Effects of Saturated Steam Softening on Radial Gradient-Dependent Hygrothermal Deformation and Compressive Response of Bamboo

Bamboo’s inherent radial gradient gives rise to heterogeneous hygrothermal deformation and mechanical responses, yet the effects of saturated steam softening across the radial thickness of the bamboo culm remain unclear. Eight radial layers of untreated bamboo (UB) and hygrothermally softened bamboo (HSB) were subjected to tests of anatomical structure, water absorption, hygrothermal deformation, and longitudinal and transverse compression in this study. The fraction of the vascular bundle area decreased from 67.67% in the outer layer to 10.08% in the inner layer, and water uptake in UB increased from 19.80% to 77.55%. Softening increased water absorption by 4.30–18.21 per hundred. Tangential and radial swelling coefficients generally decreased inwards, whereas dimensional swelling strains showed different radial trends, and the tangential-to-radial swelling ratio dropped from 1.70 to 0.62 in UB and from 1.46 to 0.59 in HSB; thus, the main direction of swelling had shifted. The most prominent adjacent-layer swelling mismatch occurred at L2–L3, and after softening, the radial mismatch increased by 52.40%. The largest dimensional swelling differences occurred between L1 and L2. Temperature-dependent deformation also varied with direction and radial position. Under longitudinal compression, the peak strain of L8 increased from 1.399% to 2.473%; transverse compression of L1 showed a 31.10% increase in peak strain but a 40.12% decrease in pre-peak work density. These findings highlight the layer-dependent effects of saturated steam softening and provide experimental evidence for assessing deformation compatibility and controlling potential damage during steam-assisted bamboo flattening.