In this paper, mechanisms of self-healing in cementitious materials, i.e. autogenous self-healing, self-healing based on mineral admixtures, self-healing based on bacteria and self-healing based on ...adhesive agents, are reviewed. Literature shows that all mechanisms of self-healing are effective, to some extent, under some particular conditions. It reveals that not any particular method of self-healing is the best, but one can be the most suitable for a particular situation. For better application of self-healing concept in engineering practice, favorable situations for self-healing in cementitious materials are summarized. The required environmental conditions for each self-healing mechanism are analyzed. Additional costs for realizing self-healing in concrete structures are also discussed. Based on the aforementioned aspects of self-healing in cementitious materials, perspectives for further research on application of self-healing in engineering practice are proposed.
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•For application of self-healing concept, favorable situations for self-healing in cementitious materials are summarized.•The required environmental conditions for each self-healing mechanism are analyzed.•Additional costs for realizing self-healing in concrete structures are discussed.•The studies of self-healing in cementitious materials in-situ are much far from satisfactory.•The repeatability of self-healing of a crack under repeated loads are still unknown.
It has been reported by previous studies that cracks in cementitious materials can be healed by further hydration of unhydrated cement particles. However, by now, neither the physicochemical process ...nor the potential of self-healing due to further hydration is completely understood. In this paper, in order to gain insight into self-healing by further hydration, healing behaviors due to further hydration were characterized and quantified. The mineralogy of healing products was qualitatively determined and the percentage of each mineral was specified. The formation of healing products as a function of time was quantified as well. Moreover, self-healing of microcracks was simulated by a reactive transport model. The calculated filling fraction by the healing products in microcracks was consistent with the experimental results. The healing process slowed down markedly after 300h. In addition, in younger cement pastes, larger amounts of unhydrated cement lead to greater filling fraction of microcracks.
The effects of Ca/Si ratio, aluminum and magnesium on the carbonation behavior of calcium silicate hydrate (C-S-H) were investigated by using X-ray powder diffraction (XRD), nuclear magnetic ...resonance (NMR) and thermogravimetric analyzer (TGA). The results showed that the Ca/Si ratio, Al/Si ratio and Mg/Si ratio had a significant influence on the structure, carbonation products and carbonation resistance of C-(M)-(A)-S-H. The mean chain length of silicate chains in C-S-H increased as the Ca/Si ratio decreased. Aluminum uptake in C-S-H increased the content of bridging silicate tetrahedron (Q
). A cross-linked structure (Q
) appeared when magnesium uptake in C-S-H. The carbonation product of C-S-H was vaterite if the Ca/Si ratio was lower than 0.87. The carbonation products of C-S-H were vaterite and calcite if the Ca/Si ratio was higher than 1.02. C-M-S-H had more polymerized units, stronger bond strength and better carbonation resistance than C-S-H.
Electrically switchable magnetization is considered a milestone in the development of ultralow power spintronic devices, and it has been a long sought-after goal for electric-field control of ...magnetoresistance in magnetic tunnel junctions with ultralow power consumption. Here, through integrating spintronics and multiferroics, we investigate MgO-based magnetic tunnel junctions on ferroelectric substrate with a high tunnel magnetoresistance ratio of 235%. A giant, reversible and nonvolatile electric-field manipulation of magnetoresistance to about 55% is realized at room temperature without the assistance of a magnetic field. Through strain-mediated magnetoelectric coupling, the electric field modifies the magnetic anisotropy of the free layer leading to its magnetization rotation so that the relative magnetization configuration of the magnetic tunnel junction can be efficiently modulated. Our findings offer significant fundamental insight into information storage using electric writing and magnetic reading and represent a crucial step towards low-power spintronic devices.
The coupled effects of external multi-ions and wet-dry cycles in sea water on the evolution of autogenous self-healing in cement paste were investigated. It was found that Mg2+, rather than Cl− and ...SO42−, in sea water plays a primary role in the healing process. The percentage of brucite in the reaction products of self-healing was more than 50%. The closure ratio of a 400-μm-wide crack after healing in sea water for 56 days was 2.6 times that of a crack healed in tap water. Furthermore, wet-dry cycles with wetting and drying periods that are each 12 h can significantly increase the healing efficiency by promoting the formation of brucite and calcite. Moreover, the promotional effect of external ions and wet-dry cycles on self-healing was more obvious in a crack with a larger width that facilitates the entrance of external ions during immersion and the evaporation of water in wet-dry cycles.
Cracks, caused by shrinkage and external loading, facilitate the ingress of aggressive and harmful substances into concrete and indeed reduce the durability of the structures. It is well known that ...self-healing of cracks can significantly improve the durability of the concrete structure. In this research, self-healing of cracks was proposed to be realized by providing extra water for further hydration of unhydrated cement particles. In order to provide theoretical guidance for the practice, self-healing by providing extra water to promote further hydration was simulated. The simulation was based on water transport theory, ion diffusion theory and thermodynamics theory. In the simulation, self-healing efficiency under different conditions as a function of time was calculated. The relationship between self-healing efficiency and the amount of extra water from the broken capsules was determined. According to the results of the simulation, the amount of extra water can be optimized by considering self-healing efficiency and other performances.
The digital image correlation (DIC) technique was applied to the analysis of displacement fields along the surface cracks of ultra-high-performance fiber-reinforced concrete (UHPFRC) under a bending ...load. Analysis was performed on the fiber bridging forces acting on a unit area of cracked sections belonging to different regions (perfect bonding region, partial debonding region, and full debonding region), according to the fiber distribution characterization and DIC data. By using the DIC technique, the visualization and quantification of the UHPFRC fracture properties could be determined. The length of the uniform zone increased, whereas the fracture process decreased under the external load of the limit of proportionality (LOP). With the appearance of macro-cracks under the external load of the modulus of rupture (MOR), the length of the uniform and fracture process zones increased as the fiber content increased. However, the localization zone decreased. The steel fiber volume fractions had limited influence on the tensile strain at cracking and the complete debonding of fibers.
Injured mature CNS axons do not regenerate in mammals. Deletion of PTEN, the negative regulator of PI3K, induces CNS axon regeneration through the activation of PI3K-mTOR signaling. We have conducted ...an extensive molecular dissection of the cross-regulating mechanisms in axon regeneration that involve the downstream effectors of PI3K, AKT and the two mTOR complexes (mTORC1 and mTORC2). We found that the predominant AKT isoform in CNS, AKT3, induces much more robust axon regeneration than AKT1 and that activation of mTORC1 and inhibition of GSK3β are two critical parallel pathways for AKT-induced axon regeneration. Surprisingly, phosphorylation of T308 and S473 of AKT play opposite roles in GSK3β phosphorylation and inhibition, by which mTORC2 and pAKT-S473 negatively regulate axon regeneration. Thus, our study revealed a complex neuron-intrinsic balancing mechanism involving AKT as the nodal point of PI3K, mTORC1/2 and GSK3β that coordinates both positive and negative cues to regulate adult CNS axon regeneration.
To elucidate the effect of aggregate size and volume on the non-uniform strain distribution in concrete, drying shrinkage of mortar and concretes were determined with 3D digital image correlation ...(3D-DIC). The distribution of shrinkage displacements and strains in mortar and concrete were analyzed. The results show that 3D-DIC makes it possible to measure non-uniform displacement distributions initiated by shrinkage in mortar and concrete. The non-uniformity became more remarkable with drying time. The presence of aggregates larger than 5 mm in concrete have locally changed the displacement and strain fields. Aggregates within 5–25 mm make non-uniform strain of concrete more fluctuant, especially when the aggregate size is larger than 10 mm. The maximum and minimum principal strain distributions became more heterogeneous with decreasing volume of aggregates.
Cracks in reinforced concrete provide preferential access for aggressive substances into the concrete. Therefore, the corrosion of reinforcement bars is accelerated. Besides, carbonation, sulfate ...attack, and alkali–silicate reaction take place deep inside the concrete. Fortunately, from previous experiments, it was found that cracks in concrete can be healed with water and Ca is a main chemical element of the reaction products of self-healing. However, the ion concentrations in water can be various depending on the sources of water. There is still a lack of information on the effect of ion concentrations on self-healing. In this article, the effect of Ca2+ ions on self-healing was investigated experimentally. Ca(OH)2 was added into water as a healing agent. Self-healing behavior of cracks with saturated Ca(OH)2 solution was explored and compared with that with distilled water. In order to gain deeper insight into the mechanism, the reaction products of self-healing were characterized by energy-dispersive spectroscopy, Fourier transform infrared spectroscopy, and thermogravimetric analysis. In addition, the filling fraction of cracks as a function of time was determined by means of backscattered electron image analysis. The efficiency of self-healing induced by saturated Ca(OH)2 solution was evaluated and compared with that with distilled water.
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