Concrete scarf stone doesn’t mean a lot to you? If you are creative, this should change urgently, with the scarf stones you can build more and faster than with any other building material. Also, build solid and resilient, they are called shuttering blocks because they replace the shuttering with their concrete filling.
The concrete formwork stone is a pretty ingenious invention that allows you to make very creative concrete structures with a minimum of effort and cost and in a fairly short time. Without having to deal with formwork timber, its construction, dismantling, cleaning, etc., this stone could become your favorite building block.
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What are shuttering blocks?
When you work with concrete, you usually use formwork, wooden boards that have to be set up and possibly treated with formwork oil (release oil) so that the concrete and formwork timber can be easily separated later. In which the concrete is poured, when it is solid, this formwork is removed again.
This conventional formwork must be laboriously cleaned after removal so that it can be used again. They are usually only used once or twice by construction companies; in private construction projects, they would have served their purpose anyway. Even if the shuttering boards (with the rarely environmentally friendly oils) are hopefully recycled somehow, an enormous waste of material, about 25,000 tons of the mostly still mineral shuttering oil alone is used in construction every year.
But there are also lost formwork, specially made molds that remain in the foundation and do not have to be stripped, cleaned, and transported away. Recently, more and more solutions in this direction have been devised, if possible even more economical and practicable for users, such as special flat lattice mats with a hinged folding system as permanent formwork.
But there is already a perfect “lost formwork”: formwork blocks are cast concrete blocks that are laid dry and then filled with concrete, so they are, so to speak, their lost formwork.
Concrete shuttering blocks can be processed quickly and easily, have tongue and groove joints and result in smooth, closed, very fine-grained visible surfaces.
Concrete formwork blocks are a very simple building material to pull up walls, quickly and efficiently, statically quite stable, and with good physical properties.
Depending on the type of construction and surface structure, the stones are set dry and walled up with mortar from the groups of mortar normally used in masonry construction, possibly also with thin-bed mortar, in a running bond (half-offset).
The cavities are filled with concrete, quality class B 5, B 10, consistency KP (plastic), KR (soft), or KF (flowable) – according to the new DIN 1045-2, the flowability ranges from F1 to F6. The cavities are either filled with flowing concrete, which no longer needs to be mechanically compacted, or filled with drier concrete, which is compacted with an internal vibrator.
The formwork blocks usually have transverse recesses through which the filler concrete flows across and thus connects the blocks with one another. Depending on the type of stone, concrete is poured in layers or for the entire structure.
The walls made of formwork stones have a high mass with good static load capacity, the wall structure is very economical, usually, neither additional reinforcement nor formwork is required. The soundproofing of such walls is also comparatively good: A wall 17.5 cm thick has a sound reduction index of 59 dB (for comparison: three-pane insulating glass, depending on its thickness, 34 to 43 dB).
Ideas for buildings
This type of “self-filling Lego” can of course be used for an incredible number of buildings, for example around the house for:
- Strip foundations
- Storage and ancillary rooms
- Retaining walls
These are the “official ideas”, but you can do a lot more with scarf stones in your home, like a shelf. The stones can be changed in color or left in a modern gray.
In the garden, scarf stones can be used for creative privacy screens that are filled with soil and planted in the respective upper stones, for garden walls or walls in the garden area, as a garden bar, and for entire bedding landscapes with a pond.
These interesting stones are sure to give a lot of ideas for all kinds of small buildings.
Formwork blocks are offered in different sizes:
- The stones have a standard width of 50 cm
- One of the construction heights offered is 20 cm and 25 cm (standard)
- You need 10 or 8 of these scarf stones per square meter of wall surface
|Construction height||Wall thickness||Amount of filling concrete / m²|
The stones are each manufactured and monitored according to certain standards relevant in the construction sector, for example:
- Wall thicknesses 11.5 and 15 cm: Guideline for non-standardized concrete products
- Wall thicknesses from 24 to 42 cm: DIN 11622 T.22: “Fermentation silos and slurry tanks: concrete shuttering blocks”
- All other wall thicknesses: approval Z-17.1-404 from the Institute for Structural Engineering
These were the standard dimensions normally offered, there are special sizes up to slightly round-shaped blocks and formwork blocks in different colors. You can simply ask at the regional concrete block factory. Exactly fitting cover plates are offered for the colored version so that you can place planters on a wall along a path.
All of the above-mentioned formwork stones are available in the most commonly used quality B25, which usually meets the requirements for small private buildings.
Quality and prices
Scarf blocks are offered in different concrete strength classes, B15, B25, B35:
- B15 quality is mostly used for standard applications for basements and foundations, with normal concrete that would be strength class C12 / 15
- B25 quality is mostly used in the visible area, it is highly compressed and dimensionally accurate and corresponds approximately to C20 / 30 concrete
- B35 is a quality for particularly heavy loads; its strength can be compared with normal concrete C30 / 37
The prices for concrete blocks naturally depend on the size and wall thickness. The common concrete shell stone is the 25 cm stone, for which the prices of various common thicknesses are therefore given below, the 20 cm stones are each a little cheaper:
- Formwork block 15 cm from 1.50 €, 8 pieces = 1 m² = 12.00 €
- Formwork block 20 cm from 1.75 €, 8 pieces = 1 m² = 14.00 €
- Formwork block 24 cm from 1.90 €, 8 pieces = 1 m² = 15.20 €
- Formwork block 30 cm from € 2.15, 8 pieces = 1 m² = € 17.20
- Formwork block 40 cm from € 3.15, 8 pieces = 1 m² = € 25.20
If you do not want to buy from the manufacturer, but rather individual scarf stones in the hardware store, you may get some seldom-used intermediate sizes sooner than from the manufacturer, but the prices increase somewhat:
Hornbach offers 17.5 cm stones, which are ideal for some horticultural projects, but a stone costs € 2.75 (from 60 pieces € 2.45). The same applies to the 24 cm formwork blocks, which cost € 2.95 as individual blocks.
Obi is calling for about the same prices for these concrete blocks in very small purchase quantities, and they have the 36.5 blocks on offer at a unit price of € 2.99.
This results in a price for hardware store bricks of approx. € 22.00 / m².
End stones, special sizes, colored scarf stones each cost a little (or quite a bit) more, and of course, you have to add any reinforcement (depending on the static requirements) and the flowing concrete (quality also according to static calculation).
Special features when laying
Formwork blocks can be processed without complex formwork, but afterward a little more laborious when sealing. Sealing is usually necessary to prevent moisture from getting through the joints. For garden walls without static load, it might not be that tragic sometimes, but that looks pretty ugly after a few years. You can seal outside walls with mineral sealing slurry (sand + cement, available in hardware stores as a powder to mix).
The sealing slurry can be applied very quickly with a thick brush (brush).
Tip: If there is a lot of moisture on the wall (e.g. clay slope behind it), the sealing slurry should be followed by a bitumen coating.
Reinforcement consists of stabilizing the wall with steel. If you are planning smaller structures without a structural engineer: Even if the stones interlock with tongue and groove, in case of doubt, the level base (foundation?) And the reinforcement is decisive for stability, even a small wall of one meter has to and should withstand a lot of pressure to be reinforced.
General building authority approval (depending on size) is required for masonry made from shuttering blocks.
Laying and filling the shuttering blocks at a glance:
- Stack the wall or wall made of formwork blocks
- Put the first row of stones in mortar group III along a tension cord
- Check with the spirit level, unevenness can be leveled out by moving the stones
- If the first layer is exactly straight, the next formwork blocks can be placed dry in the bond
- Make sure that the shuttering blocks sit tightly on top of one another without any gaps
- Now the reinforcement that the structural engineer has calculated is put in place
- This structural steel significantly increases the strength and resilience of the wall
- Before concreting, the paving stones are thoroughly pre-wetted
- Pour flowing concrete (is ordered in the concrete plant and delivered by concrete mixer)
- The flowing concrete is poured in via crane and concreting bucket or with a concrete pump
- Flow concrete is self-compacting, you are welcome to add additional compression at critical points (poke around in the flowing concrete with a stick)
- The flowing concrete now has to harden, the final strength is reached after 28 days
By the way, you can also produce flow concrete yourself on site, this is just concrete with good flowability and cohesiveness, the consistency of which is adjusted by adding a superplasticizer. This is not the easiest concrete to mix yourself, because it has to become so flowable that it compacts itself or by light poking, but on the other hand, it has to hold together so well that it does not separate itself from any necessary slight shaking or other vibrations, but there are good instructions for that too.
According to the old, often cited DIN 1045, there were four consistency ranges: stiff (KS or K1), plastic (KP or K2), soft (KR or K3, soft regular or normal consistency), and flowable (KF). In these old standards, concrete with medium consistency was highlighted and referred to as concrete with regular consistency (KR). Such concrete is a soft concrete that can be conveyed, processed, installed, and compacted easily.
The applicable DIN 1045, like the European standard EN 206, defines seven consistency ranges (very stiff, stiff, plastic, soft, very soft, flowable, very flowable), which are defined by slump classes (F1 stiff to F6 very flowable) and compression classes (C0 very stiff to C3 soft).
When you work with concrete, you should protect your skin and eyes and avoid any contact with the concrete as far as possible. Concrete contains strongly alkaline cement, which irritates the skin and mucous membranes.
All in all, scarf stones are a great material for impatient people, with them you can build walls and walls with very little effort and still have a really solid and resilient wall after filling with concrete. Here it was only about scarf stones made of concrete, but today there are also scarf stones made of completely different materials which ensure a pleasant room climate and energetically have passive house quality, for creative homeowners, the versatile stone is one of the most exciting building materials.