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Concrete in saline or coastal settings exposed to freezing temperatures is frequently affected by coupling actions of sulfate assault and freeze–thaw degradation, reducing the service life of concrete structures significantly. This study conducted an accelerated freeze–thaw cycle test in pure water and Na2SO4 solution with a mass proportion of 5% to examine the coupling impact of sulfate freeze–thaw on the frost resistance of composite limestone powder (CLP) concrete. Combined with SEM and XRD methods, the performance degradation mechanisms of composite limestone powder (CLP) concrete in coupling sulfate freeze–thaw conditions were analyzed with a microscopic point of view. The findings demonstrated that limestone powder has a filling effect but the activity is low. When the content is 10~20%, the chemical response is higher than the physical response. The pozzolanic effect of fly ash and slag can improve the pore structure and improve the compactness of concrete. The "superposition effect" of limestone powder, fly ash, and slag can improve the frost resistance of CLP concrete. The scenario of salt freezing cycles has negative effects that are worse than those of water freezing cycles on the antifreeze performance of CLP concrete, including apparent morphology, mass loss, relative dynamic modulus of elasticity, and compressive strength. Sulfate's activation effect boosts slag's activity effect, which significantly promotes the antifreeze performance of concrete subjected to salt frozen cycles over water frozen cycles. The freeze–thaw damage model of CLP concrete under coupling sulfate freeze–thaw is established through theorem analysis and experiment statistics, laying a theoretical framework for the popularization and use of this concrete. [ABSTRACT FROM AUTHOR]

The tightly spliced truss-reinforced plate-honeycomb flat beam is a novel type of composite beam, with the advantages of high rigidity, high bearing capacity, and ease of construction. In this study, on the basis of the performance in bending tests, the numerical analysis method is used to study the influence of the honeycomb hole–height ratio, and section height on the bearing capacity of the honeycomb composite flat beam. On this basis, the simplified method to calculate the ultimate bending capacity of the new honeycomb composite flat beam was proposed. The research results show the failure modes of the specimens are mainly divided into two states, including the deflection exceeding the limit and the concrete flange plate separating from the steel beam. The tightly spliced truss-reinforced plate-honeycomb flat beams have good ductility, of which the average value reaches 8.2. The simplified method proposed in this article for calculating this type of honeycomb composite beam has an error of less than 10% in terms of bending bearing capacity, which has advantages over the double T-shaped steel method. The calculation method and design suggestions proposed in this study provide a basis for the research and application of this type of composite flat beam. [ABSTRACT FROM AUTHOR]

This paper presents a joint mode for a vertical grouting sleeve and a partially distributed reinforced composite connection. The seismic performance of four hybrids precast shear walls with a shear span ratio of 1.83, an axial compression ratio of 0.20, and partially connected vertically distributed rebar was studied by pseudo-static tests. The connection ratios of the reinforced concrete with a vertical distribution were 100%, 67%, 50%, and 33% and were compared with cast-in-place shear walls. It was found that both the precast and cast-in-place specimens demonstrated bending and shear failure modes. Both ends of the mortar layer were cracked under compression, and steel connection failure occurred under the ultimate load. The load-displacement hysteresis curve was bow-shaped and relatively full; the yield load increased from 11.4 to 16.6, and the peak load increased from 4.2 to 6.8 compared with those of the cast-in-situ specimen. The strength degradation performance of the precast specimen was slightly weaker than that of the cast-in-place specimen. The ductility coefficient of the precast specimens was approximately 4, and the ultimate displacement angle was between 1/30 and 1/32, which was larger than the limit value of elastic-plastic interlayer displacement of shear wall structures required by the code. The reduction in the number of vertically distributed rebar connections led to a decrease in the bearing capacity, stiffness, ductility, and energy dissipation, but the impact was very small. [ABSTRACT FROM AUTHOR]

In the process of service, reinforced concrete structures have to bear both load and multi-environmental factors. The deterioration of reinforced concrete beams is critical to the durability, safety, and sustainability of reinforced concrete structures. The main aim of the present research is to determine the degradation mechanism of reinforced concrete beams subjected to sustained loading and multi-environmental factors. Reinforced concrete beam specimens were prepared, loaded and then exerted multi-environmental factors. At the end of each degradation period, the degradation of concrete (chemical contents of concrete beam surfaces, carbonation depth, compressive strength and maximum cracks) and the corrosion of steel bars (corrosion ratio and tensile strength) were continuously measured. Moreover, degraded reinforced concrete beams were flexural loaded in four-point bending failure tests. The degradation mechanism of reinforced concrete beams subjected to sustained loading and multi-environmental factors was analyzed. Thus, this study can promote a comprehensive understanding of reinforced concrete beams subjected to sustained loading and multi-environmental factors. [ABSTRACT FROM AUTHOR]

The air shafts of coal mines contain solid, liquid, and gaseous substances that corrode concrete under conditions of relatively high temperature and humidity. The coupling of various factors decreases the concrete strength and thus the bearing capacity of the air shaft lining. In this study, concrete corrosion tests were carried out by simulating the complex environment of a coal mine air shaft lining to study the variation in the concrete stress-strain curve. For corroded concrete specimens subjected to very low stress, the microcracks and holes were closed, generating a large deformation, indicating severe concrete corrosion. The longer the corrosion time was, the gentler the initial slope of the curve. Finally, the uniaxial compression constitutive model of corroded concrete was established based on the Weibull distribution of three parameters, and the model curve was compared with the experimental curve. This model can not only predict the change of the stress-strain curve of high-strength concrete in a coal mine air shaft environment but also provide theoretical reference for the application of high-strength concrete in an air shaft environment. [ABSTRACT FROM AUTHOR]

The shaft, a key engineering facility in coal mining, serves as an important construction channel for materials and personnel during the development of new mines. With the progression of coal mining, various harmful media will appear in the natural environment of the shaft. Through environmental detection in seven coal mines across eastern China, this paper discovers that the internal environment of the reinforced concrete shaft wall contains multiple media that are harmful to rebar and concrete. The gas media mainly contains H2S, HCl, SO2, NO2, and CO2; the liquid media contains Ca2+, Mg2+, SO42-, and HCO3; and the solid media contains SiO2, CaO, Al2O3, Na2O, Fe2O3, etc. It remains hot and humid for a long time. The external environment of the shaft wall has harmful ions, such as Na+, K+, SO42-, and Cl-, and consistently faces a high geo-stress; the concrete was corroded by the CO2 in the harmful media through neutralization, which led to physical or chemical changes within the cementing material. Various acidic media produces soluble corrosion in the concrete. The calcium salt generated by the reaction dissolves in the flowing water and separates from the concrete, gradually reducing the alkalinity and strength of the concrete. Sulfate produces expansive corrosion in concrete, which causes concrete to suffer expansive damages and, coupled with the lasting geo-stress on the shaft wall, speeds up the deterioration of rebar and concrete in the wall, posing a serious threat to the safety and service life of the shaft. [ABSTRACT FROM AUTHOR]

The improved effect of the bonding behavior of concrete and reinforcement is a critical indicator for estimating the validity of strengthening measures. The bonding behavior of unconfined or textile reinforced concrete (TRC)-confined concrete and a plain or deformed reinforcement was comparatively analyzed via the pull-out tests, and the influence of chloride wet-dry cycles also was considered on the bonding behavior. Moreover, the bonding behavior was further revealed from the view of energy. The study results indicated that the bonding failure of specimens with a deformed reinforcement was changed after TRC confinement, showing a certain ductile failure characteristic, and the integrity of the confined specimens was ensured at failure. The impact of chloride attack on the bonding behavior of concrete and a plain or deformed reinforcement was different under the unconfined or TRC-confined condition. The deterioration mechanism of the bonding behavior needs to be further investigated when the reinforcement is corroded seriously. The results from the energy analysis are consistent with the pull-out test results. [ABSTRACT FROM AUTHOR]

Because of the existence of beams, columns, shear walls, and other structural components, the external wall of a building is often composed of two parallel structures, wherein one is a concrete structure and the other is made of bricks. Therefore, to determine the economical thermal insulation thickness of an external wall, one can not only consider a single structure. Mathematical models of economical thermal insulation thickness considering and not considering the effect of non-homogeneity of external walls were established based on the method of life-cycle cost. The economical thermal insulation thickness and economic benefits of a building wall in Beijing that is composed of two parallel structures with EPS thermal insulation were comparatively analyzed. The comparisons were carried out with two different heat sources and three different concrete structure-to-external wall ratios. The results show that non-homogeneity of external walls have a considerable influence on the economical thermal insulation thickness and economic benefit of an insulating external wall. The economical thermal insulation thickness under the consideration of non-homogeneity of external walls will become thicker. However, the increase in life-cycle cost is not significant, whereas the increase in life-cycle saving is very significant, and the payback period becomes shorter. The effectiveness of the investment and the energy efficiency revenue without consideration of the effect of non-homogeneity of external walls are underestimated. Thus, a method that considers this effect can obtain more reasonable economical insulation thicknesses and economic benefits. [ABSTRACT FROM AUTHOR]

The interfacial bonding behavior of textile reinforced concrete (TRC)-confined concrete and corroded steel bars was studied by central pull-out testing using three types of specimens (not strengthened, strengthened after corrosion and strengthened before corrosion). The results show that without the TRC constraint, the ultimate bond strength between the concrete and corroded steel bar decreases with increasing corrosion ratio, especially after the concrete cracks due to reinforcement corrosion, at which point the ultimate bond strength rapidly decreases. However, the ultimate bond strength of the specimens strengthened after corrosion can be three times greater than the strength of the specimens not strengthened when the concrete cracks due to reinforcement corrosion. For the specimens strengthened before corrosion, the TRC strengthening layer effectively limits the decrease in the ultimate bond strength between the corroded steel bars and concrete, which results in specimens with a high interfacial bond strength at high corrosion ratios. Thus, compared to the other two types of specimens, this case comparatively yielded the best results. Finally, for all three types of specimens and according to the experimental data, a calculation model of the ultimate bond strength was established considering changes in the corrosion ratio. [ABSTRACT FROM AUTHOR]

When the energy saving rate of existing residential buildings renovation is determined, the thermal performances of external walls, windows, and roof interact with each other. Therefore, it is necessary to study the determination of economical insulation thickness of building envelopes considering the interaction among building envelope performances. The objective function and its bound of envelope thermal performance optimization in the energy-saving renovation of existing residential buildings in severe cold and cold zones in China were established. It is the conditional extremum problem and can be solved through Lagrange’s method of multipliers to determine the economical insulation thickness of external walls and roofs simultaneously. The method is proved to be feasible by an existing residential building in Beijing. When the same window types are selected, the energy-saving renovation program of the building envelope determined by the Lagrangian optimization method can produce the minimum investment in insulation, minimum investment payback period, and the largest net present value (NPV) of the life cycle. [ABSTRACT FROM AUTHOR]

Three single reinforcement concrete (RC) frames, including 1 reference specimen and 2 specimens strengthened with shear walls, were fabricated and subjected to low cyclic loadings, in order to evaluate seismic performances of strengthened single frame structures with exterior cantilevers. Through comparison and analysis of failure mode, hysteretic behavior, skeleton curve, energy dissipation, strength, and stiffness degradation of the tested frames, the validity of the shear wall-based reinforcement method for single frames was verified. Test results indicate that the stiffness and load-bearing capacities of strengthened frames increased considerably in comparison with the reference frame. A "strong column-weak beam" failure pattern was observed on the cantilever side, and the failure of the shear wall was always prior to the column, which can increase the structural redundancy and improve the failure mechanism and seismic performance of an existing single frame. [ABSTRACT FROM AUTHOR]

X-ray computed tomography was applied to continuously track the migration process of water in unsaturated cement pastes in situ. It was found that the process of water uptake starts with early rapid growth and a subsequent slowly increasing period, followed by a relatively stable stage. The rate of water penetration was accelerated with a decrease in slag incorporation. An analytical equation was developed to establish the relationship between the depth and the mass coefficient of capillary suction and the obtained predicted values were found to be in excellent agreement with the experimental values observed via the gravimetric method. [ABSTRACT FROM AUTHOR]

This article presents the mechanical properties of polyvinylchloride-coated fabrics with Precontraint® technology. First, groups of off-axial tests are carried out, and the corresponding strength criteria are discussed. Then, the biaxial tests are carried out on the material without initial loading and the material after cyclic loading, and the corresponding stress–strain surfaces are studied. Finally, the effects of temperature on the mechanical properties are studied. Results show that this material is typically orthotropic and its strength can be predicted by the Tsai–Hill strength criterion. The differences between the mechanical properties of the warp and the weft are not significant, because the Precontraint® technology used in the processing reduces significantly the discrepancies, compared with common plain woven fabrics. The material without initial loading is nonlinear and viscoelastic, and after cyclic loading it becomes linear and its stiffness increases. The biaxial tensile behaviors are affected by biaxial loading significantly, and then different constitutive relations should be conducted regarding to different service conditions. With temperature increasing, the tensile strength and the elastic modulus gradually decreases, and the breaking strain increases. [ABSTRACT FROM AUTHOR]

The issue of steel bar corrosion has seriously affected the working performance of concrete structures. Reinforcement is an effective measure to restore the mechanical properties and prolong the service life of concrete structures. Determining the bond properties between textile-reinforced concrete (TRC)–confined concrete and a corroded steel bar is one of the main issues in evaluating the effect of TRC reinforcement. Based on the results of electrochemical corrosion and pullout tests, the bond properties between concrete and a steel bar under unconfined and TRC–confined conditions were comparatively analyzed in terms of the corrosion-induced cracking, failure mode, bond-slip curve, bond strength, and bond failure mechanism. The empirical calculation formula of the ultimate bond strength between unconfined concrete and a corroded steel bar was obtained, and the confinement effects of TRC and fiber-reinforced polymer (FRP) were compared by collecting data from the relevant literature. The research results indicated that TRC confinement can effectively improve the bond properties between concrete and a corroded steel bar and limit the development of corrosion-induced cracks. Furthermore, TRC confinement controlled any further crack expansion during the pulling out of the steel bar. After TRC confinement, the brittle failure characteristic of the bond behavior between concrete and a corroded steel bar was improved, and the failure mode was changed from splitting to splitting–pullout. The confinement effect of TRC using a lower fiber-reinforcement ratio is similar to that of FRP. [ABSTRACT FROM AUTHOR]