Formwork is a structure made of panels, struts and stops, which is used to give concrete and reinforced concrete products their shape. If we talk about construction, this system is necessary when pouring any type of foundation, but the largest structures are needed when installing a strip monolithic foundation. Formwork is also used to create reinforcing belts in masonry walls made of building blocks. In the same buildings, a reinforced belt is often needed at the top to create a solid base for attaching the roofing system. It is also formed using formwork. This design will also be needed when pouring concrete paths or concreting a blind area, and for some other types of work.
Calculation of formwork for the foundation
In this case, calculation means the calculation of the required thickness of the boards or boards of the structure, taking into account the distance between the supports, the specific gravity of the mixture being poured and the resistance ability of the mold material. The traditional, simplest formwork design consists of panels installed at the bottom of a pre-dug trench, held in a vertical position by special supports.
The choice between stakes or braces depends on the volume of the mixture being poured and the type of soil: for small foundation strips (250...400 mm) and dense soil that is not prone to shedding or sliding, stakes are sufficient. If in doubt, it is better to give preference to braces, since they are able to hold the panels in place even with increased load.
Formula for calculating the thickness of shields (minimum)
Here
- G is the load of the liquid mixture on the formwork, it is assumed to be maximum and is defined as the product of the specific weight of concrete and the thickness of the poured layer G = qx H. The specific gravity is q = 2500 kg/m3, H is the height of the foundation strip in meters;
- n is the compaction coefficient of the concrete mixture. If vibration compaction is not used, the coefficient is equal to one;
- l – distance between shield supports, in meters;
- T – load resistance of the board material; for wood it is assumed to be 8 x 105 kg/sq.m.
Taking into account the difficulty of independently determining the grade of concrete, respectively, its density, as well as the lack of accurate data on the resistance of the material of formwork panels, calculations are carried out approximately, for average values. Typically, the thickness of the board is taken to be 25 mm.
The second part of the calculation is determining the required number of boards of a given thickness and the number of fasteners. For these calculations, it is necessary to know the length of the foundation strip and the height of the fill. The length of the tape is multiplied by two, since the shields are located on both sides. When installing a strip foundation with a step, the calculation is carried out twice - for the first and second stages of pouring. The amount of lumber is determined based on the data on which board for the foundation formwork will be used (dimensions).
Drawing up a plan similar to that shown in the illustration will help you navigate a more accurate calculation of the required length of the form, the number of connecting and supporting parts.
Where to buy?
Of course, you can make a purchase at a hardware store that sells lumber, but if you want to save a little, then you should purchase materials directly from the manufacturer. There are sawmills in every city and there you can either purchase ready-made boards or order their production according to given parameters. In addition, there you can also decide on the cost of formwork boards and their delivery to the site. If the production facilities of a forestry department are located in your city, then you can purchase even better material there.
Also, if the length of the boards is not important to you, you can contact a furniture manufacturer, where you can often find the remains of boards that are quite suitable for constructing formwork.
What materials can formwork be made from?
Formwork can be made from various materials. Among them are:
- Metal. This type of material is very expensive, but at the same time the most versatile. It is made from steel sheets, the thickness of which is about 1-2 mm. The main disadvantage of this material is its high cost, so it is used extremely rarely.
- Reinforced concrete. This type of formwork is also expensive. It is made of concrete slabs of different thicknesses. When using it, the consumption of concrete for pouring is significantly reduced. However, the main disadvantage of this material is its high weight, which requires the use of special heavy equipment.
- Expanded polystyrene. This type of formwork is also not cheap, but there is no need to remove it after the concrete has gained strength. It is made from ready-made polystyrene foam blocks and is quite easy to install. The main disadvantage of this material is its high cost and possible difficulties in the process of selecting some complex elements.
- Available materials. Formwork can also be made from slate, corrugated sheets and other suitable materials. The advantage of such formwork is its low price, however, among the disadvantages one can note the complexity of assembling the structure, there is a risk of concrete mortar leaking through the formed cracks, it is necessary to use additional wedges and supports.
- Wood. This type of formwork material is widespread and inexpensive. Most often, cut boards or sheet plywood are used to create formwork. It is easy to assemble, but additional reinforcement elements must be installed. This type of formwork is best used for beginners in construction.
Polyethylene film
Used as a waterproofing material. The film is not equivalent, it can be replaced with roofing felt, roofing felt.
Film
It is recommended to use polyethylene film in several cases:
- If after dismantling the formwork you do not plan to throw away the boards. If new lumber was used, it can be used again in building a house. But if traces of cement are found on the surface, it is better to abandon the boards; when cutting, you can significantly dull the knives on the surface planer.
- If a waterproofing layer is installed on the surface of the formwork, it will not be deformed, which means that dismantling will be easier.
Formwork structure made of boards and plywood
Properly connected panels should not have any gaps between them. To fasten wooden panels, a system of ties, spacers, slopes, clamps and stops is installed.
The installation of the formwork structure must be carried out in such a way that its internal surface does not have any protrusions, but is smooth and even. The developed standard technologies for assembling board or plywood formwork for foundations recommend using boards and plywood with a thickness of at least 10 mm for panels.
Plywood with a thickness of at least 10 mm is attached to the timber frame
If the foundation has a large width, then the lateral pressure of concrete can be withstood by boards made of boards with a thickness of 30 to 40 mm. The frame of the shields and stiffening ribs are prepared from wooden beams with a cross-section of 40x60 mm. Fasteners for enclosing structures are made from the remains of boards and timber. Other suitable materials are also used.
After installing wooden or plywood panels, carefully check the surface of the formwork for cracks and irregularities. If any are found, they need to be eliminated.
First, boards and plywood are cut to the size of the panels. Boards or sheets of plywood are laid on the laid out bars. Then all the elements are connected with nails or self-tapping screws. The finished planes are framed with timber and stiffening ribs are attached to them.
To attach the formwork, you must choose only high-quality reinforcement
After this, the enlarged elements are installed in the design position and the installation of fasteners begins. Support boards are placed along the formwork installation line, the pointed lower ends of the support beams rest against the ground, and the upper ends hold the formwork.
Fence walls are placed along the entire perimeter of the marking, securing them with evenly distributed fasteners. The panels are connected to each other by pieces of timber using nails or self-tapping screws. The opposite walls of the fence are connected with U-shaped clamps. Clamps are made from boards and timber.
Such structures prevent the formwork walls from breaking apart under the pressure of the poured concrete mixture. On the outside, slopes are made of timber, the pointed part of which rests on the ground, and the other end supports the upper part of the shield.
Removed board panels can be disassembled into boards and beams, then they are used for other work. Dismantled plywood sheets can be used again to construct a foundation at another facility.
Permissible deviations
The hardened mass can be processed for roughness, insulated, covered with waterproofing, but it will no longer be possible to eliminate geometric deviations (especially towards a smaller thickness). A mismatch along the axes of load-bearing walls can stop all construction or require new fundamental design solutions.
SNiPs establish permissible deviations for formwork:
The frame, assembled from any number of parts (a solid plane of plywood or a set of boards 0.1 m wide), of different heights and lengths of a straight section, is equipped with a sufficient number of stops; if necessary, metal squares are placed on the corners.
Basic information about strip foundations
Strip foundations can be used in the construction of houses with or without basements. Either way, you get significant savings when choosing this type of foundation.
When constructing a basement, it is advisable to build a foundation in a ready-made pit using double-sided formwork; in this case, the foundation will play the role of basement walls.
A simpler method is to pour the structure into specially prepared trenches. In this case, you can save on the cost of formwork, because it is enough to install only its outer part, which, when using modern materials (formwork using extruded foam), will also become a reliable element of insulation and waterproofing.
The calculation of foundation parameters must be carried out by a civil engineer who will be able to calculate all design parameters depending on the condition of the soil, the presence of groundwater and other factors.
The entire technology of pouring a strip foundation can be divided into several stages:
- Preparatory and excavation work
- Preparing the base, installing formwork
- Manufacturing of reinforcing frame
- Pouring concrete mixture
Let's look at all these stages in more detail.
Preparatory and excavation work
The main issue of preparation is the calculation and purchase of necessary materials. If you decide to prepare the concrete mixture yourself, which will help somewhat reduce the cost of pouring a strip foundation, but will significantly increase the time of its construction, you need to bring the required amount of crushed stone, sand, and cement to the construction site.
In addition, before starting work, it is necessary to purchase formwork material and reinforcement; all this can be imported during the excavation process. In addition, to prepare concrete, you need to make sure you have a concrete mixer, which, in extreme cases, can be rented. But we would still recommend using ready-made concrete mixtures, which will greatly simplify the work.
Construction of a clay castle
As already mentioned, concrete can be laid either in trenches or in a foundation pit. In addition to the fact that constructing a pit will simplify the work on arranging the basement, in this case it is possible to arrange additional waterproofing of the building by installing a clay castle over the entire area, which will significantly reduce the access of groundwater to the building.
When performing excavation work, it is necessary to constantly monitor elevations; this will be the basis for the stability of the entire building.
In addition, trenches must be monitored along the axes of the building. To do this, they resort to the so-called cast-off, it is performed as follows. 2 m are retreated from the lines of the future walls, supports are mounted to which the boards are attached. Thus, we obtain a structure that follows the entire contour of the building, which facilitates excavation work.
Foundation stripping
Possible mistakes
If the formwork is made incorrectly, errors are made in its installation, or it is poorly fixed, the strength of the foundation deteriorates. In severe cases, filling has to be started all over again. Therefore, it is important to initially correctly establish the foundation form and secure it well.
Protrusion of shields during pouring
If the formwork swells during the pouring stage, work is stopped immediately. In the problem area, concrete is removed with shovels. To prevent it from flowing back, cross boards are nailed inside the mold along its entire width. The concrete is thrown at a distance of 1.5 m from the accident site.
Important!
The concrete is thrown away very quickly. Otherwise, it is impossible to correct the error. Therefore, several people work at once.
After the pressure on the shields is reduced, they are gradually straightened. For convenience, use a jack and wooden beams. You cannot hit the formwork with a hammer. Vibration causes the concrete solution to “float” strongly and further disrupt the geometry of the structure. The shield is leveled gradually. Fastenings or wooden boards break due to too sudden movements. After leveling the formwork, it is immediately fixed in the correct position and the reliability of the supports is carefully checked.
In some cases, the formwork does not stick out easily, but falls during pouring. Then the spilled concrete is removed, the site is cleared and all the work begins again. The foundation form is carefully secured again with a large number of braces and studs so that it does not fall apart.
The shift appeared the next day
If the bulge does not appear immediately, the concrete is allowed to harden completely. Only after final hardening are the wooden panels removed. Then the resulting mound is knocked down with a puncher. You cannot remove the formwork until the concrete mass has completely hardened and remove the convexity with a shovel. Such actions provoke the appearance of cracks in other parts of the base. As a result, its strength is greatly reduced.
When building a strip foundation, proper assembly and installation of formwork is very important. All work on its installation is carried out especially carefully and carefully, strictly following the technology and not skipping a single stage. The strength of the foundation largely depends on the securely assembled structure.
Types and types of strip foundation
Concrete strip foundations are divided into 3 groups by type:
- Not buried. It is located at ground level on a bed of crushed stone and sand.
- Shallow. Designed for panel, frame, foam block or wooden houses. Depth of burial – 50-70 cm.
- Recessed. Used for houses with basements, buildings with heavy weight, and in areas with heaving soil. The laying depth is below 50 cm of the soil freezing level.
The buried option is rarely used - the amount of concrete for the foundation is required very large. In places with cold climates, the laying depth can be more than 2 m.
When deciding which type to choose for construction, pay attention to 4 indicators:
- the load that the base will experience;
- required height of the base;
- ground water level;
- depth of soil freezing.
By type, concrete strip foundations are divided into prefabricated and monolithic. The first consists of individual elements produced in a factory, from which the structure of the desired configuration is assembled. With the same thickness as a monolithic one, it is inferior in strength by almost a third. Among the advantages of this option are the absence of the need to independently calculate the amount of concrete for a strip foundation, ease of assembly and the ability to install blocks at intervals to save money.
Monolithic is made directly at the construction site: cement mortar is mixed and the strip foundation formwork is concreted. The construction of this option is more labor-intensive, but it is stronger than the prefabricated one and is cheaper.
If you have a choice between different types of foundation, you can determine which will be better by comparing the characteristics and features of each
So, when choosing between a strip foundation or a monolithic slab, several parameters are taken into account:
- Soil type. For weak and non-heaving soils, tape is optimally suited. On heaving areas, it is more advisable to use a stove.
- Time. Installation of the slab is possible at any time of the year; after installation, construction can continue. It is necessary to pour a strip foundation at above-zero temperatures, after which you need to wait until the concrete gains strength.
- Life time. The service life of the slab declared by the manufacturers is up to 100 years, but it can be shortened due to high load, unfavorable operating conditions and under the influence of other factors. A prefabricated strip foundation retains its integrity for an average of 50-75 years, a monolithic one - up to 150.
- Budget. What will cost less – a strip foundation or a monolithic slab? The cost of the tape is several times lower than the slab.
It is impossible to unequivocally answer which is better - a strip foundation or a monolithic slab. To build a house on heaving soils in a short time, the second option would be preferable. On slightly heaving soil, tape is used, which is cheaper and easier, since installation does not require special equipment.
The construction process consists of 5 main stages - preparatory work, installation of formwork, reinforcement and concreting and maintenance of concrete
In order for the resulting structure to have the necessary strength, it is important to know what grade of concrete can be used for a strip foundation, as well as to follow the construction technology
Filling in layers
It is important to calculate this process according to plan. If the foundation depth is 1.5 meters, it is enough to divide the pouring into three stages of 50 cm each
Follow these instructions:
- knit reinforcement for the entire volume of the formwork;
- pour concrete along the height;
Pouring the foundation in layers
- after 7 hours, remove the top layer of cement laitance on the surface. It should be removed, since when it hardens, this layer becomes very fragile. After removal, the surface will become rough, which will increase adhesion to the next layer;
- after three days, raise the structure and place it at a higher level. Leave the pipes in concrete;
- Refill the exposed formwork.
Installation of wooden formwork under a strip base on stable soil
To install such a structure, use an edged board 3-4 cm thick and 10-15 cm wide, you can also use plywood 0.5-1.5 cm thick.
Wood formwork assembly process
Do-it-yourself installation of this structure is done in this way: first, guide boards are placed in the pit on the ground side and fixed with pegs driven into the ground, then the shields are attached so that their ends coincide with the edges of the guide boards. The shields must be positioned using a level or plumb line. The panels are secured with wooden slopes from the inside of the formwork.
When assembling this structure with your own hands, you must take into account that a similar structure will be on the other side, so you need to strictly adhere to the dimensions of the foundation tape. Such actions will make it possible to avoid gaps at the joints. To maintain the same width of the tape around the entire perimeter, spacers must be used in increments of 0.5 m.
There is a second option for installing formwork in stable soil. It involves the construction of the specified building structure only for the ground part of the foundation. In this embodiment, the underground part of the form for pouring the foundation will be the walls of the trench, which must be covered with waterproofing material
It is important that they are dug strictly in compliance with the vertical level
Shields shaped like a base are fixed along the edges of the trench using braces made from the remains of boards. The braces can be nailed to wooden stakes or driven into the soil at a distance of at least 1 m from the formwork panels. They are attached to the shields using self-tapping screws in the places where the transverse bars are located. When fixing the shields, you need to adhere to the level so that the shields do not deviate vertically. To control the degree of deviation, a level or rule is used. The deviation of the boards from the vertical is allowed no more than 0.5 cm per 1 m of its height.
Watch a video showing how to use a permanent wooden structure.
After installing this structure, its inner walls are covered with waterproofing material so that an overlap is formed at the bottom of the trench. It will prevent the waterproofing material from breaking when pouring the concrete mixture.
Purpose
Formwork for a strip or monolithic foundation is a form into which concrete mortar is poured. It is made in the form of a shield from wooden boards or plywood, materials with the addition of polymers. As a result, a container is formed, the shape of which coincides with the concrete strip or monolithic base. The panel structure is erected at the casting site according to the dimensions indicated on the working drawings.
Traditionally, the structure is built from edged boards. More convenient is special formwork made of polymer materials. It is intended for reusable use. To install the formwork structure, durable material is used so that it can withstand the pressure that occurs when the cement mortar hardens. The form must be airtight so that the solution does not leak out through the cracks.
The procedure for reinforcing a strip foundation
The existing scheme for reinforcing a strip foundation for building a house requires following several mandatory rules:
- Application when carrying out reinforcement of rods of class A400
- Minimizing the use of welding when connecting rods, as this technology helps to weaken the sections
- At the corners, the frame can only be tied; the use of welding at the corners is not recommended
- The protective layer of concrete used in a particular situation must be at least 4 centimeters to protect metal elements from the negative effects of the environment and corrosion
- The use of smooth reinforcement is not recommended
- During laying, concrete must not be able to get stuck between the rods, which will eliminate control over failure due to too frequent placement of metal rods
Detailed step-by-step instructions with photos and videos provided for clarity will help you create a reinforcement frame. A significant advantage of reinforced strip foundations is the combination of high reliability and affordable cost. Steel and concrete are high-strength materials.
This is important for creating a foundation for construction on any soil, except for naturally stable and reliable rocks. In other situations, reinforcement of any foundation will eliminate damage caused by stress
How is it formed
When carrying out work such as reinforcing a strip foundation, the drawings include three groups of rods:
- Working rods used for laying along the belt
- Horizontal elements arranged transversely
- Vertical options, transverse
The task of transverse reinforcement is to connect all working elements into a single unit of reliable, innovative working rods that is stable in use. They are often called clamps.
An important feature of the work is the use of SNP and other regulatory specialized documents for activities such as reinforcement of strip foundations. When calculating, SNiP 52-01-2203 is used
In this regulatory document it is easy to find all the necessary calculations for creating reinforcement for the strip foundation of a small country house.
What requirements for concrete are determined by regulatory documents?
If you follow the order of creating reinforcement, it is important to comply with the mandatory requirements for the concrete used in a particular work. When creating a strip foundation with your own hands at the site of future construction, it is worth considering that the main characteristics of the strength of concrete structures include resistance to axial compression, readiness to withstand tension and not respond to transverse fracture
Reliability correction factors can vary from 1 to 1.5
When creating a strip foundation with your own hands at the site of future construction, it is worth considering that the main characteristics of the strength of concrete structures include resistance to axial compression, readiness to withstand tension and not respond to transverse fracture. Reliability correction factors can vary from 1 to 1.5.
