Materials and Structures最新文献

Alkali-activated slag (AAS) is a low-carbon construction material and exhibits high mechanical strength and good fire resistance. However, compared to ordinary Portland cement, AAS has a greater problem in terms of resistance to carbonation. In this study, three reactive MgO and Mg(OH)₂ were used to enhance the carbonation resistance of AAS mortars. It was found that both reactive MgO and Mg(OH)₂ were able to improve the carbonation resistance of the AAS mortar, and the reactive MgO was more effective than Mg(OH)₂. Increasing the reactivity and dosage of MgO can significantly enhance the carbonation resistance of AAS mortar, resulting in a 70.5% reduction in carbonation depth. The compressive strength, phase composition, and microstructure before and after carbonation were tested, which showed that the highly reactive MgO had a greater accelerating effect on the hydration of AAS. It can not only significantly improve the compressive strength of AAS mortar but also limit the diffusion of CO₂ into the mortar. Meanwhile, the introduction of higher reactive MgO produced more hydrotalcite with a laminar structure, which could absorb a large amount of CO₃²⁻. In addition, the incompletely hydrated MgO could react directly with CO₂ during the carbonation process to achieve the purpose of carbon sequestration.

A liquid accelerator is a key component in shotcrete; however, it often faces challenges such as high alkali content and insufficient long-term concrete strength. This study utilizes polyaluminum sulfate as the primary raw material to develop an environmentally friendly, alkali-free, and fluorine-free accelerator (FAF). Through orthogonal experiments and cement compatibility tests, the effects of FAF on setting time, 1 day compressive strength, and 28 days compressive strength ratio of cement paste were analyzed, leading to the determination of the optimal FAF mixing ratio. The coagulation-promoting mechanism of FAF was further examined using X-ray diffraction (XRD) and scanning electron microscopy (SEM) results. Additionally, concrete slab tests were conducted to assess the impact of different FAF dosages on concrete rebound rates and mechanical properties, with the optimal dosage range found to be 4–8%. Field trials indicated that, at a 6% FAF dosage, shotcrete rebound rates on tunnel arch walls and roofs were 10.3% and 13.5%, respectively. The composition of the FAF does not contain alkali metal salts, which reduces the risk of alkali-aggregate reactions in concrete and minimizes the potential health risks to construction personnel, aligning with the global trend towards sustainable construction practices.

While it is widely accepted that the steel-concrete interface (SCI) plays an important role in governing the long-term durability of reinforced concrete structures, the understanding about the primary features of the SCI that influence corrosion degradation mechanisms has remained elusive. This lack of knowledge can be attributed to, firstly, the complex heterogeneous nature of the SCI, and secondly, the absence of established experimental techniques suitable for studying the relevant SCI features. Here, we use focused ion beam—scanning electron microscopy (FIB-SEM) nanotomography to obtain high-resolution 3D tomograms of the SCI. Five tomograms, spanning volumes ranging from 8000 to ({200,000},{upmu hbox {m}^{3}}), of both non-corroded and corroded SCIs were acquired. The achieved voxel size falls within the range of 30–50 nm, which captures capillary pores highly relevant for moisture and ion transport. Potential pitfalls when applying the FIB-SEM technique to the SCI are highlighted, including aspects related to the electron detectors. We present an image processing pipeline that reduces artifacts and generates tomograms segmented into solid matrix and pore space. Furthermore, to characterize the SCI pore structure, diffusion tortuosity and porosity profiles. The analysis showed that there is a pronounced anisotropy in the pore structure. This work demonstrates that the FIB-SEM technique can be applied to acquire high resolution tomograms of the SCI pore structure, which can be digitally analyzed to inform transport models of the SCI.

This study investigates the impact of varying steel fiber (SF) content (0%, 0.8%, 1.0%, and 1.2% by volume) on the mechanical and durability properties of 3D-printed Ca(OH)₂-activated geopolymer concrete (GPC). The addition of 1.2% SF improved flexural strength by 69% at 7 days and 16% at 28 days, while tensile strength more than doubled to 3.75 MPa at 28 days. Although compressive strength remained unaffected at 43 MPa, SF enhanced interlayer bond strength by 20%, which is crucial for layer cohesion in 3D-printed structures. Additionally, the elastic modulus increased by 7%, contributing to improved stiffness. Durability assessments, including autogenous shrinkage and self-induced stress, indicated a slight reduction in shrinkage of SF-reinforced samples, with no significant effect on self-induced stress. Microstructural analysis using scanning electron microscopy (SEM) and X-ray micro-computed tomography (µCT) demonstrated the crack-bridging behavior of steel fibers, enhancing ductility and fracture resistance. There was a slight increase in porosity (5.34%) of SF-reinforced samples without negatively affecting their mechanical properties. Notably, SF improved early-age toughness and controlled crack propagation across printed layers, addressing a critical challenge in 3D-printed concrete. The novelty of this work lies in successfully reinforcing 3D-printed Ca(OH)₂-activated GPC with recycled steel fibers, enhancing mechanical properties, interlayer bonding, and durability without compromising printability. This study offers a sustainable reinforcement strategy for 3D printing in construction.

This study investigated the interactive effects of pre-damage, water boundary conditions, and internal frost damage on concrete at dual-scale. The pre-damage included pre-cracking, which has not been studied experimentally before, and pre-compressive damage. Concrete specimens underwent pre-damage and had varied water boundary conditions during Freeze-Thaw Cycles (FTC). At the macro-scale, wedge-splitting tests combined with Digital Image Correlation (DIC) were conducted to assess post-FTC strength and fracture behaviour. At the meso-scale, X-ray CT scanning was employed to identify internal crack patterns. Results reveal that at the macro-scale, significant interaction between pre-damage and frost damage reduced splitting tensile strength compared to the internal frost damage alone. Besides, increased water exposure during FTCs reduced both splitting tensile strength and compressive strength, with a less pronounced reduction in splitting tensile strength. It also led to a diffuse crack pattern and increased tensile ductility. At the meso-scale, specimens subjected to the interactive effects of pre-damage and internal frost damage exhibited cracks along aggregate-cement interfaces and within the cement paste. Reference specimens displayed no internal cracks, while specimens exposed to only FTCs showed only cracks along aggregate-cement interfaces. Full submersion of specimens during FTCs induced more internal cracks than solely water on top. These findings on the interactions between pre-damage, water boundary conditions, and internal frost damage offer insight into the causes of frost damage, vital for the design and assessment of concrete structures in frost-prone environments. Furthermore, the results of these dual-scale tests can be used as a test case for the development of upscaling numerical models describing heat transfer and frost degradation in concrete.

Marine sand has gained significant interest among researchers as a potential solution to the shortage of river sand for construction purposes. However, using marine sand as a fine aggregate in mortar has yielded contradictory results. To investigate the underlying reasons for this phenomenon, an experimental study was conducted to study the influence of marine sand with different characteristics on the mechanical performance of mortar. The shape and size of marine sand significantly impact its loose bulk density, voids and water absorption. The properties of marine sand mortar include workability, hardened density, flexural strength, and compressive strength. Beach marine sand, with its fine and uniform particles, requires higher water content, resulting in lower workability. This reduction in workability leads to decreased mechanical strength due to the increased voids within the mortar. However, marine sand’s sub-angular to angular shape contributes to mechanical strength by bonding with the cement paste and through the interlocking action between sand particles. Despite this, it has been found that the presence of voids within the mortar is the dominant factor contributing to its low mechanical strength. The flexural strength of marine sand mortar was reduced by 16.9%–49.3% compared to river sand mortar, while the compressive strength decreased by 20.9%–64.9%. One important finding is that marine sand that contains impurities such as coral skeletons and seashell fragments significantly reduces the mechanical performance of marine sand mortar. Based on this observation, it is evident that not all marine sand is suitable for use as fine aggregate in mortar.

Iron sulfide concentrations and mineral associations triggering the internal deterioration of concrete structures are still enigmatic. Incidences of internal sulfate attacks induced by iron sulfide-containing concrete aggregates appear worldwide. Severe cases are reported from Canada, the United States of America, and Ireland. Moreover, conservative limits for the total sulfur content of aggregates increased the need to dispose of otherwise high-quality resources for concrete production. The maximum threshold values for total elemental sulfur in the European standard EN-12620 for concrete aggregates are (le) 1 wt.(%), and as little as (le)0.1 wt.(%) if the non-stoichiometric iron-sulfide pyrrhotite (Fe_(1-x)S) is present in the rock. This study investigates the potential of scanning electron microscopy-based automated mineralogy for mineral classification and the quantitative quality assessment for concrete aggregate material. Identifying the stoichiometrically closely related disulfide pyrite and monosulfide pyrrhotite is emphasized. The iron/sulfur ratio and greyscale variations in the electron backscatter images between pyrite and pyrrhotite were tested as additional differentiation criteria when acquiring mineral mapping and point-of-interest analysis. The added greyscale criterion yielded a better distinction between the two chemically similar phases. A good correlation was achieved when comparing results from energy-dispersive X-ray spectroscopy in automated mineralogy with wavelength-dispersive spectroscopy point analyses on the electron microprobe. Semi-quantification of the chemical data from automated mineralogy was computed for the total sulfur content in the petrographic samples. The total sulfur content of bulk samples, investigated by high-temperature combustion and inductively coupled plasma atomic emission spectroscopy, was consistent with the semi-quantitative results of automated mineralogy.

This recommendation specifies a method for measuring passive elastic waves and assessing damage to concrete members such as decks and girders. To visualize internal damage, acoustic emissions (AE) and elastic wave velocity are employed. Firstly, based on the data detected by AE sensors, the location of the AE sources is estimated. Then, the velocity distribution in the concrete is evaluated. Accordingly, the internal damage of concrete deck can be evaluated quantitatively by applying a simple procedure using two evaluation axes, which are AE source density and elastic wave velocity in the concrete. Thus, the local deterioration of the concrete member can be classified into several stages that represent the damage levels of internal defects.

Replacement of Portland cement is a practical strategy to reduce concrete manufacturing CO₂ emissions. However, this approach typically results in a diminished portlandite content in the hardened mix, elevating the risk of carbonation-induced corrosion in steel-reinforced concrete. Carbonation is frequently studied by exposing the samples to elevated CO₂ levels (1% and 20%). However, the carbonation process and its by-products might differ markedly under natural conditions. In the context of RILEM TC 281-CCC ‘Carbonation of Concrete with SCMs’, a comprehensive three-year natural carbonation study on mortar samples was carried out across three laboratories. Samples were made with commercially available cement (CEM I, CEM II/B-V, CEM III/B). This study examined two natural carbonation scenarios: one in a regulated climate chamber and the other outdoors, protected from direct rainfall. The progression of carbonation was determined using a phenolphthalein indicator and compared to optical pH measurements. The phase composition was analysed by X-ray diffraction, attenuated total reflectance Fourier transform infrared spectroscopy, and thermogravimetric analysis. Additionally, the CO₂ capture in three-year-old naturally carbonated samples was assessed and contrasted against the reactive CaO content. The thermogravimetric analysis data revealed a non-linear relationship between the portlandite content in the uncarbonated zone and the carbonation rate. A reduced clinker content leads to lower pH values in carbonated and uncarbonated zones. Notably, samples containing CEM II displayed the largest formation of CaCO₃ which, divided by the theoretical maximum amount of CaCO₃ from reactive CaO, signifies the highest degree of carbonation among the cement types studied.

3D concrete printing is an innovative technology poised to transform the construction industry by enabling the automated, layer-by-layer creation of structures directly from digital models. This approach offers numerous advantages over traditional construction methods, including reduced labor costs, faster build times, and the ability to produce complex geometries with high precision. However, unlike conventional mold-cast concrete, 3D printable concrete must support itself without external formwork, posing significant challenges related to material deformation during the printing process. Uncontrolled deformation can lead to structural instability, design deviations, and cumulative errors. Traditional methods for monitoring the geometrical quality of 3D-printed concrete are often insufficient in accuracy and efficiency. Recent advancements in artificial intelligence (AI) present new opportunities for addressing these challenges. AI-assisted methods leverage machine learning to analyze large datasets, enabling more accurate predictions and real-time monitoring and control of deformation during the 3D printing process. In this paper, we explored the application of AI-assisted methods for real-time deformation analysis in 3D concrete printing. Specifically, the Yolo-v5 algorithm, an AI-assisted object detection technique, was employed for the computer vision of extruded concrete filaments. Several quantitative metrics were proposed, including the layer height, layer angle, and curvature. In addition, the rheological properties of 3D-printed concrete were measured to refine the computer vision analysis results. Through experimental validation, we demonstrated the effectiveness of the developed AI-assisted computer vision system in monitoring the 3D concrete printing process.