This study investigates the size effect on the compressive strength of foamed concrete at the mesoscale level combining X-ray computed tomography (X-CT) and a discrete lattice model. Image segmentation techniques and X-CT were employed to obtain virtual specimens comprising hydrated cement paste and air voids. The lineal-path function and pore size distribution was used to characterise the air void structure. A two-dimensional lattice fracture model of foamed concrete considering different wet densities was established. The model was verified experimentally at a wet density of 700 kg/m³ and then used to predict the strengths of specimens with wet densities of 600 and 800 kg/m³. Square and rectangular specimens (slenderness ratio = 2) with widths of 10, 20, 40, 70.7, and 100 mm were investigated. Results show that the air void structure significantly influences the observed size effect on the compressive strength in the investigated size range. A random forest regressor was used to predict the compressive strength of the foamed concrete; the regressor yielded satisfactory results. Finally, existing analytical size effect models were used to fit the simulated strength. Although good fitting was achieved, special attention should be given to the applicable range and physical meaning of fitted empirical parameters.

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The classically lattice model assumes the local elements behave elastic brittle, neglecting the ductility of the mortar matrix. This leads to the simulated load⁃displacement response more brittle than the realistic. To solve the aforementioned issue, a piece⁃wise approach was introduced to describe the elastic⁃plastic constitutive relation of lattice element. The fracture process and the load⁃displacement response were obtained through the sequentially⁃linear solution approach. The model was calibrated using the uniaxial tension and compression tests. It is found that the model can precisely simulate the fracture process and load⁃displacement response. Moreover, the model was used to model the size effect in uniaxial tension and the influence of the specimen’s slenderness and boundary confinement on the fracture behavior under compression. It offers a new theoretical method and approach for studying the fracture of concrete.

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An engineered cementitious composite (ECC) belongs to a type of high-performance fiber-reinforced materials. Fiber alignment causes the anisotropy of such materials. Herein, the influence of the fiber orientation on water and ion penetration into an ECC was studied. Fiber alignment was achieved using an extrusion approach. Water absorption, sorptivity, chloride penetration resistance, sulfate attack resistance, and freezing–thawing resistance of specimens with fiber aligned horizontally (AH), vertically (AV), and randomly (R), corresponding to the direction of the exposure surface that was studied. The results showed that fibers oriented perpendicular to the water path delayed water migration into the ECC matrix. The sorptivity was significantly affected by the fiber direction. The sorptivity of the AH specimens was 35% and 13% lower than that of the AV and R specimens, respectively. After 180 days of exposure, the chloride penetration depth of the AH specimens was 5.7 mm, which is 13.6% and 20.8% lower than that of the AV and R specimens, respectively. The sulfate ingress profile indicates that the fiber–matrix interface oriented perpendicular to the penetration path can effectively delay sulfate migration. The fiber orientation also influences the compressive strength gain under immersion conditions (Na2SO4 solution, Na2SO4 + NaCl solution, and water). Compared with the AH and R specimens, the AV specimens are more sensitive to the immersion condition. In contrast, the fiber orientation has no significant effect on ECC specimens under freeze–thaw cycles. These findings indicate that controlling the fiber alignment and orientation in an ECC can improve its durability under certain exposure conditions.@en

This work presents a study of mechanical properties of foamed concrete at the meso-scale based on a combination of X-ray computed tomography (XCT) technique and a discrete lattice type fracture model. The microstructure of the foamed concrete with different densities was obtained by XCT technique and binarized as two-phase (pore/solid) materials. The parameters (e.g., porosity, pore diameter and spacing distribution) of foamed concrete air‐void structure were characterized. The virtual specimens were subjected to computational uniaxial compression, Brazilian splitting and three-point bending test to calculate strengths and elastic modulus. The mechanical properties of solid phase were derived from the recent outcome of micromechanical models. Two types of element input parameters were used to investigate the influence of the input parameters on the simulated results. The modelling results (strength value and fracture pattern) were compared with the experiments. It shows that, without further calibration, the lattice model can predict the mechanical strength and crack pattern with good accuracy. The fracture toughness KIC was derived using three-point bending strength and the average pore diameter. The results indicate that the presence of air-void structure increases the brittleness and reduces the fracture toughness of the foamed concrete.@en

Micro-cubes of hardened cement paste with the lengths of 100 μm and 200 μm were prepared by micro-dicing and thin-sectioning techniques. The compressive strength and splitting strength of these specimens were measured by a nanoindenter equipped with different types of tips. Considering the heterogeneity nature of the material in a micro-scale, we examined 100 specimens via a statistic analysis. The results show that the two-parameter Weibull distribution can be used to represent the probability distribution of both compressive strength and splitting strength in a micro-scale. The specimens with a higher water-to-binder ratio have a higher strength and a lower variability. Furthermore, the specimen size has an influence on the measured strength. The bigger specimens have lower strength and variability. The reported average value and variability of strengths obtained in this work are greater than those of the concrete in a meso-scale, but lower than those of the hydration product in a nano-scale.@en

A combination of laboratory experiments and numerical simulations at multiple length scales can provide in-depth understanding of fracture behaviour of hydrated cement paste (HCP). To that end, the current work presents a numerical study on compressive failure of hydrated cement paste (HCP) at the micro-scale. Virtual specimens consisting of various phases were obtained using a combination of X-ray computed tomography and image segmentation techniques. The discrete lattice fracture model was used for the deformation and fracture analysis of the specimens subjected to uniaxial compression. The input local mechanical properties of each individual phase were taken from the literature in which a micro-scale compression test was conducted for the calibration of the same type model. The influence of slenderness ratios (1 and 2), water-to-cement ratios (0.3, 0.4 and 0.5 respectively) and lateral confinement of the specimen ends (free and restricted) on the failure behaviour were investigated. It has been shown by the current study that the stress–strain response cannot be completely separated from the used boundary conditions. The proposed model provides an effective tool to understand the compressive fracture behaviour of cement paste at the micro-scale.@en