Characterization of Concrete Slab–Granular Subbase Interface Friction in Jointed Plain Concrete Pavements

Reliable characterization of friction at the concrete slab–granular subbase interface is important for the analysis and design of jointed plain concrete pavements. Although previous studies have investigated slab–subbase friction using push-off tests, systematic comparisons of interface condition, slab thickness, movement rate, and repeated loading within a consistent experimental framework remain limited. This study aims to contribute to this understanding through an experimental investigation of the frictional response of concrete slabs supported on a compacted granular subbase. Push-off tests were conducted on 18 concrete slabs with thicknesses of 100, 150, and 200 mm, considering two interface conditions: direct contact between the concrete and granular subbase (rough interface) and separation by a polyethylene sheet (smooth interface). Three movement rates (6, 9, and 12 cm/h) and successive loading cycles were considered. The experimental results were evaluated in terms of friction resistance force (FRF), displacement, and apparent coefficient of friction. A Design of Experiments (DOE) screening approach, supported by Pareto charts and response contour plots, was also employed to assess the influence of the investigated parameters. The results indicate that interface condition is the dominant factor affecting friction resistance. For 100-mm-thick slabs, changing from a smooth to a rough interface increased the initial FRF by 203.1–220.0%, depending on the movement rate. Increasing slab thickness generally increased FRF but reduced the apparent coefficient of friction, whereas successive loading cycles progressively reduced friction resistance. The influence of movement rate was comparatively less pronounced. These findings quantify the influence of interface conditions and loading parameters on slab–subbase friction and provide experimental data for subsequent pavement modelling and design studies.