This paper presents an experimental study of the separated behavior of short ( L/H=3) high strength concrete-filled rectangular hollow section (RHS) tubes concentrically loaded in compression to failure. A total of 50 specimens were tested. Experimental results showed that the concrete strength influenced the failure pattern of the specimen. The height-to-breadth ratio of the rectangular tube (varying from 1.0 to 1.6) had no evident influence on the ultimate bearing capacity of the specimen. Then based on the experimental results, a numerical separation method was successfully used to separate the compressive load carried by the steel tube and the core concrete. The equivalent One-Dimensional nonlinear stress-strain models of the steel and the confined concrete were suggested, which can be used to determine the overall behavior of the high strength concrete-filled RHS tubes. The stress-strain models have been used to numerically analyze the behavior of high strength concrete-filled RHS tubes. The numerical results are compared with the experimental results and they agreed well with each other.

Among the techniques for converting stacked coal gangue to reusable material, one of the most effective ways is to use coal gangue as a coarse aggregate in green concrete productions. The physical and chemical properties of rock and spontaneous-combustion coal gangue are generally suitable for being used as a coarse aggregate in green concrete. Coal gangue concrete is not recommended to be used in subsurface structures, as its water absorption law would be changed under a large replacement ratio. The mechanical performance of coal gangue concrete is degraded by raising the replacement ratio. Over-low and -high concrete grades are not suggested to be used as coal gangue aggregate, unless extra admixtures or specific methods are used. The influence of coal gangue on the durability of coal gangue concrete is remarkable, resulting from the porous structure of coal gangue that provides more transmission channels for air and liquid in concrete, but is beneficial for thermal insulation. The usage of coal gangue in structural concrete members is still limited. The mechanical behavior of some structural members using coal gangue concrete has been reported. Among them, concrete filled steel tubes are a preferable configuration for using coal gangue concrete, regarding both the mechanical and durability performance.

Dealing with mineral tailings is one of the most important topics for solving the environmental problems in the mining industry. Among the techniques converting stacked molybdenum tailings to reusable cleaner products, one of the most effective ones is to use molybdenum tailings in concrete productions. The physical properties including density, microscopic structure and finesse module, and chemical composition of molybdenum tailings are similar with those of natural sand. The radionuclide assessment of molybdenum tailing meets the requirements for using as structural materials. Therefore, Molybdenum tailing is suitable to be used as the replacement of cement and fine aggregates in mortar and concrete. Based on the results of strength and duration performance comparison, the usage of molybdenum tailing as a replacement of natural sand is a more feasible way than that of ground super-fine molybdenum tailings in cement binder. It is feasible to use molybdenum tailings as fine aggregate in the preparation of structural concrete. When the amount of ground super-fine molybdenum tailings replacing cement is less than 10%, it is beneficial to improve the freeze-thaw and carbonization resistance of the concrete.

An experimental study of the behaviour of high strength concrete-filled slender rectangular hollow section (RHS) tubes under the combined actions of axial compression and bending moment is presented. A total of 26 specimens were tested. The main parameters considered in the test were slenderness ratio, depth-to-breadth ratio, steel-to-concrete area ratio and eccentricity-to-depth ratio. The experimental results showed that the ultimate capacity of test specimens decreased rapidly with increase of slenderness ratio and eccentricity-to-depth ratio. For the specimens with depth-to-thickness ratio larger than 50, local buckling failure occurred prior to the ultimate capacity. A nonlinear analysis program, BC, was developed to analyze the behaviour of high strength concrete-filled RHS tubes. A comparison for ultimate capacity showed that the theoretical results agreed very well with the experimental results. The depth-to-breadth ratio (varying from 1.0 to 1.6) showed little influence on the stability capacity of the specimens. The comparison of the experimental results with the results calculated by the design equations in the codes, such as LRFD, EC4 and AIJ showed that EC4 closely predicts the ultimate capacity of the specimens.

The current codes and specifications use the equivalent beam-column method to predict the in-plane stability capacity of concrete-filled steel tubular (CFST) arches. Using this method, the effects of the rise-span ratio and the nonuniform moment on the stability capacity of CFST parabolic arches are not considered. This may induce a difference between calculation results and actual structural stability capacity. To study the effects of the rise-span ratio and the nonuniform moment, six fixed CFST parabolic arches, with rise-span ratios of 1/4.5, 1/6, and 1/9, were tested and analyzed in this study. The failure patterns and mechanisms were studied. The confining effect of the steel tube to the concrete at failure was investigated. The influence of the rise-span ratio and loading conditions on the in-plane stability of fixed CFST arches was also considered. A nonlinear elastic-plastic finite-element (FE) model was used to predict the buckling behavior of the fixed CFST parabolic arches. Based on the test and the FE results, a design equation is proposed for the design of in-plane stability of fixed CFST parabolic arches under combined bending and compression.

Ultra-high-performance concrete (UHPC) with a low steel fiber volume fraction offers lower material costs than UHPC with typical steel fiber volume fractions, and has the potential to mitigate the ductility degradation of rebar-reinforced UHPC (R-UHPC). This study explores the reinforcement effect on the tensile behavior of UHPC with a low fiber volume fraction with the aim of facilitating more cost-efficient UHPC applications. The axial tensile behavior of 30 UHPC specimens with low fiber volume fractions at different reinforcement ratios was tested through direct tensile tests. The findings indicate that adopting UHPC with a low fiber volume fraction can significantly mitigate the ductility deterioration of rebar-reinforced UHPC (R-UHPC), and both increasing the reinforcement ratio and decreasing the fiber volume fraction contribute to the improvement in ductility. The failure modes of R-UHPC are determined by the ratio of reinforcement ratio and fiber volume fraction, rather than a single parameter, which also means that R-UHPC with different parameters may correspond to different methods to predict tensile load-bearing capacity. For UHPC with a fiber volume fraction low to 0.5%, incorporating steel rebars gives superior multi-crack cracking behavior and excellent capacity to restrict the maximum crack width. Increasing the fiber volume fraction from 0.5% to 1.0% at the same reinforcement ratio will yield little benefit other than an increase in tensile load-bearing capacity.