The use of additive manufacturing in self-healing cementitious materials: A state-of-the-art review

Sayadi S., Chang Z., He S., Schlangen E., Mihai I.C., Jefferson A.

This paper describes the development of a discrete lattice model for simulating structures formed from self-healing cementitious materials. In particular, a new approach is presented for simulating time dependent mechanical healing in lattice elements. The proposed formulation is designed to simulate the transient damage and healing behaviour of structures under a range of loading conditions. In addition, multiple and overlapping damage and healing events are considered. An illustrative example demonstrates the effects of varying the healing agent curing parameters on the computed mechanical response. The model is successfully validated using published experimental data from two series of tests on structural elements with an embedded autonomic self-healing system. The meso-scale model gives detailed information on the size and disposition of cracking and healing zones throughout an analysis time history. The model also provides an accurate means of determining the volume of healing agent required to achieve healing for all locations within a structural element. The importance of the information provided by the model for the design of self-healing cementitious material elements is highlighted.

Wan Z., Xu Y., He S., Chen Y., Xie J., Šavija B.

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 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.

van Overmeir A.L., Šavija B., Bos F.P., Schlangen E.

With the introduction of 3D concrete printing, research started on how to include reinforcement in 3D printed structures. Initial studies on the implementation of strain hardening cementitious composites (SHCC) as self-reinforcing printable mortars have shown promising results. The development of this new type of SHCC comes with additional challenges. Where SHCC by itself is already a complex material engineered to achieve specific micromechanical behaviour under tensile loading, its application in 3D printing techniques imposes even more requirements - the so-called ‘printability’ requirements. The question that rises for the development of this new material is how to achieve printability without losing strain hardening capacity. This paper investigates the influence of raw materials and additives, such as silica fume, limestone powder, viscosity modifying agents and water, on the fresh and hardened mechanical properties of printable SHCC, by improving on a previously developed mixture. The fresh material mixtures were subjected to slump flow tests to analyse their applicability for 3D printing. In hardened state, the mixtures were tested on their compressive strength and flexural strength to assess their potential for strain hardening capacity. Finally, two mixtures were selected for printing. The mixtures were assessed on print quality and buildability by the deployment of a buildability test. Furthermore, the printed elements were mechanically tested at 28 days, on compressive strength, flexural strength and uniaxial tensile strength and strain. It was concluded that the silica fume content and water to solid ratio are relevant variables for 3DP-SHCC optimization. The study has yielded two 3DP-SHCC mix designs that display significant strain hardening capacity and good printability properties.

Fernández F., Jarabo R., Asensio E., Guerrero A.

The conventional construction sector is the one that generates CO2 emissions, as well as waste generation. Faced with this problem, 3D printing has positioned itself as an alternative. Therefore, in recent years, interest in cement-based material for 3D printing has increased in the construction sector as a partial or total replacement for conventional construction methods. However, 3D printing, despite being a novel technique, has some drawbacks, one of the biggest threats being the generation of cracks or microcraks that appears by the transport or the design of the 3D printed figures. These cracks can cause major structural and durability problems in the final application. Numerous materials have been developed to meet the requirements of 3D printing. Nevertheless, there are few publications on materials that are able to be printed in 3D and have an autogenous self-healing capacity. Therefore, in this study, we are working on the development of an Engineered Cementitious Composites (ECC) material, also known as Strain Hardening Cementitious Composites (SHCC) that has the characteristics to achieve structural integrity, durability, reliability, and robustness of 3D printing (ECC-A3D). This paper describes the experimental procedure of an ECC material in two different environments (at room temperature, 34 ± 2% RH and 20 ± 2 °C, and curing chamber 98 ± 2% RH and 20 ± 2 °C). The characterization of the ECC-A3D material is studied by fresh properties, consistency, open time, extrudability and buildability and hardened properties, compressive and flexural strength up to 90 days. A 3D printing material has been achieved that reaches a value of 16.54 MPa and 50.82 MPa in flexural and compression at the hydration age of 28 days at room temperature, and 17.13 MPa and 58.26 MPa in curing chamber. The self-healing behavior of ECC is evaluated by three non-destructive methods: absorption and sorptivity tests, optical microscopy, and micro-computed tomography, confirming a reduction in absorption and crack healing during hydration time.

Ahmadi K., Mousavi S.S., Dehestani M.

Riordan C., Anglani G., Inserra B., Palmer D., Al-Tabbaa A., Tulliani J., Antonaci P.

The use of capsule-based technology for self-sealing and self-healing cementitious systems has been extensively investigated for both macro- and microencapsulated additions. In this study, macrocapsules, produced using a novel technique were characterised and compared, evaluating mechanical triggering, bonding with the cementitious matrix, and self-sealing efficiency upon integration into cementitious mortar specimens. Macrocapsules containing a commercially available water repellent agent were produced in two ways. Stereolithographic additive manufacturing (3D printing) was used to produce novel rigid acrylate macrocapsules as well as alumina ones. Cementitious macrocapsules produced with a rolling technique were also used as a comparison. The capsules were characterised in terms of watertightness, water uptake, and shell morphology. Following this, the capsules were integrated into cement mortar prisms and subjected to controlled cracking by three-point bending to evaluate the triggering and subsequent self-sealing effect. The results highlighted influential process parameters that can be optimised and explored for further capsule-based self-sealing in structural applications.

Lim T., Cheng H., Hu J., Lee Y., Kim S., Kim J., Jung W.

Studies on self-healing capsules embedded in cement composites to heal such cracks have recently been actively researched in order to improve the dimensional stability of concrete structures. In particular, capsule studies were mainly conducted to separately inject reactive healing solutions into different capsules. However, with this method, there is an important limitation in that the probability of self-healing is greatly reduced because the two healing solutions must meet and react. Therefore, we propose three-dimensional (3D) printer-based self-healing capsules with a membrane structure that allows two healing solutions to be injected into one capsule. Among many 3D printing methods, we used the fusion deposition modeling (FDM) to design, analyze, and produce new self-healing capsules, which are widely used due to their low cost, precise manufacturing, and high-speed. However, polylactic lactic acid (PLA) extruded in the FDM has low adhesion energy between stacked layers, which causes different fracture strengths depending on the direction of the applied load and the subsequent performance degradation of the capsule. Therefore, the isotropic fracture characteristics of the newly proposed four types of separated membrane capsules were analyzed using finite element method analysis. Additionally, capsules were produced using the FDM method, and the compression test was conducted by applying force in the x, y, and z directions. The isotropic fracture strength was also analyzed using the relative standard deviation (RSD) parameter. As a result, the proposed separated membrane capsule showed that the RSD of isotropic fracture strength over all directions fell to about 18% compared to other capsules.

Wan Z., Zhang Y., Xu Y., Šavija B.

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.

Nguyen M., Fernandez C.A., Haider M.M., Chu K., Jian G., Nassiri S., Zhang D., Rousseau R., Glezakou V.

De Nardi C., Gardner D., Cristofori D., Ronchin L., Vavasori A., Jefferson T.

Recently, the development of 3D mini-vascular networks has demonstrated their ability to facilitate self-healing in concrete structures. These 3D printed polylactide (PLA) hollow ligament tetrahedral shaped units (TETs) can heal multiple occurrences of damage by releasing a single-component healing agent stored within. To improve the healing efficacy of the concrete-TET system whilst overcoming the potentially short shelf life of single-component healing agents, the TETs design was modified. TETs with co-axial hollow ligaments (d-TETs) suitable for storing bi-component healing agents were manufactured. Different healing agents, including epoxy resin (i.e. cycloaliphatic epoxy resin plus hardener) and sodium silicate in isolation or in combination with either nanosilica or nanolime solution were explored. The d-TETs were effective in storing these bi-component healing agents without them undergoing premature mixing and curing. Moreover, the d-TETs successfully ruptured and released healing agents at a crack width of 0.35 mm. After one damage-healing cycle, significant strength and stiffness recovery was achieved: d-TETs hosting sodium silicate and nano-lime yielded the best strength recovery (25%), whilst most other sodium silicate-based healing agent combinations demonstrated stiffness recoveries of approximately 40%. This demonstrates the efficiency of the d-TETs concept as a system, which allows strength and stiffness recovery despite the limited volume of healing agent.

Xi B., Huang Z., Al-Obaidi S., Ferrara L.

Ultra High-Performance Concrete (UHPC) has superior mechanical properties, including high compressive strength, tensile strain hardening behavior, and self-healing capacity. However, there has been limited focus on developing predictive models for UHPC's self-healing properties, despite extensive research in the aforesaid respect. While multi-physics modeling has made progress in predicting the coupled chemical, physical, and mechanical phenomena in cement-based materials, data-driven models, including Artificial Intelligence (AI) and Machine Learning (ML), are gaining popularity in predicting some concrete properties. In this study, a machine learning model was developed to predict UHPC's self-healing performance using three meta-heuristic algorithms, i.e., whales optimization algorithm (WOA), grey wolf optimization (GWO), and flower pollination algorithm (FPA), combined with extreme gradient boosting tree (Xgboost). The dataset used for the model was obtained from original experimental tests on UHPC's crack sealing performance under sustained through crack tensile stress and exposure to various aggressive environments for up to six months. The model's predictive performance was assessed using four mathematical indicators. The regression error characteristic (REC) and Taylor diagrams also showed the optimal models’ performance were found to be consistent and reliable across different optimization algorithms. SHapley Additive exPlanation (SHAP) results revealed that exposure time and crack width were most critical features for predicting self-healing performance. The study demonstrated the potential of using machine learning for predicting UHPC's self-healing performance and provided insights into the most critical factors affecting the process.

Fernández F., Jarabo R., Asensio E., Guerrero A.

The conventional construction sector is the one that generates CO2 emissions, as well as waste generation. Among all new construction techniques, 3D printing has emerged as an alternative since it is capable of mitigating these inconveniences and has a number of advantages, such as speed of construction and design flexibility. As a result, interest in 3D printing has increased in recent years as a total or partial replacement in the construction sector. Nevertheless, 3D printing has some drawbacks; one of the biggest threats is the generation of cracks or microcracks that appears during the transport or the design of the 3D printed figures. These cracks can cause major structural and durability problems in the final applications. Numerous materials have been developed to meet the requirements of 3D printing. One of the possible developing materials is the ECC (Engineered Cementitious Composites) since it has the capacity for autogenous self-healing, that is, the material presents in its composition the capacity to reduce and heal the possible cracks that are generated. The main goal is to evaluate the self-healing behavior of the 3D printed material (ECC-A3D). This material has the characteristics to achieve structural integrity, reliability, and robustness of 3D printing. In previous stages of research, an ECC-A3D material has been developed having the characteristics to achieve the structural integrity, reliability, and robustness of a 3D printed material. This paper describes the experimental procedure of the evaluation of the self-healing behavior of the ECC-A3D in two different curing conditions (at room temperature, 35 ± 2% RH and 20 ± 2 °C, and curing chamber at 98 ± 2% RH and 20 ± 2 °C). Hardened properties are researched via compressive and flexural for up to 90 days. Meanwhile, the self-healing behavior of ECC is investigated by mechanical recovery test, and absorption and sorptivity tests. This study presents the results of the tests in a hardened state and shows the results that confirm the autogenous healing of ECC material in two different environments.

Šavija B.

Self-healing concrete has shown excellent potential in improving the durability of (reinforced) concrete structures and reducing the need for their repair and maintenance. This has been further substantiated by several successful full-scale demonstrator projects. Nevertheless, industrial uptake of the technology is lagging behind, mainly due to the higher initial cost compared to traditional concrete. In addition, it is well known that some self-healing mechanisms can have detrimental effects on properties of concrete, such as e.g., the compressive strength, making some engineers sceptical about practical applicability. With these two issues in mind, one might wonder: shouldn’t we simply apply self-healing concrete only where it is needed? This has been done in the past in so-called hybrid structures, in which self-healing concrete was used in the cover zone as a stay-in-place mold, while traditional concrete was used as infill. Additive manufacturing (3D printing) techniques offer additional possibilities in selective placement and optimization of self-healing concrete composites. Additive manufacturing provides unprecedented freedom in design and optimization of structures at virtually no additional cost. This could allow customizing the placement of self-healing agents based on structural design and loading considerations of a given structure. In this talk, recent developments and potential applications of different additive manufacturing techniques for design and fabrication of self-healing concrete will be discussed.

Rengaraju S., Al-Tabbaa A.

Self-healing technologies provide the long-term resilience of concrete structures by enabling self-diagnose and self-repair of damages (aging cracks, cyclic load damages, and corrosion-induced cracks). However, self-healing technologies require special additives and materials in addition to the ones in conventional concrete. Hence, it is often perceived to have higher environmental impacts, and therefore, it is necessary to understand the same. This study is aimed to analyse the life cycle assessment (LCA) of concrete with microcapsules produced by different techniques to investigate the sustainability of these concretes. Two microcapsule techniques, namely complex coacervation and membrane emulsification, were studied at the laboratory scale and then projected to the industrial scale. The analysis shows that the concrete with microcapsules does not adversely impact the emissions in the production stage if supplementary cementitious materials are used. Further, if the beneficial effects of the self-healing technologies are considered in the use phase, the impacts are much lower. Thus, this assessment gives meaningful insights by identifying major impacts in the production of self-healing technologies and helps to improve their design and application in concrete.

Vangansbeke E., Shields Y., De Belie N., Van Tittelboom K., Tsangouri E.

The tracking of healing on concrete slabs where dense crack patterns are formed under bending is reported using Acoustic Emission (AE) and Ultrasound Pulse Velocity (UPV). Additively manufactured polymeric networks are designed to distribute a polyurethane agent through capillary actions and under pressure to the open cracks, formed in the slabs. It is shown that the crack pattern is controlled by the geometry of the vascular networks that are positioned near the steel reinforcement. The activation of both conventional linear and interlinked web-shaped networks is monitored by AE, however in both cases the load at which the initial cracks form is lower in series with embedded networks compared to the reference series, an indication of an overall weakening effect. The area where the healing agent circulates is larger (300x400 mm2) than past tests on beams, but only local healing is evident by UPV mapping. An indirect proof of cracks filling with stiffened agent is provided by the AE pencil-lead breaking test, as the amplitude recovery after healing can be linked to crack closure. This preliminary work evaluates the design of 3D printed vascular networks, but also explores the potential of AE and UPV as inspection tools in healing studies.

Song K., Liu J., Zhao J., Zhang Q.

Zhang Y., Lu P., Fang G., Dong B., Hong S., Wang Y., Li J., Fan S.

Yen E., Mishra G., Iqbal M.I., Namakiaraghi P., Shields Y., Van Tittelboom K., De Belie N., Farnam Y.(.