Diy monolithic overlap: calculation, preparation and installation of formwork

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In this article, you will learn how to make your own reinforced concrete floor over brick walls. The text contains detailed calculations of materials. The article will tell you about different methods of formwork, the rules of reinforcement and the cost of material and work.

Do-it-yourself monolithic overlap. Calculation, preparation and installation of formwork

In the previous article I talked about the fact that there are three ways to install formwork – inventory, homemade and combined. The first two of them are fundamentally different and we will consider each step by step.

Calculation of a monolithic floor

Initial data

Regardless of the chosen method of formwork installation, the same high-quality result and respect for the dimensions are required. Therefore, there is a set of requirements and calculations that are the same for both methods..

Suppose we need to arrange a monolithic reinforced concrete floor over a rectangular building with an internal load-bearing wall. Internal dimensions of the premises: 5×3.7 m and 5×2.5 m. Ceiling height 2.7 m.

Calculation of concrete

The overlap area will be equal to:

  • S = 5 (2.5 + 3.7) + 1 x 0.4 = 31.4 m2

With a thickness of 200 mm, the volume of concrete will be:

  • V = 31.4 0.2 = 6.28 m3

The overlap mass will be:

  • M = 6.28 2500 = 15 700 kg = 15.7 tons

Do-it-yourself monolithic overlap. Calculation, preparation and installation of formwork

Reinforcement calculation

For the reinforcement of the floor, the frame of the A3 O 16 reinforcement is adopted from two mirror meshes with a step of 180 mm. The number of longitudinal bars in one mesh is the overlap width divided by the step:

  • Nprod = 5000/180 = 27.7 = 28 pcs.

Length of longitudinal bars in one mesh:

  • Lprod = Nprod A = 28 6.6 = 184.8 = 185 m

The number of transverse bars in one mesh is the overlap length divided by a step of 180 mm:

  • Npoper = 6600/180 = 36.6 = 37 pcs.

Length of cross bars in one mesh:

  • Lpoper = Npoper В = 37 х 5 = 185 m

Total length of rods in one mesh:

  • L1 = Lprod + Lpoper = 185 + 185 = 370 m

The total length of the bars in the floor frame:

  • Ltotal = L1 2 = 370 x 2 = 740 m

1 m2 overlap accounts for 740 / 31.4 = 23.5 linear meters. m fittings.

1 m3 overlap accounts for 740 / 6.28 = 117.8 linear meters. m fittings.

Do-it-yourself monolithic overlap. Calculation, preparation and installation of formwork

Plane material quantity

When using plywood to create a table plane, it is necessary to calculate the number of whole sheets (1220×2440 mm, 3 m2), based on the linear dimensions of the premises. This is done to reduce waste:

  • N = Spom / Sleaf = 31.4 / 3 = 10.5 pcs.

In total, you need 10 whole sheets of film faced plywood. The remaining space can be sewn up with a cheaper material – ordinary plywood or board.

If a 100×25 board is adopted as the main material of the plane, then its volume will be:

  • Vboards = 31.4 0.025 = 0.785 m3

Number of inventory beams

The specificity of precast formwork is that it consists of girders on which the beams rest. The step of the crossbars is no more than 1.2 m, they are placed along the room. The spacing of the beams is 400–600 mm, and they usually run across. The number of lines of the support beam will be equal to the width of the room divided by the step. In our case:

  • N1 = 3.7 / 1.2 = 3.08, we take 3
  • N2 = 2.5 / 1.2 = 2.08, we take 2

In total, there are two lines of crossbars in each room. These are 10 beams of 2.8 m each.

Do-it-yourself monolithic overlap. Calculation, preparation and installation of formwork

Cross beams are calculated based on the width of the rooms. In our case – 10 pcs. 3.6 m and 10 pcs. by 2.4 m.

Number of racks

Telescopic inventory racks are installed only under the crossbars with a step of 800–1000 mm. In our case – five lines of 5 meters – 25 meters. We divide by a step – we get the quantity:

  • Nracks = 25 / 0.8 = 31.25 = 32 pcs.

There is one tripod and one unvilka (“crown”) for each stand.

Advice.Select the maximum height of the racks. This will give convenience when marking and setting the horizon. The stand should not be extended to the maximum – the more liner in the bar, the stronger it is.

Installation of inventory formwork

To mount an inventory table, you will need a minimum set of hand and measuring tools – a hammer, tape measure, rule / level, cord, markers. To strap the perimeter, you will need a drill with dowels BM O 6 100 mm.

Attention! Make sure the table stand is installed firmly. Settlement under one pillar during the application of a load to the table (placing concrete) can cause destruction of the entire structure and injury to workers.

The initial condition for all options is that the walls are laid with high quality, the top row is laid out “in the horizon” and can serve as a guide.

Operating procedure:

1. Move the horizon line to eye level along the walls.

2. Along the inner perimeter of the walls, at a height of 20 mm below the edge of the wall, fix a board 100×25 mm. Dowel pitch – 500-800 mm.

Attention! The perimeter strapping is not a support, but a beacon, which will also facilitate dismantling and prevent the cement milk from leaking.

3. Extend the struts to the required length and secure with a lock or stop while the jack nut is tightened. The approximate height of the rack is the distance from the floor to the top of the wall minus 20 mm (plywood), minus 200 mm (beam) and minus 200 mm (crossbar). In our case: 2700 – 420 = 2280 mm.

4. Install crossbar posts along each room. On one crossbar line – 3 racks (with tripods and univils).

5. Install the inventory beams into the universal forks along the length of the crossbar. Overlap of beams on the support – 300 mm.

Do-it-yourself monolithic overlap. Calculation, preparation and installation of formwork

6. Bring the cross beams up and spread them flat with a step of 400–600 mm on the crossbars.

7. Transfer the horizon mark using the hydro level to the fixed part of the rack (sleeve).

8. Measure the required distance from the level mark to the lower plane of the crossbar. Make an L-shaped template.

9. Set the height of the crossbars according to the template using the sleeve nuts.

10. Install the formwork beams in the design position (on the edge).

11. Lay out the plywood sheets along the beams, fixing them at the corners with 50–70 mm nails. Or sew up the plane with a board.

Do-it-yourself monolithic overlap. Calculation, preparation and installation of formwork

Attention! There is no need to overly firmly fasten the plywood to the beams – vertical loads do not act on the fasteners. At the same time, an excess of nails will significantly complicate dismantling. Plywood joints should be on the beam, especially longitudinal.

12. Cover the remaining areas with consumables..

13. Check the integrity of the formwork, block holes and holes if necessary.

14. Install the remaining racks with a step of 800 mm under the crossbars.

Sometimes the plane is covered with a thin layer of technical oil – this greatly facilitates dismantling..

Attention! When using oil, make sure that it does not get on the fittings.

At this, the installation of the inventory table of the floor formwork is completed and all other operations – reinforcement, flanging, concreting – coincide with any other method.

Installation of self-made formwork

The technology of installing timber and lumber formwork largely depends on the experience of the master. There is no specific technology here, since the method is considered handicraft. Often, lumber is used for the racks and beams of the table, intended for the rafter system or floors of the building..

There are a number of requirements that must be observed when assembling the formwork yourself..

Racks should be made of solid wood – beams or boards. Splicing “overlap” is not allowed. It is allowed to build up “in the end” on intermediate racks.

Section and spacing of racks:

  • 75×75 mm – 800 mm
  • 100×100 – 1000 mm
  • 120×120 – 120 mm
  • 150×150 – 1600 mm
  • 180×180 – 1800 mm
  • 200×200 – up to 2000 mm

Bolt thickness and pitch *:

  • 75 mm – 600 mm
  • 100 mm – 800 mm
  • 120 mm – 1000 mm
  • 150 mm – 1400 mm
  • 200 mm – 1800 mm

Thickness and spacing of formwork beams:

  • 40 mm – 400 mm
  • 50 mm – 500 mm
  • 60 mm – 600 mm

And so on at a rate of 1 to 10.

* Assumes a reasonable width of a plank or beam of 100-200 mm.

Operating procedure:

1. Carefully measure the height from the floor to the top of the wall.

2. Make U-shaped elements (“frames”) from the material for posts and crossbars. The uprights must be fixed 300 mm from the edge of the ledger.

3. Install them at a given step (see above), unsealing with temporary slopes.

Do-it-yourself monolithic overlap. Calculation, preparation and installation of formwork

4. Check the correctness of installation, compliance with the horizon.

5. Install additional posts and secure the structure with permanent slopes.

6. Unfold the formwork beams with a given step (see above).

Do-it-yourself monolithic overlap. Calculation, preparation and installation of formwork

7. Make the flooring from the selected material (plywood or board). If the flooring is supposed to be boardwalk, lay polyethylene on it.

Note.All other things being equal, the quality of the overlap (mark, plane) made with the help of self-made formwork will always be lower compared to the inventory one. However, this is not so important, since the ceiling is mainly made suspended.

Floor reinforcement

The principle and schemes of reinforcement are given in the previous article..

Do-it-yourself monolithic overlap. Calculation, preparation and installation of formwork


After creating the reinforcement cage, the outer board should be made. To do this, pull the cord at the desired height and install the sides made of plywood or panels on the BM 150 mm dowels.

Floor concreting

The concrete is poured in one go with vibration. For more details, see our article..

Dismantling the formwork

The slab formwork is removed no earlier than 28 days from the moment of concreting. This is the most dangerous part of the job, especially if the ceiling height exceeds 2.5 meters. Disassemble the table with great care, gradually removing the racks and staying in a safe place.

Cost 1 m3 reinforced concrete floor

Name Unit rev. Quantity in 1 m3 Unit price, rub. Price 1 m3
Concrete M100 (B12.5) cub. m 1 2500 2500
Armature A3 O 16 run. m 117.8 25 3000
Total for material 5500

Prices for installation and concreting services

Formwork rental (tentatively) complete set 80 rubles / day / sq. m
Concreting with inventory formwork 2500 rubles / cubic meter m
Concreting with self-made formwork 3000 rub / cubic meter.
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