Self-healing Cementitious Composites Research Articles

Autolytic capsules incorporating alkali-activated slag system for self-healing cementitious composites

The optimization of healing materials used in capsules for effective crack self-healing has garnered significant attention. This study focused on the development of a novel capsule using a full-solid-waste alkali-activated slag system. Instead of traditional cement clinker and chemical agents, healing materials such as red mud, calcium carbide residue, phosphogypsum, and ground granulated blast furnace slag were used. The capsules were autolytic due to the retardation and weak expansion properties of gypsum. The healing mechanisms were mainly attributed to the formation of ettringite and calcium silicate hydrate. Crack width, crack area, and strength recovery tests were used to thoroughly analyze the self-healing behavior of the capsules. Detailed characterizations were conducted to understand the healing products and the evolution process. The results showed that cracks with initial widths less than 350 μm were well-healed, with a healing ratio ranging from 55% to 100% for all cracks (initial widths <600 μm). The cementing characteristics of the healing products were significantly increased by the formation of substantial ettringite, which led to an enhanced strength recovery ratio. Crucially, the composition of the healing products closely resembled the hydrates in the matrix, suggesting that the healed mortar had a high level of system compatibility. These results highlighted the potential of the developed alkali-activated slag capsule in promoting self-healing of cracked cementitious composites.

Microcapsule-enabled self-healing concrete: A bibliometric analysis

With the development of self-healing technology, the overall properties of the microcapsule-enabled self-healing concrete have taken a giant leap. In this research, a detailed assessment of current research on the microcapsule-enabled self-healing concrete is conducted, together with bibliometric analysis. In the bibliometric analysis, various indicators are considered. The current state of progress regarding self-healing concrete is assessed, and an analysis of the temporal distribution of documents, organizations and countries of literature is conducted. Later, a discussion of the citations is analyzed. The research summarizes the improvements of microcapsule-enabled self-healing cementitious composites and provides a concise background overview.

Monitoring the self-healing evolution of cementitious mixtures with superabsorbent polymers through air-coupled ultrasound

Self-healing cementitious composites including superabsorbent polymers (SAPs) have been studied as an alternative to costly, labour-intensive repairs. Whereas the assessment of the healing ability can be performed through a variety of testing methodologies, an in-situ evaluation requires the adoption of a non-destructive technique in order to maintain the present infrastructure. Additionally, the assessment method should be easily applicable for reasons of time efficiency. Thanks to their sensitivity to the elastic properties, ultrasonic waves have enabled the evaluation of the regained mechanical performance, without interfering with the microstructure. While coupled ultrasonic measurements are an established procedure, the use of air-coupled ultrasound provides the advantage of omitting the transducers’ coupling. As the coupling is a time-consuming task and imposes a certain variability between performed measurements, the adoption of air-coupled ultrasound would lead to a significant improvement of the evaluation method. Therefore, a comparison between both techniques using various mortar mixtures was performed.

Bacterial Viability in Self-Healing Concrete: A Case Study of Non-Ureolytic Bacillus Species.

Cracking is an inevitable feature of concrete, typically leading to corrosion of the embedded steel reinforcement and massive deterioration because of the freezing-thawing cycles. Different means have been proposed to increase the serviceability performance of cracked concrete structures. This case study deals with bacteria encapsulated in cementitious materials to "heal" cracks. Such a biological self-healing system requires preserving the bacteria's viability in the cement matrix. Many embedded bacterial spores are damaged during concrete curing, drastically reducing efficiency. This study investigates the viability of commonly used non-ureolytic bacterial spores when immobilized in calcium alginate microcapsules within self-healing cementitious composites. Three Bacillus species were used in this study, i.e., B. pseudofirmus, B. cohnii, and B. halodurans. B. pseudofirmus demonstrated the best mineralization activity; a sufficient number of bacterial spores remained viable after the encapsulation. B. pseudofirmus and B. halodurans spores retained the highest viability after incorporating the microcapsules into the cement paste, while B. halodurans spores retained the highest viability in the mortar. Cracks with a width of about 0.13 mm were filled with bacterial calcium carbonate within 14 to 28 days, depending on the type of bacteria. Larger cracks were not healed entirely. B. pseudofirmus had the highest efficiency, with a healing coefficient of 0.497 after 56 days. This study also revealed the essential role of the cement hydration temperature on bacterial viability. Thus, further studies should optimize the content of bacteria and nutrients in the microcapsule structure.

A resilience assessment framework for microencapsulated self-healing cementitious composites based on a micromechanical damage-healing model

In this paper, a resilience assessment framework for microencapsulated self-healing cementitious composites is proposed based on a micromechanical damage-healing model. A 3D micromechanical analytical model is constructed to analyze the performance evolution during the damage-healing process of self-healing concrete. The resilience assessment of microencapsulated self-healing concrete is defined by virtue of the residual stiffness, self-healing effect on stiffness and damage cumulative on stiffness, which corresponds to three main features of resilience; namely, the robustness, recoverability and adaptability. The assessment results indicate that the release of healing agents within microcapsules and healing process of extended microcracks allows the microencapsulated self-healing concrete to have higher resilience than conventional concrete. Moreover, a parameter sensitivity analysis is conducted to investigate the influence of the healing efficiency, the applied initial damage and the fracture toughness of the repaired microcrack on resilience of microencapsulated self-healing concrete. The results indicate that higher healing efficiency and applied initial damage leads to high resilience, and fracture toughness of the repaired microcrack makes less difference to the results. The findings of this paper lay a theoretical foundation for the resilience design of self-healing material layer of underground structures.

Direct ink writing of vascularized self-healing cementitious composites

Direct ink writing of cementitious materials can be an alternative way for creating vascular self-healing concrete by intentionally incorporating hollow channels in the cementitious matrix. In this study, a 3D-printable fibre reinforced mortar was first developed. Three groups of specimens were fabricated using direct ink writing, where the two top and bottom printing layers were printed with different printing directions. The macrostructure of the hardened specimens was studied using CT scanning. Four-point bending tests were carried out to investigate the initial flexural strength and the strength recovery after healing with injected epoxy resin. Furthermore, water permeability test was used to evaluate the healing potential of the samples. The results from CT scanning show that printing direction influences the actual volumes of hollow channels and the volume of small pores which are a consequence of the deposition process. The hollow channels of all samples were squeezed by the upper layers during the printing process, and the longitudinally printed samples were the most affected. When printing direction changes from longitudinal to transverse, the initial flexural strength decreases. Similarly, the average permeability of the cracked samples increases when the printing direction changes from longitudinal to transverse. Although the healing effectiveness regarding flexural strength is remarkable for all specimens, it was only possible to perform a single healing process as hollow channels were then blocked by the epoxy resin. The rough surface of the hollow channels is inferred to make it difficult to extract the epoxy resin out of the specimens.

Autogenous self-healing of polypropylene fiber reinforced ettringite-based composite

This study intends to advance the poor knowledge on the ability of an ettringite-based binder to obtain autogenous self-healing and its relevant mechanism. We developed a high-performance polypropylene fiber-reinforced cementitious composite using a slag-based ettringite binder and investigated its autogenous healing property under water-saturated conditions. Cracks were created via four-point bending loads on prismatic specimens at different crack opening displacements (i.e., 0.5, 1, and 2 mm in total crack opening). After curing times, the mechanical retention and recovery of the damaged composite were compared with the sound sample. We found that the material effectively heals micro cracks (i.e., under 100 μm) after 1 month of aging, while larger cracks could not completely close during the total curing times (up to 3 months). The healing products were mainly CaCO3 and ettringite observed through both experiments and thermodynamic modelling. These crystals mainly formed from the carbonation of ettringite and other AFm phases in aqueous conditions. The strength retention slightly reduced (about 10%) after healing, however, the composite still exhibited deflection-hardening behavior. Findings show the high potential of ettringite-based binder to be a matrix for self-healing cementitious composites. Further improvements, nevertheless, are needed to increase the healing rate and strength recovery of the composite.

The effect of oxygen and water on the provision of crack closure in bacteria-based self-healing cementitious composites

This study determined the impact of oxygen and water on the crack healing performance of bacteria-based self-healing cementitious composites (BBSHCCs). Oxygen and water are considered essential to BBSHCCs using non-ureolytic bacteria, but in practice, the availability of either, for even short periods of time, cannot be guaranteed in the immediate aftermath of a crack forming. In this study, four healing regimes, (i) daily wet-dry cycle, (ii) weekly wet-dry cycle, (iii) semi-submerged and (iv) a deposition medium, were utilised to compare the impact of oxygen and water on the performance of BBSHCCs. Furthermore, an oxygen-releasing coating (ORC) containing calcium peroxide was evaluated and used in this study to further understand the function of oxygen in bacteria-based self-healing. Results showed that bacteria-based self-healing was only effective in wet conditions that also permit oxygen to penetrate the crack. Where there was a possibility for the composite to dry, after cracking, despite being wet some of the time the self-healing was far less efficient. It was observed that an overdose of nutrients and calcium in the healing environment resulted in early rapid healing, followed at a later stage by the dissolution of the initially formed precipitates. Moreover, it was found that the ORC contributed significantly to the recovery of both crack width and water tightness of samples. These findings will aid the development of appropriate test standards for self-healing cementitious composites and for the design and implementation of such cementitious composites in practice.

Aerobic non-ureolytic bacteria-based self-healing cementitious composites: a novel approach without added calcium precursor

Self-healing concrete has been widely researched to reduce the cost of repairing and maintaining concrete infrastructure. Microbially induced calcite precipitation (MICP) is a promising solution that uses bacteria to produce calcite within cracks and seal them, preventing further deterioration. However, protecting the self-healing agents, including calcium precursors, bacteria, and growth nutrients, from the concrete matrix can be challenging, and encapsulation methods can lead to strength loss, slowed cement hydration, and complicated manufacturing. Therefore, in this study, to reduce the need for protective shells and their negative impact, we investigated the role of aerobic non-ureolytic bacteria in the healing process and determine the feasibility of inducing calcite precipitation without extra added calcium precursor in the concrete matrix. This study investigated the self-healing efficiency of this novel bacteria-based self-healing cementitious composites (BBSHCC) via crack observation, permeability test and compositions’ analysis. Samples at different curing ages were prepared as well to clearly indicate the impact of minerals of cementitious composites on the microbial activities. The novel BBSHCC samples, consisting solely of bacteria and nutrients, demonstrated exceptional self-healing ratios in terms of crack closure and water tightness regain. These ratios exceeded 95% and 80%, respectively, after 28 days of healing, irrespective of the curing ages. This demonstrates the high potential of using calcium minerals naturally present in the cement matrix as a calcium source for aerobic non-ureolytic bacteria Bacillus cohnii to activate biomineralization and achieve healing. Notably, with increasing curing age of the novel BBSHCC, the rate of crack closure decreased, which was likely due to decreased accessibility of calcium for biomineralization. Additionally, healing products generated by biomineralization tended to initially form locally around cementitious composites, especially in mature samples. Further analysis of the cementitious composites near the healed crack revealed a large presence of portlandite, which was suggested to be a result of biomineralization.

Development of novel self-healing strain-hardening cementitious composites (SH2CC) for dynamic cyclic loading conditions using mineral and polymer admixtures

Dynamic cyclic loading commonly encountered in engineering practice can pose great threat to the integrity and durability of concrete infrastructures. Ductile materials with enhanced self-healing performance could provide a promising solution under such conditions. This study proposes and investigates, for the first time, as self-healing SHCC (SH2CC) for dynamic reversed-cyclic (DRC) applications through incorporating a mineral substitute (MS) comprising of reactive MgO and quicklime and triethanolamine (TEA) additive in a high-volume-fly-ash matrix. Different DRC preloads were considered. Results indicated that applying 5% MS combined with 1.5% TEA additive showed optimal crack width control below 12 μm under up to 5 cycles of DRC preloading and the highest potential for mechanical strengthening of modulus of rupture, first cracking strength and flexural stiffness reaching up to 160%, 137% and 210% respectively after healing. Moreover the highest crack sealing efficiency was observed up to 88% under 14d water immersion. Microstructural analysis revealed that MS produced additional hydrates and promoted more carbonates to form in cracks during healing, while TEA effectively paused hydration under air curing but resumed the process upon water contact for massively enhancing crack healing (especially mechanical strengthening). Combining MS and TEA in SH2CC produced complementary healing products both in cracks and fibre-matrix interfaces indicating better filling and bridging of the cracks and demonstrating great potential for enhancing healing performance under aggressive loading conditions.

Self-healing cementitious composites with a hollow vascular network created using 3D-printed sacrificial templates

Additively manufactured vascular networks have great potential for use in autonomous self-healing of cementitious composites as they potentially allow multiple healing events to take place. However, the existence of a vascular tube wall may impede with the healing efficiency if it does not rupture timely to release the healing agent. The issue of vascular material design has therefore been a major topic of research. To overcome this, dissolvable Polyvinyl Alcohol (PVA) filament is adopted in this study to fabricate the vascular networks. Fabricated networks are coated with wax, placed in cementitious mortar and removed upon hardening, thereby leaving a network of hollow channels. Different printing directions were expected to affect the dissolvability of printed structures and were therefore fabricated and tested. Different shapes (i.e., 2D and 3D) of vascular networks were printed and embedded in the cementitious mortar. Four-point bending tests and permeability tests were performed to investigate the healing efficiency. Multiple healing cycles were applied in the cracked specimens. The results show that the vertically printed PVA tubes with wax coating have good dissolution behaviour. As expected, the existence of vascular networks decreases the initial flexural strength of the specimens. In terms of healing efficiency, excellent mechanical and water tightness recovery were achieved when using epoxy resin as the healing agent. The mechanical recovery after the first healing process is higher than the following healing process. The watertightness of the cracked samples keeps decreasing with the increase of healing cycles. Specimens embedded with 3D vascular networks have higher healing potential than those utilizing 2D vascular networks.

Mechanical characteristics of fiber-reinforced microcapsule self-healing cementitious composites by orthogonal tests

This study combined the microcapsule self-healing system with fiber reinforced concrete (FRC), and investigated the effects of microcapsule content, mineral admixture type, mineral admixture content, and fiber type on tensile and compressive strength, ductility, and self-healing properties of the fiber-reinforced microcapsule self-healing cementitious composites by the orthogonal test method. For the uniaxial tensile and cubic compression tests, the optimum tensile strength group consists of a 2.2 mass ratio of GGBS, PE fiber, and no microcapsules. The optimum compressive strength group consists of a 2.2 mass ratio of GGBS, hybrid fiber, and no microcapsules. Microcapsule content and fiber type are the main factors affecting tensile properties, and an increase in the microcapsule content will reduce the tensile strength and compressive strength of the matrix. The optimum tensile strain group consists of 10% microcapsule content and a 2.2 mass ratio of GGBS and PE fiber. The results of the energy-based steady-state crack propagation test show that the fiber type is the most critical factor affecting the ductility, and the tensile ductility of the PE fiber group is higher than that of the PVA fiber group. The scanning electron microscopy/energy dispersive X-ray spectrometry (SEM/EDS) diagram proved the healing effect of the fiber-reinforced microcapsule self-healing cementitious composite.

Chloride Ion Corrosion Resistance of Innovative Self-Healing SMA Fiber-Reinforced Engineering Cementitious Composites under Dry-Wet Cycles for Ocean Structures

To evaluate the chloride ion corrosion resistance of proposed innovative self-healing concrete based on shape memory alloys (SMA) and engineering cementitious composites (ECC), a total of 2 kinds of 22 specimens were prepared. Chloride ion corrosion tests of self-healing SMA-ECC concrete under dry-wet cycles were carried out. It was found that the chloride ion erosion depths of SMA-ECC were significantly smaller than that of MC, and the growth rate of erosion depth of SMA-ECC was obviously smaller than that of MC after 15 dry-wet (dry and wet) corrosion cycles. The chloride ion content of SMA-ECC vanished at the erosion depth more than 10 mm, which was consistent with the test result of AgNO3 solution color-rendering test. Test results indicate that, compared to marine concrete (MC), SMA-ECC has a better chloride ion corrosion resistance behavior. Moreover, the chloride ion concentration of SMA-ECC at a chloride ion erosion depth of less than 10 mm decreased more significantly than that of MC, indicating that almost all chloride salt solution reacted in the outer layer of SMA-ECC, which is consistent with the conclusions of 4.1 and 4.2. Finally, based on the erosion distribution of chloride ions and Fick’s second law, a calculation model describing the relationship between the apparent chloride ion diffusion coefficient and the boundary condition of the chloride ion content was proposed.

Improved anti-corrosion and mechanical aspects of reinforced cementitious composites with bio-inspired strategies

The device of self-healing cementitious composites is a most anticipated approach for repair of unavoidable cracks through biological intrusions. The bio-inspired self-healing composites systematically heal the cracks and control the penetration of corrosive ions towards the embedded steel. However, the effect of bacterial additions on the electrochemical performance of embedded steel holds several questions and needed to be explored further. Therefore, this research was carried out to assess the effect of a calcifying bacterial strain “Bacillus safensis” and its carrier media, biochar, on mechanical and transport properties along with the corrosion performance of embedded steel in cementitious composites. The boosted strain energy storing potential with 25.5% and 38.18% enhanced flexural and compressive strength than control sample, respectively was observed. Additionally, about 89% gain in compressive strength was observed after 28 days of cracking at 85% of ultimate compressive strength. Moreover, a significant increase in relative healing degree was seen during ultrasonic pulse velocity measurements of uncracked and cracked samples. Furthermore, the healing precipitate was recognized as calcite through scanning electron microscopy and Fourier transform infrared spectroscopy. The lower sorptivity trends also indicated the denser microstructure with minimum water uptake capability and porosity which lead towards minimum corrosion damage in the embedded steel. Besides, the corrosion inhibition efficiency was noted as 95.18% compared to the control sample. Thus, the proposed combination of biotic and abiotic materials would be a good solution for recycling waste and enhancing mechanical and durability prospects of reinforced cementitious composites.

Influence of printing direction on 3D-printed vascular based self-healing cementitious composites

Compared with other self-healing mechanisms, embedding vascular networks in cementitious matrix enables repairing wider cracks and performing multiple healing cycles. For vascular-based self-healing cementitious composites, additive manufacturing (AM) allows fabricating vascular structures with complex geometry. For Fused Filament Fabrication (FFF), printing direction has great influences on the properties of the 3D-printed vascular network. To timely release the healing agent when cracks occur, selecting the proper printing direction for the vascular network in self-healing concretes is necessary. In this work, two commercial printing filaments, i.e., polylactic acid (PLA) and polyvinyl alcohol (PVA), are used to create the vascular structures. The influence of printing direction on the dissolution of 3D-printed PVA structure is investigated. Besides, the flexural strength and water tightness of samples with PLA vascular printed in different printing directions are compared. After the 4-point bending tests, epoxy resin is manually injected into the vascular networks to seal the cracks. It is found that the strength recovery and the permeability of the cracked specimens are influenced by the printing direction due to the different crack closure after unloading.

Monitoring of self-healing cementitious materials through contactless ultrasound

Self-healing cementitious composites provide an alternative to labour-intensive and costly manual repairs. While a cementitious blend possesses an inherent ability to repair its own damage through autogenous healing, an enhancement of the self-healing capacity can be obtained through the inclusion of superabsorbent polymers (SAPs). The implementation of such innovative materials within the construction industry requires proper evaluation methods to ensure a safe environment for the user. Over the past few years, contact ultrasonic measurements have proven their potential in assessing the self-healing progress. The sensitivity of ultrasonic waves to the elastic properties of the material under study allows for a direct link with the regained mechanical performance. Additionally, its non-destructive nature enables in-situ evaluations. However, the coupling of the sensors leads to a certain variability in the obtained results, as the application of the sensors is not identical between measurements. In an effort to increase the reliability of the results, contactless ultrasound can be applied, which is investigated in the present research.

Avaliação de concreto autocicatrizante através do encapsulamento de metasilicato de sódio em perlita e argila expandidas

abstract: Investigating the behavior of self-healing cementitious composites is necessary to know alternatives that can be applied in structures increasing their life service. Therefore, this study evaluated concrete self-he aling from the use of expanded perlite (EP) and expanded clay (EC) capsules impregnated with a sodium metasilicate solution. These materials were used to substitute natural aggregates in concrete in proportions of 0 wt·%, 15 wt·% and 30 wt·% which were cured in humid or submerged environments. Cracking was induced with a flexural test and a closing with cicatrization product. was evaluated and measured visually with a software. Capillary absorption tests indicated a reduction in the porosity of samples which incorporated self-healing materials, considering it as an important property related to durability. Samples with EP achieved 100% self-healing with 15% substitution. Crack filling was achieved in cracks up to 0.43 mm wide. Samples with EC achieved 50% crack recovery under humid curing and 90% under submerged curing. It was concluded that incorporating the sodium silicate allowed improvements to fissure sealing and it is an alternative to produce self-healing concrete in Brazil. EP was more effective than EC as encapsulating material. Despite that, the EP did not impact the compressive strength due to its small size and better packing of the mixture, Also, EP presented higher healing percentage when comparing with samples containing EC.

Evaluation of Self-Healing Efficiency of Microcapsule-Based Self-Healing Cementitious Composites Based on Acoustic Emission

Microcapsule self-healing technology is one of the effective methods to solve the durability problem of cement-based composites. The evaluation method of the self-healing efficiency of microcapsule self-healing cement-based composites is one of the difficulties that limits the self-healing technology. This paper attempts to characterize the self-healing efficiency of microcapsule self-healing cement-based composites by acoustic emission (AE) parameters, which provides a reference for the evaluation of microcapsule self-healing technology. Firstly, a kind of self-healing microcapsules were prepared, and the microcapsules were added into the cement-based composites to prepare the compression samples. Then, the specimen with certain pre damage was obtained by compression test. Secondly, the damaged samples were divided into two groups. One group was directly used for compression tests to obtain the damage failure process. The other group was put into water for healing for 30 days, and then compression tests were carried out to study the influence of self-healing on the compression failure process. During the experiments, the AE signals were collected and the AE characteristics were extracted for the evaluation of self-healing efficiency. The results show that the compression pre damage test can trigger the microcapsule, and the compression strength of the self-healing sample is improved. The failure mechanism of microcapsule self-healing cement-based composites can be revealed by the AE parameters during compression, and the self-healing efficiency can be quantitatively characterized by AE hits. The research results of this paper provide experimental reference and technical support for the mechanical property test and healing efficiency evaluation of microcapsule self-healing cement-based composites.

Compressive properties of self-healing microcapsule-based cementitious composites subjected to freeze-thaw cycles using acoustic emission.

Microcapsule self-healing technology is an effective scheme to improve the durability of cementitious composites. In this paper, the compressive properties of microcapsule-based self-healing cementitious composites after freeze-thaw cycles were studied using acoustic emission (AE), and the changes in AE characteristics, compressive strength, mass loss rate, and electric flux of microcapsule-based self-healing cementitious composites with different microcapsule contents and freeze-thaw cycles were studied. The results show that if the content of the microcapsule is appropriate, with the increase in the number of freeze-thaw cycles, the AE hits will generally increase first and then decrease, and the early AE events will also decrease. Because of the different contents of microcapsules, the improvement effect and defect effect change dynamically with the number of freeze-thaw cycles, which is also reflected in the dynamic process of compressive strength. After 100 freeze-thaw cycles, the compressive strength of self-healing cementitious composite samples with 5% content of microcapsules and 3% content of microcapsules is the highest. The changes in mass loss rate and electric flux are similar to the AE characteristic parameters, which further verifies the results of AE. The research results of this paper provide experimental data and experimental methods for the engineering application of microcapsule self-healing cement-based composites in cold areas.

Polydopamine@carbon nanotube reinforced and calcium sulphoaluminate coated hydrogels encapsulating bacterial spores for self-healing cementitious composites

Bacterial spores have been applied to develop self-healing cementitious composites, while protections of endospores in concrete are required. Herein, one reinforced calcium alginate hydrogel with a protective shell was designed to encapsulate bacterial spores. The hydrogels were reinforced by highly dispersed carbon nanotubes (CNTs) coated with polydopamine, which led to more spherical shaped hydrogels, hence calcium sulphoaluminate (CSA) cement can be uniformly coated on the hydrogels to form the protective shell. With the reinforcement of CNTs and the coating of CSA cement, the hydrogels were conducive to germination, growth and activities of bacteria even after the hydrogels endured long-term exposure to strongly alkaline and calcium concentrated solution. Due to these positive effects of the modified hydrogels on spores, concrete with spores encapsulated in the modified hydrogels obtained enhanced self-healing performance in terms of crack width and permeability reductions as more biological mediated calcium carbonate was precipitated within cracks.