Insulated Swedish plate foundation: do-it-yourself USP

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If you are pursuing the goal of building a house with a zero energy balance, then one of the most attractive types of bases will be USB. The technology of its device has already been brought to technical perfection and tested over decades of operation, today we will describe it in all details..

Insulated Swedish plate foundation

Insulated slab construction

Unlike a conventional monolithic slab, the USHP foundation has a number of design features that provide it with outstanding stability and heat conservation indicators. Another characteristic feature of the Swedish slab is the use of premium quality materials. This implies significant financial investments, but the result is definitely worth it: with a service life of more than 50-70 years, the developer receives a finished floor with almost deaf insulation and the ability to establish a multi-storey building even on very weak and heaving soils with a high GWL.

Insulated Swedish plate foundation

The Swedish plate is insulated with a special range of products made of extruded polystyrene foam. Since even with a relatively small thickness, the slab can have a significant dead weight and withstand the mass of the building up to 2-3 floors, the insulation material must be highly resistant to deformation during compression – from 200 kPa. Cheaper PSB cannot boast of sufficiently high strength indicators.

Elimination of manifestations of frost heaving under the foundation is achieved by a continuous belt of insulation around the perimeter of the foundation and the device of a blind area that diverts water. Thermal insulation under the blind area is usually 50–70 mm, with the thermal conductivity of insulating materials not exceeding 0.035 W / m * K. With the same parameters, the thickness of the insulation layer of the slab itself can reach 200–250 mm. The maximum deformation of a high-quality insulation under full load at this thickness is about 10-15 mm.

Insulated Swedish plate foundation

Another feature of the UST is increased strength and spatial rigidity, which is achieved due to the special configuration of the lower part of the foundation. Along the perimeter and under the load-bearing walls, the slab has protruding ribs that evenly distribute the load over the entire area of ​​the support and give it a very high rigidity. Even during the construction of a two-story stone house, the force of pressure on the ground rarely exceeds 0.6-0.8 kgf / cm2, accordingly, the building will stand steadily even on moisture-saturated sandy loam, peat soil and plastic clay.

In view of the features described above, the main task in the construction of the USP is reduced to ensuring that the slab does not experience deformations under the weight of the building structures. In general practice, the height of the stiffeners ranges from 2 to 5 times the thickness of the slab. Moreover, if the span between the ribs exceeds 50–70 values ​​of their thickness, the slab is reinforced either with a thicker reinforcement scheme, or by the device of additional intermediate ribs.

Excavation and preparation

In reality, there are many configurations of the Swedish slab, differing in thickness and reinforcement pattern. However, this does not change the essence of the technology for a very extraordinary reason, which is quite difficult for an average person to accept. The fact is that the durability and stability of the USB is provided not by the design of the slab itself, but due to the correct preparation of the base under it..

Insulated Swedish plate foundation

It all starts with removing the fertile soil layer or digging a deeper pit, if a basement floor is planned under the house. In this case, the bulk of the soil is removed in an area larger than the slab itself. In each direction from the planned marking of the foundation, you need to retreat 40-50 cm plus the depth of the slab, multiplied by about 1.35-1.5. This need is due to the fact that a dense incompressible bedding is prepared both under the slab and under the blind area, which easily “releases” water. In this case, the width of the blind area is always determined by the depth of the foundation, because the spread of the liquid when it seeps through the backfill occurs in a fan. Thus, the deeper the depth, the wider the wet spot. The radius of this spot for the backfill material under the slab is about one and a half times the depth.

Insulated Swedish plate foundation

After removing the soil, the pit is cleaned up manually and covered with a needle-punched geotextile, then backfilling is carried out. The first is sand – high-quality river sand, without clay inclusions and with the lowest possible compaction coefficient, that is, fractions from 1.3 to 2 mm. Sand is poured in layers of 50–70 mm and rammed by a vibration method with a slab mass of about 100–120 kg. The thickness of the sand bed is at least 20 cm, but in general it can reach 2–2.5 times the nominal slab thickness. Building a thicker layer of sand usually doesn’t make sense.

Insulated Swedish plate foundation

If groundwater may appear at the resulting mark, a system of drainage pipes wrapped in geotextiles is laid in the pit along the perimeter to avoid siltation. To provide the necessary slope, small gutters are undermined in the sand, while the general plane remains strictly horizontal. After the installation of the drainage system, the pit is covered with geotextile with a density of 250 kg / m2, then gravel is filled with a fraction of 15 to 30 mm. In the best case, granite crushed stone is used, which is poured in layers with a gradual decrease in the fraction up to 10-15 mm.

The function of the gravel pad is to drain the bottom of the slab and distribute the load. Backfilling is carried out up to the mark at which it is planned to support the edges of the slab. The depth of support is determined by the thickness of the slab with stiffening ribs, assuming that the plane of the finished floor will be 20-25 cm above the adjacent soil.

Insulated formwork device

The Swedish plate has a solid insulation belt of the lower plane without cold bridges. Arranging such a thermal protection scheme is quite simple for a flat slab foundation, however, the presence of ribs makes its own adjustments. Especially for this purpose, special products for the formation of permanent formwork are produced.

The formation of external stiffening ribs is carried out by means of L-shaped trays, which are set along the perimeter of the slab and aligned along the marking cords and level. The outer edge of the trays determines the total thickness of the slab and rib, while the inner one is formed by hand using slabs that are cut in place. The necessary strength of the formwork to withstand the loads during pouring is provided by external decks made of sheet materials, supported by stakes driven into the ground along the upper and lower chords.

Insulated Swedish plate foundation

When the rib formwork is assembled, the space between them is filled up with careful compaction by flushing or vibration. Backfilling can be done with sand or fine road gravel, there is not much difference in that. In order not to disturb the geometry of the formwork, temporary plastic lintels are inserted into the trays.

Insulated Swedish plate foundation

When filling the cavities between the ribs, it is not brought out flush with the inner walls. Instead, the inner walls protrude by the thickness of the insulation boards used. After the outer belt of thermal insulation is assembled, the formwork is covered with a profiled-type waterproofing membrane. In the corners, the hydro-barrier is carefully trimmed and placed on top of each other with an overlap of 150-200 mm, snapping in the pimpled locks.

On top of the waterproofing, an internal insulation belt is mounted, represented by 2-3 layers of EPSP, 50-70 mm each. In this case, the size of the trays is reduced by the thickness of the insulation, which must be foreseen in advance. Fixation of EPS is usually not carried out, since the formwork is constructed the day before pouring or a day earlier. In strong winds, the slabs can be tied together with small portions of universal glue or pressed down with pressure until the reinforcement cage is assembled.

Reinforcement USHP

The insulated Swedish plate contains a small amount of reinforcement, but it is correctly distributed in the thickness of the concrete for the highest quality perception of loads. The assembly of the reinforcing cage begins with the edges: U-shaped clamps of smooth structural reinforcement with a diameter of about 8 mm are laid in them, the size of which is selected taking into account protective concrete layers of 50 mm on each side. The tails of the clamps are released 20-25 cm above the upper line of reinforcement of the common plane.

Insulated Swedish plate foundation

The slab itself is reinforced with two layers of a mesh of rods from 8 to 14 mm in two rows, while the thickness of the rods in each layer is different. Due to the fact that the main load on the plane of the slab comes from the resistance of the soil, the main working reinforcement, which takes tensile loads, is located in the upper zone and the thickness of the rods is higher here. The lower row is made of thinner reinforcement, but with a smaller cell size, it is necessary to impart monolithic strength, it is also used as an installation system for attaching communications.

Insulated Swedish plate foundation

When laying the mesh, it is knitted in place, placing longitudinal rods on distance chairs, providing a protective layer from below about 40 mm. Transverse rods are laid on top, all intersections are tied with wire. To securely fix the upper row, U-shaped anchors are tied to the lower mesh, to the upper tails of which the longitudinal bars of the main reinforcement are attached with wire. After the upper mesh is completely tied, the rib reinforcement cages are slightly raised, the tail ends are folded and tied to the bars of the upper reinforcement row.

Laying of communications

The construction of the USP is carried out in such a way that all the necessary communications or channels for their laying remain in the slab. In order not to overlook anything, here is the maximum list of what can be hidden in the thickness of concrete:

  1. Underfloor heating pipes or heating cable;
  2. Entering water into the house;
  3. Waste canals with access to bathrooms and risers;
  4. Plumbing for household needs;
  5. Lead-in electrical cable in a protective sheath or only sheath with a cord for pulling;
  6. Electricity outlets for street lighting and household needs;
  7. 2–3 spare channels for pulling communication lines or additional cables.

Insulated Swedish plate foundation

It should be borne in mind that the hidden laying of communications when placing metering units inside the building may require the registration of an act of hidden work. So that the communications cannot be damaged during pouring, they are laid exclusively inside rigid shells, the most budgetary example of which is technical HDPE pipes made from recycled materials. So that the pipes are not squeezed by the mass of concrete, they are muffled and spool valves are cut in to pump air under a pressure of 3–3.5 atm.

Insulated Swedish plate foundation

Pouring and processing concrete

The advantage of the Swedish slab is that the concrete work takes place in one stage, therefore the construction speed is one of the highest. A real Swedish slab requires factory-made concrete. This requirement is connected not so much with the need to ensure the exact value of the grade, but with the need to fill the entire slab at one time without the formation of cold seams..

Insulated Swedish plate foundation

Since the Swedish slab is installed on a drained site, concrete of strength class B20 and higher is suitable for its manufacture, but without any special claims for frost resistance. The discharge of the concrete mix begins from the center of the formwork so that the walls of the trays experience the dynamic impact of water hammer only at the final stage of pouring. As the form is filled, the concrete is carefully compacted with a deep vibrator, while due to the relatively small thickness of the slab, there is no need to fear delamination.

Insulated Swedish plate foundation

The leveling of a concrete slab can be carried out as a manual rule followed by grinding, or it can be immediately brought to zero using a vibrating screed. The finished floor surface in both cases is ready for the installation of most light coatings, from linoleum to parquet board.

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