowie die Gas- und Wasserpermeabilität sowohl an gerissenen als auch rissfreien Prüfkörpern aus textilbewehrtem Beton (TRC) untersucht. Eine deutliche Steigerung der Wasseraufnahme bei Proben mit unbeschichteten Textilien konnte mit Zunahme der Garnfeinheit beobachtet werden. Bei den gerissenen Proben besteht eine ausgeprägte Abhängigkeit der Transportraten von Flüssigkeiten und Gasen zu den relevanten Risscharakteristika (kumulative Risslänge, Rissbreite). Weiterhin sind Selbstheilungseffekte von feinen Rissen infolge einer zyklischen Wasserbeaufschlagung beobachtet worden. Hierbei kam es zu einer deutlichen Reduzierung der Transportraten sowohl für Wasser als auch für Gase. Eine speziell entwickelte Permeabilitätsmesskammer zur Durchführung von In- Situ-Messungen ermöglicht Untersuchungen zum Stofftransport unter axialer Zugbelastung bei verschiedenen Dehnungszuständen.

Yes
Cracks in concrete composites, whether autogenous or loading-initiated, are almost inevitable and often difficult to detect and repair, posing a threat to safety and durability of concrete infrastructures, especially for those with strict sealing requirements. The sustainable development of infrastructures calls for the birth of self-healing concrete composites, which has the built-in ability to autonomously repair narrow cracks. This paper reviews the fabrication, characterization, mechanisms and performances of autogenous and autonomous healing concretes. Autogenous healing materials such as mineral admixtures, fibers, nanofillers and curing agents, as well as autonomous healing methods such as electrodeposition, shape memory alloys, capsules, vascular and microbial technologies, have been proven to be effective to partially or even fully repair small cracks. As a result, the mechanical properties and durability of concrete infrastructure can be restored to some extent. However, autonomous healing techniques have shown a better performance in healing cracks than most of autogenous healing methods that are limited to healing of cracks having a narrower width than 150 µm. Self-healing concrete with biomimetic features, such as self-healing concrete based on shape memory alloys, capsules, vascular networks or bacteria, is a frontier subject in the field of material science. Self-healing technology provides concrete infrastructures with the ability to adapt and respond to the environment, exhibiting a great potential to facilitate the creation of a wide variety of smart materials and intelligent structures.
The full-text of this article will be released for public view at the end of the publisher embargo on 16 May 2021.

Crack formation in concrete structures due to various load and non-load factors leading to degradation of service life is very common. Repair and maintenance operations are, therefore, necessary to prevent cracks propagating and reducing the service life of the structures. Accessibility to affected areas can, however, be difficult and the reconstruction and maintenance of concrete buildings are expensive in labor and capital. Autonomous healing by encapsulated bacteria-based self-healing agents is a possible solution. In this study, a novel bacteria-based healing agent was investigated in order to test the self-healing efficiency of the specimens in comparison to the commonly used healing agents made of lactic acid derivatives. (PLA). The novel integrated healing agent is a non-toxic, biodegradable dissolved inorganic carbon substrate derived from wastewater that has been used as an encapsulation material for spores of cohnii bacteria in the Bacillus gene family and for nutrients made up of yeast extract. This dissolved inorganic carbon substrate is a bioplastic made by the bacteria in wastewater known as alkanoate derivatives (AKD). To assess the effect of these healing agents on the mortar characteristics, quantification and characterization of the self-healing were conducted. The quantification of the self-healing efficiency was performed by various experimental techniques such as light microscopy, water permeability, chloride ion permeability, and thermogravimetric analysis whereas the material characterization was investigated by x-ray diffraction and environmental scanning electron microscope. Moreover, a statistical analysis was performed to study the correlation of self-healing efficiency between various experimental techniques. The incorporated dosages of healing agents adopted were 2.6% and 5% by weight of cement. Complete immersion in water was considered to be the condition of treatment of the specimen for healing at two separate periods of 28 and 56 days. The crack widths investigated were in the range of 0.04 to 0.8 mm. The quantification and characterization tests indicated that bacterial containing mortar series especially PLA and AKD at 5% dosage displayed a higher self-healing performance and an indication of precipitated calcium carbonate in the crack mouth. However, the results from the chloride migration test didn’t show any influence by the self-healing healing agents. Furthermore, the statistical analysis identified a major impact of the internal crack geometry on the difference in self-healing ratios, also in the cases where effective crack width is equal.
Sprickbildning i betongkonstruktioner p.g.a. olika laster och lastoberoende faktorer som leder till förkortning av livslängden är mycket vanligt förekommande. Reparation och underhåll är därför nödvändiga för att förhindra att sprickorna propagerar och reduceras konstruktionernas livslängd. Möjligheterna att komma åt de skadade partierna kan dock vara svåra och reparationerna är vanligen både arbetsintensiva och kostsamma. Självläkning med ingjutna bakteriebaserade, självläkande tillsatser är en möjlig lösning på problemet. I denna studie undersöktes en ny bakteriebaserat självläkande tillsats för att prova den självläkande förmågan i jämförelse med vanligt förekommande självläkande tillsatser av mjölksyrederivat (PLA). Den nya integrerade självläkande tillsatsen är en giftfri, biologiskt nedbrytningsbar, oorganisk kolsubstratslösning utvunnen ur avloppsvatten, en tillsats som har använts som ett inkapslingsmaterial för sporer från cohnii-bakterier från bacillussläktet och från näringsämnen framställda ur jästextrakt. Denna kolsubstratslösning är en bioplast framställd ur avloppsvatten och känd som alkanoatderivat (AKD). För att bestämma effekten av dessa självläkande tillsatser på cement bruks egenskaper genomfördes kvantifiering och karakterisering av självläkningen. Kvantifieringen av självläkningens effektivitet utfördes genom olika experimentella metoder såsom ljusmikroskopi, vattengenomsläpplighet, kloridjonstransport och termogravimetriska analyser medan materialkarakteriseringen utfördes med röntgendiffraktion och svepelektronmikroskop (ESEM). Vidare genomfördes en statistisk analys för att undersöka korrelationen mellan olika experimentella metoder. De doser av självläkande tillsatser som användes var 2,6 och 5 % av cementvikten. Fullständig nedsänkning i vatten ansågs vara den lämpligaste lagringen för självläkning under två olika tidsperioder på 28 respektive 56 dygn. De sprickbredder som studerades låg i intervallet 0.04 till 0.8 mm. Försöken kring kvantifiering och karakterisering indikerade att bruken innehållande bakterier, i synnerhet 5 % PLA och AKD, utvecklade en högre form av självläkande beteende och en förekomst av kalciumkarbonat i sprickspetsen. Resultaten från försöken kring kloridtransport visade emellertid inga tecken på någon effekt från de självläkande tillsatserna. Vidare identifierades i den statistiska analysen att inre sprickbildning har stor betydelse för självläkningseffekten även i fall där den effektiva sprickbredden är lika stor.

Los materiales autosanables son materiales con la capacidad de reparar sus daños de forma autónoma o con ayuda mínima de estímulos externos. En el campo de la construcción, el desarrollo de elementos autosanables aumentará la durabilidad de las estructuras y reducirá las acciones de mantenimiento y reparación. Los elementos de hormigón armado presentan frecuentemente pequeñas fisuras (< 0.3 mm), no relevantes mecánicamente, pero que pueden suponer un punto de entrada para agentes agresivos. El hormigón tiene una cierta capacidad de autosanación, capaz de cerrar pequeñas fisuras, producida principalmente por la hidratación continuada y la carbonatación. Estudios recientes han intentado mejorar dicha capacidad y diseñar productos específicos para conseguirla. Estos productos incluyen, entre otros, aditivos cristalinos, agentes micro o macroencapsulados, e incluso el uso de bacterias. Los aditivos cristalinos (CA) son un tipo de aditivo para hormigón que se considera que aporta propiedades de autosanación. No obstante, la falta de conocimiento sobre su comportamiento limita su uso. Además, los métodos para evaluar la autosanación en hormigones no están estandarizados todavía. Esto complica la realización de un análisis crítico de los diferentes productos y métodos de evaluación propuestos en la literatura. Para responder a esta falta de conocimiento, los objetivos de esta tesis son: 1) estudiar y proponer procedimientos experimentales para evaluar los fenómenos de autosanación en hormigón y, 2) evaluar experimentalmente las mejoras producidas al introducir aditivos cristalinos. Esta tesis incluye como ensayos para la determinación de la autosanación: la evaluación del cierre de fisuras, la permeabilidad al agua, flexión a tres puntos y absorción capilar. Además, se han realizado varias campañas experimentales para validar los ensayos propuestos. Posteriormente, estos ensayos se han utilizado para analizar la influencia de varios parámetros, incluyendo entre otros: presencia de aditivos cristalinos, nivel de daño, tiempo necesario para el sanado, composición del hormigón y condiciones de sanado. Finalmente, se analizan los efectos producidos al añadir aditivos cristalinos en hormigón en la fluidez, resistencia e hidratación. Los resultados muestran que el cierre de fisuras es un ensayo eficaz y sencillo para evaluar la autosanación. Sin embargo, la orientación de la fisura durante el sanado ha resultado ser de gran importancia, y no considerar este aspecto puede llevar a conclusiones engañosas. El ensayo de permeabilidad al agua propuesto en este trabajo presenta una buena estabilidad y es fácil de implementar en laboratorios. Además, las relaciones obtenidas entre los parámetros de fisura y la permeabilidad del agua han confirmado la relación cúbica indicada en la literatura. Este trabajo muestra que analizar la eficiencia de autosanado mediante el cierre de fisuras puede llevar a una sobreestimación de la capacidad de sanación, comparada con los resultados obtenidos mediante permeabilidad. Los ensayos de sorptividad resultaron fáciles de implementar, sin embargo, los resultados obtenidos mostraron una alta dispersión y sensibilidad a las variaciones en las fisuras producidas durante el proceso de prefisuración. En cuanto a la evaluación de la recuperación mecánica, los resultados muestran que la evolución de las propiedades del hormigón con el tiempo es un parámetro que debe considerarse, especialmente en fisuras de edades tempranas. En este trabajo se ha obtenido que los aditivos cristalinos potencian las reacciones de autosanación, pero tienen una capacidad limitada. La proximidad de los CA a la industria es un punto positivo para su inclusión como un nuevo tipo de aditivo de hormigón. Sin embargo, los resultados obtenidos en esta tesis indican que se necesitan más análisis para determinar sus efectos completos en hormigón, especialmente con respect
Self-healing materials are materials with the capability to repair their damage autonomously or with minimal help from an external stimulus. In the construction field, the development of self-healing elements will increase the durability of structures and reduce their maintenance and repair actions. Reinforced concrete elements frequently suffer small cracks (< 0.3 mm), not relevant mechanically, but they can be an entrance point for aggressive agents. Concrete has a natural self-healing capability able to seal small cracks, produced by the continuing hydration and carbonation processes. Recent studies have attempted to improve that healing capability and to design specific products to achieve it. These products include, among others: crystalline admixtures, micro- or macro-encapsulated agents, and even the use of bacteria. Crystalline admixtures (CA) are a concrete admixture that is claimed to provide self-healing properties. However, the lack of knowledge on their behavior and self-healing properties limits their usage. In addition, the methods to evaluate the self-healing capability of mortar and concrete are not standardized yet. This complicates the performance of a critical analysis of the different self-healing products and evaluation methods found in the literature. In order to answer to this lack of knowledge, the objectives of this thesis are: 1) to study and propose experimental procedures in order to evaluate self-healing in concrete and, 2) to evaluate experimentally the self-healing enhancements produced when introducing crystalline admixtures. This thesis includes the following tests for the determination of the self-healing: the evaluation of crack closing, water permeability, three point bending tests and capillary absorption test. In addition, several experimental campaigns have been performed with the objective of validating the proposed tests. Afterwards, these methods have been used to analyze the influence of several parameters, including among others: the presence of crystalline admixtures, the damage extent, healing time needed, concrete composition and healing conditions. Finally, the effects that crystalline admixtures produce in concrete are analyzed in terms of slump, strength and hydration. The results show that crack closing is an effective and simple method to evaluate self-healing. However, the orientation of the crack during healing is of great importance, and disregarding this aspect may lead to misleading conclusions. The water permeability method proposed in this work has good stability and it is easy to implement in concrete laboratories. Moreover, the relations obtained between crack parameters and water permeability confirmed the cubic relation, as reported in the literature. This work shows that analyzing healing efficiency by means of crack closing tends to overestimate self-healing if compared with the results obtained by means of water permeability. Sorptivity analysis tests were easy to implement, however, the results obtained in this work showed high dispersion and sensitivity to the variations of the cracks introduced during the precracking process. Regarding the evaluation of mechanical recoveries, the results show that the evolution of concrete properties with time is a parameter of importance that, therefore, should be considered, especially for early age cracks. In this work, crystalline admixtures have been reported as an enhancer of self-healing reactions, but with a limited capacity of enhancement. The proximity of CA to the industry is a positive point to their inclusion as a new type of admixture for concrete. However, the results obtained in this thesis indicate further analyses are needed to determine their full effects on concrete, especially regarding self-healing.
Els materials autosanables són materials amb la capacitat de reparar els seus danys de forma autònoma o amb ajuda mínima d'estímuls externs. En el camp de la construcció, el desenvolupament d'elements autosanables augmentarà la durabilitat de les estructures i reduirà les accions de manteniment i reparació. Els elements de formigó armat presenten freqüentment fissures menudes (< 0.3 mm), no rellevants des del punt de vista mecànic, però poden suposar un punt d'entrada per a agents agressius. El formigó té una capacitat de autosanació capaç de tancar fissures menudes, produïda principalment per la hidratació continuada i la carbonatació. Estudis recents han intentat millorar eixa capacitat i dissenyar productes específics per aconseguir-la. Aquests productes inclouen, entre d'altres, additius cristal·lins, agents micro- o macroencapsulats, i fins i tot l'ús de bacteris. Els additius cristal·lins (CA) són un tipus d'additiu reductor per formigó que es considera que proporciona propietats de autosanació. No obstant, la manca de coneixement sobre el seu comportament limita el seu ús. A més, els mètodes per avaluar la autosanació de formigons encara no estan estandarditzats. Açò complica la realització d'una anàlisi crítica dels diferents productes i mètodes d'avaluació proposats a la literatura. Per respondre a aquesta manca de coneixement, els objectius d'aquesta tesi són: 1) estudiar i proposar procediments experimentals per avaluar els fenòmens d'autosanació en formigó i, 2) avaluar experimentalment les millores produïdes en introduir additius cristal·lins. Aquesta tesi inclou com assajos per a la determinació de l'autosanació: l'avaluació del tancament de fissures, la permeabilitat a l'aigua, flexió a tres punts i absorció capil·lar. A més, s'han realitzat diverses campanyes experimentals per validar els assajos proposats. Posteriorment, aquests assajos s'han utilitzat per analitzar la influència de diversos paràmetres: presència d'additius cristal·lins, nivell de dany, temps necessari per a la sanació, composició del formigó i condicions de sanació. Finalment, s'analitzen els efectes produïts en afegir additius cristal·lins en formigó en fluïdesa, resistència i hidratació. Els resultats mostren que el tancament de fissures és un assaig eficaç i senzill per avaluar l'autosanació. No obstant això, l'orientació de la fissura durant la sanació ha resultat ser de gran importància, i no considerar aquest aspecte pot portar a conclusions enganyoses. L'assaig de permeabilitat a l'aigua proposat presenta una bona estabilitat i és fàcil d'implementar en laboratoris. A més, les relacions obtingudes entre els paràmetres de fissura i la permeabilitat a l'aigua han confirmat la relació cúbica de la literatura. Aquest treball mostra que analitzar l'eficiència de l'autosanació amb el tancament de fissures pot sobreestimar la capacitat de sanació, comparada amb els resultats obtinguts-dues mitjançant permeabilitat a l'aigua. Els assajos de sorptivitat van resultar fàcils d'implementar, però, els resultats obtinguts en aquest treball van mostrar una alta dispersió i sensibilitat a les variacions en les fissures produïdes durant el procés de prefissuració. Pel que fa a l'avaluació de la recuperació mecànica, els resultats mostren que l'evolució de les propietats del formigó amb el temps és un paràmetre d'importància que, per tant, s'ha de considerar, especialment per fissures primerenques. En aquest treball s'ha obtingut que els additius cristal·lins potencien les reaccions d'autosanació, però tenen una capacitat limitada. La proximitat dels CA a la indústria és un punt positiu per a la seva inclusió com un nou tipus d'additiu de formigó. Tanmateix, els resultats obtinguts en aquesta tesi indiquen que calen més anàlisis per determinar els seus efectes complets en formigó, especialment pel que fa a l'autosanació.
Roig Flores, M. (2018). Self-healing concrete: efficiency evaluation and enhancement with crystalline admixtures [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/100082
TESIS

Neste trabalho desenvolveremos um modelo estatístico em uma escala micrométrica de interações entre as componentes do sistema que pretende descrever a propagação de trincas em materiais submetidos a tensões cíclicas. Apesar de sua extrema simplicidade, este modelo é capaz de reproduzir um resultado experimental bastante difundido entre engenheiros e especialistas, conhecido como lei de Paris, cujo enunciado estabelece que a taxa de crescimento de uma trinca sob carregamento cíclico é proporcional a uma potência da variação em seu correspondente fator de intensidade de tensões sendo largamente utilizada em aplicações práticas. Estamos particularmente interessados em estudar a introdução de desordem em determinados parâmetros associados ao material investigando as modificações impostas por este tipo de abordagem ao comportamento estatístico do modelo. Nossos principais resultados serão obtidos numericamente a partir de uma aproximação do tipo campo efetivo que ignora a correlação existente entre as diversas trincas que podem se formar ao longo do sistema durante o processo. Simulações numéricas do modelo serão igualmente consideradas ao analisarmos situações mais gerais do processo de propagação em que efeitos associados à regeneração de trincas podem desempenhar um importante papel na descrição do comportamento mecânico de um material.
In this work we consider a statistical model in a micrometric scale of interactions between the components of the system which intends to describe the failure of materials subjected to cyclic-load fatigue. Although quite simple, this model is able to reproduce an important experimental result widespread among engineers and experts, known as Paris law, which states that the growth rate of a crack at subcritical load is proportional to a power of the change in its stress-intensity factor and it is largely used in engineering practice. We are particularly interested to study the introduction of disorder in some parameters of the material investigating the modifications caused by this kind of approach in the statistical properties of the model. Our main results will be obtained numerically assuming an effective-field like approximation which neglects the correlation between the different cracks emerging throughout the system during the breaking process. Numerical simulations of the model are also performed in order to describe more general situations of propagation where the effects of crack self-healing can play an important role in the material strength.

La rupture différée des fibres SiC de type Hi-Nicalon à l’échelle multifilamentaire, des minicomposites de type Hi-Nicalon/PyC/SiC et Hi-Nicalon/type PyC/B4C a été étudiée à l’aide de moyens d’essais spécifiques et innovants. Des essais de fatigue statique sous air aux très hautes températures (900°C-1300°C) avec mesure des déformations ont ainsi pu être réalisés sur ces différents matériaux. Les résultats expérimentaux obtenus (durée de vie, déformation, lois de comportement en traction) ont permis de comprendre et de modéliser les mécanismes responsables de la rupture différée aux différentes échelles : - Les fils Hi-Nicalon rompent par mécanisme de fissuration lente activé par l’oxydation du carbone libre des fibres. Le mécanisme de fissuration est perturbé par la formation rapide d’oxyde SiO2 à partir de 1000°C : pour les faibles contraintes, la cinétique de fissuration lente est ralentie par formation d’oxyde protecteur empêchant l’accès de l’oxygène aux fissures ; pour les fortes contraintes, la rupture des fils est prématurée à cause de collages inter-fibres (fibre-oxyde-fibre). A 1200°C, le mécanisme de fluage semble être à l’origine de la rupture quasi-instantanée du matériau pour des contraintes supérieures à 200 MPa. - Les minicomposites Hi-Nicalon/type PyC/SiC rompent par mécanisme de fissuration lente ralenti par la présence de matrice SiC et par la formation d’oxyde SiO2 limitant l’accès de l’oxygène aux fibres. le mécanisme de fluage est observé à partir de 1200°C mais il n’a jamais été responsable de la rupture du matériau. - Les minicomposites Hi-Nicalon/type PyC/B4C rompent par mécanisme de fissuration lente ralenti par formation d’oxyde B2O3 à 900°C pour les fortes contraintes. Pour les autres températures et pour les faibles contraintes à 900°C le mécanisme de rupture est la diminution rapide du diamètre des fibres à cause de l’augmentation de la cinétique d’oxydation des fibres par l’oxyde B2O3. Des modèles analytiques basés sur ces différents mécanismes permettent de prévoir la durée de vie du matériau en prenant en compte les incertitudes de mesure et la variabilité des résultats de durée de vie
Delayed failure of SiC Hi-Nicalon multifilament tows (500 fibers), minicomposites Hi-Nicalon/type PyC/SiC and Hi-Nicalon/type PyC/B4C was investigated in static fatigue, in air, at high temperatures (900°C – 1300°C) using specific and innovative devices. Static fatigue tests with measure of strain were performed on these materials. The experimental results (lifetime, strain, tensile behavior) have helped to understand and model the mechanisms responsible for the delayed failure at the different scales: - Hi-Nicalon tows rupture is caused by subcritical crack growth mechanism activated by oxidation of free carbon in the fibres. This phenomenon is disrupted by fast oxide SiO2 formation over 900°C: subcritical crack growth kinetic slows down for low stresses because of protective oxide formation which prevents the cracks from oxygen; For high stresses, the lifetime of Hi-Nicalon tows is weaker because of fibers interactions (fiber-oxide-fiber). At last, creep seems to cause the rupture of the tows for stresses over 200 MPa at 1200°C. - Hi-Nicalon/type PyC/SiC minicomposites break by subcritical crack growth slowed down by the SiC matrix and by the SiO2 formation which limit the access of the oxygen to the fibers. Creep occurs at 1200°C but it isn’t responsible of the rupture. - Hi-Nicalon/type PyC/B4C minicomposites break by subcritical crack growth slowed down by the formation of B2O3 oxide at 900°C for high stresses. The rupture is caused by the fast decrease of the diameter of the fibers at the other temperatures and for low stresses at 900°C. The oxidation kinetic of the fibers increases because of the dissolution of silica coating by B2O3 oxide. Analytical modeling was performed to schedule the lifetime of these materials and the variability of the experimental results is studied

碩士
國立交通大學
土木工程系所
102
This study aims to develop self-healing concrete by using SCA (self-curing admixtures) and other chemical materials. There are three stages. The first stage is to find out the best SCA type and dosage to be used with water-proofing admixtures. The second stage is to study the effect of curing condition under a fixed mix proportions to determine the mechanical properties and durabilities and the interaction of them. The third stage is to split the specimens and investigate the self-healing performance. The results showed that under wet curing condition, the combined admixtures can accelerate the strength development and reached 90 day strength of the controlled specimens at 28 days. The water-proofing admixture did not show similar results and the strength was even lower than the controlled group at early ages. On the durability test, the electrical resistance and RCPT of the SCA group showed better results than all other groups. On the self-healing, the combined SCA and water-proofing admixture showed the best results and can reduce the crack width by 2mm at 56 days. Keywords: concrete ,self-healing, self-curing

Yes
The paper presents a new healing system that uses pre-tensioned hybrid tendons to close cracks in cementitious structural elements. The tendons comprise an inner core, formed from aramid fibre ropes, and an outer sleeve made from a shape memory PET. During the manufacturing process, the inner core of a tendon is put into tension and the outer sleeve into compression, such that the tendon is in equilibrium. A set of tendons are then cast in a cementitious structural element and heat activated once cracking occurs. This triggers the shrinkage potential of the PET sleeve, which in turn releases the stored strain energy in the inner core. The tensile force thereby released applies a compressive force to the cementitious element, in which the tendons are embedded, that acts to close any cracks that have formed perpendicular to the axis of the tendons. Details of the component materials used to form the tendon are given along with the tendon manufacturing process. A set of experiments are then reported that explore the performance of three different tendon configurations in prismatic mortar beams. The results from these experiments show that the tendons can completely close 0.3 mm cracks in the mortar beams and act as effective reinforcement both before and after activation. A nonlinear hinge-based numerical model is also described, which is shown to be able to reproduce the experimental behaviour with reasonable accuracy. The model is used to help interpret the results of the experiments and, in particular, to explore the effects of slip at the tendon anchorages and the amount of prestress force that remains after activation. It is shown that, with two of the tendon configurations tested, over 75% of the prestress potential of the tendon remains after crack closure.
UK-EPSRC (Grant No. EP/P02081X/1, Resilient Materials 4 Life, RM4L).
The full-text of this article will be released for public view at the end of the publisher embargo on 17 Oct 2021.

Yes
The presence of cracks has a negative impact on the durability of concrete by providing paths for corrosive materials to the embedded steel reinforcement. Cracks in concrete can be closed using shape memory polymers (SMP) which produce a compressive stress across the crack faces. This stress has been previously found to enhance the load recovery associated with autogenous selfhealing. This paper details the experiments undertaken to incorporate SMP tendons containing polyethylene terephthalate (PET) filaments into reinforced and unreinforced 500 × 100 × 100 mm structural concrete beam samples. These tendons are activated via an electrical supply using a nickelchrome resistance wire heating system. The set-up, methodology and results of restrained shrinkage stress and crack closure experiments are explained. Crack closure of up to 85% in unreinforced beams and 26%–39% in reinforced beams is measured using crack-mouth opening displacement, microscope and digital image correlation equipment. Conclusions are made as to the effectiveness of the system and its potential for application within industry.
EPSRC for their funding of the Materials for Life (M4L) project (EP/K026631/1) and Costain Group PLC for industrial sponsorship of the project and author

The properties of laser, microplasma and GTAW welds on representative gas turbine blade materials are disclosed. Proprietary filler materials and technology were used to clad multipass welds onto IN738, RenéN5 and CMSX4 alloys which were then subject to vacuum heat treatment before testing. It was found that welds with a bulk content of boron up to 0.6 wt. % demonstrated a capability to heal cracks adjacent to the fusion line (HAZ cracks) and they exhibited superior tensile and stress-rupture properties at a temperature of 982°C. Welds that comprised 1.5 to 2% silicon had superior oxidation resistance at a temperature of 995°C. Combined alloying of welds with moderate amount of boron and silicon produced a unique combination of both high mechanical and oxidation properties. Healing of HAZ cracks took place during post weld heat treatment at a temperature exceeding the solidus temperature of the weld metal eutectics but below of a solidus temperature of the base material. It was found that boron and silicon additives reduced welding pool solidification temperature and increased the solidus–liquidus range. At this temperature a partial re-melt of eutectics occurred allowing healing of HAZ and weld solidification cracks while weld geometry was supported by a continuous framework of high temperature dendrites. This allows the tip repair of turbine blades manufactured of precipitation strengthened superalloys that are normally prone to weld cracking.

Self-healing is the most valuable phenomenon to overcome the integrity decreases which are caused by the damages in service. Thus, self-healing should be automatically occurred as soon as the damages occur, and the healed zone should have high integrity before damaging. In the case the structural ceramics, the severest flaw is surface cracks which are possible to be introduced by crash, fatigue, thermal shock and corrosion during service. Thus, the self-healing of surface cracks in the structural ceramics is an important issue to ensure the structural integrity of the ceramic components. The present author and coworkers succeeded to endow the excellent self-healing ability to structural ceramics by using the high temperature oxidation of the dispersed silicon carbide particle. In the present paper, the healing mechanism, behavior and advantages will be introduced. Moreover, our latest self-healing ceramics will be also reported.

The problem of cracking in steam turbines and compressor blades is one of the major problems associated with their design. Not only in these areas, but many other parts are suffering from the same problem of cracking due to excess stress, fatigue and high temperatures. One of the recent solutions to this is the use of specialized alloys, ceramics and metal matrix composites with improved high temperature strength and fatigue limits, but these materials will also suffer from cracking. Another potential method of improving the fatigue life of steam turbine and compressor blades is through the use of self-healing metals. There has been a recent interest in making self-healing metal composites that heal any cracks with little or no human interaction. One of the ways to achieve this is to send a healing agent (generally a low melting alloy or uncured resin) through hollow passages made in the matrix. When a crack appears, the healing agent flows into the crack, solidifies and closes the crack, effectively healing it. Recently, researchers at UW-Milwaukee have extended the concept of self-healing to metals, and have synthesized self-healing aluminum and solder alloys. This method may be used for the blades of steam turbines and compressors. The hollow passages can be made in a similar method that cooling passages of the gas turbine blades are made. Proper material choice will result in good bonding between the healing agent and the walls of a crack. This study deals with the advantages, ease of use and other considerations for this method to be used in practice. The main problems that need to be over come are materials selection, loss of strength due to addition of hollow channels, and the need to develop methods to initiate healing by flow of the healing agent into a crack. Plausible solutions to these are also discussed.

Self-healing shape memory polymers possess the ability to heal macro and micro cracks by autonomic processes or when subjected to a suitable external stimulus. Recent advancements in the field have shown that the healing capabilities of self-healing polymers can be improved, thus yielding to high healing efficiencies. Depending on the application, the efficiency may refer to shape fixity, shape recovery ratio, dimensions recovery, strength regain, crack healing, etc. Based on test results, it is established that there is an intrinsic correlation between pre-strain levels, shape fixing and free shape recovery of samples programmed above the glass transition temperature (Tg). For samples programmed at multiple temperatures (above and below the glass transition temperature), the absence of lateral and 3D confinements lead to poor to no crack healing. Better compressive strength properties were, however, achieved by samples programmed at higher temperatures above Tg.

Carbon fiber reinforced composites have been used in a wide range of applications in aerospace, mechanical, and civil structures. Due to the nature of material, multiple types of structural damage including micro matrix cracks and delaminations can significant degrade the integrity and safety of composites. It is difficult to detect and repair such damage since they are always barely visible to the naked eye. This paper presents the development of self-sensing and self-healing functions in order to detect damage progression and conduct in-situ damage repair in composite structures. Carbon nanotubes, which are highly conductive materials, are uniformly distributed within epoxy to develop the self-sensing capability. Shape memory polymer is used in the hot spot to obtain the self-healing capability. The developed multi-functional material is applied to carbon fiber reinforced composites for the autonomic detection and heal the matrix cracking. Experimental results showed that the developed composite materials are capable of detecting and healing the matrix cracks and delaminations. The developed self-healing material has the potential to be used as a novel structural material in mechanical, civil, aerospace applications. It can be used to detect and in-situ repair matrix damage induced by low velocity impacts and fatigues.