Discrete modelling of long-term flexural serviceability of cracked fibre-reinforced concrete using single-fibre/interface and matrix creep data

Serviceability under sustained actions often governs the design of fibre-reinforced concrete. We present a discrete framework for predicting long-term flexural serviceability that requires only matrix creep and single-fibre/interface creep data. Cracks are modelled as strong discontinuities, and fibres are discretely embedded with crack-activated Kelvin-chain elements, so viscoelastic effects arise only when a fibre intersects a crack. The fibre viscoelastic mechanism is prescribed a priori by fibre stiffness (slip/pull-out for steel; tensile elongation for compliant polymers) and by the instantaneous bridging stress across the crack. Because the creep–stress relation is non-proportional, fibres are grouped by stress level, each with distinct rheological parameters that update with crack opening and orientation; angle-dependent crack intersections are handled automatically. Calibration relies solely on single-fibre/interface creep together with matrix creep, eliminating the need for composite-level creep campaigns in many cases. Validation against steel- and polypropylene-reinforced members under sustained loading shows good agreement for time-dependent crack widths and deflections, consistent with slip/pull-out dominance in steel systems and tensile elongation in polymer systems. This reduces the experimental burden for introducing new fibre systems and enables serviceability assessment when only interface-scale data are available. The framework is readily implementable within discrete-crack finite-element formulations, supporting serviceability assessment across fibre types and stress ranges.