The Importance of Tension Softening (of Concrete) in Modelling of Tension Piles-Paper

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The paper examines how concrete tensile strength and tension softening affect the design and modelling of tension piles. Conventional design often assumes concrete cracks as soon as tension develops and assigns all tensile resistance to reinforcement. The authors argue this is overly conservative: concrete can carry tension before cracking, and bond between cracked concrete and steel continues to transfer load through tension stiffening.<br /><br />The study uses finite-element modelling with PLAXIS’s Concrete constitutive model, which represents tensile and compressive hardening and softening using a fracture-energy-based smeared-crack approach. The model is applied first to an unconfined, fixed-end pile and then to a reinforced bored pile in a typical profile of fill, soft clay and stiff alluvial soil. Soil–pile interfaces allow slip and separation, while reinforcement is modelled separately.<br /><br />Results show that concrete contributes substantially to pile stiffness and tensile capacity. In the unconfined example, concrete cracks before the steel yields. In the soil-supported pile, reinforcement helps confine the concrete, limits crack propagation and enables load sharing between concrete and steel. The reinforced pile therefore carries more tension and sustains greater deformation than either concrete or steel considered alone. Without reinforcement, cracking is concentrated near the pile head, and the pile cannot effectively transfer tensile loads along its shaft.<br /><br />The authors conclude that neglecting concrete’s tensile contribution can lead to unnecessary overdesign. Finite-element analysis using a suitable concrete model can help assess cracking, tension softening and tensile-strength utilization, supporting more economical pile and reinforcement design. The model’s parameters can be calibrated using standard concrete tests.

Keywords

tension piles

concrete tensile strength

tension softening

finite-element modelling

PLAXIS Concrete model

fracture energy

tension stiffening

soil-pile interaction

reinforced bored pile

crack propagation