Accounting for concrete creep in the analysis of reinforced concrete structures using the nonlinear deformation model of SP 63.13330

Introduction. Concrete creep is a defining factor in the analysis of the stress-strain state (SSS) of reinforced concrete structures, causing increased deflections and intense stress redistribution between concrete and reinforcement. Traditional calculation approaches using the Non-linear Deformation Model (NDM), based on simplified adjustments to concrete stress-strain curves, have limited applicability when calculating systems with complex loading histories, in stress relaxation problems, and when modeling strengthening under load.

Aim. To develop and verify numerical algorithms for accounting for creep within the NDM structure based on a creep measure, ensuring the separate isolation of instantaneous and long-term strain components. Materials and Methods. An approach is proposed that is implemented at the level of elementary section areas by introducing creep in the form of initial strains. Three algorithms were developed, differing in the scheme of rheological process approximation: direct iterative accounting, integral accounting using the trapezoidal method, and step-bystep time integration with sequential updating of reference stresses. Numerical implementation was carried out in the “NDM+” software complex. Verification was conducted based on test results of 16 axially compressed reinforced concrete prisms (Ghent University) conducted over 12 years, with varying reinforcement percentages (0–6 %) and average concrete stress levels (0–15 MPa).

Results. Comparison with experimental data showed that algorithms based on the trapezoidal method ensure high accuracy (error 1–7 %) throughout the entire observation interval (4000 days). It was established that simplified iterative accounting leads to an underestimation of strains at late stages by up to 12 % due to the accumulation of methodological error during stress relaxation approximation. The nature of force redistribution was numerically confirmed: for specimens with moderate reinforcement, an increase in reinforcement stresses of up to 2.36 times was recorded, with a simultaneous decrease in concrete stresses by a factor of 1.2.

Conclusions. The developed algorithms represent an advancement of the provisions of the non-linear deformation model, offering a tool for a more detailed SSS analysis of structures constructed in stages or strengthened under load. The proposed approach complements existing engineering methodologies, ensuring a more rigorous physical accounting for the separation of strain components and providing engineers with flexibility in using various rheological models.

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