Auxetic cementitious cellular composites (ACCCs) possess advantageous mechanical properties in static tests, such as high fracture resistance and efficient energy dissipation. However, little attention has been given to understanding the impact resistance of ACCCs. In this study, two typical elliptical-shaped ACCC specimens, P25 and P50, were designed with major axis lengths increased by 25 % and 50 %, respectively, compared to the reference P0 with circular holes. The specimens were architected through additive manufacturing (AM) assisted casting, and subjected to low-velocity impacts from Schmidt hammer with a consistent initial impact energy. Their impact resistance was assessed based on impact responses, including rebound value, absorption energy, localized damage in the impact zone, crack propagation, and peak reaction force during impact. Besides single impact tests, multiple impact tests were conducted until specimens failed. Their impact results were compared with those of the reference (P0). A high-speed camera was further used for Digital Image Correlation (DIC) to analyze strain distribution of the specimens during the brief impact period. Furthermore, a numerical model considering strain rate effects was developed to simulate the impact behavior of ACCCs, demonstrating good agreement with experimental data. On this basis, a parametric analysis was performed to evaluate the effects of impact energy, relative density, specimen size, and RVE size on impact resistance. Both experimental and numerical results indicate that ACCCs demonstrate superior impact resistance compared to the reference (P0). They exhibit mitigated localized damage in the impact zone and increased contact stiffness. Moreover, ACCCs show greater endurance under multiple impacts and higher accumulated energy absorption until failure. This enhanced performance is attributed to auxetic behavior, which draws more material into the impact zone for dispersing energy and reducing localized damage, thereby maintaining overall structural integrity. Specifically, P50 exhibits higher impact resistance than P25 due to the enhanced auxetic behavior resulting from its greater aspect ratio. This creates a greater bending moment to enable more ligaments to dissipate energy through rotation-induced plastic deformation, thereby reducing localized damage. Considering the widespread availability of cementitious materials, this study highlights the potential of ACCCs for lightweight, high-performance protective structural materials for impact mitigation in infrastructure.

Portland cement paste has a highly heterogenous evolving microstructure that complicates the development of stronger and greener cementitious materials. Microstructure is the fundamental input of multiscale studies on material behaviors. Herein, we propose a conditional generative AI framework for synthesizing high-fidelity 3D microstructures of hydrating cement paste (1–28 days) with varying water-to-cement ratios and Blaine fineness values. A latent diffusion transformer, operating within a compact two-stage latent space derived via a vector quantized variational autoencoder, efficiently captures and reproduces experimentally measured microstructural patterns. Statistical analyses confirm strong consistency in grey value distributions, micromechanical properties, hydration phase evolution, and particle size distributions, with only minor boundary-related discrepancies. Validation using a pretrained classifier further corroborates the fidelity of generated microstructures. This approach provides a robust tool for realistic cement paste microstructure generation, supporting multiscale modeling and advancing the design of sustainable cementitious materials.

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.

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.

Auxetic cementitious cellular composites (ACCCs) exhibit desirable mechanical properties (e.g., high fracture resistance and energy dissipation), due to their unique deformation characteristics. In this study, a new type of cementitious auxetic material, referred to as peanut shaped ACCC, has been designed and subsequently architected using additive manufacturing techniques. Two peanut shaped ACCCs specimens with different pseudo-minor axes have been tested under uniaxial compression with Digital Image Correlation (DIC) to assess their compressive behavior, peak strength, Poisson's ratio, and energy dissipation capacity. Additionally, cyclic tests were conducted to investigate their compressive resilience properties, further elucidated through microstructural analysis using a digital optical microscope. The mechanical test results were also compared with those of previously developed elliptical-shaped ACCCs. Furthermore, a numerical model was used to simulate the mechanical behavior of peanut shaped ACCCs under uniaxial compression, and showed a good agreement with the experimental data. The auxetic behavior observed in peanut shaped ACCCs arises from the rotation of sections facilitated by fiber bridging at the ligament of adjacent holes within the cementitious unit cell. In comparison to elliptical-shaped ACCCs, peanut shaped ACCCs can exhibit a slightly more negative Poisson's ratio and mitigate stress concentration. The reduction of stress concentration enables peanut shaped ACCCs to dissipate substantial energy, showcasing enhanced ductility and toughness. In cyclic tests, peanut shaped ACCCs exhibit superior recoverable deformation elasticity, attributed to robust fiber bridging capacity. The exceptional mechanical properties exhibited by peanut shaped ACCCs offer a scalable solution for developing energy-absorbent and multifunctional cementitious materials for smart infrastructure.

The early-age viscoelasticity of alkali-activated slag concrete (AASC) is critical for its early-age cracking proneness and long-term performance, particularly regarding creep and internal stress development. This study employs an innovative approach to quantify the early-age viscoelastic behavior of AASC, utilizing a Temperature Stress Testing Machine to conduct compressive, repeated and minutes-long creep tests, covering the curing age from 6 h till 28 days. This study is based on the linear theory of viscoelasticity and the Boltzmann superposition principle. A double power law function is employed to model creep and to further predict the internal stress of restrained AASC. It is demonstrated that the double power law function accurately captures the short-term creep of AASC, enabling reliable predictions of early-age stress accumulation and relaxation. Overall, this study highlights the pronounced viscoelasticity of AASC and the effectiveness of the experimental and modelling approaches used to quantify it.

Monitoring of gradual increase in elastic modulus of concrete over time is crucial for designing structures exposed to early age loading and predicting long-term deformations, such as creep. Two primary methods are used to assess elastic modulus: the static method, involving compression tests, and the dynamic method, utilizing approaches like EMM-ARM (E-modulus Measurement through Ambient Response Method), impact-echo, and ultrasonic approach. The static method, although destructive, yields the static or secant modulus, directly applicable for structural design. However, it cannot be utilized to track changes in elastic modulus within the existing structure caused by factors such as hydration, freeze-thaw, or chemical attack. In contrast, the non-destructive dynamic method can monitor these changes in the existing structure. Yet, the elastic modulus obtained through this method, known as the dynamic elastic modulus, represents the initial tangent modulus and is generally higher than the static modulus. To estimate the static elastic modulus through the non-destructive ultrasonic approach, we propose a new signal processing technique using direct wave interferometry (DWI) in this study. To validate the elastic modulus estimated through this technique, embeddable ultrasonic sensors are installed in the specimen within the temperature stress testing machine (TSTM). The experimental results show that the elastic modulus derived from the newly proposed DWI-based ultrasonic approach consistently provides more accurate estimates of the static elastic modulus compared to the UPV-based dynamic elastic modulus. The relative errors between the DWI-based and compression test-based elastic moduli on 7-day is 2.6 %. This approach also enables the tracking of static elastic modulus changes due to freeze-thaw cycles or chemical attacks.

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.

This paper presents a comprehensive investigation on the positive potential of steel slag (SS) to mitigate the autogenous shrinkage of alkali-activated slag (AAS) while maintaining a reasonably high strength. Changes of the physicochemical properties of AAS with the addition of SS were examined in terms of hydration heat, autogenous shrinkage, chemical shrinkage, internal relative humidity (RH) and mechanical behaviors. The microstructure of AAS-SS systems was characterized using X-ray diffraction, thermogravimetric analysis and nitrogen adsorption techniques. The shrinkage mechanism and quantification approach of the AAS-SS systems were discussed, in addition to a sustainability assessment. The results indicate that the 7-day autogenous shrinkage of AAS paste was decreased by 16 %, 35 % and 42 % when SS was incorporated by 15 %, 30 % and 45 % respectively, owing to the obviously slower hydration and higher internal RH at the early age. Meanwhile, the inclusion of SS substantially mitigates the chemical shrinkage and reduces the pores below 50 nm, thereby significantly decreasing the capillary pressure associated with smaller water-filled pore sizes. Substitutions of blast furnace slag by up to 45 % SS enable to reduce CO2 emissions by 18.4 kg/m3 and decrease autogenous shrinkage by 42 % without obvious compromise in the loss of elastic modulus and compressive strength.

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.

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.

Carbonation of alkali-activated slag (AAS) materials has been primarily concerned in atmospheres with gaseous CO₂. This study, by contrast, highlights that AAS pastes would also be carbonated under tap water immersion. Calcite is the main CO₂-bear phase in both sodium hydroxide- and sodium silicate-activated AAS pastes, and the paste pre-cured for a longer curing period shows more severe carbonation. Additionally, calcium carbonate can densify the deteriorated microstructure of sodium hydroxide-activated paste caused by long-term leaching. The indentation modulus of pastes subjected to tap water immersion is higher than those under deionized water immersion. The uptake of CO₃^2- by hydrotalcite (Ht) and gels is also detected, resulting in the formation of Ht-CO₃ and decalcification of gels. Due to the synergistic effect of leaching and carbonation, a characteristic layered distribution of pastes close to the exposure front is observed, comprising the carbonated layer, transitional (carbonated + leached) layer, and leached layer, progressing from the outermost to the inner regions. Eventually, the kinetics of underwater carbonation, as well as the discrepancy between dry and underwater carbonation, is revealed.

Vascular self-healing concrete (SHC) has great potential to mitigate the environmental impact of the construction industry by increasing the durability of structures. Designing concrete with high initial mechanical properties by searching a specific arrangement of vascular structure is of great importance. Herein, an automatic optimization method is proposed to arrange vascular configuration for minimizing the adverse influence of vascular system through a reinforcement learning (RL) approach. A case study is carried out to optimize a concrete beam with 3 pores (representing a vascular network) positioned in the beam midspan within a design space of 40 possibilities. The optimization is performed by the interaction between RL agent and Abaqus simulation environment with the change of target properties as a reward signal. The results illustrates that the RL approach is able to automatically enhance the vascular arrangement of SHC given the fact that the 3-pore structures that have the maximum target mechanical property (i.e., peak load or fracture energy) are accessed for all of the independent runs. The RL optimization method is capable of identifying the structure with high fracture energy in the new optimization task for 4-pore concrete structure.

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.

Rebar-reinforced coarse aggregate ultra-high-performance concrete (R-CA-UHPC) has been used in the construction of new structures and strengthening of deteriorated aged infrastructures, and it inevitably sustains tension. To study the tensile behavior of R-CA-UHPC members, axial tensile tests for dog-bone-shaped specimens were designed and conducted. The investigated variables included reinforcement ratio in terms of rebar quantity/diameter, and concrete type (CA-UHPC vs. normal concrete). The test results showed that the improved rebar/CA-UHPC bond property prevents the emergence of splitting cracks, but intensifies the crack localization for CA-UHPC and strain concentration for rebar after yielding. Moreover, the restrained effect of rebar on free shrinkage of CA-UHPC leads to a decrease in the first cracking strength for R-CA-UHPC members. Based on the established development functions of elastic modulus, autogenous shrinkage, and tensile creep for CA-UHPC, the restrained effect was quantified according to Dischinger's-differential-equation-based theoretical analysis. Finally, the models to predict the first cracking stresses/strains and the yielding loads of the R-CA-UHPC members were developed and validated.

Since the introduction of cementitious materials, shrinkage-induced earlyage cracking (EAC) has emerged as a significant issue that negatively influences the function, durability, and aesthetics of concrete structures like dams, tunnels, and underground garages. This thesis aims to develop new experimental and modelling techniques that help resolve this longlasting issue, with a particular emphasis on the EAC induced by AD (AD). Unlike the thermal and drying deformation which are induced by heat and moisture transport, respectively, the AD is an intrinsic behavior caused by the self-desiccation of the hydration of cementitious materials. The ADinduced EAC risk is especially high when it comes to modern (or future) cementitious materials, such as high-performance concrete, ultra-highperformance concrete, and alkali-activated slag concrete.

In the current study, experiments and numerical simulations were carried out to investigate the cracking behavior of reinforced concrete beams consisting of a very thin layer (i.e., 1 cm in thickness) of SHCC in the concrete cover, tension zone. 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. The study involved testing hybrid SHCC/concrete beams that have various types of interfaces. The results were compared to the control reinforced concrete beams that do not have SHCC in the cover. Four-point bending tests were performed with the beams and Digital Image Correlation (DIC) was utilized to track the development of crack pattern and crack width. Results show that hybrid beams possessed similar load bearing capacity but exhibited a significantly improved cracking behavior as compared to the control beam. With a 1-cm-thick layer of SHCC, the maximum crack width of the best performing hybrid beam exceeded 0.3 mm at 53.3 kN load, whereas in the control beam the largest crack exceeded 0.3 mm at 32.5 kN load. The hybrid beam with the proposed new interface formed 10 times more cracks in SHCC than the hybrid beam with a simple smooth interface and had an average crack width less than 0.1 mm throughout the loading. The lattice model has successfully showcased its ability to predict and offer valuable insights into the fracture behavior of hybrid systems. The simulation results indicate that the presence of a weak interface bond, coupled with mechanical interlocking, can effectively facilitate the activation of SHCC, resulting in the formation of more cracks and a delayed progression towards the maximum crack width. As the volume ratio of SHCC used in the hybrid beams is only 6%, the current study highlights the strategic use of minimum amount of SHCC in the critical region to efficiently enhance the performance of hybrid structures.

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.

This paper investigates the influence of temperature on autogenous deformation and early-age stress (EAS) evolution in ordinary Portland cement paste using a recently developed Mini Temperature Stress Testing Machine (Mini-TSTM) and Mini Autogenous Deformation Testing Machine (Mini-ADTM). In the Mini-TSTM/ ADTM, CEM I 42.5 N paste with a water-cement ratio of 0.30 was tested under a curing temperature of 10, 15, 20, 25, 30, and 40 °C. X-Ray diffraction (XRD) tests were conducted to measure the amount of ettringite and calcium hydroxide, which reveals the micro-scale mechanisms of autogenous expansion. The applicability of the Maturity Concept (MC) for the prediction of autogenous deformation and relaxation modulus under different temperatures was also examined by the experimental data and the viscoelastic model. This paper leads to the following findings: 1) The autogenous deformation of ordinary Portland cement paste is a four-stage process comprising the initial shrinkage, autogenous expansion, plateau, and autogenous shrinkage; 2) Higher temperature leads to higher early-age cracking (EAC) risk because it accelerates the transitions through the first three stages and causes the autogenous shrinkage stage to start earlier. Moreover, higher temperatures also result in increased rates of autogenous shrinkage and EAS in the autogenous shrinkage stage; 3) Autogenous expansion and plateau are attributed to the crystallization pressure induced by CH. Temperature-dependent CH formation rates determine the duration of the plateau stage; 4) Low-temperature curing can delay but not completely prevent the EAC induced by autogenous deformation; 5) The MC cannot predict the autogenous deformation at different temperatures but can be used to calculate the relaxation modulus, which in turn aids in EAS prediction based on autogenous deformation data.