Self-healing concrete, with its ability to autonomously repair damages, holds promise in enhancing its structural durability and resilience. Research on self-healing concrete in the past decade has advanced in understanding the mechanisms behind healing, exploring various healing agents, and assessing their effectiveness in concrete structures. However, the full potential of self-healing concrete remains untapped unless its effects are effectively integrated into the design practices of reinforced concrete structures. Realizing this challenge, this paper synthesizes the current research progress and discusses the possibilities to consider self-healing into design codes. The focus was placed on two specific benefits of applying self-healing concrete: one centered on durability and the other on mechanical performance. Specifically, the effect of self-healing on impeding chloride penetration into cracked reinforced concrete was discussed first. Modifications of parameters in existing predictive models based on different types of healing approaches were recommended. Furthermore, the possible impact of the self-healing capacity in mitigating the stiffness reduction of concrete was also discussed. Equations that can describe the stiffness regained due to healing action are presented. In each part of the case study, limitations and challenges still hindering standardization and wider application in the construction field are discussed.

The microstructure of cement paste determines the overall performance of concrete and therefore obtaining the microstructure is an essential step in concrete studies. Traditional methods to obtain the microstructure, such as scanning electron microscopy (SEM) and X-ray computed tomography (XCT), are time-consuming and expensive. Herein we propose using Denoising Diffusion Probabilistic Models (DDPM) to synthesize realistic microstructures of cement paste. A DDPM with a U-Net architecture is employed to generate high-fidelity microstructure images that closely resemble those derived from SEM. The synthesized images are subjected to comprehensive image analysis, phase segmentation, and micromechanical analysis to validate their accuracy. Findings demonstrate that DDPM-generated microstructures not only visually match the original microstructures but also exhibit similar greyscale statistics, phase assemblage, phase connectivity, and micromechanical properties. This approach offers a cost-effective and efficient alternative for generating microstructure data, facilitating advanced multiscale computational studies of cement paste properties.

Adding hydrated lime (CH) into blended cement incorporating high volume of Supplementary Cementitious Materials (SCMs) is a viable method to provide the necessary calcium hydroxide for the pozzolanic reaction, thereby improving the mechanical performance at later stages. However, the effects of relatively small dosages of CH on the rheological properties and resulting microstructure of limestone-calcined clay cement (LC3) remain unclear. This paper aims to investigate the influence of a small CH addition on the fresh and hardened properties of LC3 systems, in which Portland cement is largely replaced (80 wt%) by limestone and calcined clay. The results indicate that the additional CH notably reduces the water film thickness, leading to increased dynamic yield stress, plastic viscosity and re-flocculation. A delay in the elasticity development and static yield stress evolution within the first 1.5 h was observed with the addition of 2.5 wt% CH, attributed to the initial dissolution of CH, which is mitigated by using 10 wt% CH. Furthermore, additional CH accelerated early-age hydration and facilitated long-term pozzolanic reactions, resulting in the increased amount of C-(A)-S-H gel and AFm phases, and reduced porosities after 7 and 28 days. These chemical effects could well compensate the high air void content caused by the high viscosity, and therefore contributes to mortars with higher compressive strengths than plain LC3 at later ages.

Uncontrolled cracking in reinforced concrete structures accelerates durability issues by creating pathways for external agents to penetrate the matrix, often leading to costly repairs and reduced service life. This dissertation addresses these challenges through the development of a self-healing strain-hardening cementitious composite (SHCC) designed specifically for application in the concrete cover zone.

This thesis adopted a multi-faceted methodology. First, a self-healing SHCC material was developed, featuring bacteria-embedded polylactic acid (PLA) capsules to realize controlled microcracking and robust healing. Next, the research introduced a localized application strategy to address the cost-effectiveness of this material. By applying the self-healing SHCC exclusively to the concrete cover zone, the region most critical to durability, this approach minimizes unnecessary use of healing agents, balancing performance with economic viability. To validate the concept, experimental and numerical analyses were conducted to evaluate the performance of hybrid beams with self-healing SHCC covers. Furthermore, different manufacturing methods, including prefabrication and 3d printing, were explored. Lastly, design strategies were proposed to incorporate the self-healing benefits into structural service life models. The feasibility of the developed system was demonstrated at full scale by applying it in the construction of a tramline.

The study revealed that the incorporation of PLA capsules into SHCC significantly improved crack-healing efficiency while maintaining critical tensile properties. It was found that the fibre/matrix bond properties were enhanced by the addition of the HA. As a result, the addition of healing agents reduced residual crack widths by up to 70%, ensuring faster and more robust healing under varied conditions.

At the structural level, hybrid beams with SHCC covers exhibited enhanced performance. Beams with SHCC applied in the bottom cover zone demonstrated improved flexural behaviour, with controlled crack patterns and reduced crack widths, attributed to the optimized interface condition between the SHCC cover and concrete core. A novel type of SHCC/concrete interface that features a weakened chemical adhesion, but an enhanced mechanical interlock bonding was developed to facilitate the activation of SHCC. Similarly, hybrid beams with lateral SHCC layers showed a notable increase in shear resistance under critical loading conditions. Numerical simulations supported these experimental findings, revealing the importance of the interface condition between SHCC cover and concrete core.

For the developed self-healing cover system to be applied in structures, it is necessary to consider the implications of healing during the design process. Analysis of this thesis shows that, by refining existing engineering models to include the impact of cracks, it becomes possible to predict and design the healing effects under specific scenarios.
To further demonstrate the self-healing cover concept, the developed self-healing SHCC was applied in a full-scale construction project where stringent requirements for tensile performance and crack healing properties are essential. The project showcased the feasibility of large-scale mixing, pumping, and application of the self-healing SHCC system.

This thesis contributes to the field of self-healing concrete by advancing material performance, structural application techniques, and design integration. By focusing on localized and practical implementations, the research bridges the gap between experimental advancements and full-scale applications where traditional solutions do not meet demands. The findings underscore the potential of self-healing concrete to extend the service life of structures without imposing substantial additional costs.

Due to the gradual diffusion of CO₂ under natural exposure, areas with varied degrees of carbonation exist at different depths from the surface of slag-rich cement paste. While extensive research has been dedicated to investigating the fully carbonated zone as identified by phenolphthalein spray, the transitional zone, located between the fully carbonated and the uncarbonated regions, has received comparatively less attention. This study thus aims to address this research gap by exploring its microstructural, micromechanical, and mineralogical properties. The results reveal that carbonation-induced damage extends beyond the fully carbonated zone as identified by phenolphthalein. Particularly in the transitional area close to the carbonated zone, nanoindentations results reveal that micromechanical properties of this area are even lower to that of the fully carbonated zone. In addition, mineralogical investigation suggest that the depth of carbonation stays within the range where slag-containing blends loses its green coloration. By comparing specimens with different slag composition, it was found that the depth of this faded green area can be an important indicator to assess the carbonation resistance of slag-containing blends.

Concrete-to-concrete interfaces are brittle and reinforced with steel to ensure force transfer and provide ductility. Recent research in ceramics and polymers shows that by implementing intricate interlocking geometries, named bistable interlocks, toughness can be added to inherently brittle materials and their connections. In this research, the “bistable interlock” concept is applied to cementitious materials (strain-hardening cementitious composites, SHCC) offering a novel approach to increase the toughness of concrete interfaces. A bistable interlock mechanism is achieved by geometrically designing double-radii surface morphologies that can lock into two hardening positions under tensile loads and is combined with material hardening interlock of SHCC. The investigation focused on the effects of interface shape (straight vs. curved) and geometric characteristics (key length and diameter of interface keys), and interface treatments (as-cast, lubricated, and prefabricated). The findings highlight the critical role of interface treatment. Specimens with an untreated, strong interface were unable to activate bistable behavior, primarily failing due to key rupture. Lubricated interfaces facilitated key pullout, demonstrating in curved specimens up to 80% higher energy absorption compared to untreated specimens. The tensile strength of the architectured interface reached about 30% of the SHCC strength, whereas its deformation capacity was doubled. These results underscore the potential for customized, tough connections and their application in the design of precast concrete components.

Auxetic cementitious cellular composites (ACCCs) exhibit hinge-type recoverable deformation during auxetic behavior phase, a rare pseudo-elastic property in cementitious materials. However, their low load-bearing capacity during this phase restricts their use in high-load applications. This study developed ACCCs using strain-hardening cementitious composites (SHCCs) with short (SHCC-SS) and long (SHCC-LS) softening tails, fabricated by additive manufacturing-assisted casting. Uniaxial compression tests employing Digital Image Correlation (DIC) evaluated their compressive behavior, peak strength, Poisson's ratio variation, and energy dissipation. Cyclic tests after pre-compression assessed their recoverable deformation resilience, with fiber bridging at joint cracks examined using digital optical microscope. Results were compared to a reference using fiber-reinforced cementitious materials with strain softening (SS). Compared to the reference (SS), ACCCs using SHCC mixtures exhibit superior load-bearing capacity and stable auxetic behavior under compression. After self-contact, they maintain a negative Poisson's ratio up to a considerably high compressive strain, preventing splitting failure and preserving structural integrity. This is because incorporating SHCC enables greater joint rotation by promoting multiple cracks with strain hardening, which delays primary crack formation and reduces its opening. During cyclic tests, P1-shaped ACCCs with SHCC-LS and SHCC-SS enhance the elasticity modulus of recoverable deformation by 4.8 and 3.0 times, respectively, compared to SS. SHCC-LS outperforms SHCC-SS in compressive resilience due to its prolonged softening tail, which improves fiber bridging in primary cracks and increases rotational stiffness in hinge joints. SHCC mixtures with initial strain hardening and extended softening enable scalable design of advanced auxetic cementitious materials across various load levels.

This paper presents a state-of-the-art review on the application of additive manufacturing (AM) in self-healing cementitious materials. AM has been utilized in self-healing cementitious materials in three ways: (1) concrete with 3D-printed capsules/vasculatures; (2) 3D concrete printing (3DCP) with fibers or supplementary cementitious materials (SCMs); and (3) a combination of (1) and (2). 3D-printed capsules/vascular systems are the most extensively investigated, which are capable of housing larger volumes of healing agents. However, due to the dimension restraints of printers, most of the printed vasculatures/capsules are in small scale, making them difficult for upscaling. Meanwhile, 3DCP shows great potential to lower the environmental footprint of concrete construction. Incorporation of fibers and SCMs helps improve the autogenous healing performance of 3DCP. Besides, 3D-printed concrete with hollow channels as the vasculature could further improve the autonomous healing and scalability of self-healing cementitious materials. Finally, possible directions for future research are discussed.

The article ‘Effect of matrix self-healing on the bond-slip behavior of micro steel fibers in ultra-high-performance concrete’, written by Salam Al-Obaidi, Shan He, Erik Schlangen and Liberato Ferrara, was originally published in volume 56, issue 9, article 161 without Open Access.

This study investigates the structural behaviour and self-healing performance of hybrid reinforced concrete (RC) beams, enhanced with a 1.5-cm-thick self-healing cover composed of bacteria-embedded strain hardening cementitious composite (SHCC), for its potential in crack width control and crack healing. The research focuses on the performance under both flexural and shear loading, examining aspects such as load-bearing capacity, surface crack pattern, crack propagation between layers, and healing effectiveness. Results demonstrate the successful activation of the healing function, alongside improvements in structural performance. Under flexural loading, hybrid beams exhibited greater load-bearing capacity and significantly improved crack control ability. The maximum crack width of the hybrid beams exceeded 0.3 mm at 124.7 kN load, whereas in the control beam the largest crack exceeded 0.3 mm at only 59.8 kN load. Under shear loading, while the influence of the cover on structural capacity was minimal, it notably improved post-peak ductility and energy dissipation. Interface delamination was not observed in both cases. The results of the current study demonstrate the potential of delivering the self-healing mechanism precisely where it is most needed, which presents a scalable and economically viable strategy for integrating self-healing technology into standard construction practices.

Temperature Stress Testing Machine (TSTM) is a universal testing tool for many properties relevant to early-age cracking of cementitious materials. However, the complexity of TSTMs require heavy lab work and thus hinders a more thorough parametric study on a range of cementitious materials. This study presents the development and validation of a Mini-TSTM for efficiently testing the autogenous deformation (AD), viscoelastic properties, and their combined results, the early-age stress (EAS). The setup was validated through systematic tests of EAS, AD, elastic modulus, and creep. Besides, the heating/cooling capability of the setup was examined by tests of coefficient of thermal expansion by temperature cycles. The results of EAS correspond well to that of AD, which qualitatively validates the developed setup. To quantitatively validate the setup, a classical viscoelastic model was built, based on the scenario of a 1-D uniaxial restraint test, to predict the EAS results with the tested AD, elastic modulus, and creep of the same cementitious material as the input. The predicted EAS matched the testing results of Mini-TSTM with good accuracy in 6 different cases. The viscoelastic model also provided quantitative explanations for why variations in early AD do not influence the EAS results. The testing and modelling results together validate the developed Mini-TSTM setup as an efficient tool for studying early-age cracking of cementitious materials. At the end, the potential limitations of the Mini-TSTM are discussed and its applicability for concrete with aggregate size up to 22 mm is demonstrated.

Early-age cracking risk induced by autogenous deformation is high for cementitious materials of low water-binder ratios. The autogenous deformation, viscoelastic properties, and stress evolution are three important factors for understanding and quantifying the early-age cracking risk. This paper systematically reviewed the experimental and modelling techniques of the three factors. It is found that the Temperature Stress Testing Machine is a unified experimental method for all these three factors, with a strain-controlled mode for stress evolution, hourly-repeated loading scheme for viscoelastic properties, and free condition for autogenous deformation. Such unified method provides basis for developing various models. By coupling a hydration model for volume fractions of hydrates, a homogenization model for upscaling of viscoelastic properties, and capillary pressure theory for self-desiccation shrinkage, a unified model directly mapping the mix design to the early-age stress can be constructed, which can help optimize the mix design to reduce the early-age cracking risk.

Engineered cementitious composite (ECC) is widely employed in engineering due to its high toughness and ductility. The Interfacial Transition Zone (ITZ) between the fibers and the matrix plays a vital role in influencing the strength and durability of ECC. This study introduces a numerical method to simulate fiber pull-out behaviors, specifically the fiber debonding and slipping. A birth-death method is proposed to account for the mechanical transition from fiber debonding stage to slipping stage. The contributions of various phases in the ITZ are explicitly considered. Furthermore, nanoindentation tests and Backscattered Electron (BSE) imaging were conducted to determine the microstructures of ITZ and local mechanical properties of each phase within the ITZ. A series of fiber pullout experiments with polyvinyl alcohol (PVA) fibers were conducted to calibrate and validate the model. Subsequently, the validated model was employed to explore the influence of w/c ratios, fiber orientations and bonding properties on the interfacial behavior. The microstructure-informed model proposed herein effectively predicts fiber pull-out behavior, facilitating a thorough exploration of fracture mechanisms throughout the pull-out process, and serves as the basis for multiscale modeling of ECC.

This study aims to experimentally investigate the autogenous deformation and the stress evolution in restrained high-volume ground granulated blast furnace slag (GGBFS) concrete. The Temperature Stress Testing Machine (TSTM) and Autogenous Deformation Testing Machine (ADTM) were used to study the macro-scale autogenous deformation and stress evolution of high-volume GGBFS concrete with w/b ratios of 0.35, 0.42, and 0.50. The early-age cracking (EAC) risk (quantified by stress-strength ratio) and stress relaxation were analyzed extensively based on ADTM and TSTM results. Furthermore, Environmental Scanning Electron Microscopy (ESEM), X-ray Diffraction (XRD), and Mercury Intrusion Porosimetry (MIP) were conducted to explore the micro-scale origin of the autogenous deformation of high-volume GGBFS concrete, which supports the observations on the macroscale measurement of TSTM/ ADTM tests. This study finds that the ettringite formation in the first two days results in autogenous expansion, which can delay the appearance of tensile stress. The magnitude of autogenous expansion depends on the compatibility of ettringite content and pore size. The w/b ratio of 0.42 turns out to be optimal because it produces the highest amount of ettringite and results in the highest autogenous expansion. In comparison, the w/b ratio of 0.35 introduces significant autogenous shrinkage after the expansion peak and therefore corresponds to a high early-age cracking risk.

The current study investigates short-term and long-term crack-healing behaviour of mortars embedded with bacteria-based poly-lactic acid (PLA) capsules under both ideal and realistic environmental conditions. Two sets of specimens were prepared and subjected to different healing regimes, with the first set kept in a mist room for varying short durations (i.e., 1 week, 2 weeks, 3 weeks and 8 weeks) and the second set placed in an unsheltered outdoor environment for a long-term healing process (i.e., 1 year). Alteration of microstructure because of self-healing was characterized by backscattered electron (BSE) imaging and energy dispersive X-ray spectroscopy (EDS) via crack cross-sections. Results show that visible crack healing enabled by bacteria began after 2 weeks in a humid environment. The healing products initially precipitated at crack mouths and gradually moved deeper into cracks, with the precipitated calcium carbonate crystals growing larger over time. After 8 weeks, healing products can be found even a few millimetres deep inside cracks. Observations of crack healing in a realistic environment revealed significant differences compared to healing under controlled conditions. While no healing products can be found at crack mouths, a substantial healing process was observed throughout the entire crack depth. It is likely that the environmental actions such as rainfall and/or freeze and thaw cycles may have worn away the healing products at crack mouths and thus led to a deeper ingress of oxygen into cracks, which promoted the activation of healing agents and associated calcium carbonate precipitation deep inside a crack.

This paper presents an experimental study on the interface bonding properties of polyvinyl alcohol (PVA) fiber in alkali-activated slag/fly ash (AASF) pastes. Three interface bonding properties (i.e., the chemical bonding energy Gd, the initial frictional bond strength τ0, and slip-hardening behavior) were determined using single-fiber pullout tests. The microstructure and chemical composition of the reaction products in the fiber/matrix interfacial transition zone (ITZ) and the nearby matrix were also characterized to reveal the influence of PVA fiber to its surrounding matrix. It is found that Gd increases primarily with increasing Ca/(Si+Al) ratio of C-(N-)A-S-H gel. Unlike that in cementitious materials, the inclusion of PVA fiber in AASF pastes promotes the formation of a high-Ca C-(N-)A-S-H phase rather than crystalline portlandite near the fiber surface. This study provides useful guidance for tailoring the interface bonding properties of AASF and also the development of high-performance composites such as strain-hardening geopolymer composites.

In the current study, experiments were carried out to investigate the cracking behaviour of reinforced concrete beams consisting of 1-cm-thick layer of Strain Hardening Cementitious Composite (SHCC) in the concrete cover zone. The hybrid SHCC/concrete beams with different types of interfaces were tested and compared with control reinforced concrete beams without a SHCC layer. A new SHCC/concrete interface that features a weakened chemical adhesion but an enhanced mechanical bonding was also developed to facilitate the activation of SHCC. The beams were tested in four-point bending configuration, while Digital Image Correlation (DIC) was used to evaluate crack pattern development and crack widths. Results show that hybrid beams possessed similar load bearing capacity but exhibited an improved cracking behaviour as compared to the control beam. The maximum crack width of the best performing hybrid beams exceeded 0.3 mm at approximately 53.3 kN load, whereas in the control beam it exceeded 0.3 mm at only 32.5 kN load. It is thus expected that the hybrid beams developed in the current study will possess an improved durability and enhanced self-healing potential as a result of having smaller cracks, leading to an extended service life at the expense of minimal additional cost.

In 3D concrete printing, fast structuration is a prerequisite for ideal buildability. This paper aims to study the impact of inorganic additives, i.e., CaCl₂ and gypsum, on structural build-up and very early-age hydration of limestone-calcined clay-cement (LC3) pastes within the first 70–80 min. Results show that, increasing the dosage of CaCl₂ or gypsum can accelerate storage modulus G' and static yield stress evolution with time, as well as increase chemically bound water (H) content and total specific surface area (SSA_total). Furthermore, good correlations were found between G' and H content, as well as static yield stress and the ratio of free water content to SSA_total. The acceleration by CaCl₂ can be attributed to stimulating C₃S and C₃A hydration and promoting crystal formation, i.e., ettringite, portlandite, and Friedel's salt. Additionally, the increase in gypsum percentage led to a large amount of unreacted gypsum in the system, resulting in an increase in SSA_total.

In the current study, experiments were carried out to investigate the structural performance of reinforced concrete (RC) beams with a self-healing cover zone. The cover zone consists of a 1.5-cm-thick layer of bacteria-embedded strain hardening cementitious composite (SHCC) for a combination of crack width control and crack healing. The aim is to bring together two emerging technologies (i.e., self-healing and strain-hardening) that show great potential for realizing highly efficient concrete structures. RC beam without the self-healing cover was also prepared as the control specimen for comparison purposes. The experimental program includes loading the beams to failure in four-point bending configuration and sawing the beams to segments for crack pattern analysis and crack healing. Results show that the beams with self-healing cover exhibited a 45-60% improvement in structural capacity. The crack patterns of the hybrid beams were also largely modified. While the reference beam formed only a few major cracks, the hybrid beams formed around 40 fine cracks in the constant bending moment region with an average crack width smaller than 0.2 mm even at maximum load. By having an improved cracking behavior and an enhanced self-healing capacity, it is expected that the beams with a self-healing cover will possess an extended service life at the expense of minimal additional cost.

Limestone-calcined clay-cement (LC3), as one of the most promising sustainable cements, has been under development over the past decade. However, many uncertainties remain regarding its rheological behaviors, such as the metakaolin content of calcined clay. This study aims to investigate the effect of increasing the content of fine-grained metakaolin in calcined clay on the rheology of LC3 pastes. Rheological behaviors and early-age hydration of studied mixtures were characterized using flow curve, constant shear rate, small amplitude oscillatory shear and isothermal calorimetry tests. Results show that increasing the content of fine-grained metakaolin decreased flowability but promoted structural build-up and early-age hydration. These phenomena can be attributed to the decrease of mean interparticle distance caused by the increased amount of fine-grained metakaolin, which may enhance colloidal interactions, C-S-H nucleation and direct contact between particles. Overall, modifying the fine-grained metakaolin content is a feasible approach to control the rheology of LC3 pastes.