Evaluation of axial stress‐axial strain behavior of high‐strength fiber‐reinforced lightweight aggregate concrete ( HSFRLC ) confined with spiral stirrups under axial compression

Abstract The axial stress–strain models of confined high‐strength fiber‐reinforced lightweight concrete (HSFRLC) were important key inputs for analytical programs to predict the full‐range loading deformation of confined HSLC and HSFRLC. The descending branch of the axial stress‐axial strain curves of low‐ and middle‐confined HSLC and HSFRLC columns confined with stirrups under axial compression had obvious transition points and residual stress point, which the existing prediction models did not considered. This paper investigated the axial stress–strain behavior of HSLC and HSFRLC confined by spiral stirrups under axial compression, a total of 12 confined HSLC columns and 12 confined HSFRLC columns were tested, with key parameters including stirrup volumetric ratio, steel fiber content, and concrete strength. The results showed that confined HSLC columns shifted from shear failure under low stirrup volumetric ratios to expansion dominated failure at high stirrup ratios; by contrast, all HSFRLC columns experienced expansion failure governed by the combined lateral confinement from spiral stirrups and steel fibers. The axial stress–strain responses of both material types featured four clear stages: elastic stage, elastoplastic stage, sharp stress drop stage, and gradual stress decline stage. As the stirrup volumetric ratio raised from 0.9% to 2.5%, the normalized peak stress, peak strain and residual stress of confined specimens increased by up to 46%, 67%, and 108%, respectively; the addition of 2% volume fraction steel fiber further improved the normalized residual stress by up to 64%. Based on the test database, a complete segmented axial stress–strain constitutive model was established, with explicit formulas proposed for the transition point and residual stress point on the descending branch. The predicted‐to‐test ratios mean values for peak stress and peak strain reached 0.950 and 1.004, respectively, nearly all predicted values fall within the 0.85–1.15 bandwidth of test measurements, demonstrating that the proposed model achieved higher accuracy and reliability for describing the full‐range mechanical behavior of confined HSLC and HSFRLC.