A hollow sphere slab (Dutch: “bollenplaatvloer”) is a biaxial voided reinforced concrete floor system in which hollow plastic spheres are embedded between layers of reinforcement. By replacing non-structural concrete in the core of the slab, this system significantly reduces weight and material consumption while maintaining structural performance.
How It Works
The system consists of:
- Bottom reinforcement layer — a mesh of steel reinforcing bars forming the tension zone
- Hollow plastic spheres — lightweight, recycled polypropylene balls placed in a regular grid between the reinforcement layers
- Top reinforcement layer — a second mesh completing the compression zone
- Concrete — poured around and between the spheres, encapsulating them within a solid slab
The spheres displace concrete from the neutral zone of the slab — the area that contributes little to structural strength — reducing self-weight by up to 35 percent compared to a solid slab of equivalent span and load capacity.
Advantages
- Weight reduction — 25 to 35 percent lighter than a solid slab, reducing loads on columns and foundations
- Material savings — up to 20 percent less concrete and proportionally less steel
- Longer spans — the reduced self-weight allows spans of 15 metres or more without intermediate supports
- Flat soffit — no downstand beams required, simplifying services routing and reducing floor-to-floor height
- Sustainability — lower carbon footprint due to reduced concrete and steel volume
Key performance figures for a hollow sphere slab compared with a solid concrete slab. s
Application
Hollow sphere slabs are used in:
- Multi-storey commercial and residential buildings
- Car parks and parking garages requiring long, unobstructed spans
- Office buildings where flexible, column-free floor plates are desired
- Public buildings such as schools, hospitals, and sports halls
Design follows standard reinforced concrete codes, with the voided section accounted for in bending, shear, and punching shear calculations. The system is compatible with post-tensioning for even longer spans.
Related Terms
- Hollow-core slab
- Waffle slab
- Post-tensioned slab
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