Flexural and Shear Behaviour of Post-Tensioned Paper-Tube Voided Slabs and a Design Criterion for Tendon Substitution

Voided slabs reduce self-weight by removing concrete near the neutral axis, but the interaction between continuous cylindrical voids and post-tensioning remains poorly understood because the voids restrict the tendon position, and hence, its available lever arm. This study examines 32 full-scale one-way slab specimens, comprising 20 flexural and 12 shear tests, with solid and paper-tube voided sections, three formwork configurations, and post-tensioned variants. Archived load records were reconciled against the raw load-cell signal and cross-checked using reinforcement-strain measurements. In flexure, voids occupying 33–42% of the gross section as drawn (22.7–34.9% on a nominal two-tube basis) changed peak load by only +3.7% on average, consistent with the limited sensitivity of flexural strength to concrete removal near the neutral axis. Post-tensioning produced no systematic increase in peak load because the tube geometry restricted the tendon depth and reduced its lever arm relative to the bars it replaced. A closed-form, section-level screening check was developed to quantify this geometric limitation. In the two controlled shear pairs, the solid specimens yielded in flexure before peak, whereas their voided companions failed in diagonal tension; voiding reduced peak load by 19.2% and 38.0%, whereas the measured work to peak decreased by 81.9% and 91.3%. These results indicate that continuous cylindrical voids may impair pre-peak deformation capacity more severely than peak shear resistance at low shear-span-to-depth ratios.