Incorporating encapsulated phase change materials into mass concrete can effectively mitigate temperature rise, yet the capsule breakage during the mixing process commonly leads to the direct interaction between phase change materials and cementitious materials. The influence of this direct interaction on the microstructural development and mechanical performance of cementitious materials remains insufficiently understood. This study investigated experimentally the early-age mechanical properties, shrinkage and creep of cement pastes with direct incorporation of polyethylene glycol (PEG). The microstructures of PEG-incorporated cement pastes were also examined using mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and Fourier transform infrared (FTIR) spectroscopy. The results show that increasing PEG content from 0% to 20% reduced the semi-adiabatic peak temperature by 7.1% and 38.7%, and correspondingly prolongs the time to reach the peak temperature by 29.0% and 155.1%, respectively. While the hydration-retarding effect of PEG leads to increased porosity and a coarser pore structure at early ages, the subsequent precipitation of solid PEG particles within the pores exerts a filling effect, which partially offsets these microstructural deficiencies. A longer curing age or a lower PEG content yields a higher compressive strength. When the PEG content increases from 0% to 10% and 20%, the compressive strength decreases by 14.0%∼20.6% and 23.6%∼66.0%, respectively. However, both flexural strength and bending elastic modulus of PEG-modified cement pastes exhibit an initial increase followed by a decline at later ages. Moreover, the addition of 10% and 20% PEG reduces the shrinkage of cement paste under sealed condition by 11.5% and 34.7%, and specific creep by 10.5∼22.6% and 27.6∼49.0%, respectively. These findings provide a better understanding of the early-age cracking mechanism of mass concrete incorporating PEG as a phase change material.

Characterizing the time-dependent behavior of prestressed concrete beams is crucial for service safety and durability. This study investigates the effects of prestress levels and cracking on the time-evolving deformation of partially prestressed concrete (PPSC) beams, where "partially prestressed" refers to members that permit limited tensile stress or cracking under service loads. Four test beams were fabricated at two initial prestress levels (sigma pe=0.6 fptk and 0.5 fptk). For each prestress level, one beam was preloaded to induce flexural cracks (precracked), and the other served as a reference (initially uncracked). Sustained loading on the four beams lasted 240 days in two phases: Phase I at a sustained-load ratio of 0.33 (applied sustained load/ultimate flexural capacity), during which two reference beams were uncracked; Phase II increased the sustained-load ratio to 43 %, slightly above cracking load, leading to minor cracks in the reference beams but less severe than those in precracked beams. Key properties, including prestress loss, strain distribution, deflection, and crack evolution (captured via digital image correlation, DIC), were monitored and analyzed. Results show that the strain-, curvature-, and deflection-based creep coefficients are consistently interrelated. The DIC-based damage factor reliably captured crack evolution under sustained loading, and an exponential correlation was established between the creep-reduction coefficient of cracked beams and the damage factor quantified from DIC. The findings clarify how prestress level and cracking influence the time-dependent deformation of PPSC beams, and provide a basis for incorporating damage factor into long-term deformation prediction.

To achieve ratio optimization for alkali-activated fly ash–slag cementitious materials (AAFS), hydration characteristics, microstructure evolution, hydration kinetics, thermodynamic analysis, and strength regression modeling were systematically investigated with slag content, silicate modulus, alkali–solid ratio, and water–binder ratio as variables. The results indicate that appropriately increasing the slag content and water–binder ratio can accelerate the reaction rate of AAFS to varying degrees, enhance the polymerization degree and yield of binding phases, refine the pore structure, and thereby improve mechanical performance; systems with a high alkali–solid ratio exhibit reaction retardation at an early stage but accelerated reaction in the later period, which gives rise to a pronounced strength difference between 7d and 28d; an excessively high silicate modulus impedes the hydration process, which keeps the strength of AAFS at a consistently low level. Meanwhile, the phase composition and evolution trend of hydration products predicted by thermodynamic simulation are in good agreement with the quantitative results from SEM image segmentation. Furthermore, the established strength regression model shows high predictive accuracy, demonstrating that the strength development of AAFS is closely correlated with its hydration characteristics and pore structure evolution.

The restrained stress of concrete structures usually varies due to the changes in temperature, humidity, and external loads at early age. Both creep and creep recovery have a great influence on the accurate assessment of stress development and cracking risk of concrete structures. However, the creep and recovery behavior of early-age cementitious materials under different stress levels is still not fully understood. This study measured the minute-long creep and creep recovery of cementitious materials with various water-to-cement ratios (0.3, 0.4, and 0.5), curing ages (1, 7, and 14 days), and sand contents (0%, 25%, and 50%) under different compressive stress levels (30%, 60%, 75%, and 90%). Then a two-scale finite-element (FE) model was proposed to predict the elastic modulus and creep behavior. The results show that stress level affects the creep property more significantly than the creep recovery. The creep recovery and specific creep ratio tends to decrease with increasing stress level. Most of the short-term creep is recoverable under low stress level. The proposed two-scale FE model can give a good prediction of elastic modulus. However, the predicted creep of some cementitious materials may exhibit a certain deviation from the measured one. The short-term nonlinear creep coefficient increases with increasing stress level, which can be reasonably captured by a nonlinear creep model. These findings can provide insight into the creep mechanism of early-age cementitious materials and improve the prediction accuracy of creep property under different stress levels.

This paper develops a model to calculate carbon emissions during the construction period of dredging projects. Carbon emission quotas for various types of dredgers and auxiliary vessels in different construction conditions and geotechnical soil types during the dredging project’s construction period are established, as well as the power consumption quota for management activities. Taking the construction of the main project of the cross-river channel from Shenzhen to Zhongshan (S09)’s foundation trench excavation and channel dredging, the Thilafushi Island reclamation project in Malé, and the second phase of the southern section of the Guangzhou Port Area channel maintenance project (2022–2023) as case studies, the validity of the quotas is verified. During the construction period, under the same dredging soil quality and the same working condition level, the carbon emissions of different types of dredgers are different. Conversely, under different dredging soil qualities and different working condition levels, the carbon emissions for the same dredger or auxiliary vessel are different. The carbon emissions of each dredger or auxiliary vessel increase with the increase in the ship’s specifications. The carbon emissions of dredging projects are huge, with direct carbon emissions accounting for 97%, and indirect carbon emissions from equipment deployment and management activities accounting for 3%, among which the carbon emissions from electricity consumption in management activities account for only 0.3%.

Cement concrete has been used for the construction of vacuum tunnel in the hyperloop transportation system. However, concrete behavior in vacuum environment is largely unexplored. This paper carried out systematic research on the mechanical properties and microstructures of concrete with and without surface coating in vacuum environment. Compressive and flexural tensile tests were conducted on concrete cylinders exposed to vacuum for different periods. Scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), and thermogravimetric analysis (TG) were employed to evaluate the effects of vacuum exposure on cement hydration and microstructural characteristics. The test results indicate that the vacuum exposure at early ages of concrete inhibits cement hydration reactions and deteriorates the microstructure and mechanical properties of concrete. However, vacuum exposure after 28 d's curing in standard conditions had beneficial effects on concrete due to the densification of the matrix. It was also found that the surface coating significantly enhanced the flexural tensile strength of concrete in both atmospheric and vacuum environment. In vacuum environment, the surface coating effectively inhibits moisture evaporation and the formation of porous structure of hydration products, but weakens the positive effects of later age-vacuum exposure on concrete. It was concluded that both vacuum exposure and surface coatings had dual effects on the mechanical properties and microstructures of concrete.

The deterioration of the interlayer interface of a double-block ballastless track is affected by the environmental temperature and moisture conditions, which will have a negative effect on its service life. Composite specimens with interlayer interfaces of double-block ballastless track were fabricated and deteriorated by an accelerated method, i.e., immersed in saturated ammonium chloride solution with various temperatures for different times. Then, the deterioration condition and mechanical properties of the composite specimens were investigated experimentally by a universal material testing machine and acoustic emission technique. The automatic sensor test (AST) method is capable of assessing the deterioration condition of the interlayer interface based on the relative wave velocity. The deterioration depth of the interlayer interface tends to increase with increasing solution temperature and immersion time. Both the solution temperature and immersion time have a negative impact on the splitting tensile strength and direct shear strength. A linear relation is found between the splitting tensile strength (direct shear strength) and the cumulative AE energy released at the fracture moment. The damage factor defined by the cumulative AE energy for most composite specimens is no greater than 0.2 before they are going to be fractured but increases sharply to 1.0 at the fracture moment.

Interfacial transition zone (ITZ) is a weak part around aggregate in concrete, which could threaten the macroscopic properties of concrete such as strength, stiffness, deformation, and durability. However, it is difficult to quantify the ITZ effect on the macroscopic behavior of concrete only by experimental methods due to the tiny thickness of ITZ. This study employed finite element (FE) method to investigate the ITZ effect on the elastic modulus, creep, and shrinkage. A mesoscale three-phase FE model consisting of homogeneous matrix phase, aggregates and ITZ around them was developed for concrete in this study. The influences of the FE model dimension and the meshing size on the calculated results were first discussed. Then the influence of the thickness, viscoelasticity and shrinkage of ITZ on the macroscopic properties of concrete were evaluated based on the FE calculation and grey relation analysis. Finally, the measured elastic modulus, creep and shrinkage of concrete with different contents of clay on coarse aggregates were used to validate the FE model. It is found that the calculated results of the two-dimensional (2D) FE model are in good agreement with those of the threedimensional (3D) FE model under the linear viscoelastic assumption. The meshing size has a minor effect on the calculated results when it is 0.2-2 times of the ITZ thickness. The influence of ITZ properties on the macroscopic properties of concrete will become more significant as the ITZ thickness increases. Moreover, the elastic modulus of ITZ has a greater impact on the effective properties of concrete than the thickness, creep, and shrinkage of ITZ. The findings in study can contribute to a better understanding of ITZ effect on the macroscopic properties of concrete and provide references to the reasonable consideration of ITZ in numerical studies.

Many in-service concrete structures are normally exposed to varying temperature condition. The time-dependent and non-uniform temperature distribution in concrete structures could affect the accurate assessment of their electrical properties. Therefore, the influences of temperature change over time and the spatial variation in temperature on the measured electrical resistance of cementitious materials are still in need of investigation. The coupled thermal-electrical responses of two types of cementitious materials under varying temperature history were investigated by a series of thermal and electrical tests in this study. Then a coupled thermal-electrical finite element (FE) model was developed to simulate the coupled thermal-electrical responses. The results show that the input direct current has a minor effect on the measured electrical resistance by embedded four-probe method and the specimen temperature when it is no greater than 3 mA. The relationship between the measured temperature at 10-mm depth of the specimen and the apparent resistance is well captured by a linear equation. The coupled thermal-electrical finite element model can predict well the developments of specimen temperature and apparent electrical resistance under varying temperature history. The FE results reveal that the non-uniform temperature variation results in the redistribution of electrical current within the tested specimen. However, electrical field between the two inner electrodes is always uniform when the embedded four-probe method is used, which is not affected by the non-uniform temperature variation.

The vacuum-based maglev train is an emerging transportation system currently under development worldwide. While concrete is a technically viable material for constructing vacuum tunnels, its mechanical performance in this unique environment remains unclear. This paper investigates the behavior of concrete in vacuum using a large customized vacuum tube. The results show that concrete undergoes significant water loss in vacuum, resulting in impeded cement hydration and porous hydration products with a foam structure. Vacuum exposure during the early ages of concrete has negative effects on mechanical properties. However, once the cement hydration reaches a high level of maturity, further vacuum exposure has little influence. Compared with atmospheric conditions, the shrinkage of concrete in vacuum is significantly increased. These findings provide a valuable experimental database and new insights for the use of concrete in vacuum-based maglev trains.

为评估跳仓法施工对大体积混凝土早期应力的影响,基于有限元方法分析不同因素下单块大体积混凝土楼板的早期应力,采用灰色关联度方法确定不同因素的影响程度,并对新白云站大体积混凝土楼板的跳仓法施工进行仿真分析.结果表明:入模温度、混凝土绝热温升值、楼板平面尺寸、楼板厚度、环境对流系数、垫层弹性模量对大体积混凝土楼板早期应力的影响程度依次降低;大体积混凝土楼板内部的钢筋可降低其早期应力,但对其影响可忽略不计;减小大体积混凝土楼板的仓块尺寸可有效降低其早期应力;仓块的浇筑顺序会影响大体积混凝土楼板早期应力的发展,有规律的浇筑顺序可使混凝土受力趋于均匀,以减少应力集中现象.

The thermal conductivity (λ-value) of cementitious materials with steel fibers is not always better than that of plain cementitious materials due to the interface between the steel fiber and cementitious materials and the increased porosity. A mesoscale finite element (FE) model considering both fiber-matrix interface and pore effects was developed to quantify the influences of porosity, pore water saturation degree, fiber-matrix interface property, fiber content, fiber length, fiber shape, and fiber orientation on the λ-value of cementitious materials, which can explain well the various relationships between the λ-value of cementitious materials and steel fiber content reported in the literature. In the FE model, cementitious materials containing steel fibers were divided into two scales where Scale I included solid phase and pore, and Scale II was made up of the homogenous composites from Scale I and steel fibers. To achieve a more accurate representation of the pore structures, the pores at Scale I were created randomly without specifying their geometry. The fiber-matrix interface in cementitious materials at Scale II was modelled as a zero-thickness interface with constant interfacial thermal conductance. It is found that the λ-value of cementitious materials decreases linearly with increasing porosity but increases linearly with increasing pore water saturation degree. The influence of steel fiber on the λ-value of cementitious materials depends on the fiber-matrix interface property. When the fiber content or fiber length increases, the λ-value will increase for good fiber-matrix interfaces with high interfacial thermal conductance; while poor interfaces with low interfacial thermal conductance lead to an opposite trend. Fiber geometry has a limited impact on the λ-value. The fiber orientation can affect the λ-value of cementitious materials. When the fiber rotational angle with respect to the direction of temperature gradient increases, the λ-value of cementitious materials with the same fiber-matrix interface property will decrease.

The microstructures of seawater cement-based materials normally are influenced by the salinity in seawater, which could result in different properties. The thermal conductivity (?-value) of seawater cement-based materials is a key input parameter for the heat transfer analysis. However, few studies have been conducted on this issue. This study investigated the ?-value of seawater cement-based materials using experimental and numerical methods. The influences of water-to-cement (w/c) ratio, curing age, and sand content on the ?-value of seawater cement-based materials were examined experimentally. Then a three-dimensional mesoscale finite-element model was established to assess the influences of pore size and porosity on the ?-value of cement-based materials. It was found that the ?-value of seawater cement-based materials increases with increasing curing age or sand content, and a low w/c ratio contributes to a high ?-value, similar to that of the tap-water cementitious material. The ?-value of seawater cement paste with curing ages of 1 and 7 days was slightly greater than that of the tap-water cement paste. However, no significant difference was found between the ?-values of the 28-day seawater and tap-water cement pastes. Pore size had a negligible influence on the ?-value. The ?-value of cement-based materials decreased with increasing porosity, and the decrease in ?-value of cement-based materials with pores filled with air was more significant than that with pores filled with water, indicating that the pore-water saturation has a great effect on the thermal conductivity.

This book provides information on characterizing the microstructure and mechanical properties of cementitious materials at microscale.

Numerical simulations are performed to investigate the mechanical behavior of concrete in uniaxial compression test and tensile splitting test. Especially, the digital image processing (DIP) techniques are utilized to differentiate the interface types on the fracture surface of the specimen that is already split, which involves transforming the color image of the fracture surface into a binary image through graying, binarization, and opening operation processing. The boundary data obtained from the binary image is further processed by mirrored comparison and contour simplification, and is finally imported into the numerical model to reconstruct the different interface regions on the splitting surface. Different material properties are assigned to the corresponding interface regions through intensive programming with Python, whereby the influence of heterogeneity on the splitting process is analyzed through the established numerical model. Through comparison with homogeneous model, it is found that the heterogeneous model is capable of reproducing a more genuine fracturing process, with respect to the fracture initiation locations and splitting sequence of different interfaces. The proposed modeling schemes are valuable for potential researches exploring the failure process of the materials such as concretes and rocks whose mechanical behavior is influenced by their inherent heterogeneity.

Incorporation of fiber into cementitious materials would affect their mechanical and creep properties. The macro creep test is normally time-consuming and is difficult to evaluate the influence of the microstructures fully. This study investigated experimentally the elastic modulus and flexural creep of mortars with different volume fractions of steel fibers. Then a mesoscale finite element (FE) model consisting of mortar and randomlydistributed steel fibers was developed. The results show that the elastic modulus of mortars increases with increasing volume fraction of steel fiber, but the specific flexural creep exhibits an opposite trend. The influence of volume fraction of steel fiber on the specific flexural creep is more significant than that on the elastic modulus. The mesoscale FE model developed in this study is capable of capturing well the development of elastic modulus and flexural creep of mortars with different volume fractions of steel fibers. The simulated results reveal that fiber orientation is an important factor affecting the elastic modulus and flexural creep of steel fiber-reinforced mortars. The flexural creep (elastic modulus) of steel fiber-reinforced mortar increases (decreases) with increasing fiber rotational angle with respect to the stress direction. Moreover, both the elastic modulus and flexural creep of the mortar containing randomly-distributed steel fibers are equivalent to those of the mortars with steel fibers distributed by a fixed rotational angle of 30 degrees when the volume fraction of steel fiber ranges from 1% to 3%. The findings in this study are expected to provide references to further investigations of elastic and creep properties of fiber-reinforced cementitious materials.

Incorporating a suitable amount of sulfoaluminate expansive additive (SEA) into cementitious materials can compensate the shrinkage deformation. However, the influence of SEA on the relative humidity inside cementitious materials is still unclear. This study conducted a series of tests to investigate the mechanical properties, internal relative humidity, and shrinkage of early-age cement mortars containing different contents of SEA. Then finite element simulations of the drying of cement mortars with different contents of SEA were conducted to derive the moisture diffusion coefficient. The results reveal that the amount of expansive ettringite generated in cement mortar tends to increase with increasing SEA content, which can decrease the porosity of cement mortar and refine its pore structures. Both the compressive strength and flexural strength of cement mortar decrease with increasing SEA content. The decrease in mortar strengths is more significant during the later age. The mortar shrinkage under both sealed and drying conditions decreases with increasing SEA content. However, the compensation effect of SEA on the shrinkage deformation is less effective under drying condition. The relative humidity inside the cement mortar under sealed condition would decrease by 9.3% at 36 days when 8% SEA was used. While the decrease would be 3.4∼7.2% when the cement mortar was exposed to drying condition with RH of 60%. Moreover, since SEA densifies the microstructures and refines the pores in mortar, the moisture diffusion coefficient of mortar decreases with increasing SEA content.

混凝土结构物服役期间受到荷载、环境温湿度、化学侵蚀等作用其性能会发生劣化,交流阻抗技术具有快速无损、重复性高等优点,是研究水泥基材料微观结构和性能演化的一种可行方法.总结了交流阻抗的工作原理以及用于表征水泥基材料结构性能的等效电路模型,并对目前研究涉及的电化学工作站选择、试件制备方法、试验参数设置、交流阻抗谱拟合软件等试验细节进行了归纳分析,最后评述了现有的应用成果,以期为水泥基材料交流阻抗的试验研究提供参考.

In view of the difficulty and limitation of the direct experimental measurement of water permeability, particularly for high-strength modern dense concrete, this chapter summarizes the existing techniques and methods to measure the pore parameters and to predict the intrinsic permeability of cementitious materials, including the MIP, BSE, X-ray CT scanning pore measuring techniques, and the Katz-Thompson, the general effective media (GEM), and the Navier–Stokes method for permeability prediction. The predicted intrinsic permeabilities based on the measured microstructure parameters are compared to the experimental results, and the applicability of different techniques and prediction methods are discussed. It is noted that the computed permeability is more suitable as a comparison tool for selecting mixture under the conditions of using the same technique and calculating method. The content of this chapter can provide an insight into the feasibility of the measuring techniques and the predicting method for quantifying the permeability of cementitious materials.

In recent years the application of concrete material in environment with very low temperatures is on the rise. However, systematic knowledge on the mechanical behavior of concrete at low temperatures are not readily available for most researchers and engineers. In the 1970s to 1980s the cryogenic behavior of concrete was intensively studied by researchers from western countries, but after that relevant research activities decreased until it regains researchers' interests in recent years. Boosted by the rapid development of economy in China, Chinese researchers start to focus on this topic again and have contributed some new knowledge in this respect. To help researchers and engineers design, analyze and construct concrete structures in the cryogenic environment, this paper presents a literature review on the mechanical properties of concrete subjected to low temperatures and cryogenic freeze-thaw cycles. First the effects of low temperatures on the main properties of concrete including compressive strength, tensile strength, elastic modulus, etc., are reviewed and the existing mathematical models are summarized and compared. In the second part, the concrete damage induced by cryogenic freeze-thaw cycles is discussed, and emphasis is put on the mechanism of cryogenic freeze-thaw damage. The complied conclusions by the review paper can facilitate and safeguard the application of concrete structures at very low temperatures.