On the Long-term Deflection Behavior of Timber–concrete Composite Slabs

Abstract

This paper investigates the long-term behavior of cross-laminated timber (CLT)–concrete composite (TCC) slabs through a large-scale parametric study using a two-layer finite-element model with interface springs to represent partial interaction. The study isolates the influence of four parameters—concrete shrinkage (modeled via relative humidity, RH), composite factor (γ), concrete creep coefficient (φ), and timber creep coefficient (K_def)—on time-dependent deflections. A total of 36,864 analyses were performed for a 6 m span under quasi-permanent loading, with results normalized to the least-deflecting case in each subset.
For a representative configuration (CLT 120 mm, concrete 50 mm) and RH = 60%, the normalized 50-year deflection increases by approximately 22% with higher shrinkage, by 6% when reducing γ from 0.9 to 0.3, by 8% when increasing φ from 1.5 to 3.0, and by 12% when increasing K_def from 0.6 to 1.0. Across configurations, shrinkage is consistently the dominant driver. Moreover, shrinkage-induced deflection scales approximately linearly with the distance from the centroid of the concrete layer to the composite neutral axis (Z_na), highlighting geometry as a primary design lever.
The design implications are as follows: 1. prioritize low-shrinkage concrete mixes; 2. avoid intentionally reducing γ, as global stiffness and overall performance deteriorate; and 3. select layer proportions that minimize Z_na while balancing the stiffness gains and shrinkage sensitivity associated with thicker concrete toppings.