Reversible Strengthening of Traditional Timber Members with Nano-SiO2-Modified CFRP in High-Humidity Environments

Timber-structure buildings are a common traditional architectural type in China whose value has shifted from residential use toward cultural heritage preservation, historical research, and cultural-tourism exhibition. In subtropical regions such as Fujian and Hunan, these structures are exposed to year-round humid conditions that promote moisture-induced decay, making routine maintenance and strengthening essential. The scientific gap addressed here is that the durability–reversibility coupling of nano-SiO2-modified CFRP–timber systems under sustained high-humidity exposure has not been systematically quantified, and no single-variable dose–response framework currently exists for isolating the contribution of nano-silica modification from connector effects. Conventional fiber-reinforced polymer (FRP) strengthening techniques are irreversible in application and prone to interfacial debonding in high-humidity environments. This study proposes a recoverable strengthening system employing nano-silica-modified carbon fiber-reinforced polymer (modified CFRP) composites applied to a detachable bolt-sleeve connection structure. Five specimen groups (blank control, unmodified CFRP, and M-CFRP-1, M-CFRP-3, M-CFRP-5 with 1.0%, 3.0%, and 5.0% nano-silica mass fractions) were subjected to accelerated aging for 120 days at 90.0 ± 3.0% relative humidity, followed by shear, compressive, and flexural tests complemented by SEM, DMA, moisture absorption measurement, thermal conductivity testing, one-way ANOVA with Tukey’s HSD post hoc test, ISO 13788 Glaser condensation risk analysis, and 1D heat-balance modeling. The M-CFRP-3 group exhibited the lowest equilibrium moisture content (6.8%), the highest retention of flexural (82%), shear (78%), and compressive (82%) strengths, and a diurnal inner-surface temperature swing of only 4.8 °C. The M-CFRP-3 envelope eliminated surface condensation throughout the diurnal cycle (dew point 19.15 °C), while the bare-timber envelope experienced 4.08 h/day of condensation risk; a 1D heat-balance analysis confirmed that the combined effect of narrowed moisture-induced conductivity gap and CFRP thermal inertia accounted for the observed temperature swing reduction. Disassembly of the bolt-sleeve system limited timber substrate damage to below 15% and supported three complete disassembly–reassembly cycles. The application of modified CFRP with 3 wt% nano-silica combined with a detachable bolt-sleeve connection simultaneously achieves durability, reversibility, and hygrothermal stability. This study advances a quantitative single-variable framework linking nano-SiO2 dose to moisture-barrier, mechanical, and hygrothermal responses, providing a transferable methodology for designing reversible strengthening systems for heritage timber in humid climates. providing a feasible strategy for the maintenance and strengthening of traditional timber-structure buildings in high-humidity climatic regions.