FBS block foundation: pros and cons

Recommendation points

• Organization of work execution
• Bearing qualities and stability of the foundation
• Preferred soil types
• Varieties of foundation blocks
• Combined device systems
• Warming and waterproofing problems

Prefabricated foundations are quite a rarity in domestic individual housing construction. Nevertheless, such a base arrangement system is not devoid of merit. Today we will analyze the construction of FBS foundations, the benefits of using this technology and possible pitfalls awaiting an inexperienced developer.

Organization of work execution

Prefabricated concrete foundations are used where monolithic construction is impossible or undesirable for a number of reasons: from the considerable remoteness of the concrete unit and unfavorable climatic conditions to the simple unwillingness of the developer to face the difficulties of formwork, reinforcement and pouring of deeply buried structures.

At the same time, the FBS foundation has a much higher construction speed with a significant reduction in labor costs. However, the technological complexity of erecting a prefabricated base for a house can be even higher than with monolithic concreting. Requires the involvement of earthmoving and loading equipment, a careful calculation of the order of laying blocks and adherence to the sequence of construction work.

The general procedure for constructing a foundation from a FBS is as follows:

1. Layout of the site according to the construction project and the binding scheme.
2. Excavation works: removal of soil to the depth of freezing, formation of inclined walls of an open pit according to the design prism of collapse.
3. Pouring a monolithic base, or a cushion or foundation.
4. Sequential laying of blocks with their binding with cement mortar and reinforcing mesh at the corners and junctions.
5. Exposure of the foundation until the completion of shrinkage.
6. Filling of the reinforced seismic belt and the device for overlapping the first floor.
7. Erection of the walls of the first floor.
8. Installation of waterproofing and foundation insulation.
9. Backfilling of soil.

The composition and order of work may differ slightly. In particular, with a high groundwater level, artificial drainage of the construction site can be performed, but, depending on the hydrogeology of the site, the drainage system can be laid both at the initial stage and immediately before backfilling. However, after studying the features of the construction of prefabricated block foundations, most of the questions about the technology and the stages of their construction will disappear by themselves..

Bearing qualities and stability of the foundation

The lack of solidity of prefabricated foundations significantly reduces their structural strength. The main destructive effect comes from lateral deformations of the soil and frost heaving forces. Despite the fact that block foundations are well suited for compressive loads, their resistance to displacement is very low if the base does not experience a sufficiently high degree of pressure from the building structures located above. In turn, the absence of a common reinforcing cage makes the prefabricated foundation very susceptible to bending and twisting forces..

There are two main rules in the construction of prefabricated foundations:

1. Their location is entirely in the freezing soil layer or below the freezing depth, which excludes the lifting of the structure by the forces of frost heaving.
2. Elimination of lateral action from the ground until the moment when the compressive effect is high enough so that the friction forces between the blocks do not allow their displacement.

At the same time, it is also required to level the difference in the density of the soil layer on which the foundation rests. This is done by installing blocks on a monolithic reinforced concrete tape-grillage or slab, which acts as a load distribution from soil resistance.

Preferred soil types

Prefabricated foundations are not suitable for subsiding and highly heaving soils due to the weak degree of resistance to bending loads. At the same time, one should not confuse the cases when the supporting base is assembled from the FBS and the options for using blocks for the construction of the basement walls. In the latter case, the supporting function is performed not by the blocks themselves, but by the NZLF tape or a monolithic plate under them.

Laying foundation blocks on a monolithic slab

The construction of a foundation from blocks according to the basic technology is shown for dry and hard-plastic underlying soils, the bearing capacity of which, in terms of the bearing area and the mass of the building, provides a reliability coefficient of the order of 1.2–1.4. In general, we can recommend laying a prefabricated foundation without a grillage with a soil bearing capacity of at least 4.5 kg / cm².

The device of a monolithic armopoyas for laying FBS blocks

If the density of the underlying soil provides a bearing capacity of the order of 3.8-4 kg / cm² (sandy loam, fatty clays) or conversion to mass and bearing area does not guarantee a positive coefficient of reliability, the foundation should be laid on top of sand and gravel preparation of a trapezoidal profile. With a safety factor of less than 1 FBS, it can only be laid on a grillage or, at least, an extended foundation.

Varieties of foundation blocks

The FBS range is not very wide. According to GOST 13579–78, with a block height of 30 or 60 cm, their width can be from 30 to 60 cm in 10 cm increments. The length of the blocks ranges from 90 to 240 cm, while the standard seam of the binder has already been incorporated into the length and height dimensions, that is, in reality blocks are 20 mm shorter and lower.

FBS block sizes

Foundation blocks are not reinforced by default, except for embedded mounting loops. Concrete of strength class from B7.5 to B15 is used as the main material. At the same time, when making custom-made blocks according to standard shapes, there are no restrictions for laying linear or mesh reinforcement in concrete, or using a mixture with special parameters, for example, with increased frost resistance..

FBS block with cladding

For the construction of concrete foundations and bearing basement walls, wall foundation blocks (FBS) are used, in some cases they are made of heavy concrete. Varieties of blocks made of silicate or expanded clay concrete are unsuitable for load-bearing and critical structures. In turn, the foundation blocks can be marked with FBV (with the presence of a longitudinal groove) or FBP (with voids in the manner of a cinder block).

Combined device systems

So, in the classical version, the FBS foundation is incredibly simple and even primitive: the lower and upper reinforcing belt, between which there can be an arbitrary number of rows of blocks, but usually no more than 4–5. This type of foundation is optimal for buildings without a basement and a technical subfloor, that is, the blocks are supported on all sides by soil, and their extrusion inward is excluded.

The device of the upper armored belt of the foundation made of FBS

In the presence of a basement or basement, the horizontal compression loads can be compensated in two ways:

1. Reinforcement of joints with 8 mm reinforcing mesh to compensate for the internal stretch zone.
2. Installation of internal lintel walls.

It is also possible to lay FBS on unstable soils, but, as already mentioned, only if there is a load distribution system – pile-grillage or strip foundation. If desired, the cutouts of the FBV slabs can be used to equip precast monolithic foundations. In this case, the cutouts of the blocks serve as trays for embedding with bridges or pouring chords, due to which a high-quality perception of bending loads is ensured. The same goals can be achieved using conventional FBS blocks, between which, after 1–2 rows, reinforced belts are cast along the attached formwork..

Combined foundation: tape and FBS blocks

Warming and waterproofing problems

The main disadvantage of prefabricated foundations made of FBS is considered to be their low localizing ability. If it is necessary to equip a dry basement, the joints between the blocks are a serious problem, and these are far from cold seams of monolithic concreting.

There are several ways to solve it. The main one is cutting the outer sides of the joints and laying a bentonite cord in them. However, even such efforts will not be enough if special concrete with low water absorption was not used in the production of blocks..

In the general case, the insulation of prefabricated foundations is applied by surfacing using high-quality sheets based on cross-linked polypropylene or fiberglass. In this case, the hydrobarrier must be temporarily secured in the upper zone of the foundation above the GWL level, otherwise there is a high risk of delamination due to atmospheric influences, because the foundation will remain open almost until the completion of construction.

It is also possible to solve the problem of defrosting low-grade concrete by insulating the outer surfaces of the foundation and blind area. For these purposes, a conventional inexpensive low-density PSB is used, which is temporarily glued to the blocks over the insulation, and then supported by backfill soil..