In engineering design, the strength analysis of concrete structures heavily depends on the compressive strength of hybrid fiber reinforced concrete (HFRC), which also has an impact on the stability and safety of the structure. The objective of this study is to develop a unified empirical model that can quickly estimate the compressive strength of hybrid fiber reinforced concrete under multiaxial compression. To measure the multiaxial compressive strength of hybrid fiber reinforced concrete, 108 cylindrical specimens and 225 cubic specimens were designed for conventional and true triaxial testing, respectively. Two typical stress paths, i.e., proportional loading and constant restraint loading, were employed to simulate the multiaxial compressive strength of hybrid fiber reinforced concrete, and stress ratio- and confinement pressure-dependent formulas were proposed to calculate the strength correspondingly. Based on the validation against the available test results, it has been demonstrated that the empirical model can effectively predict the axial strength of hybrid fiber reinforced concrete. The test findings reveal that the constraint pressure considerably affects the compressive strength of concrete, and steel fiber can further improve these capabilities significantly.

Foliated metamorphic rock exists widely in the surrounding rock of tunnel structures, underground utility caverns and mountain civil structures, and its laminated foliation structure induces obvious anisotropic time-dependent creep deformation, which seriously threatens the long-term service safety of underground building support systems. The strength and time-dependent deformation of foliated phyllite vary sharply with bedding dip angles, and long-term creep easily triggers large convergence, secondary-lining cracking and structural intrusion failure of tunnel structures. In this work, multi-stage uniaxial compression creep tests were carried out on sericite phyllite specimens with foliation dip angles of 0°, 30°, 45° and 90° using a programmable rock creep testing apparatus, aiming to investigate the anisotropic creep behaviors of phyllite surrounding rock for tunnel engineering. The anisotropic P-wave velocity characteristics of phyllite under different bedding angles were first analyzed to quantify the directional structural difference in the rock matrix. Based on the creep test curves, the full-stage deformation laws including instantaneous strain, decelerating creep, steady-state creep and accelerating creep were systematically summarized for specimens with different bedding orientations. Combined with macroscopic creep failure modes, phenomenological analysis was performed on the anisotropic creep damage behavior controlled by foliation weak planes. The results demonstrate that inclined bedding phyllite presents the most severe creep deformation and steady-state creep rate, and three typical creep failure modes (cross-bedding shear failure, tensile splitting failure, bedding slip shear failure) correspond to horizontal, vertical and inclined foliation specimens respectively. This study quantitatively reveals the time-dependent anisotropic mechanical properties of foliated soft rock, establishes a systematic experimental database and provides experimental basis for long-term stability prediction, and enriches the rheological research system for the durability evaluation of underground civil structures.

The use of recycled aggregate concrete (RAC) enables the valorization of construction waste and supports carbon-reduction strategies. However, long-term service-induced deterioration means that recycled aggregates and their interfacial transition zones inevitably contain defects, which severely restrict the safe application of RAC in load-bearing structures. Notably, although RAC reduces embodied carbon by recycling construction waste, steel tube manufacturing introduces an additional carbon footprint; such carbon trade-offs can be well compensated by the improved structural efficiency and extended service life of steel-confined concrete, achieving superior whole-life carbon benefits. In the present study, a steel-tube-confined recycled aggregate concrete (STCRC) composite system is proposed. Through designed external confinement, the stress state of the internal concrete is altered from uniaxial compression to triaxial compression, thereby enhancing its axial load-bearing capacity. Axial compression tests were performed on 36 short column specimens of steel-tube-confined concrete (STCC) composed of C30 aggregate concrete and Q235 steel tubes with three wall thicknesses (4.5 mm, 6 mm, 8 mm). Further parametric finite element analyses with 16 calculation cases were conducted to quantify the effects of higher concrete strength grades (C40 and C50) and of steel tube strength grades. The evolutionary characteristics of the load-displacement response, the axial stress–lateral strain relation, and the lateral strain–longitudinal strain were systematically investigated across various parameters. Test outcomes indicate that steel tube confinement significantly restrains lateral dilation of RAC and enhances its ductility and ultimate bearing capacity, with higher confinement efficiency observed for RAC than for natural aggregate concrete (NAC). Numerical results further identify the differing sensitivities of NAC and RAC to variations in tube wall thickness, steel yield strength, and concrete strength grade. Using combined experimental and numerical datasets, a peak stress modification factor is proposed, and a tailored stress–strain constitutive model for STCRC is developed and validated. The research findings provide theoretical guidance for the design of axially compressed short columns made of prefabricated recycled concrete.