The decision to use a hollow-core slab is made early on—usually in the preliminary design phase, when spans, column grids, and slab thicknesses are determined. Later, it can only be changed with considerable effort. This article examines the conditions under which the use of hollow spheres as displacement elements is actually cost-effective, where its limitations lie, and what is important in the tendering process.
What a hollow core ceiling is
A hollow-core slab is a reinforced concrete slab in which plastic spheres displace a portion of the concrete. These spheres are fixed between the upper and lower layers of reinforcement, placing them in a zone where the concrete is subject to minimal structural stress: compressive and tensile forces are absorbed by the outer edges of the cross-section, while the neutral axis in the center of the slab essentially bears its own weight. It is precisely this concrete that is replaced. The load-bearing capacity remains unchanged, but the self-weight is reduced. With Euro-Matic's 225 mm hollow spheres, this method can save up to 35 percent of the concrete material and reduce the slab's self-weight by 25 to 35 percent. This principle is internationally established and has been used for decades in building construction, administration, and industrial facilities.
When the investment pays off
The economic advantage doesn't arise from the slab itself, but rather from its subsequent effect on the entire supporting structure. Because the slab becomes lighter, the loads that columns, walls, and foundations have to bear are reduced. These structural elements can be made slimmer—in multi-story buildings, the effect is multiplied with each additional floor. This is most evident in point-supported flat slabs with large, column-free spans. In these cases, the proportion of the slab's own weight to the total load is particularly high, and the savings are correspondingly substantial. A rule of thumb for the early project phase: the larger the span and the more floors, the more worthwhile it is to consider. With small spans, thin slabs under approximately 25 centimeters, or highly open floor plans, however, the advantage often outweighs the additional costs.
Where hollow spheres are not used
Not every area of the ceiling is suitable for displacement bodies—and this is where careful planning is essential. High shear forces act in the bearing areas around columns, and there is a risk of punching shear. In these areas, the cross-section remains solid: the hollow sections are omitted, resulting in a solid concrete column head. The same applies to areas with concentrated loads, cantilevers, and zones where openings, suspensions, or heavy fixtures are planned.
The position of the spheres themselves is also crucial: they must be secured against buoyancy, as fresh concrete would otherwise push them upwards. Manufacturer systems address this using cages or modules that simultaneously ensure adequate concrete cover and maintain the distance to the reinforcement. The design of the slab, the definition of the solid areas, and the verification of shear capacity must always be the responsibility of a structural engineer—based on the applicable approval for the system used.
What weight reduction means for the construction process
Lighter ceiling elements are easier to handle on the construction site, reducing transport, crane utilization, and assembly time. For cast-in-place concrete slabs, the reduced concreting process shortens cycle times. At the same time, the number of concrete deliveries decreases—a significant factor in logistics planning for larger construction projects and one that becomes increasingly important when ready-mix concrete is in short supply.
The material and CO₂ balance
Cement production accounts for approximately eight percent of global CO₂ emissions. Therefore, every cubic meter of concrete not used in construction directly impacts a building's environmental footprint. With Euro-Matic's hollow spheres, one kilogram of plastic replaces up to 100 kilograms of concrete—a ratio that makes the material used in the displacement sphere negligible compared to the mass saved.
This is relevant for projects with sustainability certification: DGNB, LEED, and BNB all assess material usage and the embodied energy of the structure. A documented reduction in the amount of concrete can be directly credited in these systems. Anyone already conducting a life cycle assessment should therefore include the hollow-core slab in their consideration of options early on—not just after the structural design is complete.
What matters in the tender
| Punkt | Why he counts |
|---|---|
| Approval of the system | Basis for dimensioning and acceptance; must correspond to the planned application |
| Massive areas defined | Column heads, load introductions and penetrations belong in the detailed design. |
| Buoyancy control | Stability during concreting must be addressed through design. |
| Concreting sections | Placement and compaction require coordinated layer heights. |
| material properties | Sphere geometry, wall thickness, and plastic type influence displacement volume and robustness. |
Frequently asked questions
Is the ceiling losing its load-bearing capacity?
No, provided the design is carried out correctly. Only concrete from the core area, which is subject to low structural loads, is displaced. The design is carried out by the structural engineer after the approval of the respective system.
How does a hollow-core ceiling behave in the event of a fire?
The decisive factors are the concrete cover of the reinforcement and the verification according to the system approval. Since the plastic bodies are completely encased in the concrete and the load-bearing cross-sectional edges remain unaffected, the usual fire resistance classes can be achieved.
Is the system also suitable for prefabricated parts?
Yes. Hollow core elements are used in cast-in-place concrete slabs as well as in precast concrete slabs and prefabricated elements. The choice primarily influences the type of positioning aid and the coordination of the concreting sections.
From what ceiling thickness does its use become worthwhile.
The diameter of the displacement body must match the slab thickness, as sufficient concrete cross-section must remain at the top and bottom. For 225 mm spheres, correspondingly thicker slabs are required; the specific requirements are detailed in the system approval.
Hollow core slabs are not a universal solution, but for large spans, point-supported flat slabs, and multi-story buildings, they are an effective lever—saving material, reducing loads across the entire structure, and measurably improving the CO₂ balance. Crucially, this option should be examined early on, and the solid sections should be planned meticulously.
Background information on the principle and material savings can be found in the article "Plastic Hollow Spheres in Concrete Construction ". Technical data for the spheres used is available on the page " 225 mm Hollow Spheres in Concrete Construction" . To discuss a specific project, please contact us via the contact form.
