Self-healing concrete is characterized as the capability of concrete to fix its cracks on its own autogenously or autonomously. It not only seals the cracks but also partially or entirely recovers the mechanical properties of the structural elements. This kind of concrete is also known as self-repairing concrete. Because concrete has a poor tensile strength compared to other building materials, it often develops cracks in the surface. These cracks reduce the durability of the concrete because they facilitate the flow of liquids and gases that may contain harmful compounds. If microcracks expand and reach the reinforcement, not only will the concrete itself be susceptible to attack, but so will the reinforcement steel bars.[1][2] Therefore, it is essential to limit the crack's width and repair it as quickly as feasible. Self-healing concrete would not only make the material more sustainable, but it would also contribute to an increase in the service life of concrete structures and make the material more durable and environmentally friendly.[3][4]
Plants and animals have the capacity to seal and heal wounds. In all plants and animals examined, firstly a self-sealing phase and secondly a self-healing phase can be identified. In plants, the rapid self-sealing prevents the plants from desiccation and from infection by pathogenic germs. This gives time for the subsequent self-healing of the injury which in addition to wound closure also results in the (partly) restoration of mechanical properties of the plant organ. Based on a variety of self-sealing and self-healing processes in plants, different functional principles were transferred into bio-inspired self-repairing materials.[9][10][11] The connecting link between the biological model and the technical application is an abstraction describing the underlying functional principle of the biological model which can be for example an analytical model[12] or a numerical model. In cases where mainly physical-chemical processes are involved a transfer is especially promising.There is evidence in the academic literature[13] of these biomimetic design approaches being used in the development of self-healing systems for polymer composites.[14]The DIW[clarification needed] structure from above can be used to essentially mimic the structure of skin. Toohey et al. did this with an epoxy substrate containing a grid of microchannels containing dicyclopentadiene (DCPD), and incorporated Grubbs' catalyst to the surface. This showed partial recovery of toughness after fracture, and could be repeated several times because of the ability to replenish the channels after use. The process is not repeatable forever, because the polymer in the crack plane from previous healings would build up over time.[15]Inspired by rapid self-sealing processes in the twining liana Aristolochia macrophylla and related species (pipevines) a biomimetic PU-foam coating for pneumatic structures was developed.[16] With respect to low coating weight and thickness of the foam layer maximum repair efficiencies of 99.9% and more have been obtained.[17][18][19] Other role models are latex bearing plants as the weeping fig (Ficus benjamina), the rubber tree (Hevea brasiliensis) and spurges (Euphorbia spp.), in which the coagulation of latex is involved in the sealing of lesions.[20][21][22] Different self-sealing strategies for elastomeric materials were developed showing significant mechanical restoration after a macroscopic lesion.[23][24]
Wikipedia Bio References Build 527 Crack
For the first method, fragile glass capillaries or fibers are imbedded within a composite material. (Note: this is already a commonly used practice for strengthening materials. See Fiber-reinforced plastic.)[70] The resulting porous network is filled with monomer. When damage occurs in the material from regular use, the tubes also crack and the monomer is released into the cracks. Other tubes containing a hardening agent also crack and mix with the monomer, causing the crack to be healed.[64] There are many things to take into account when introducing hollow tubes into a crystalline structure. First to consider is that the created channels may compromise the load bearing ability of the material due to the removal of load bearing material.[71] Also, the channel diameter, degree of branching, location of branch points, and channel orientation are some of the main things to consider when building up microchannels within a material. Materials that don't need to withstand much mechanical strain, but want self-healing properties, can introduce more microchannels than materials that are meant to be load bearing.[71] There are two types of hollow tubes: discrete channels, and interconnected channels.[71]
According to a 1996 study by H. L. Erlich in Chemical Geology journal, the self-healing ability of concrete has been improved by the incorporation of bacteria, which can induce calcium carbonate precipitation through their metabolic activity.[118] These precipitates can build up and form an effective seal against crack related water ingress. At the First International Conference on Self Healing Materials held in April, 2007 in The Netherlands, Henk M. Jonkers and Erik Schlangen presented their research in which they had successfully used the "alkaliphilic spore-forming bacteria" as a "self-healing agent in concrete".[119][120] They were the first to incorporate bacteria within cement paste for the development of self-healing concrete.[121] It was found that the bacteria directly added to the paste only remained viable for 4 months. Later studies saw Jonkers use expanded clay particles[122] and Van Tittlelboom use glass tubes,[123] to protect the bacteria inside the concrete. Other strategies to protect the bacteria have also since been reported.[124] Even microcapsule based self-healing applications has been extended on bio-based coating materials. These coatings are based on neem oil and possesses another bio-based character as it utilized vegetable oil as a core material.,[125]
Encapsulation techniques have been demonstrated to be a promising solution for cracking as highlighted by an improvement of mechanical performance and/or transport properties of material. Though the improvement of mechanical behavior has been more reported on the other host matrix as polymers [31-40], woven composites [41], or fiber reinforced composites [42], the mechanical recovery of self-healing cementitious composites containing microcapsules has become an increasing attention [43-48]. Pelletier et al. [43] reported 26% of the original load resistance of a self-healing concrete system involving internal encapsulation of a sodium silicate solution in polyurethane microcapsules present in the matrix, compared to a recovery of 10% of the references without capsules. Besides the strength recovery, the improved toughness and the attenuation of corrosion have also been obtained. The work of Nishiwaki [44-46] demonstrated that the insufficient mixing of the two part-released resins from the capsules, i.e. the healing agent and the catalyst, can result in a poor polymerization degree and therefore poor mechanical performance of the adhesive in terms of compressive strength and splitting strength. Yang et al. [47] obtained 45.8% and 30.4% increase in compressive strength of the mortar at 1d and 28d age respectively, incorporating the microcapsules with methylmethacrylate monomer and triethylborane as the healing agent and the catalyst for use in the cementitious system. Hu et al. [48] developed a cement paste involving urea formaldehyde (UF) microcapsules filled with epoxy resins to exhibit self-healing properties. They evaluated the healing efficiency by means of the compressive strength recovery of the healing specimen to that of the intact one. They found that the healing efficiency can reach to 111% for the paste with 1% of microcapsules and 60% pre-damage, defined as the ratio of the load applied to the resistance of the intact specimen, loaded in the same fashion. The role of self healing in mechanical properties of cementitious composites is thus a major concern of the present study. 2ff7e9595c
Comments