Floating Behaviour and Segregation Resistance of Lightweight Expanded Glass Aggregates in Ultra-High Performance Concrete

Lightweight Ultra-High Performance Concrete (L-UHPC), based on Ultra High-Performance Concrete (UHPC) with a compressive strength of at least 120 MPa, has attracted increasing attention as a means of reducing structural self-weight while maintaining excellent mechanical properties and durability. Despite these advantages, the large density difference between lightweight aggregates (LWA) and the cementitious matrix often leads to aggregate floating and segregation in highly flowable mixtures, resulting in poor internal homogeneity and inconsistent hardened properties. This study investigates the fresh-state stability of L-UHPC incorporating Lightweight Expanded Glass Aggregate (LEGA) with two particle size ranges (2–4 mm and 4–8 mm), characterized by their smooth, rounded morphology. Five trial mixtures were prepared by varying the water-to-binder ratio (0.18–0.25), superplasticizer dosage (0.85–2.00%), and viscosity modifying admixture (VMA) content (0–0.10%), while maintaining a constant flow spread of 250 ± 10 mm to evaluate stability independently of workability. The results showed that the incorporation of VMA significantly improved mixture stability and limiting the upward movement of lightweight aggregates, even at similar flow spread values. The optimum mixture was achieved with VMA dosage of 0.10%, which exhibited the least aggregate floating. Cross-sectional observations of hardened specimens further confirmed the fresh-state results by showing improved aggregate distribution and reduced segregation. The findings indicate that viscosity modifier plays a critical role in controlling the floating behaviour and segregation resistance of lightweight expanded glass aggregates, providing useful guidance for the design of stable and high-performance lightweight UHPC mixtures.