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Formwork is a structure of shields, spacers and stops, which serves to give concrete and reinforced concrete products a shape. If we talk about construction, then this system is necessary when pouring a foundation of any type, but the largest structures are needed when constructing a tape monolithic foundation. Formwork is also used when creating reinforcing belts in masonry walls from building blocks. In the same buildings, a reinforced belt is often needed at the top to create a solid base for attaching the roof system. It is also formed using formwork. This design will also be needed when pouring concrete paths or concreting blind areas, and for some other types of work.
Removable and non-removable
According to the principle of use, the formwork can be removable (collapsible) and fixed. As the name implies, the removable one is disassembled after the concrete gains strength above the critical one (about 50%). Therefore, it can be used several times. Depending on the material, the same kit can withstand from 3 to 8 pours, industrial options can be used several dozen, and some hundreds of times.
Fixed formwork becomes an inseparable part of the foundation. Such systems began to be used relatively recently. They are made mainly from extruded polystyrene foam. Blocks of different configurations are produced, which are interconnected using locks and metal studs. From the blocks, as from the constructor, the necessary form is typed.
The non-removable formwork made of expanded polystyrene not only gives shape, but also simultaneously serves as thermal and hydro insulation, and also has soundproofing properties. It costs a lot, but immediately solves many problems, and the time spent on building the foundation is significantly reduced.
There is another type of fixed formwork – hollow concrete blocks. They also have different configurations – wall, corner, with a radius, etc. They consist of two or three walls and several jumpers holding the walls in a certain position. They are connected to each other with locks, reinforced with rods.
formwork requirements
Since this whole system is created in order to give shape to concrete and reinforced concrete products, it must be strong and elastic enough to withstand the pressure of the mass of liquid concrete. Therefore, rather serious requirements are imposed on the materials for formwork in terms of strength. In addition, the assembled panels must have a smooth and even inner surface: it forms the foundation walls, and hydro- and / or thermal insulation materials are then fixed to them. It is easier to attach them to flat (at least relatively) surfaces.
Removable construction materials
In construction organizations, there are metal structures assembled on studs and bolts. In private construction, formwork panels are made of boards, moisture-resistant plywood and OSB. Wooden blocks are used as stops and spacers. Nobody bothers to make a metal structure, but it is very expensive and unprofitable for a one-time use.
When building a cottage or country house, boards are most often used. Any species can be used, both coniferous and deciduous. It is better to take an edged one: the solution should not ooze through the formwork, and this is unrealistic to achieve with an unedged board.
With a foundation height of up to 1,5 meters, the formwork board must have a thickness of at least 40 mm. Shields are fastened with bars of section 60 * 40 mm or 80 * 40 mm. If the height of the foundation is large – it is deep – such bars will not be enough to hold the mass of concrete. At a height of more than a meter, you need to use a bar of 50 * 100 mm or more. For assembly use nails or screws. Their length is 3/4 of the total thickness of the board and bar (for the above dimensions 60-70 mm).
Formwork is also made from plywood. There is even a special formwork, laminated with paper with synthetic impregnations. The coating has increased resistance to aggressive environments, which is liquid concrete. This material is marked with FSF (using formaldehyde glue).
The thickness of plywood for formwork is 18-21 mm. Shields are assembled on a metal or wooden frame. The wooden frame is made from a bar 40 * 40 mm, fasteners need to be used shorter – 50-55 mm. When using plywood, it will be easier to work with self-tapping screws: nails are hard to hammer.
OSB is used for this purpose infrequently, but this option also takes place. The thickness is about the same: 18-21 mm. Structurally, it is no different from plywood shields.
Select the dimensions of the sheets of these sheet materials based on the dimensions of the required formwork panels – so that there is as little waste as possible. Special surface quality is not required, so you can take low-grade materials, which are usually called “building”.
Decide for yourself what to make the formwork for the foundation: it depends on the prices for these materials in your region. The usual approach is economic: what is cheaper is used.
Do-it-yourself formwork for a strip foundation
The most voluminous is the formwork for the strip foundation. It follows the contours of the house and all load-bearing walls on both sides of the tape. When constructing a more or less large building with a large number of partitions, the cost of foundation formwork materials will be very significant. Especially with deep foundations.
Shield design and connection
When assembling the formwork with your own hands, it is important to make the shields strong: they will need to hold the mass of concrete until hardening occurs.
The dimensions of the formwork panels vary and depend on the geometry of the foundation. The height is slightly higher than the height of the foundation, you determine the length of each shield yourself, but usually it is from 1,2 to 3 m. It is inconvenient to work with very long structures, so the optimal length is about 2 m. The total length of the entire formwork should be such that they became exactly according to the marking of the foundation (do not forget to take into account the thickness of the shield).
When making formwork from boards, cut several pieces of the same length, fasten with bars and nails or self-tapping screws. When using nails, they are hammered from the inside of the shield, bent on a bar. It is easier to work with self-tapping screws: they do not need to be bent, because due to the thread they provide a snug fit of the elements. They are twisted from the inside of the shield (the one that will be facing the foundation wall).
The first and last bars are fastened from the edge at a distance of 15-20 cm. Between them, at a distance of 80-100 cm, additional ones are placed. To make it convenient to install formwork panels, two or three bars (along the edges and in the middle) are made 20-30 cm longer. They are sharpened and driven into the ground during installation.
Plywood or OSB boards are assembled on a bar frame. When assembling, it is important to strengthen the corners well. In this design, they are the weakest point. You can strengthen them with metal corners.
Do-it-yourself formwork installation
If the shields were made with several elongated bars, they must be placed along the stretched marking cords. The difficulty lies in the fact that at the same time you need to expose in a vertical plane. For fixing, you can use bars clogged at the mark and set vertically. When installing, set the plane of the shields close to these bars. They will be both support and guides.
Since the bottom of the trench or pit must be flat (compact it and level it to the level), it should be easy to set the shields horizontally. Try not to hammer them too much: it will be easier to level them later. Lower one of the corners to the level of the bedding. There should not be a gap, the solution should not flow out. Having achieved a snug fit, take the building level, apply along the shield and hammer the second edge with a hammer until the upper edge is set horizontally. You are already setting up the next shield relative to the installed one: they must be at the same level and in the same plane.
If the shields are made without long bars, at the bottom of the pit, along the marking line of the tape, a bar is fixed, which will serve as a stop. The shields are attached close to it, then fixed with the help of slopes and struts.
Strengthening – braces and stops
In order for the formwork not to fall apart under the mass of concrete, it must be fixed from the outside and from the inside.
Braces are installed outside. Supports should be at least a meter apart. Particular attention should be paid to the corners: they put stops in both directions. If the height of the shield is more than 2 meters, then one belt of stops is not enough. In this case, at least two tiers of spacers are made: upper and lower.
It is also necessary to stabilize the distance between two opposite shields. To do this, use studs made of reinforcement with a diameter of 8-12 mm, metal gaskets and nuts of the appropriate diameter. Studs are installed in two tiers: above and below, at a distance of 15-20 cm from the edge.
The length of the studs is approximately 10-15 cm longer than the width of the tape. There are two options:
- Both ends of the reinforcement are threaded. Then each stud will require two metal sealing plates and nuts.
- On the one hand, the hairpin is bent and flattened, a thread is cut with an arc. In this case, one nut is needed (there are still two plates).
The internal distance between the shields, equal to the design width of the tape, is fixed using pieces of plastic pipes. Their inner clearance should be slightly larger than the thickness of the studs.
The assembly goes like this:
- Holes are drilled in both shields with a long drill.
- A piece of pipe is installed between them.
- The pin is being threaded.
- Metal plates are installed (they will not allow the hairpin to break the shield material).
- Nuts are tightened and tightened.
You need to work together, and better – three of us. One inside between the shields installs tubes, and one person for installing studs and tightening nuts.
When removing the formwork, first unscrew the nuts and remove the studs, then dismantle the slopes and stops. Released shields are removed. They can be used further.
How to spend less
A lot of material is spent on the manufacture of formwork for a strip foundation: the boards form the entire strip on both sides. At great depths, the flow rate is very high. Let’s say right away: there is an opportunity to save money. To make only part of the formwork and pour not everything in one day, but in parts. Despite popular belief, this will hardly affect the strength of the foundation (if you know the secrets), and you can save decently. You can divide the foundation either horizontally or vertically.
Filling with layers
With a large depth of laying, it is more profitable to fill in parts horizontally (in layers). For example, the required depth is 1,4 m. You can break the pour into two or three stages. With two stages, it will be necessary to make shields with a height of 0,8-0,85 m, with three – 50-55 cm.
The order of work is as follows:
- The formwork is set up from the short panels made, the reinforcement is knitted for the entire required volume.
- Concrete is poured according to the height of the formwork.
- After 7-8 hours after pouring, it will be necessary to remove the top layer from the entire surface of the tape with a trowel. When vibrating concrete, cement milk rises. As it hardens, it becomes brittle and brittle. It is this layer that will need to be removed. As a result, the surface will be uneven and rough, and this will improve adhesion (adhesion) with the next layer of concrete.
- At a temperature of +20°C, after three days the formwork can be removed, the boards cleaned and fixed higher. When removing the shields, take out the studs. Tubes can be left in concrete. They have already become part of the monolith. Sometimes they are taken out, and the holes are filled with mortar.
- Raise the formwork higher and pour it again.
The shield is simply installed on the already “grabbed” concrete, and rests against the edges of the trench, but at a different level
When installing the second (and third, if necessary) tier, the shields find a little on the already filled area, covering the tape from the sides. In this case, the lower row of studs usually serves as a stopper and emphasis. Therefore, when installing them, put them all on the same level from the bottom edge of the shields.
The fittings are already connected, the internal studs are cut. It remains only to put the other tubes, put the studs back in place and put the outer stops and braces. It does not take much time to install the next layer of formwork.
Why does this method not affect the strength of the foundation? Because the strength of concrete is not taken into account in the calculation. She goes to reserve. In addition, the load in strip foundations is distributed along the long side. And there are no gaps in length. So the foundation will stand for a long time.
vertical division
The second way is to split the plan vertically. The foundation can be divided into two or three parts. Only you need to divide not exactly “along the line”, but to space the joints a certain distance.
In the part of the building selected for installation, install the formwork with “plugs” in those places where the part to be installed ends. Inside the installed part, knit a reinforcing cage. In this case, the bars of longitudinal reinforcement must extend beyond the formwork by at least 50 diameters of the reinforcement used. For example, a 12 mm bar is used. Then the minimum outlet outside the formwork will be 12 mm * 50 = 600 mm. The next rod is tied to this release, and one by one they will go to these 60 cm.
One important detail: when breaking down the plan of the house into parts, make sure that the “pieces” poured during this period end at different levels (see the picture).
Fill the assembled area with concrete. As in the previous method, after 7 * 8 hours it will be necessary to beat the solution, but already on vertical surfaces. Take a hammer and remove the sidewall plug, beat the cement-sand mortar down to gravel (near the formwork there will most likely be a layer of mortar without aggregate). As a result, the surface will be chipped, which is good for adhesion with the next portion of the solution.
These methods can be safely used in private construction: they are practiced in the construction of monolithic multi-storey buildings, and there the workloads on concrete walls and foundations are incomparably greater.
There is another trick. Everyone says that boards or plywood can then be used in auxiliary work. In practice, it turns out differently: it is impossible to cut wood or plywood soaked with cement. In addition, it becomes dirty and rough, and it is also unrealistic to clean and polish it: it does not “take” any grain. So, in order for the wood (and plywood, if not laminated) to remain usable, the front of the shields is covered with a dense film. It is fixed with a construction stapler and staples. If it is damaged, replacement takes very little time. Formwork improved in this way gives an almost perfectly flat foundation surface, which facilitates subsequent work on hydro- and thermal insulation.