Henglin Lv

To study the impact of limestone powder (LP) fineness on the physical and mechanical characteristics of concrete, a total of 88 sets of LP concrete mixtures were designed, in which the LP content (0~35% of the mass of cementitious materials), LP-specific surface area (350–1000 m²/kg), and water–binder ratio were used as testing factors. The experimental and theoretical approaches were performed to study the physical (slump) and mechanical properties (compressive strength and bond-slip behaviors with steel bars) of LP concrete. Secondly, the compressive activity index (CAI) was introduced as a measurement for quantifying the compressive activity of LP. The results indicate an optimal improvement in the physical and mechanical properties of LP concrete when the LP content is equal to or less than 15% and the specific surface area is equal to or less than 600 m²/kg. An increase in LP content will result in a drop in the bond strength between concrete and steel bars; in contrast, an increase in LP-specific surface area will enhance the bond strength. Furthermore, CAI can better reflect the role of LP in concrete, which provides a theoretical basis for the application of LP in concrete.

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.

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.

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.

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.

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.

The effects of hydroxypropyl methyl cellulose ether, starch ether, bentonite, and redispersion emulsoid powder on the working and mechanical properties of fresh dry-mixed mortar were studied. The results show that hydroxypropyl methyl cellulose ether has the greatest impact on the consistency and water retention of ordinary dry-mixed mortar and that redispersion emulsoid powder reduces the water action and starch ether has essentially no effect on water retention. It also shows that the time of mortar condensation when mixed with hydroxypropyl methyl cellulose ether is the longest, followed by redispersion emulsoid powder and bentonite. Starch ether can slightly, but not obviously, extend the setting time of cement mortar. Hydroxypropyl methyl cellulose ether has the greatest impact on the mechanical properties of ordinary dry-mixed mortar, followed by redispersion emulsoid powder, starch ether, and bentonite. As the water retention increases, the setting time of the mortar also increases. The use of water as a thickening material has a retarding effect on the mortar, increases the water-retention rate, and increases the retarding effect. Moreover, increasing the content of the chemical admixtures decreases the strength of cement mortar.