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Cementitious Capillary Crystallization Waterproofing Material (CCCW), as an efficient self-healing agent, can effectively repair damage in concrete structures, thereby extending their service life. To address the various types of damage encountered in practical engineering applications, this study investigates the impact of different mixing methods for CCCW (including internal mixing, curing, and post-crack repair) on the multi-dimensional self-healing performance of concrete. The self-healing capacity of concrete was evaluated through water pressure damage self-healing tests, freeze–thaw damage self-healing tests, mechanical load damage self-healing tests, and crack damage self-healing tests. The results show that the curing-type CCCW mixing method exhibited the best self-healing effect in repairing water pressure, freeze–thaw, and load damages, with corresponding healing rates of 88.9%, 92.7%, and 90.5%, respectively. The internally mixed CCCW method was also effective for repairing load damage in concrete, while the repair-type CCCW mixing method demonstrated the weakest repair effect on these types of damage. For concrete with induced pre-existing cracks, the internally mixed CCCW method, after 28 days of water-immersion curing, exhibited a significantly higher crack self-healing ability, with a self-healing ratio of 333.8%. Optical microscopy observations revealed that the crack surfaces were almost fully sealed, with a substantial deposition of white crystalline material at the crack sites. Further analysis using scanning electron microscopy (SEM) and X-ray Diffraction (XRD) provided insights into the surface morphology and phase characteristics of the self-healed cracks, indicating that calcium carbonate (CaCO3) and calcium silicate hydrate (C-S-H) were the main products responsible for crack healing. [ABSTRACT FROM AUTHOR]

Abstract The self-healing of concrete through the use of bacteria is an ecological, sustainable, and environmentally friendly method that can replace time-consuming and costly repairs. The aim of this research was to isolate and select spore-forming bacilli with potential for concrete self-healing. Soil samples from the limestone mines of Simbal, Trujillo, La Libertad, Peru were used. These samples were processed to primarily isolate spore-forming bacteria. The self-healing potential of these bacteria was evaluated through their ability to produce the urease enzyme, grow at an alkaline pH, and produce calcium carbonate crystals in a culture medium with urea and CaCl2 at pH 8.0. The produced crystals were visualized with scanning electron microscopy and their calcium content was confirmed through EDS analysis. The isolated bacteria were identified by 16 S rRNA gene amplification by PCR as Bacillus sp and Streptomyces sp. It was observed that Bacillus sp. produces CaCO3 crystals with vaterite morphology, while Streptomyces sp. produces rhombohedral calcite crystals. It is concluded that the isolated bacteria have the potential to be used in concrete self-healing processes.

This study examined the encapsulation of Bacillus sphaericus LMG 22257 spores in biopolymeric microcapsules (MCs) for use in cement mortar, with a focus on enhancing self-healing properties. Biopolymers, including alginate (ALG), chitosan (CTS), carboxymethyl cellulose (CMC), and ALG/CMC blends at various ratios, were employed to fabricate the MCs through ionotropic gelation and freeze–drying. The physicochemical properties of MCs, including size, morphology, and swelling behavior under simulated concrete conditions, were assessed. Among these, ALG/CMC-MCs exhibited superior characteristics and demonstrated the highest urea hydrolysis activity when incorporated into the mortar, indicating optimal spore protection. Despite an initial decrease in compressive strength, the ALG/CMC blend with an ALG:CMC mass ratio of 6:4 achieved a crack healing efficiency of 96.7% over 28 days under cyclic wet-dry conditions. These findings highlight the potential of biopolymer encapsulation for embedding functional microorganisms in construction materials, contributing to a more durable and sustainable infrastructure. [ABSTRACT FROM AUTHOR]

Soil contamination, land desertification and concrete cracking can have significant adverse impacts on sustainable human economic and societal development. Cost-effective and environmentally friendly approaches are recommended to resolve these issues. Microbially induced carbonate precipitation (MICP) is an innovative, attractive and cost-effective in situ biotechnology with high potential for remediation of polluted or desertified soils/lands and cracked concrete and has attracted widespread attention in recent years. Accordingly, the principles of MICP technology and its applications in the remediation of heavy metal–contaminated and desertified soils and self-healing of concrete were reviewed in this study. The production of carbonate mineral precipitates during the MICP process can effectively reduce the mobility of heavy metals in soils, improve the cohesion of dispersed sands and realize self-healing of cracks in concrete. Moreover, CO2 can be fixed during MICP, which can facilitate carbon neutrality and contribute to global warming mitigation. Overall, MICP technology exhibits great promise in environmental restoration and construction engineering applications, despite some challenges remaining in its large-scale implementation, such as the substantial impacts of fluctuating environmental factors on microbial activity and MICP efficacy. Several methods, such as the use of natural materials or wastes as nutrient and calcium sources and isolation of bacterial strains with strong resistance to harsh environmental conditions, are employed to improve the remediation performance of MICP. However, more studies on the efficiency enhancement, mechanism exploration and field-scale applications of MICP are needed. [ABSTRACT FROM AUTHOR]

The causes of cracks in concrete are varied, and regardless of their origin, these cracks invariably have a detrimental impact on the durability of concrete structures and escalate their maintenance costs. This paper presents a comprehensive review of current knowledge regarding the methods of self-healing in concrete, ranging from autogenic and improved autogenic self-healing to the autonomous self-healing of concrete. Particular emphasis is placed on the methods of autonomous concrete self-healing: the bacterial healing method, the crystalline hydrophilic additives healing method, and the capsule-based self-healing method. The hypothesis is that applying these self-healing methods could potentially prevent damages or cracks in concrete caused by freeze–thaw cycles, thereby extending the lifespan of concrete structures. The mechanism of action and current achievements in the field are provided for each method. [ABSTRACT FROM AUTHOR]

The research on the self-healing of concrete promoted by crystal admixtures provides a new direction for the green development of construction industry. In this study, three crystal admixtures, i.e., l-aspartic acid (LAA), Na 2 SiO 3 and nano-silica (NS), were screened from the experimental results in terms of promotion of hydration, fast reaction, and effective filling. The three components were mixed as concrete self-healing agents, and the experimental ratio was designed by response surface method. Finally, the optimal ratio was selected according to the visual healing effect. The durability of mortar with the optimum ratio of self-healing agent was studied by the experiment of resistance to chloride ion and sulfate attack; its preliminary heat release was studied by hydration microcalorimeter. The microscopic analysis was carried out by X-ray powder diffraction (XRD), thermogravimetric analysis (TG), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) test and mercury injection method (MIP). The experimental results showed that the best healing effect was achieved when the dosage of LAA, Na 2 SiO 3 and NS reached 0.5 %, 0.4 % and 0.15 % of the cement mass. In the process of healing, the early healing products are mainly calcium carbonate, and the late healing products are mainly ettringite and calcium silicate hydrate (CSH), and the healing agent effectively reduces the porosity and improves the pore structure, making the concrete matrix more dense. • The maximum crack healing rate of 263µm crystal mixed with self-healing agent was obtained by self-healing experiment. • The self-healing agent can not only repair the cracks, but also improve the pore structure. • The healing process was monitored and theoretical models of different stages of healing were established. [ABSTRACT FROM AUTHOR]

This study aims to evaluate self-healing properties and recovered dynamic moduli of engineered polypropylene fiber reinforced concrete using non-destructive resonant frequency testing. Two types of polypropylene fibers (0.3% micro and 0.6% macro) and two curing conditions have been investigated: Water curing (at ~25 Celsius) and air curing. The Impact Resonance Method (IRM) has been conducted in both transverse and longitudinal modes on concrete cylinders prior/post crack induction and post healing of cracks. Specimens were pre-cracked at 14 days, obtaining values of crack width in the range of 0.10-0.50 mm. Addition of polypropylene fibers improved the dynamic response of concrete post-cracking by maintaining a fraction of the original resonant frequency and elastic properties. Macro fibers showed better improvement in crack bridging while micro fiber showed a significant recovery of the elastic properties. The results also indicated that air-cured Polypropylene Fiber Reinforced Concrete (PFRC) cylinders produced ~300 Hz lower resonant frequencies when compared to water-cured cylinders. The analyses showed that those specimens with micro fibers exhibited a higher recovery of dynamic elastic moduli. [ABSTRACT FROM AUTHOR]

A damage-plasticity constitutive theory for zero-thickness interfaces is presented aimed at predicting time-dependent self-healing phenomena in concrete. The material model is based on fracture-energy concepts and accounts for the time evolution of concrete porosity induced by the self-healing mechanism. The accuracy and soundness of the proposed interface model are demonstrated through comparative analyses of model predictions against experimental results on three point beam tests performed up to crack opening and then, up to complete failure, before and after been subjected to different exposure conditions, respectively. A wide range of experimental tests was considered in this work for both calibration and validation purposes of the proposed interface model. [ABSTRACT FROM AUTHOR]

This study aims to evaluate self-healing properties and recovered dynamic moduli of engineered polypropylene fiber reinforced concrete using non-destructive resonant frequency testing. Two types of polypropylene fibers (0.3% micro and 0.6% macro) and two curing conditions have been investigated: Water curing (at ~25 Celsius) and air curing. The Impact Resonance Method (IRM) has been conducted in both transverse and longitudinal modes on concrete cylinders prior/post crack induction and post healing of cracks. Specimens were pre-cracked at 14 days, obtaining values of crack width in the range of 0.10–0.50 mm. Addition of polypropylene fibers improved the dynamic response of concrete post-cracking by maintaining a fraction of the original resonant frequency and elastic properties. Macro fibers showed better improvement in crack bridging while micro fiber showed a significant recovery of the elastic properties. The results also indicated that air-cured Polypropylene Fiber Reinforced Concrete (PFRC) cylinders produced ~300 Hz lower resonant frequencies when compared to water-cured cylinders. The analyses showed that those specimens with micro fibers exhibited a higher recovery of dynamic elastic moduli.

• Introducing polyurea as a new material for encapsulation of bacteria for self-healing cement paste. • Successfully encapsulation of bacteria and nutrition in polyurea microcapsules. • Characterization of synthesized polyurea capsules by destructive and non-destructive methods. • Self-healing of cement paste by using encapsulated bacteria in polyurea capsules – a proof of concept. Cracks in concrete increase permeability and exacerbate the detrimental effects of weathering and potential chemical attacks. Common repair techniques involve the use of sealant agents or utilization of biotechnology methods that can self-heal the cracks. In this study, a polyurea polymer because of its adaptable mechanical properties needed for encapsulation (e.g. moisture barrier and spore immobilisation) and its fast curing reaction is proposed as a potential suitable shield and carrier for Bacillus pseudofirmus to produce calcium carbonate (CaCO3) crystal. Synthesis and encapsulation of the bacteria in the polyurea were performed using in-situ polymerization. Chemical characterization and microstructural analysis of the produced polyurea capsules show that the Bacillus pseudofirmus bacterial spores and nutrition were successfully encapsulated without affecting its chemical structure. The bacterial growth further was confirmed by integration of several morphological analyses. The polyurea microcapsules carrying the bacteria were well-mixed in a cement paste. The hardened specimens were cracked using a three-point bending apparatus to artificially trigger the healing effect of the bacteria capsules. The morphology and chemical characterization confirmed precipitation of calcium carbonate around the cracked zone. Polyurea encapsulation was found to be a practical approach to protect bacteria included in cementitious material for the purpose of self-healing. [ABSTRACT FROM AUTHOR]