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This paper investigates damage and healing of cementitious materials based on fly ash (FA) with a special focus on the effect of repeated loading on the self-healing performance. In total, 90 prism specimens and 33 cylinder specimens were prepared and tested. Variables including the FA replacement ratio and preloading level were considered. Different methods were adopted to evaluate the self-healing performance, i.e., mechanical tests, acoustic emission (AE) analysis, water absorption measurement, and scanning electron microscope (SEM) observation. Test results revealed that an appropriate amount of cement replaced by FA was beneficial to the self-healing performance of the specimens subjected to a high level of repeated preloading, where the strength increased by 11% and the water absorption rate decreased by 12%, in comparison with that of the undamaged specimens. This was attributed to the active hydration of FA, which was confirmed by an energy dispersive X-ray spectroscopy (EDS) analysis. This study extends understanding of the self-healing performance when subjected to repeated loading and provides some useful suggestions for the self-healing technique based on mineral additives. [ABSTRACT FROM AUTHOR]

Self-healing of cracks in cementitious composites is of great significance to improve the serviceability of concrete structures. In this study, poly urea–formaldehyde (PUF) microcapsules enclosing epoxy resins were synthesized. The damage- and healing process of cement paste incorporating microcapsules was in situ detected by acoustic emission (AE) technique, in which passive AE and active AE were combined to provide complementary information about the damage and crack formation. The two representative AE signals, i.e., matrix cracking and debonding of the interface, were used as calibration for further AE post-processing analysis. The effects of the concentration of microcapsules and the level of pre-damage were investigated. The results revealed the distinguished cracking mechanisms according to the differentiated feature of the signals in terms of the temporal and spectral AE descriptors. The plot of average frequency (AF) versus RA index (rise time/amplitude) confirms that the cracking modes contribute to the characteristic spectrum. [ABSTRACT FROM AUTHOR]

The effective mechanical properties of matrices healed by embedded microcapsules were investigated using Eshelby theory based on the microscale healing mechanisms, referring to the mechanical properties of the adhesive and the proportion of cracks that could be healed. This was determined by measuring the water absorption and vacuum saturation of precracked matrices. The microcapsules or cracks should not exceed 4 vol.% to ensure that approx. 90% of the modulus is retained even if the cracks cannot be reattached at all. Both the healed cracks and unhealed cracks contribute to the anisotropy, with a greater contribution from the unhealed cracks. The elastic modulus of the adhesive has a more significant influence on the anisotropy as the applied load increases. The measured stiffness recovery indicates that the cracks were only partially healed. This value corresponds to a healing efficiency index of 0.2–0.4 for the present conditions. [ABSTRACT FROM AUTHOR]

The effects of the addition of minerals of various compositions (i.e., silica-based materials, chemical expansive agents, swelling minerals and crystalline components) on the self-healing performances of cementitious materials were investigated. The self-healing capabilities were assessed by performing water permeability tests, quantifying the widths of the surface cracks and studying the water absorption of mortars that were pre-cracked by either splitting or compression. The results showed that the cracks that appeared early on healed more efficiently when they were cured in still rather than flowing water. High pHs and high temperatures accelerate crack healing. The healing efficiency can be further improved by utilizing a combination of minerals rather than a single mineral. A self-healing mechanism was discussed by combining these results with micro-observations. The precipitation of calcium carbonate, which is aided by higher pH values and higher calcium ion contents, was found to be the main contributor to the healing of surface cracks. [ABSTRACT FROM AUTHOR]

Capsule-based self-healing of cementitious materials is a promising solution to improve the serviceability and to prolong the longevity of concrete structures. The successful use of mechanical triggering microcapsules to achieve cracks healing is still challenging and is closely dependent on the breakage of capsules upon crack formation. In this study, self-healing microcapsules were synthesized via complex coacervation using a styrene maleic anhydride (SMA) shell and an epoxy resin adhesive. The SMA/gelatin bi-layer wall formed with gelatin as the out layer for better trigger sensitivity under crack formation. The effects of various preparation parameters, referring to stirring rate, temperature, core-shell ratio and pH, have been investigated by characterizing the microcapsule morphology, shell thickness, size distribution, and chemical nature. The morphology demonstrates the encapsulation ability of SMA/gelatin and the spherical shape facilitates the rupture under crack formation. The elastic modulus and hardness of the microcapsules were determined using nano-indentation to assess the interfacial bonding and likelihood of mechanical triggering. Self-healing functionality of the developed microcapsules were validated in paste according to the in situ Acoustic Emission measurement. [ABSTRACT FROM AUTHOR]

Highlights • The self-healing capability of steam-cured mortar was investigated for the first time. • AE signals can distinguish damage stage between healing products and host matrix. • The amount and energy of healing products can be qualitatively assessed by AE signals. Abstract The objective of this work is to investigate the self-healing performances of steam-cured mortars with mineral additives (e.g., calcium-based materials, chemical expansive agents and crystalline components). The self-healing capabilities of crack was evaluated by transport tests and acoustic emission (AE) analysis for cracked mortars. The two representative AE parameters, i.e., amplitude and energy, were used to reveal the characteristics of healing products. The results showed that the improvement of transport properties of cracked steam-cured mortars were more effective compared to standard-cured mortars. The healing efficiency can be further improved by utilizing mineral additives. The differences of damage stage between healing products and host matrix can be distinguished by AE signals. Upon reloading of steam-cured mortar with mineral materials, the number of amplitude were much larger and the cumulative energy was much higher compared to other mortar. It indicated that the steam-cured mortar with mineral materials possessed larger amount and much higher energy healing products in the crack to some extent, which were well consistent with that of transport properties and microscopic observation. [ABSTRACT FROM AUTHOR]