How to choose a fixed slate formwork for a foundation

How to choose a fixed slate formwork for a foundation How to choose a fixed formwork from slate for a foundation

The standard technology of building formwork for a foundation cannot be called simple and at the same time a cheap process, primarily due to the fact that you need to use a lot of edged boards. With the rising cost of sawlogs and commercial timber, more convenient and cheaper forms of formwork have to be sought. Alternatively, permanent formwork can be used to equip the base, and it is not necessary to buy expensive foam or expanded polystyrene. Let's talk about fixed slate formwork.

The most popular and affordable types of permanent formwork

Among the large number of new materials that are suitable in terms of cost and parameters, the most attractive are cement-based slabs with various fillers of mineral and organic origin.

At the moment, the list of the most interesting building sheet materials that are suitable for creating permanent formwork includes:

  • Pressed types of boards based on wood waste products.
  • Fiber cement and wood concrete sheet materials.
  • Various slate modifications.

Please note that a significant difference in the use of non-removable formwork technology is the requirement for resistance to long-term exposure to the ground, which greatly reduces the range of new materials that are suitable for use.

Building codes do not recommend the use of wood or wood-based materials for structures that have been in damp ground for a long time.

Firstly, the structure of a porous-type tree begins to swell and becomes a pleasant environment for the development and decay of pathogenic microflora. Secondly, wood destruction significantly reduces the effectiveness of foundation waterproofing. For this reason, in practice, non-removable timber formwork is not used.

Applying slate for the base

How to choose a fixed slate formwork for a foundation If you have the need to make a permanent formwork from a simple and easy-to-handle material, it is required to use waterproof plywood or OSB boards. Like slate, such materials have excellent moisture resistance, and in different cases it is quite easy to make a non-removable frame on your own, without outside help.

The weight of a square meter of plywood is 3 times less than the weight of sheet slate with a similar thickness, but the price of the latter is approximately 2 times less than the price of pressed plywood sheet or OSB. Apparently, this is the reason why a slate flat sheet is chosen as a material for non-removable formwork much more often than pressed wood.

Foundation formwork based on asbestos-cement slate

Asbestos-cement slate has the highest resistance of the available cement-based materials.The technology of creation does not provide for the use of wood or organic materials, respectively, slate can be in the ground for an unlimited amount of time without changes in properties. Initially, slate was intended for use as a roofing material that is resistant to weathering and temperature extremes.

It is excellent for roofing applications, but has a certain limitation for use in load-bearing structures.

Of all types of slate for fixed formwork, it is ideal for flat slate of at least 1 cm. The absence of waves and the correct dimensions of the geometry simplify the use of slate as a fixed formwork. The use of asbestos fibers as a filler and a cement base provides excellent adhesion of the material of the permanent formwork to the mass of the concrete foundation. In addition to the listed advantages, asbestos slate also has certain disadvantages that prevent its widespread use as a material for fixed slate formwork.

  1. Sheets based on asbestos cement have very poor flexibility, and it can burst under load, even for no apparent reason. Usually this behavior is due to poor quality or a violation of production technology.
  2. The use of chrysolite fibers will lead to the fact that asbestos-cement type slate is able to absorb moisture to a significant extent, but it will not collapse and will not lose its bearing capacity. This feature significantly worsens the waterproofing of the concrete foundation surface.
  3. The material is very heavy and inconvenient, for example, the mass of a slate sheet of the most popular size 1.

    5 * 3 meters is almost 90 kg.

The technology of creating non-removable formwork with the use of flat slate largely coincides with the construction process of a wooden form for casting the base. At the first stage, it is required to dig and level the walls of the trench to the maximum. In order to prevent the slate sheet from pinching under concrete pressure, the soil wall should be as flat as possible and have a position that is close to vertical. The bottom of the pit will be carefully compacted, and also covered with sand and crushed stone.

If possible, the bottom of the formwork is covered with a concrete preparation with a thickness of 5 to 7 cm.

The biggest problem in the use of slate is the difficulty of fixing it and reliable fastening to the rest of the elements and sheets of the frame in a fixed formwork. To simplify the installation process, wooden blocks with dimensions of 3 * 3 cm are attached to the lower and upper edges of the horizontal type of slate sheet by means of self-tapping screws.To mount the slate sheets in the trench, it is necessary to hammer vertical racks of wood with a thickness of at least 5 cm in advance along the walls of the trench, with a step that is equal to the sheet length.

After laying the sheets on the walls that have been prepared, the slate must be attached to the racks using soft wire and self-tapping screws.

At the upper edge, the sheets must be fastened with wooden planks so that the slate does not fall and does not move until the end of the assembly process of the formwork. After that, lay the reinforcement frame, install spacers between the uprights of the vertical type. To give the structure an additional degree of rigidity, parallel sheets are required to be fastened with a heavy wire of a soft type. Trenches, sinuses must be carefully filled with a mixture of clay and sand so that the slate sheets do not mix from their original position. In the process of pouring, concrete is required to be fed in small portions, simultaneously tamping the backfill with manual rammers.

The entire mass of concrete must be leveled along the length of the base using a rammer and a shovel.

Please note that it is strictly forbidden to pour fixed sheet-type slate formwork with a salvo discharge from a mixer or concrete mixer. In 9 out of 10 cases, this procedure leads to the fact that the walls of the material are cracked.

Asbestos-free materials for non-removable formwork

Asbestos-cement type slate is not the only possible material for making a permanent formwork of concrete casting. The low strength of the mechanical type of chrysolite slate forces us to abandon its use in favor of the most convenient cement-bonded and fiber compositions for processing.

Moreover, if desired, the formwork can be made of wood concrete, fiber cement slate or polymer sand material.

The price of such materials is slightly higher than that of asbestos-cement slate, but in many cases their use justifies the invested money. The non-removable type of formwork made of CBPB, that is, cement-bonded particle board, is gaining incredible popularity. High resistance to moisture, excellent strength and workability of the material at a relatively low price make it an effective substitute for all types of slate in order to equip non-removable formwork structures.

How to choose a fixed slate formwork for a foundation

Like the asbestos-type slate, the pressed particle board has high adhesion to mixtures of concrete, which makes it possible to minimize shrinkage processes when pouring concrete into the form of DSP.

The main difference between DSP is in technology and production structure. A mixture of finely cut pine needles and a cement binder is pressed under enormous pressure, resulting in a strong structure with high surface hardness. If you use cellulose fiber instead of shavings, you get FTP, that is, fiber cement boards. Such compositions are excellent as a material for permanent wall formwork. Moreover, USP is used for decorative finishing of the base area and basement wall surfaces.

When using DSP, the speed of formwork installation is approximately 2 times higher than that of asbestos-cement slate.Moreover, excellent resistance to frost and moisture makes it possible to use DSP in the decoration of basement-type surfaces and walls of basements.

Such a board contains 20% of modified cement shavings, and therefore the material is relatively easy to drill, fasten with self-tapping screws and screws, which greatly facilitates the facing and assembly of all surfaces. DSP is often used as part of a removable formwork option. The price of DSP with a thickness of 1 cm is approximately 180 rubles per square meter.


Fixed slate formwork is almost harmless to humans and the environment, unlike expanded polystyrene foam, which means that they can be successfully applied not simply for permanent formwork, but also for finishing interior walls and surfaces of residential premises.

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