Concrete shell

Skin concrete refers to a concrete construction method in which permanent formwork-such as formwork blocks made of concrete, lightweight concrete, or polystyrene-remains as a permanent component of the finished structure. The core of these blocks is filled with in-situ concrete, resulting in a monolithic, high-strength wall. This method combines the advantages of solid concrete construction with fast, formwork-free execution.

Structure and Operating Principle

Formwork blocks are laid dry, like masonry, and their cavities are used as formwork. After reinforcing the cavities with steel bars, concrete is poured in, turning the blocks into a load-bearing composite cross-section. The exterior wall made of formwork blocks also serves thermal insulation or soundproofing functions, depending on the material used.

Polystyrene formwork blocks (so-called ICF systems = Insulating Concrete Formwork) and classic concrete formwork blocks are particularly common. ICF systems consist of two shells made of expanded polystyrene (EPS) that are connected by webs and remain in place on both sides as insulation after pouring. These systems provide excellent thermal insulation values and are suitable for Passive House and KfW standards.

The principle of permanent formwork is by no means new: as far back as the ancient Roman Empire, wall structures were built using permanent stone shells and compacted in-situ concrete-a concept that continues to this day in a modernized form. Today’s ICF systems are the result of decades of optimization in terms of insulation effectiveness, workability, and structural performance.

Applications and Advantages

Concrete-clad formwork is particularly suitable for basement and ground-floor walls, retaining walls, and buildings in areas in contact with the ground where waterproofing and load-bearing capacity are required. Unlike conventional formwork, there is no need for erecting and removing formwork; the permanent formwork can already be insulated. In residential construction, insulated concrete panels are valued when fast structural phases need to be combined with good thermal insulation.

Advantages at a glance: Easy to install even by non-specialists, good thermal insulation (with ICF systems), high load-bearing capacity due to the concrete core, good sound insulation values due to the mass of the concrete, and flexibility in floor plan design. A disadvantage is that creating cavities for utilities is more difficult than in lightweight building materials, and finishing the finished concrete core can be labor-intensive.

Another advantage of the shell concrete construction method is its suitability for self-builders with manual skills: The formwork blocks can be laid without heavy equipment, and the subsequent concrete pouring can be done with ready-mix concrete. This makes shell concrete particularly appealing to homeowners who want to be actively involved in the construction process and perform parts of the work themselves. However, the reinforcement design should always be carried out by a qualified structural engineer.

Shell Concrete Compared to Other Construction Systems

Compared to conventional reinforced concrete with permanent wood or steel formwork, shell concrete stands out primarily due to the elimination of formwork removal and its integrated insulation. Compared to aerated concrete or brick masonry, shell concrete offers higher compressive strength and better protection against water penetration, but is more expensive to construct and requires more planning.

Lightweight concrete formwork blocks (e.g., based on expanded clay or wood chips) represent an interesting middle ground: They are lighter than traditional concrete formwork blocks, offer better thermal insulation, and are easier to drill or cut. Systems such as Durisol or VELOX combine these properties with good practical construction characteristics and are established as standard methods among select carpentry and masonry firms in the Nuremberg metropolitan area.

Renovation and Existing Buildings

In existing buildings, concrete-clad construction is less common than reinforced concrete or masonry; it is primarily a method used in new construction. When purchasing older buildings, the term therefore primarily comes into play in the context of retrofitting basements or adding extensions. Anyone planning a new building or an extension using the shell concrete construction method should have the project supervised by an experienced structural engineer who is familiar with the system behavior of the formwork blocks used.

Caution is advised when performing renovation work on shell concrete walls: the reinforcing bars in the concrete core must not be damaged. Electrical wiring and water pipes should be routed outside the concrete core, within the insulation layer, or in pre-wall installations.

Practical Tip for Homeowners in Nuremberg and Franconia

In the greater Nuremberg area, concrete-clad basements are primarily used in new construction projects where high groundwater levels or highly variable soil conditions-such as in parts of the Knoblauchsland region or along the Pegnitz River-require a particularly watertight construction method. For new construction projects, we recommend consulting with a soil engineer as early as the planning phase to determine whether the water-pressure-retaining shell concrete design is cost-effective or whether a conventional waterproof concrete construction method is more economical.

For homeowners who wish to build their own home using the concrete shell construction method, we recommend working with a contractor certified for the respective ICF system. The system manufacturers (e.g., VELOX, Neopor, Durisol) offer training and lists of certified installers.

Frequently Asked Questions

Is insulated concrete construction the same as formwork block masonry?

No, even though formwork blocks are the typical building material for insulated concrete construction: The key factor is that the blocks are cast with concrete, thereby forming a composite cross-section. Pure formwork block masonry without concrete casting would be a completely different structural system.

How well does an insulated concrete wall insulate?

That depends on the type of formwork block. Polystyrene formwork blocks (ICF) achieve good U-values and can meet KfW Efficiency House standards. Classic concrete formwork blocks, on the other hand, have poorer insulation values and often require additional exterior insulation.

Can concrete-clad masonry be modified later-e.g., for utility slots?

Milling or chiseling slots in the concrete core zone is only possible to a limited extent and must be coordinated with the structural engineer, as the reinforcement must not be damaged. In the formwork block shell (e.g., polystyrene), however, installation channels are usually easy to create.

What are the costs for concrete-clad walls compared to conventional reinforced concrete?

The additional cost of the material (formwork blocks) is often offset by savings on formwork. With ICF systems, there are also the benefits of more affordable insulation and reduced construction time. Overall, the total construction costs for insulated concrete forms are generally comparable to or slightly higher than those of conventional reinforced concrete construction, while offering better energy efficiency.

Is shell concrete suitable for passive houses?

Yes, ICF systems with high-quality polystyrene formwork can achieve U-values of 0.15 W/(m²K) or better, thereby meeting the requirements of the passive house standard. This requires careful detailed planning at corners, openings, and connections to avoid thermal bridges. In this case, a certified Passive House designer should review the construction details.

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