Strip foundation. Part 4: assembling concrete block structures

Recommendation points

• Preparatory work
• Choosing foundation blocks
• How to properly organize the installation work
• Installation of strip foundation from blocks
• Strengthening the block foundation

This article is fully devoted to the installation of prefabricated strip foundations from FBS blocks. We tried to reveal all the nuances that take place in the construction of such structures. Separately, we propose to consider the classification of blocks and understand the criteria for choosing one or another of their types..

You can learn a lot of useful information about foundations by reading our previous publications:

• How to drain the site
• What type of foundation to choose
• Strip foundation. Part 1: types, soils, design, cost
• Strip foundation. Part 2: preparation, marking, earthwork, formwork, reinforcement
• Strip foundation. Part 3: concreting, final operations
• Column foundation
• Pile foundation
• Slab foundation

Preparatory work

Just like any other foundation, a prefabricated foundation made of concrete blocks needs competent design, study of soil properties, and calculation of building loads. Before starting work on its construction, you should properly plan the actions, prepare the site, mark the axes of the building. Preparatory operations do not have any major differences from the construction of monolithic belts, well, except that you definitely have to prepare a place for storing bulky materials and the passage of heavy equipment.

There is one peculiarity in earthworks – it will not work here to use a narrow trench, as is the case with some types of monolithic belts. The fact is that the block foundation has horizontal and vertical seams, and they must be properly sealed on both sides. In addition, it is often necessary to organize waterproofing, insulation, drainage, and replacement of heaving soil. Therefore, already at the height of the first row of blocks, it is necessary to have a sinus width sufficient for a person to move and perform certain manipulations. Usually the walls of the trench are made with a slope, expanding upward. A light gap of 40-60 cm at the sole is considered normal..

Another point is that there are much more requirements for the quality of the pillow. Theoretically, on sandy soils, it can not be arranged – you just have to clearly align the bottom of the trench and remove all loose inclusions and waste from it. But, since there are practically no ideal natural foundations in life, you have to make an artificial pillow.

Note that the prefabricated strip foundation is used only on non-heaving or slightly heaving soils and laying it below the freezing depth, there are no frost heaving forces acting on the sole from below and cannot be. This means that it makes no sense to make a high cushion under the sole of this foundation, which could serve as a replacement for problem natural soil with non-porous soil (sand, gravel …).

In our case, we level the bottom of the trench with sand, and that’s it. A layer of 5–10 cm must be laid in a groove from wall to wall. In the pit, the pillow should be 30 cm wider than the sole on both sides. It is very important to maintain the horizontalness of the upper edge of the leveling sand preparation, this is not a monolith – tolerances tend to accumulate in prefabricated structures. It is most convenient to level the pillow with a long rule according to the “hats” of the beacons. As such, you can use wooden wedge bars that are driven into the ground at the design height (we use a level or water level).

The pillow, bedding, sand preparation (call it what you want) must be moistened and compacted by tamping.

Choosing foundation blocks

A prefabricated foundation made of concrete elements is made of two main types of products – wall blocks and FL slabs for the sole device. Ready-made concrete elements can be used in any geological and hydrological conditions, under a building of almost any mass and configuration. In the factory design, blocks and plates are distinguished by guaranteed high strength and resistance to aggressive environments, precise geometric dimensions, and well thought-out design. All this makes it possible to reduce technological risks and the timing of the construction of the zero cycle. And this, in fact, is the advantage of prefabricated strip foundations. There are craftsmen who are trying to make foundation blocks on their own. Of course, the product comes out cheaper, but its brand will be questionable, to put it mildly, especially since some operations in artisanal conditions are almost impossible to reproduce, for example, high-quality vibrocompression or the legendary “steaming”.

Foundation blocks are made of heavy concrete (2200-2500 kg / m³), silicate concrete of medium density (up to 2000 kg / m³) or expanded clay concrete (1800 kg / m³), in accordance with GOST 13579–78. Products are not reinforced, concrete is used with strength class from B7.5 to B15. The choice of FB in terms of density depends on the loads experienced by the foundation and the required resistance to external factors. We described in detail about load calculations in the article “Strip foundation. Part 1: types, soils, design, cost ”. In practice, the most widespread are blocks of heavy concrete, as the most durable.

By design, there are three main types of blocks:

• FBS – solid foundation blocks – the most versatile;
• FBP – hollow, U-shaped, which are used to construct unloaded basement walls;
• FBV are solid blocks with cutouts for laying communications and laying lintels under the ceiling of basements.

The dimensions of the foundation blocks are standardized. Their height is usually 580 mm. The lengths of FBS blocks are 880, 1180, 2380 mm, FBP – 2300 mm, FBV – 880 mm. The width of the foundation blocks can be selected depending on the loads on the base and the thickness of the walls – it is 400/500/600 mm, solid FBS products can have a width of 300 mm. Many manufacturers have at their disposal molds with adjustable width and length, so the developer can order the production of non-standard elements of the prefabricated foundation (as a rule, these are small additional blocks), which makes it possible to obtain an exact configuration without “embedding” large gaps. The block geometry is marked in decimetres with rounding (length / width / height). For example, “FBS12.4.6-T” means that before us is a solid block of heavy concrete (silicate would be denoted by the index “C”, expanded clay concrete with the letter “P” – porous) 1180 mm long, 400 mm wide, 580 mm high.

Sometimes the block width, even 600 mm, turns out to be small in order to correctly transfer the load to the natural base. Then it makes sense to use a plate of the FL brand as a pillow, which allows you to reduce the pressure on each square centimeter of soil and apply a FB of a smaller width (the main thing is that the bearing capacity of the tape is sufficient). These are trapezoidal reinforced concrete products made of heavy concrete with a density of 2200-2500 kg / m³. The width of the FL element can be from 600 to 3200 mm, and the length is 780/1180/2380/2980 mm.

Separately, sole plates are distinguished by their resistance to aggressive media (permeability class):

• “P” – reduced permeability
• “H” – normal permeability
• “O” – especially low permeability

Depending on the ultimate loads on the base (pressure is taken into account at a certain wall thickness), FL slabs are classified into 4 groups. Division into groups and basic characteristics of foundation slabs are standardized by GOST 13580-85 “Reinforced concrete slabs for strip foundations”. For example, the product “FL6.24-4-N” is a foundation slab 600 mm wide, 2380 mm long, normal permeability, the fourth group in terms of bearing capacity (as an option, 4.5 kgf / cm² with a wall thickness of 300 mm).

It is better to purchase foundation blocks from manufacturers with a good reputation who have been producing reinforced concrete products for more than one year, are ready to confirm their competence with certificates for each batch of goods. Give preference to a company that has its own laboratory that controls product quality. Please note that concrete blocks can mature naturally, under favorable conditions this happens within 28 days. Steaming speeds up the hardening process, but is not a prerequisite for a quality product.

In any case, all elements of the precast foundation must be inspected. Checked:

• deviations from the linear size (10–15 mm according to GOST are already considered critical);
• straightness of the upper horizontal plane;
• rectangularity of products;
• the integrity of the embedded elements – mounting loops;
• the presence and nature of cracks, shells, sagging;
• correct storage (wooden substrates must be used);
• correct markings.

How to properly organize the installation work

Before starting the installation, it is necessary to draw up a plan for the installation of all concrete products. Then you do not have to think during work which block to put where, and you can calculate the exact number and specification of elements. The plan is created for each row separately, by the way, the block foundation is usually made of three rows or more. Keep in mind that some distance can “eat” the joints of the blocks, they can also accelerate the free space, if it is small. Due to the horizontal joints, the foundation will be about 100 mm higher than if you simply add up the block heights. All elements must be mounted with bandaging of vertical seams.

The installation of the foundation for a large house should be planned for several days – one row per shift.

Blocks must be stored in several places, no closer than 2 meters from the walls of the trench / foundation. It is imperative to use wooden linings. The layout of the storage points depends on the ordering scheme of stacking and the characteristics of the crane that will operate on the site (boom reach, lifting capacity, etc.). Please note that pulling the blocks without full lifting (oblique tension of the lines), by dragging, is prohibited.

During the installation process, you will need a cement mortar for the “bed” and concrete for sealing vertical joints. A mortar unit should be prepared – you will need a concrete mixer, a container, bulk materials, water.

The optimal composition of the brigade is at least 5 people, excluding the crane operator. Two fitters work in a trench, they lay and level the bed of mortar, receive and lay blocks. The other two people prepare the solution and feed it into the work area. One worker acts as a rigger – he is engaged in the selection of blocks and their slinging.

Only one installer should be selected to guide the crane operator. A set of conditional gestures is determined in advance, or, if visibility is limited for some reason, uninterrupted communication is provided (for example, by radios).

If a house with a basement is being built, then first the foundation tape is mounted, and then the walls of the basements. The construction of the foundation in sections, several rows at once, is allowed to be carried out only if the crane is inside the pit, and it will need an exit.

The first two rows are mounted in a trench or on blocks, the third row and higher are recommended to be collected from the scaffold and additional platforms.

The working area is planned, cleared of foreign objects, if necessary, fenced off.

All employees must be instructed on safety when working with lifting equipment.

Installation of strip foundation from blocks

The first row of blocks, including FL slabs, are installed directly on the sand pad. They are oriented according to individual height and spatial marks located at the bottom of the excavation. Previously, on the pillow, you can draw the location of the vertical seams for each element. Many craftsmen at the corners of the foundation arrange a boundary board from a board, to which corner blocks are attached.

Blocks in the corners and located at the abutment of the load-bearing walls are lighthouse, they are installed first. The correctness of the installation of the first blocks is checked using a plumb bob stretched from the cast-off cord. Between the corner blocks, along the upper outer edge, pull the control cord – “mooring”. Intermediate concrete elements of the row are laid on it. This installation order applies to each row..

Next, the vertical seams between the blocks are sealed with concrete. To lay one standard FBS block, you will need about 10–12 liters of concrete. It is imperative that the concrete in the seam be stitched – compact with a piece of reinforcement.

Mounting loops located on the surface (there are also recessed in niches), bend with a sledgehammer, or cut off with a grinder.

The second and subsequent rows are installed on a mortar bed. The installers, who are in the pit, take the mortar and use shovels to level it on the already laid blocks, with a layer of 20–30 mm (about 50 liters of the mixture goes to an element with a length of 2380 mm). The mobility of the mortar should not exceed 5–7 cm. The bed is arranged under each foundation element separately, with a small margin – the mortar strip is made retreating 40–50 mm from the end of the block. The solution is constantly needed fresh, it is forbidden to “refresh” with water or use one that has begun to set.

The rigger selects the block according to the ordering scheme and checks its geometric dimensions, examines the mounting loops. Next, he gives a signal to the crane driver to feed the slings. After fixing the hooks in the loops, the cables are pulled, the quality control of the slinging is carried out. Then the block rises to a height of 50–70 cm from the ground, a worker with a long scraper cleans the lower plane of the foundation element from dirt, ice, concrete flows, moves 4–5 meters away and allows the product to be moved to the working area.

The installers receive the foundation block at a height of approximately 30 cm from the installation point. They, if necessary, turn it and give the command to lower the product. At a height of 10 cm from the reference plane, the block is leveled with crowbars, the lower part of the product is oriented along the edge of the lower row, and the upper edge is set along the mooring cord.

Further, the crane operator completely lowers the block (the slings are loosened), the correctness of its installation is checked by a plumb line, a long rack level and a cord. If everything is normal, the hooks of the lines are removed from the hinges, the boom is withdrawn. Boundary deviations along the center line (mooring) are considered to be 10 mm, the height difference of the upper plane of the laid block should not exceed 5 mm. If problems with the levels are found, then the foundation element is dismantled, the mortar bed is changed.

It is strictly forbidden to put crushed stone, pieces of brick, pieces of reinforcement and other objects between the blocks for the purpose of leveling. For slight alignment in the design position, it is allowed, with tensioned slings, the use of wooden wedges, which must be removed.

It is believed that the blocks of all external walls located in the ground should be equal to the internal wall, and those located above should be equal to the external side. However, it would be more logical to carry out all installation work on the outer edges, which will allow you to get a good plane for waterproofing.

The mixture protruding from the seams is cut with trowels, all joints after the installation of the blocks are carefully sealed with flush mortar – they are embroidered. If there is a need to fill the gap between the blocks or, for example, to close an artificial opening for the passage of communications, you should install the formwork and fill the area with concrete.

Strengthening the block foundation

In areas with increased seismic hazard and in areas with unstable soils, horizontal seams of a prefabricated foundation made of blocks are reinforced with a welded mesh made of smooth reinforcement with a diameter of 5–6 mm – it is recessed as it is installed in the bed from the solution. Separately, with piece rods, only the seam between the FBS block and the FL plate can be reinforced.

Another effective method to increase the strength of a block structure is to install a monolithic belt over the pillow or over the last row of blocks. This element not only connects all parts of the foundation into a single structure and effectively redistributes the loads, but also helps to correct some errors in the horizontalness of the upper plane. The height of a monolithic belt is not less than 200 mm, it is identical in width to blocks. The belt installation technology is similar to the construction of a monolithic strip foundation, here you will also need formwork, concrete (grade 250 and higher), reinforcement (4 strings of class A3 with a section of 10-14 mm), the same concreting techniques are used. Read “Strip foundation. Part 3: concreting, final operations ”. Some difficulty is caused by the installation of formwork panels, since it cannot be loosened from the ground. This is the so-called sliding formwork, which encloses the blocks with shields, and the main fixing role in its structure is played by two levels of tie rods.

After the installation of the foundation blocks, if the monolithic belt is not provided for by the project, you can almost immediately proceed with the waterproofing device and backfill the sinuses. The block foundation does not need to mature, it is already ready for the construction of walls.