Serviceability performance and time-dependent deformations of recycled aggregate concrete composite metal slabs

Whilst recycled aggregate concrete (RAC) is increasingly being used in construction of slabs, there is still limited data on the service performance of such structural elements. This article investigates the long-term behaviour of composite steel concrete slabs cast with recycled aggregate concrete (RAC). The study addresses a gap in understanding the effect of RAC in composite slabs subjected to sustained loads and human-induced vibrations. Various tests were conducted, including material testing of RAC, non-uniform shrinkage tests on small-scale slabs, and long-term tests on full-scale slabs. The experimental programme involved twelve slabs with dimensions of 0.15 m thickness x 1.0 m width, and varying lengths of 2.0 m, 3.0 m, 4.0 m or 5.0 m. The slabs were subjected to static loading, vertical sustained loading for 90 days, and human-induced vibrations tests considering normal walking, brisk walking, and jumping. The results demonstrated that all slabs met the serviceability limits outlined in current guidelines (ISO 2631-1), thus confirming the suitability of RAC for composite slab construction. The vibration tests results showed that the RAC composite slabs performed well in service conditions, with vibration accelerations for longer spans (5.0 m) being 24.3%, 8.3%, and 17.5% higher than for shorter spans (2.0 m) during normal walking, brisk walking, and jumping, respectively. Despite these increases, all slabs remained within the vibration limits of ISO 2631-1. The study also proposes a novel and practical semi-empirical equation to predict time-dependent deformations and creep, which is validated through both experimental data and finite element analysis (FEA). The results from a parametric FEA further revealed that slab vibration frequencies decreased by up to 22.8% as the span increased from 6.0 m to 12.0 m, highlighting the influence of span length on performance. The proposed semi-empirical equation predicts accurately the time-dependent deformations with a strong agreement between experimental and predicted results (Prediction/Experiment = 1.0). This study presents new experimental data on the structural and serviceability behaviour of composite slabs made with 100% recycled concrete aggregate (RCA), which is currently scarce in the literature. By advancing understanding of RAC's behaviour under realistic loading conditions, the research contributes to the wider adoption of RAC and circular economy practices in construction of slabs.