Physics-guided unsupervised degradation phase identification for semi-rigid base slab action: A deflection-basin energy dissipation index (BEDI)

Semi-rigid pavement bases carry loads primarily through slab action developed by cement hydration and lateral load transfer. However, conventional condition assessment remains largely focused on modulus-based metrics. Here, we introduce the Base Energy Dissipation Index ( BEDI ), which quantifies the deterioration of slab action from changes in deflection-basin topology associated with energy dissipation. BEDI links these changes to Westergaard’s radius of relative stiffness ( L R ) without requiring the continuum assumption that underpins conventional indices. We combined finite-element simulations based on continuum damage mechanics with composite-plate calculations incorporating interlayer slip using the Newmark–Goodman model. The analyses showed that BEDI followed changes in L R as the base transitioned from an intact, continuous slab to a fragmented structure. We then evaluated BEDI using 856 falling-weight deflectometer (FWD) measurements from eight sections of two in-service expressways in China’s Highway Climatic Zones II and III. Unsupervised Gaussian mixture modelling identified three BEDI -defined degradation states: intact continuous, critical damage and structural collapse. Degradation passed through a metastable state before transitioning abruptly to collapse. BEDI variance rose gradually and then sharply near collapse, consistent with increasing structural disorder during service. Compared with the conventional Base Damage Index ( BDI ), BEDI separated the degradation phases more clearly and corresponded more closely to engineering thresholds. It achieved 83% precision in detecting critical damage, compared with 59% for BDI . Theoretically, its stronger response to degradation-associated changes in L R also made BEDI more sensitive to early deterioration.This mechanics-informed index may improve condition assessment of semi-rigid bases and support preventive maintenance decisions.