Study of deformation characteristics of composite building materials under long-term service loading

Abstract

Deformation characteristics of composite building materials are standardised separately by matrix type: cementitious composites are tested under GOST 24544-2020, polymer composites under GOST R 57714-2017 with a 1000-hour observation period, timber-based materials under GOST R 70877-2023. A load-bearing element is assembled from materials of different groups, while the prediction of its long-term compliance rests on incompatible test durations, which introduces a bias that safety factors do not compensate. The aim is to quantify the share of the creep resource revealed by the standardised test duration for materials with different matrix structures. An author's model of creep kinetics has been developed for six model composites on a two-parameter power-law approximation; the parameters were calibrated against the 1000-hour base and extrapolated to 50 years of service. The calculated creep coefficient of heavy concrete of class B30 amounted to 2.277 against the standardised 2,3 given in Table 6.12 of SP 63.13330.2018. The share of creep accumulated by 1000 h ranges from 16.4% for polymer concrete to 29,8% for glass-fibre composite externally reinforced with carbon fibre. The residual fraction of the initial elastic modulus after 50 years was obtained as 0.220 for polymer concrete and 0,629 for the glass-fibre composite. Ranking by creep coefficient does not reproduce ranking by absolute creep strain: with the lowest coefficient of 0,590 the glass-fibre composite accumulates 1116 microstrain against 493 microstrain for heavy concrete. The exponent of the approximating function governs the prediction more strongly than the initial stress level, and the difference in test durations is transferred into structural analysis in a systematic manner.