With the development of self-healing technology, the overall properties of the microcapsule-enabled self-healing concrete have taken a giant leap. In this research, a detailed assessment of current research on the microcapsule-enabled self-healing concrete is conducted, together with bibliometric analysis. In the bibliometric analysis, various indicators are considered. The current state of progress regarding self-healing concrete is assessed, and an analysis of the temporal distribution of documents, organizations and countries of literature is conducted. Later, a discussion of the citations is analyzed. The research summarizes the improvements of microcapsule-enabled self-healing cementitious composites and provides a concise background overview.
Cement-based materials are brittle and crack easily under natural conditions. Cracks can reduce service life because the transport of harmful substances can cause corrosion damage to the structures. This review discusses the feasibility of using microbial self-healing agents for crack healing. Tubular and spherical carriers can be used to load microbial self-healing agents and protect microbes, which prolongs the self-healing time. The area self-healing ratio, permeability, mechanical strength, precipitation depth method, numerical modeling, and ultrasonic method can be employed to identify the self-healing effect of cracks. Moreover, the self-healing mechanism is systematically analyzed. The results showed that microbial self-healing agents can repair cracks in cement-based materials in underground projects and dam gates. The difficulties and future development of self-healing cracks were analyzed. A microbial self-healing agent was embedded in the cement-based material, which automatically repaired the developing cracks. With the development of intelligent building materials, self-healing cracks have become the focus of attention.
Three-dimensional concrete printing (3DCP) can proliferate the industrialization of the construction sector, which is notoriously conservative and indolent toward changes. However, the mechanical behavior of 3DCP should be characterized and modeled considering the interfaces when its performance is thoroughly compared to that of the existing concrete construction methods. This study presents an experimental and numerical investigation of uniaxial compression and three-point bending (TPB) tests on extruded 3DCP beams in different loading directions. The orientation of translational and depositional interfaces with respect to the direction of loading influenced the strength. Both the elastic and post-damage behavior of the 3DCP specimens were compared with those of the conventionally cast specimen under quasi-static loading conditions. Despite the higher compressive strength of the casted specimen, the flexural strength of the 3DCP specimens was higher. This study employed the finite element and cohesive zone models of the appropriate calibrated traction-separation law to model fracture in the notched TPB specimens. Furthermore, the real-time acoustic emission test revealed the nature of failure phenomenon of three-dimensional-printed specimens under flexion, and accordingly, the cohesive law was chosen. The predicted load−displacement responses are in good agreement with the experimental results. Finally, the effects of cohesive thickness and notch shape on the performance under bending were explored through parametric studies.
Three-dimensional printable concrete requires further development owing to the challenges encountered, including its brittle behavior, high cement requirement for the buildability of layers, and anisotropic behavior in different directions. The aim of this study is to overcome these challenges. First, three-dimensional printable concrete mixtures were prepared using silica fume, ground blast furnace slag, and metakaolin, instead of cement, to reduce the amount of cement. Subsequently, the rheological and mechanical behaviors of these concretes were investigated. Second, three-dimensional printable concrete mixtures were prepared using 6-mm-long steel and synthetic fibers to eliminate brittleness and determine the effect of those fibers on the anisotropic behavior of the concrete. As a result of this study, it is understood that printable concretes with extremely low permeability and high buildability can be achieved using mineral additives. In addition, results showed that three-dimensional concrete samples containing short steel fibers achieve fracture energies up to 36 times greater than that of plain concrete. Meanwhile, its characteristic length values, as indicators of ductility, are 22 times higher than those of plain concrete. The weakest strength was recorded at the interfaces between layers. The bending and splitting tensile strengths of three-dimensional printed plain concrete samples were 15% and 19% lower than those of casted samples, respectively. However, the addition of fibers improved the mechanical strength of the interfaces significantly.
Three-dimensional concrete printing (3DCP) is increasingly being applied in harsh environments and isolated regions. However, the effective utilization of aeolian sand (AS) resources and by-products derived from arid zones for 3DCP is yet to be fully realized. This study developed a three-dimensional (3D) printing composite using AS and magnesium oxychloride cement (MOC) from local materials. The effects of the mole ratio of MgO/MgCl2 and sand/binder (S/B) ratio on the mechanical properties such as water resistance, drying shrinkage strain, rheology, and printability, were investigated systematically. The results indicated that the optimal mole ratio of MgO/MgCl2 was 8, which yielded the desired mechanical performance and water resistance. Furthermore, the S/B ratio can be increased to three within the desired printability to increase the AS utilization rate. The rheological recovery and buildability of the 3D-printed MOC with AS were verified. These findings provide a promising strategy for construction in remote deserts.
Post-tensioning self-centering walls are a well-developed and resilient technology. However, despite extensive research, the application of this technology has previously been limited to low-rise buildings. A ten-story self-centering wall building has now been designed and constructed using the state-of-art design methodologies and construction detailing, as described in this paper. The building is designed in accordance with direct displacement-based design methodology, with modification of seismic demand due to relevant issues including higher-mode effects, second order effects, torsional effects, and flexural deformation of wall panels. Wall sections are designed with external energy-dissipating devices of steel dampers, and seismic performance of such designed self-centering walls is evaluated through numerical simulation. It is the first engineering project that uses self-centering walls in a high-rise building. The seismic design procedure of such a high-rise building, using self-centering wall structures, is comprehensively reviewed in this work, and additional proposals are put forward. Description of construction detailing, including slotted beams, flexible wall-to-floor connections, embedded beams, and damper installation, is provided. The demonstration project promotes the concept of seismic resilient structures and contributes to the most appealing city planning strategy of resilient cities at present. The paper could be a reference for industry engineers to promote the self-centering wall systems worldwide.
Quasi-rectangular shield tunneling is a cutting-edge trenchless method for constructing metro tunnels with double tubes, owing to its advantages in saving underground space and reducing ground disturbance. However, the conventional quasi-rectangular shield tunneling method is not applicable when constructing a tunnel without a center pillar, such as a scissor crossover section of a metro line. Therefore, the 0-θ tunneling method, which combines the quasi-rectangular shield and pipe jacking methods, was investigated in this study to solve the aforementioned construction challenges. This study presents a case study of the Sijiqing Station of the Hangzhou Metro Line 9 in China, in which the 0-θ method was first proposed and applied. Key techniques such as switching between two types of tunneling modes and the tunneling process control in complex construction environments were investigated. The results demonstrated that the 0-θ method can address the technical challenges presented by the post-transition line with a high curvature and a scissors crossover line. In addition, the adoption of the 0-θ method ensured that the transformation between shield tunneling and pipe jacking was safe and efficient. The ground settlement monitoring results demonstrated that the disturbance to the surrounding environment can be limited to a safe level. This case study contributes to the construction technology for a metro tunnel containing both post-transition lines with a small turning radius and a scissors crossover line. A practical construction experience and theoretical guidance were provided in this study, which are of significance for both the industry and academia.
Balance of the groundwater and ecology is crucial for controlled discharge. However, regarding the segments of tunnel boring machines (TBMs) under high water pressure, the stability of the lining structure is often reduced by excessive drain holes required to achieve this balance. The large discharge of pinholes can easily have severe consequences, such as the lowering of the groundwater table, drying of springs, and vegetation wilting. Thus, in this study, according to the fluid–structure coupling theory, a new drainage design for TBM segments was developed by considering a mountain tunnel subject to a high water pressure as a case study. The evolution characteristics, including the external water pressure of the lining, discharge volume of the segment, and groundwater-table drawdown, were investigated via numerical modeling with drain holes and pinholes. The results indicated that the optimal design parameters of drainage segments for the project case were as follows: a circumferential spacing angle and longitudinal number on one side of a single ring of 51° and 2, respectively, for the drain holes and an inclination angle and length of 46.41° and 0.25 times the grouting thickness, respectively, for the pin holes.
Deep excavations in dense urban areas have caused damage to nearby existing structures in numerous past construction cases. Proper assessment is crucial in the initial design stages. This study develops equations to predict the existing pile bending moment and deflection produced by adjacent braced excavations. Influential parameters (i.e., the excavation geometry, diaphragm wall thickness, pile geometry, strength and small-strain stiffness of the soil, and soft clay thickness) were considered and employed in the developed equations. It is practically unfeasible to obtain measurement data; hence, artificial data for the bending moment and deflection of existing piles were produced from well-calibrated numerical analyses of hypothetical cases, using the three-dimensional finite element method. The developed equations were established through a multiple linear regression analysis of the artificial data, using the transformation technique. In addition, the three-dimensional nature of the excavation work was characterized by considering the excavation corner effect, using the plane strain ratio parameter. The estimation results of the developed equations can provide satisfactory pile bending moment and deflection data and are more accurate than those found in previous studies.
A novel approach based on the upper bound theory is proposed to assess the stability of excavated slopes with spatially variable clay in undrained conditions. The proposed random limit analysis is a combination of the deterministic slope stability limit analysis together with random field theory and Monte Carlo simulation. A series of analyses is conducted to verify the potential application of the proposed method and to investigate the effects of the soil undrained shear strength gradient and the spatial correlation length on slope stability. Three groups of potential sliding surfaces are identified and the occurrence probability of each sort of failure mechanism is quantified for various slope ratios. The proposed method is found to be feasible for evaluating slope reliability. The obtained results are helpful in understanding the slope failure mechanism from a quantitative point of view. The paper could provide guidance for slope targeted local reinforcement.
