Microcapsule-enabled self-healing concrete:A bibliometric analysis

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

The Prediction of Self-Healing Capacity of Bacteria-Based Concrete Using Machine Learning Approaches

Advances in machine learning(ML)methods are important in industrial engineering and attract great attention in recent years.However,a comprehensive comparative study of the most advanced ML algorithms is lacking.Six integrated ML approaches for the crack repairing capacity of the bacteria-based self-healing concrete are proposed and compared.Six ML algorithms,including the Support Vector Regression(SVR),Decision Tree Regression(DTR),Gradient Boosting Regression(GBR),Artificial Neural Network(ANN),Bayesian Ridge Regression(BRR)and Kernel Ridge Regression(KRR),are adopted for the relationship modeling to predict crack closure percentage(CCP).Particle Swarm Optimization(PSO)is used for the hyper-parameters tuning.The importance of parameters is analyzed.It is demonstrated that integrated ML approaches have great potential to predict the CCP,and PSO is efficient in the hyperparameter tuning.This research provides useful information for the design of the bacteria-based self-healing concrete and can contribute to the design in the rest of industrial engineering.

Distribution of Calcium Carbonate in the Process of Concrete Self-healing

The complete deposition distribution process of calcium carbonate is summarized in three directions of cracks. Distribution of calcium carbonate in the self-healing process of microbial concrete is studied in detail, with the help of a variety of analytical techniques. The results show that carbonate deposits along the x-axis direction of the cracks. The farther from the crack surfaces of concrete matrix in x-axis direction, the more the content of the substrate, the less content of calcium carbonate. Gradual accumulation of calcium carbonate along the y-axis direction is like building a house with bricks. Different repair points are gradually connected, and ultimately the whole of cracks are completely filled. In the z-axis direction, calcium deposits on the surface of fracture direction, when the crack is filled on the surface, because the internal crack hypoxia in the depths of cracks hardly produces calcium carbonate.

Self-healing of Cracks in Concrete with Various Crystalline Mineral Additives in Underground Environment

Cracks can deteriorate mechanical properties and/or durability of concrete. A few studies have shown that, cracks can autogenously heal under a certain conditions besides the traditional passive repair with a deliberate external intervention. For underground concrete structures, the presence of water, as a necessity for chemical reactions of the healing additives, is beneficial to healing concrete. In this paper, a natural healing method by mineral additives was developed according to the chemical and physical characteristics of underground environment. The healing capacity of three different crystalline mineral materials classified namely, carbonate, calcium sulphoaluminate expansive agent and natural metakaolin due to permeation- crystallization, expansion and pozzolanic reaction, has been assessed from the mechanical properties, referring to the relative elastic modulus, the strength restoration, and the water permeability of the healed specimens. In addition, the morphology of the healing products in the vicinity of the crack was observed. The results indicate that the specimens incorporated with the three mineral additives show different healing capacity according to the improved mechanical properties and permeability. The permeability of the host matrix decreased a lot after crack healing by natural metakaolin followed by carbonate whereas no noticeable improvement of water permeability has been observed for the specimens mixed with expansive agent. The specimens incorporated with carbonate show the best mechanical restoration in terms of relative elastic modulus and compressive strength. Although the dominate element is CaCO3 by reaction of CO32-, either from the dissolved CO2 or from the additives, and Ca2＋ in the cementitious system to fill the cracks, the healing capacity depends greatly on the morphology and the properties of the newly formed products.

Effects of hydrogel-encapsulated bacteria on the healing efficiency and compressive strength of concrete

Microbial-induced calcium carbonate precipitation is a promising technology for self-healing concrete due to its capability to seal microcracks.The main goal of this study was to evaluate the effects of adding hydrogelencapsulated bacteria on the compressive strength and the self-healing efficiency of concrete.To achieve this objective,12 sets of mortar samples were prepared,including three different mineral precursors(magnesium acetate,calcium lactate,and sodium lactate),at two concentrations(67.76 and 75.00 mM/L),and under two different biological conditions(with and without bacteria).In addition,a set of plain mortar samples was prepared to serve as a control.For each sample set,three mortar cubes and three beams were prepared and subjected to compression and flexural strength tests.From the compression tests,it was found that the sample containing calcium lactate along with yeast extract and bacteria displayed the best results.As for the flexural tests,once cracked,the beams were subjected to 28 d of wet/dry cycles(16 h of water immersion and 8 h of drying),where the bottom crack width was monitored(at 0,3,7,14,28 d of wet/dry cycles).Once the sample with the highest healing efficiency was identified(the one containing calcium lactate and hydrogel-encapsulated bacteria),the study was scaled up to concrete specimens.Two sets of concrete cylinders(consisting of three control samples and three samples with bacteria along with calcium lactate)were tested under compression in order to evaluate the effect of the bacteria-precursor combination on the concrete mechanical properties.The samples that yielded the greatest compressive strength were the ones containing calcium lactate and bacteria,displaying an improvement of 17%as compared to the control specimen.Furthermore,a flexural strength recovery analysis was performed on the concrete specimens revealing that the control showed better flexural strength recovery than the bacteriacontaining variant(41.5%vs.26.1%)after 28 d of wet/dry cycles.A healing efficiency analysis was also performed on the cracked samples,revealing that the control displayed the best results.These results are due to the fact that the control specimen showed a narrower crack width in comparison to the bacteria-containing samples.

A state-of-the-art review of the development of self-healing concrete for resilient infrastructure

The brittleness of cement composites makes cracks almost inevitable,producing a serious limitation on the lifespan,resilience,and safety of concrete infrastructure.To address this brittleness,self-healing concrete has been developed for regaining its mechanical and durability properties after becoming cracked,thereby promising sustainable development of concrete infrastructure.This paper provides a comprehensive review of the latest developments in self-healing concrete.It begins by summarizing the methods used to evaluate the self-healing efficiency of concrete.Next,it compares strategies for achieving healing concrete.It then discusses the typical approaches for developing self-healing concrete.Finally,critical insights are proposed to guide future studies on the development of novel self-healing concrete.This review will be useful for researchers and practitioners interested in the field of self-healing concrete and its potential to improve the durability,resilience,and safety of concrete infrastructure.

Microbial-inspired self-healing of concrete cracks by sodium silicate-coated recycled concrete aggregates served as bacterial carrier

Microbially induced carbonate precipitation(MICP)is a promising technique for the autonomous healing of concrete cracks.In this study,the effect of pH on MICP was investigated.The results indicate that the MICP process was inhibited when the pH was higher than 11.Both vaterite and calcite were produced when the pH was<8,whereas only calcite was produced when the pH was>8.Recycled concrete aggregates(RCA)coated with sodium silicate have been proposed as protective carriers for microbial healing agents.Although the presence of the coated RCA resulted in a loss of the splitting tension strength of the concrete,the loaded healing agents were highly efficient in self-healing cracks.Concrete incorporated with 20%RCA loaded with healing agents exhibited the best self-healing performance.When the initial crack widths were between 0.3 and 0.4 mm,the 7-d mean healing rate was approximately 90%.At 28 d,the crack area filling ratio was 86.4%,while its water tightness recovery ratio was 74.4%and 29.8%,respectively,for rapid and slow absorption.This study suggests that RCA coated with sodium silicate is an effective method for packaging microbial healing agents and has great potential for developing cost-effective self-healing concrete.

Multifunctional, Sustainable, and Biological Non-Ureolytic Self-Healing Systems for Cement-Based Materials

Microbially induced calcium carbonate(CaCO_(3))precipitation(MICP)has been investigated as a sustain-able alternative to conventional concrete remediation methods for improving the mechanical properties and durability of concrete structures.To date,urea-dependent MICP is the most widely employed MICP pathway in biological self-healing concrete research as its use has resulted in efficient CaCO_(3) precipita-tion rates.NH_(3) is a byproduct of ureolysis,and can be hazardous to cementitious structures and the health of various species.Accordingly,non-ureolytic bacterial concrete self-healing systems have been developed as eco-friendly alternatives to urea-dependent self-healing systems.Non-ureolytic pathways can improve the physical properties of concrete samples and incorporate the use of waste materials;they have the potential to be cost-effective and sustainable.Moreover,they can be applied in terrestrial and marine environments.To date,research on non-ureolytic concrete self-healing systems has been scarce compared to that on ureolytic systems.This article discusses the advances and challenges in non-ureolytic bacterial concrete self-healing studies and highlights the directions for future research.

Effi ciency of Concrete Crack-healing based on Biological Carbonate Precipitation

The aim of this study was to improve the capacity for crack-repair in concrete by developing a new way. The self-healing agent based on biological carbonate precipitation was developed. Crack-healing capacity of the cement paste specimens with this biochemical agent was researched. Scanning electron microscopy（SEM） and X-ray diffraction（XRD） were used to characterize the precipitation in cracks.The healing efficiency was evaluated by measuring the water permeability after crack healing as well.The experimental results show that the applied biochemical agent can successfully improve the self-healing capacity of the cement paste specimens as larger cracks can be healed. The cracks with a width of 0.48 mm in the specimens with the biochemical agent are nearly fully healed by the precipitation after 80 d repair. SEM and XRD analysis results demonstrate that the white precipitation in cracks is calcium carbonate, which displays spherical crystal morphology. Meanwhile, the water permeability test result shows that the biochemical agent can significantly decrease the water permeability of the cement paste specimens, the water permeability of specimens with the biochemical agent respectively decreases by 84% and 96% after 7 d and 28 d immersion in water, however the control specimens only respectively decrease by 41% and 60%, which indicates that the bacteria-based concrete appears to be a promising approach to increase concrete durability.

Living concrete with self-healing function on cracks attributed to inclusion of microorganisms: Theory, technology and engineering applications——A review

Concrete is the most widely used composite material in civil engineering.Microbial induced calcium carbonate precipitation(MICP)is a green and environmental friendly technology,which has received extensive attention in repair of concrete cracks.This paper introduces the research progress in Southeast University research in past 16 years.In the early stage,MICP technology of urea hydrolyzed by Bacillus pasteurii was mainly investigated to repair the surface cracks and to fill large-size cracks with grouting.However,aiming at the hidden cracks that were difficult for human intervention,a new mineralization route of Bacillus mucilaginosus was proposed,which could repair faster than Bacillus alcalophilus,and the problem of ammonia emission in the repair process of Bacillus pasteurii was also solved.In addition,in order to improve the protection of bacteria and the self-healing efficiency of the later age cracks,the methods of fiber immobilization,carrier uniformly immobilization and core-shell structural immobilization had been compared and studied.The results showed that core-shell structural immobilization had good protection ability and strong designability.What’s more,the paper also summarized the characteristics of spore germination,cell activity,nucleation and biological calcium carbonate in crack zone,and introduced the application experience of microbial self-healing concrete in water conservancy projects and subway stations.

The effects of mismatch fracture properties in encapsulation-based self-healing concrete using cohesive-zone model

Finite element analysis is developed to simulate the breakage of capsule in capsule-based self-healing concrete.A 2D circular capsule with different core-shell thickness ratios embedded in the mortar matrix is analyzed numerically along with their interfacial transition zone.Zero-thickness cohesive elements are pre-inserted into solid elements to represent potential cracks.This study focuses on the effects of mismatch fracture properties,namely fracture strength and energy,between capsule and mortar matrix into the breakage likelihood of the capsule.The extensive simulations of 2D specimens under uniaxial tension were carried out to investigate the key features on the fracture patterns of the capsule and produce the fracture maps as the results.The developed fracture maps of capsules present a simple but valuable tool to assist the experimentalists in designing appropriate capsule materials for self-healing concrete.

Computational modeling of fracture in capsule-based self-healing concrete: A 3D study

We present a three-dimensional(3D)numerical model to investigate complex fracture behavior using cohesive elements.An efficient packing algorithm is employed to create the mesoscale model of heterogeneous capsulebased self-healing concrete.Spherical aggregates are used and directly generated from specified size distributions with different volume fractions.Spherical capsules are also used and created based on a particular diameter,and wall thickness.Bilinear traction-separation laws of cohesive elements along the boundaries of the mortar matrix,aggregates,capsules,and their interfaces are pre-inserted to simulate crack initiation and propagation.These pre-inserted cohesive elements are also applied into the initial meshes of solid elements to account for fracture in the mortar matrix.Different realizations are carried out and statistically analyzed.The proposed model provides an effective tool for predicting the complex fracture response of capsule-based self-healing concrete at the meso-scale.

Bio-based and nature inspired solutions: A step toward carbon-neutral economy

Bio-based and nature-inspired solutions have been investigated recently to develop sustainable,resilient,and durable construction including but not limited to roadway infrastructures.This paper reviews state-of-the-art studies on self-healing,self-cleaning and self-rejuvenating asphalt,and concrete construction.This review draws three conclusions.(1)Self-healing construction materials have the potential to significantly extend the service life of construction elements.Urban and industrial wastes such as food waste,biomass,metals have been used to create self-healing construction materials that are more environmentally friendly.(2)Self-cleaning construction materials not only remove pollution by repelling water on their superhydrophobic surface,but also cut building and infrastructure maintenance costs,while improving cities'air quality by degrading pollutants such as NOx.Pavement engineers have exploited self-cleaning characteristic to facilitate the de-icing of pavements and lengthening the service life of pavements.(3)Self-rejuvenating materials including bio-oils can revitalize materials and delay aging;bio-oils can also be used to make bio-binders,thereby reducing the need for petroleumbased binders.The optimum concentration of bio-oil for asphalt modification depends on the chemical structure of oils.Still,regardless of dosage,self-rejuvenating binders improve asphalt workability and performance at low temperatures and increase the resistance of the asphalt mix to fatigue and cracking.This review also identified critical research gaps,including(1)the lack of a reliable,unified,and standard method to accurately measure construction materials’self-healing,self-cleaning and self-rejuvenating properties;(2)the lack of longterm field performance data to conduct comprehensive life cycle assessment and life cycle analysis;(3)the lack of accurate technoeconomic analysis to facilitate market entry of abovementioned solutions.Addressing these gaps and determining contribution of nature-inspired and bio-based technologies to a carbon neutral economy along with issuing carbon certificates can facilitate the widespread application of these technologies while promoting resource conservation and sustainability.

Self Healing Asphalt Concrete-state of the Art

Similar to engineering materials used in civil engineering,aerospace engineering and other applications,an asphalt concrete also suffers from fatigue cracking due to a cyclic loading conditions on the road,which results in a decrease of the stiffness and the strength. The self healing capability has been investigated widely due to its unique contribution to the recovery of the stiffness and the strength from cracking and then to extend the service life of asphalt pavements. In this paper,general concepts of self healing and self healing asphalt concrete were introduced. Then,the parameters that influence the self healing capability of the asphalt binder and asphalt concrete were discussed. Inspired from other self healing materials,the possibilities of applying microcapsule self healing system in asphalt concrete were investigated in detail.