Do-it-yourself prefabricated strip foundation from FBS: advantages, disadvantages, planning, installation

Everyone wants to have a reliable foundation without exception. And one of the most reliable and predictable is tape. But a monolithic strip foundation is being built for a long time: it takes only three days to assemble the formwork, and you still have to knit reinforcement, then concrete and wait a few days until it gains at least half the strength. This will take at least two to three weeks. Well, if there is such a margin of time, and if it is not? There is a way out: smart people came up with the idea of ​​transferring part of the work to the enterprise – concrete blocks are made there – FBS. From them, a foundation is then recruited, which is called prefabricated tape. The foundation of FBS is easy to do with your own hands.

When using this technology, the foundation is assembled from reinforced concrete blocks of various types and sizes. Therefore, it is often called block or prefabricated. All are the same type.

Advantages and disadvantages

The most important plus of a prefabricated strip foundation from FBS is a small resource of time that is necessary for its manufacture. It only takes a few days to assemble.

The main disadvantage: it is not monolithic, therefore it can not be installed everywhere and not always. Normally, there are prefabricated tapes from blocks on dry, not prone to heaving soils. Under other conditions, a calculation is required, and it is better – a professional one.

It should be noted that prefabricated foundations must be carefully waterproofed and insulated. It is also necessary to make an insulated blind area. These are necessary conditions for their normal operation.

If we compare the tape monolithic and block foundation, then the following picture emerges:

• Monolithic is built for a long time, block – quickly.
• A monolithic tape can be made without renting special equipment (although it is difficult to do without ordering concrete). To assemble the foundation from FBS with your own hands, you will need to rent a crane, or at least a winch.
• If you make a monolithic foundation yourself, it costs less than a block one. If hired workers or a firm work, then it will cost more, rather: the amount of work is greater, and this is a considerable additional cost.
• A monolith is more durable than a prefabricated one. Strength is about 20-30% higher. This allows it to use a monolithic tape on loams and clays.

In many respects, the monolithic tape wins. But if its strength and reliability are not in demand, then its manufacture is only a waste of time. When doing work by contractors, this is also a waste of money. Therefore, if the soils have a normal bearing capacity, and the groundwater is no closer than 2 meters from the required foundation depth, it makes sense to put the house on a prefabricated foundation.

Foundation blocks: types, sizes, markings

In private construction, several types of blocks are used. For the device of a strip prefabricated foundation, only two types are most often used:

• FBS – solid building foundation block. When they talk about this type of block, they mean a reinforced concrete (with reinforcement) element without voids. These are rectangular blocks of different sizes with mounting steel hinges on the upper surface. Sometimes vertical channels are formed in the side faces, which are filled with mortar during construction. From these blocks, a foundation strip is folded.
• FL – pillow blocks. They look like a trapezoid. They are laid on a prepared base and serve to increase the bearing capacity of the foundation.
  

  

When erecting a strip foundation, it is necessary to provide for routes for laying and supplying engineering systems: water supply, sewerage, electricity, heating. It is necessary not to forget about the ventilation system and leave openings for air vents for ventilation of the underground space or basement. For this, blocks with a channel for laying communications can be useful: FBV.

The sizes and types of blocks are regulated by GOST 13579-78. For a private developer, dimensions and performance are mainly important. The photo below shows part of the standard that defines the types and sizes of blocks.

GOST marking

To make it easier to navigate the specification, the names of the blocks by the same GOST contain information about their sizes and type.

The title comes first. Further – the numbers describing the geometry in decimeters;

• the first is the length (9, 12,24);
• the second (through a dash or dot) – width (3,4,5,6);
• the third is the height (3,6);
  

  

If any length and width are found, then the FBS height is usually made 580 mm (in the marking “6”). Blocks with a height of 280 mm can be made on request.

Further, after the numbers, there is an alphabetic designation of the type of concrete used:

• T – heavy (cement-sand mixture with crushed stone). Heaviest high density block. It is this type that is used in the construction of foundations.
• P – porous with expanded clay aggregate. They have less weight, but also less strength, and are also more hygroscopic.
• C – from silicate concrete (the main binder is lime). This type of blocks is afraid of getting wet, therefore it is not used in the construction of foundations.

For example, FBS 24.4.6 -T stands for a rectangular block made of high-density reinforced concrete. Length 2380 mm, width 400 mm, height 580 mm. By analogy, you can decipher other designations.

Stacking FBS blocks

The choice of block width is determined by the thickness of the walls located above. The length of the blocks is selected so that they occupy, if possible, the entire tape. But even experienced builders make mistakes when selecting blocks: there are some unfilled areas in which even the smallest elements do not become (they are called additional). These areas are usually sealed with cement mortar bricks. If the masonry turned out to be uneven, it is then plastered: it will be more convenient to apply waterproofing and insulation.

Typically, a prefabricated strip foundation consists of several rows of blocks. Their specific number depends on the required tape height. More often it is laid below the depth of freezing of the soil. The required height of the plinth is also taken into account.

When installing concrete blocks of any type, the same rule is used as when laying bricks: the seams should not match. To do this, they are placed so that the seam of the previous row overlaps with the body of the block in the next row. Gaps (vertical seams) between adjacent elements are filled with cement-sand mortar.

To give the structure greater strength and to connect all the blocks into a single system, reinforcement is laid on top of each row. Depending on the type of soil and the weight of the building, a class A-I – A-III bar is used. The number of bars is determined by the calculation during the design, they can be from 2 to 5 pieces. When laying and connecting the bar, all the rules for reinforcing the strip foundation are observed – the binding of corners, piers occurs in the same way. The only difference is that there is only one reinforcement belt. A layer of mortar is laid on top of the reinforcing belt, and the next row of blocks is placed on it, with offset seams.

Subject to these rules, the prefabricated strip foundation will be strong and reliable.

We build a foundation from FBS with our own hands

The technology for building a prefabricated foundation, especially at the preparation stage, is not much different from other types: first we dig a foundation pit. The difference is in the lower labor intensity of manufacturing the tape itself and in the fact that there is no need to make formwork.

Preparation of foundations under a standard block foundation:

• The fertile layer of the earth is removed, taken out or stored on the site.
• Next comes the markup. If the house has a basement, the perimeter is marked, if there is no basement, the tape is marked. Marking is done with pegs and cords. The tape is marked on both sides, indicating the width of the wall.
• Next comes the earthworks. The soil is removed to the planned level.
• The bottom of the pit or trenches is leveled and pre-rammed.
  

  

• A layer of sand or gravel is filled up and well rammed using a vibrating plate. Hand tamping will not give the required compaction. Therefore, it is better to use a vibrating plate. This is especially important if the bearing capacity of the soil is not very high. Such a simple operation as a well-compacted sand and gravel bed can significantly improve it.
• Two layers of waterproofing are laid on the compacted base.
• Foundation blocks-pillows FL are being installed. They are laid out strictly horizontally. Check the correct alignment with a building level or level. The gaps between them are filled with cement-sand mortar.
• Reinforcement is laid, on top of it a layer of mortar of at least 2-3 cm.
• FBS blocks are installed with offset seams. None of the joints should match. The minimum offset is 0,4 of the height of the exposed block. If FBS is used with a height of 580 mm, then the offset must be greater than 240 mm.
• If there is another row of blocks, the armored belt is laid again and the solution is on top of it.
• Blocks are also set with offset seams.

The number of rows depends on the required height of the plinth and the required depth of the tape. Compliance with the rule of laying blocks with dressing is mandatory. Only in this way, having assembled the foundation from FBS with your own hands, you will get a reliable foundation for the entire structure.

How to lay FBS

In any row, laying starts from the corners. Then the blocks are placed in the places where the walls are removed. They are called lighthouses, and then all the rest are equal to them. The verticality of their exposure is checked, if necessary, corrections are made. Most often, the block must be lifted again, moved and put in place.

After the beacon elements are exposed, the moorings are pulled – cords, which are guided by when installing the following blocks. They mark the boundaries of the wall, and in such a way that the FBS stand symmetrically with respect to the middle of the FL block (cushion block). The maximum permissible deviation is 12 mm. All subsequent rows of blocks should also be placed exactly above the middle.

It is more convenient to pull the cords 2-3 mm from the side surfaces of the lighthouse blocks. This will make it easier to expose. Intermediate blocks are set starting from the largest: first they put everything 2,4 meters long, then 1,2 and then 0,9. The correctness of their installation is checked with respect to marking cords, verticality – with a plumb line.

How to choose the size of FBS blocks

You can determine how many blocks you need for your foundation by selecting. You can do this yourself on a plan drawn to scale. On the plan, you mark out each block on the same scale.

The block sizes for the FBS foundation are chosen based on a simple rule: to increase the strength of the structure, you need to use the largest block sizes. Therefore, first draw the blocks of 2,4 meters installed in the corners, then, if they fit, draw them at the junction of the walls. Between them, too, put the largest blocks that can fit there. Where they no longer become, put a smaller size. If there are any gaps in which even the smallest block (0,9 meters) does not fit, it’s not scary – these gaps are filled with brickwork.

In a similar way, draw the second row, not forgetting about the displacement of the seams. If necessary, draw the third one in the same way. Then count the required number of blocks of each size. Do not throw away the resulting plan: it will come in handy when installing the prefabricated foundation already on the site.

Mortar for masonry FBS

For laying foundation blocks, a standard cement-sand mortar of the M-100 brand is used. It can be obtained using different wombs of cement and the amount of sand:

• 1 parts of sand are taken for 300 part of M2,5 cement;
• for 1 part of cement M400 sand is already 3 parts;
• when using M500 sand, add 4 parts.

First, mix the dry ingredients until a homogeneous composition and color is obtained. Then gradually add 0,5 parts of water. If the solution is too thick, add water little by little in small portions. The consistency should be like that of thick sour cream: not lumpy, but not flowing either.

Do not try to increase the amount of cement. Not only will this not increase the strength of the concrete, as you would expect, but it will greatly reduce it. To gain strength, concrete needs a strictly defined amount of aggregate (in this case, sand) and water. With a lack of one and / or another, its strength will be lower. In the worst case, it will crack and crumble.