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.

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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.

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Salt (NaCl) weathering in mortar, can be mitigated by incorporating a crystallisation inhibitor (sodium ferrocyanide) during mortar preparation. However, the service-life of the inhibitor is limited, due to its susceptibility to leaching out of mortar. Encapsulating the inhibitor in chitosan-calcium alginate capsules has demonstrated controlled-release of the inhibitor and therefore, reduction in its leaching. Nevertheless, the addition of capsules may have a negative effect on mortar's properties and/or its salt-weathering resistance. In this research, natural hydraulic lime (NHL) and commercial cement-based mortars were prepared, with encapsulated inhibitor and with directly mixed-in inhibitor. Mechanical and physical properties of mortars were assessed experimentally. An accelerated NaCl-weathering test was performed to assess the durability of mortar to salt damage. The damage evolution was assessed visually and by quantifying material loss, efflorescence and leaching of the inhibitor. At the end of the test, crystal morphology inside the pores was examined using SEM. The results show that adding inhibitor, both in encapsulated and mixed-in form, had a negligible effect on the properties of NHL and cement-based mortars. Compared to the reference mortar without inhibitor, NHL-mortars with mixed-in inhibitor and encapsulated inhibitor had a better durability to salt damage, showing negligible material loss. The capsules facilitated controlled-release and reduced leaching of the inhibitor. In cement-based mortars including the reference, no damage was observed; still, the inhibitor was shown to be effective in modifying the salt crystal habit. The results show that encapsulation of the inhibitor can improve the service-life of the mortar without compromising its performance.@en

This study presents comprehensive numerical modeling methods for simulating early-age stress (EAS) relaxation in cementitious materials, based on the autogenous deformation (AD), elastic modulus, creep, and stress continuously tested by a mini temperature stress testing machine (Mini-TSTM) and a mini AD testing machine from a very early age (i.e., from a few hours to a week). Four methods for converting creep compliance to relaxation modulus were discussed in detail and used for the one-dimensional (1D) and three-dimensional (3D) simulation of stress evolution in the Mini-TSTM test. Furthermore, virtual creep and relaxation tests were conducted using an exponential algorithm with either the Kelvin or Maxwell chains to show their applicability in simulating the viscoelastic behavior of early-age cementitious materials. The results showed that the exponential algorithm with the Maxwell chain using an exponential conversion function from creep to relaxation obtains good prediction accuracy of EAS in 3D analysis. The numerical solutions of the Volterra integral of creep compliance can lead to a negative relaxation modulus, thus introducing stress calculation errors in both 1D and 3D analysis.

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Crystallisation inhibitors, such as sodium ferrocyanide (NaFeCN), are highly effective in mitigating NaCl-induced weathering in lime-based mortars; however, direct addition of NaFeCN in lime-mortars increases its susceptibility to leaching and rapid depletion, thus compromising long-term performance. Here, we present hydrogel-capsules for the controlled-release of NaFeCN within hydraulic mortars for the prolonged prevention of salt weathering. Capsules were prepared by complexing chitosan and calcium-alginate in different ratios containing different concentrations of NaFeCN. The release of NaFeCN from these capsules was measured in (1) simulated lime-mortar solution (2) from mortar specimens incorporated with calcium alginate (CA) and chitosan-calcium-alginate (Cs-CA) capsules using ultraviolet–visible light spectrophotometry and Inductive Coupled Plasma-Optical Emission Spectroscopy. Mortars containing Cs-CA capsules exhibited controlled-release of NaFeCN with four times lower effective diffusion coefficient, compared to incorporating NaFeCN directly in mortar. Conversely, mortar containing CA capsules (without chitosan) released NaFeCN rapidly. Thus, chitosan’s presence in CA is necessary for tuning NaFeCN release and the reason may be attributed to chitosan’s role in reducing CA’s permeability and chitosan’s electrostatic-attraction to ferrocyanide anions, slowing diffusion of the latter. In conclusion, using Cs-CA capsules can control the release of NaFeCN within mortar, providing a steady NaFeCN supply to prolong mortar’s resistance against salt damage.@en

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.

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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.

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Spalling is the main problem concerning safety in concrete structures under rapid heating. Despite being studied for nearly a century, its prediction remains a challenge. In addition, the main recommendations for its prevention are merely the addition of a fixed amount of polypropylene fibres, without accounting for mix design. This paper presents an experimental study focusing on the thermo-hygral mechanism contribution to spalling. The use of mercury intrusion porosimetry and X-ray microtomography for the determination of pore connectivity is considered a key parameter. The results of the experiment are used for the development and calibration of an analytical equation capable of predicting spalling in concrete samples. The equation is then validated using over 100 examples extracted from literature, achieving an accuracy of 91.2%. Based on the validation, the proposed equation is a step forward in the prediction of concrete spalling based on the mix design parameters.

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This study employs a lattice fracture model to simulate static and fatigue fracture behaviour of Interfacial Transition Zone (ITZ) at microscale and mortar at mesoscale. The heterogeneous microstructure of ITZ and mesostructure of mortar are explicitly considered in the models. The initial step involves calibrating and validating the microscopic model of the ITZ through micro-cantilever bending tests. Subsequently, this validated ITZ model serves as a constitutive law to simulate the fracture behavior of mortar at the mesoscale using an uncoupled upscaling method. The influence of microstructural features, such as w/c ratio and microscopic roughness, on the fracture behaviour of ITZ is investigated. Moreover, the effect of ITZ properties and stress level on the fracture performance and fatigue damage evolution of mortar is also studied. The simulation results for both the ITZ and mortar demonstrate good agreement with experimental results. The proposed two models provide insights into the fracture mechanisms and fatigue damage evolution in cementitious materials subjected to static and cyclic loadings.

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An asphalt joint is formed when a fresh mix is laid and compacted next to an existing layer, brings about temperature difference during compaction, and therefore requires extra care in quality control and expose to higher cracking risks. Self-healing asphalt aims to stimulate the healing capacity of asphalt mixture and prolong its service life. The main objective of this study is to develop and optimize a calcium alginate capsules healing system for an asphalt joint mix. Capsules following two different self-healing concepts were prepared, namely conventional alginate capsules and conductive alginate capsules. Microscopy, Computed Tomography (CT) and Thermogravimetry analysis (TGA) were used to investigate the performance of alginate capsules. The results show that both types of capsules have a porous structure and a stable performance under high temperature, and therefore potentially survive from the asphalt mixing and production process. These capsules will be implemented and evaluated in full asphalt mix in future research.

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Fatigue Life Monitoring is crucial to ensuring the safety and durability of engineering structures. This paper presents an innovative approach for fatigue life monitoring using a PZT (Lead Zirconate Titanate) piezoelectric sensor operating in buckling mode to measure applied cyclic strains. A significant focus is placed on the utilization of a 3D-printed 'Extension' platform upon which the PZT sensor is installed, allowing it to operate in buckling mode while subjected to cyclic strains. An analytical framework is developed to establish a direct relationship between dynamic strain values and the sensor's output, considering critical strain attributes such as amplitude and strain rate. The analytical solutions are validated through a series of experimental tests conducted under various dynamic loading conditions. Integrating the piezoelectric sensor with the 3D-printed extension demonstrates high sensitivity and serves as a passive dynamic strain measurement sensor, with promising potential for application as sensor nodes in fatigue life monitoring of engineering structures.

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Self-healing cementitious materials show potential to reduce material consumption, maintenance costs, and environmental impacts within the construction sector. This study explores the feasibility of installing a ductile-porous vascular network in a series of retaining walls under realistic construction conditions, with the objective of both assessing the efficacy of the self-healing system and addressing any constructability issues that may arise. Numerical modelling was performed first to determine a suitable mix design and wall configuration that would promote cracking, so cracks would appear without mechanical intervention. The predicted crack distribution informed the optimal network configuration. Healing and sustainability considerations are discussed, and the benefits of implementing this technology are evaluated. When comparing a single maintenance activity using a vascular network versus manual repair, there is no significant benefit of using a vascular network. However, environmental impacts are substantially reduced when using a vascular network once multiple repair actions are considered.

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Polyvinyl alcohol fiber reinforced engineered cementitious composite (ECC) using piezoelectric polymer film has attracted significant interest due to its energy harvesting potential. This work provides a theoretical model for evaluating the energy harvesting of bendable ECC using surface-mounted polyvinylidene fluoride (PVDF). In the mechanical part, concrete damage plasticity model based on the explicit dynamic analysis was utilized to simulate the dynamic flexural behavior of ECC beam under different dynamic loading rates. The mechanism of force transfer through the bond layer between the PVDF film and ECC specimen was simulated by a surface-surface sliding friction model wherein the PVDF film was simplified as shell element to reduce computational cost. Then, the electromechanical behavior of the piezoelectric film was simulated by a piezoelectric finite element model. A simplified model was also given for a quick calculation. The theoretical model was verified with the experimentally measured mechanical and electrical results from the literature. Finally, a parametric analysis of the effects of electromechanical parameters on the efficiency of energy harvesting was performed. The verified theoretical model can provide a useful tool for design and optimization of cementitious composite systems for energy harvesting application.@en

Engineering structures, such as bridges, wind turbines, airplanes, ships, buildings, and offshore platforms, often experience uncertain dynamic loadings due to environmental factors and operational conditions. The lack of knowledge about the load spectrum for these structures poses challenges in terms of design and can lead to either over-engineering or catastrophic failure. This research introduces a robust and innovative device, analogous to a "Fitbit" for structures, capable of measuring complex loading conditions throughout the structure's lifespan. The proposed approach involves developing a middleware, referred to as an "extension," which facilitates the transfer of mechanical deformation to a piezoelectric sensor. This approach overcomes challenges associated with directly attaching piezoelectric sensors to the structure's surface such as rupture possibility in higher strain and attaching on rough surfaces. The feasibility study primarily focuses on validating the performance of the extension and monitoring variation trends. The ultimate objective is to develop an Internet of Things (IoT) sensor node capable of measuring applied cyclic loads. To achieve this goal, an electronic system and embedded software will be developed to capture the complex load spectrum and convert it into a fatigue damage index for predicting the structure's fatigue life. The collected data will be transmitted to the user through a wireless communication platform. The proposed sensor design is versatile, allowing for both attachment and embedding and is demonstrated here for monitoring fatigue in engineering structures.

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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.

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The present paper reports an overview of mechanochromic self-reporting thin-ply hybrid composite sensors, which are designed to visually indicate overload in structures. These sensors, made from combinations of high-strain and low-strain materials, change appearance earlier than the final fracture, providing a clear visual cue of damage like delamination or strain overload. They can be used for various applications, for example, overload monitoring, and to detect barely visible impact damage in composites.@en

In circumstances with wastewater and seawater, the behavior of multi-ions including calcium, chloride and others in concrete attracts attention. The present study investigated the multiple mechanisms that could happen under the special field situation above, including calcium leaching, chloride transport and multi-ion coupling. To realize the interactive ingress of multi ions, the simulation method for the processing of the individual mechanisms and the mutual influences is adopted. The distributions of the diversified ions are analyzed with the influence of the interfacial transition zone. The time-spatial distribution of porosity and its evolution mechanisms are investigated by considering the interaction with calcium ions in both pore solution and solid phase. The results indicate that calcium leaching would dominantly speed up chloride transport due to the coarsened pore structure, while the multi ions electrochemical coupling effect would facilitate calcium leaching in the early stage but subtly delay calcium leaching in the later stage.@en

The implementation of extrusion-based 3D concrete printing (3DCP) in large-scale constructions is currently limited by concerns regarding rheology control and the sustainability of this process. To address these issues, this study presents an approach to develop limestone-calcined clay-based cementitious (LC3) materials accelerated by Ca(NO₃)₂ solution in an inline static mixer-based 3DCP setup. Using this approach, a printable mixture containing only about 275 kg/m³ of Portland cement was formulated that can exhibit a good buildability performance and a 28-day compressive strength of over 30 MPa. Additionally, the effects of adding Ca(NO₃)₂ solution on the initial setting time, structural build-up, inline buildability, early-age hydration, and compressive strength of LC3 materials were investigated and discussed. Results show that the addition of Ca(NO₃)₂ solution improved the buildability and accelerated initial setting as well as the structuration due to the promoted ettringite precipitation and C–S–H nucleation. Furthermore, compressive strength at 7 and 28 days was improved through increasing the Ca(NO₃)₂ dosage, which can be attributed to the formation of NO₃-AFm and the increase in the amount of C–S–H gels.

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Crystallisation due to commonly occurring salts like sodium chloride (NaCl) is a known cause of damage in the built environment. Use of crystallisation inhibitors is a potential solution to reduce salt decay in building materials. Researchers have reported lower damage when sodium ferrocyanide (NaFeCN), a known NaCl crystallisation inhibitor, is mixed in fresh mortar. However, the high solubility of NaFeCN in water could make it susceptible to leaching and thus diminish its effect over time.@en

Plasters and renders used in historic monuments are vulnerable to degradation caused by salt weathering. Crystallisation inhibitors (molecules/ions that alter salt crystallisation) mixed into mortars have shown promising results in mitigating salt damage by inhibiting salt crystallisation, promoting salt transport to the evaporating surface, and modifying crystal habit. However, past research suggests that inhibitors easily leach out from mortars, meaning their long-term positive effect is lost. Encapsulation of an inhibitor within a mortar is a potential solution to minimise leaching. Herein, capsules composed of a polyelectrolyte complex of calcium alginate coated in chitosan are investigated for the controlled diffusive release of sodium ferrocyanide, a known NaCl crystallisation inhibitor. Capsules with varying chitosan-calcium alginate ratios are prepared using the extrusion dripping technique. The release of the inhibitor from capsules in solutions of various pH values ranging from 7–13 is investigated. Results show that increasing the capsule’s chitosan to calcium alginate ratio reduces the inhibitor release for all studied solution pH values compared to pure calcium-alginate capsules. Therefore, a controlled inhibitor release can be obtained by tuning the chitosan-alginate ratio. In future, additional tests will be performed to find suitable capsule compositions for optimising their performance when mixed in mortars.@en