Laying the foundation has long been traditional in the construction of any building, it ensures its stability, reliability, protects the building from unforeseen soil displacements. The fulfillment of these functions concerns, first of all, the correct installation of the foundation, in compliance with all possible nuances. This also applies to the correct use of reinforcing elements in the structure of a reinforced concrete foundation, so today we will try to reveal all the subtleties of the selection and installation of reinforcement for the foundation.
Every builder understands that ordinary concrete without special reinforcing elements is not strong enough in its structure - especially when it comes to large loads from large-sized buildings. The base plate has a dual role of containing loads: 1) from above - from the building or structure and all the elements inside it; 2) from below - from soil and soil, which under certain conditions can change their volumes - an example of this is the heaving of the soil due to the low level of freezing of the soil.
Concrete itself is capable of absorbing enormous compressive loads, but when it comes to tension , it clearly needs additional reinforcing or fixing structures. In order to avoid serious damage to the structure and increase its service life, developers have long ago developed a type of laying a reinforced concrete foundation, or laying concrete together with reinforcing elements.
The most obvious plus in laying a foundation with reinforcing elements is its strength. Iron, steel or fiberglass (we will look at the types below) provides additional reliability and integrity for the entire installation, reinforcement fixes concrete in a given position, evenly distributes the load and pressure to the entire base.
A separate minus of the use of reinforcing parts is that the foundations of this type are installed much longer , their installation is more complicated, more equipment is required, more stages of preparation of the territory and more workers. Not to mention the fact that the selection and installation of reinforcing elements has its own codes of rules and norms. However, it is difficult to talk about the minuses, since almost no one now uses the foundation without reinforcing parts.
The general parameters on which the technician should rely when choosing fittings are:
- the potential weight of the building with all add-ons, frame systems, furniture, appliances, basement or attic floors, even with a load of snow;
- type of foundation - reinforcing elements are installed in almost all types of foundation (it is monolithic, pile, shallow), however, when installing a reinforced concrete foundation, it is most often understood as a strip variety;
- the specifics of the external environment: average temperature values, level of soil freezing, soil heaving, groundwater level;
- type of soil rocks (the type of reinforcement, as well as the type of foundation, is highly dependent on the composition of the soil, the most common are loam, clay and sandy loam).
As you may have noticed, the choice of reinforcement for the foundation is subject to the same external influences as the foundation itself, and therefore must take into account all the rules and regulations for installation.
As already mentioned, the installation of reinforcement in a reinforced concrete foundation is regulated by a separate set of rules. Technicians use the rules edited by SNiP 52-01-2003 or SP 63.13330.2012 under paragraphs 6.2 and 11.2, SP 50-101-2004, some information can be found in GOST 5781-82 * (if we are talking about using steel as a reinforcing element). These codes of practice can be difficult for the novice builder to perceive (taking into account weldability, ductility, corrosion resistance), however, adherence to them is the key to the successful construction of any building. In any case, even when hiring specialized workers to work at your facility, the latter should be guided by these standards.
Unfortunately, only the basic requirements for foundation reinforcement can be distinguished:
- working rods (which will be discussed below) should be at least 12 millimeters in diameter;
- as for the number of working / longitudinal rods in the frame itself, the recommended figure is from 4 or more;
- relative to the pitch of the transverse reinforcement - from 20 to 60 cm, while the transverse rods should be at least 6-8 millimeters in diameter;
- reinforcement of potentially dangerous and vulnerable to stress places in the reinforcement occurs through the use of foots and paws, clamps, hooks (the diameter of the last elements is calculated based on the diameter of the rods themselves).
Choosing the right fittings specifically for your building is not an easy task. The most obvious parameters for choosing reinforcement for the foundation are the type, class, and also the grade of steel (if it is specifically about steel structures). There are several varieties of reinforcing elements for the foundation on the market, depending on the composition and purpose, profile shape, manufacturing technology, and features of the foundation load.
If we talk about the types of reinforcement for the foundation based on the composition and physical properties, then there are metal (or steel) and fiberglass reinforcement elements. The first type is most common, it is considered more reliable, inexpensive and tested by more than one generation of technicians. However, nowadays, reinforcing elements made of fiberglass are also increasingly common, they appeared in mass production not so long ago, and many technicians still do not risk using this material in the installation of large-sized buildings.
There are only three types of steel reinforcement for the foundation:
- hot rolled (or A);
- cold-deformed (VR);
- cable car (K).
When installing the foundation, it is the first type that is used, it is durable, elastic, and resistant to deformation. The second type, which some developers like to call wire, is cheaper and is used only in individual cases (usually - reinforcement of strength class 500 MPa). The third type has too high strength characteristics, its use in the base of the foundation is impractical: both economically and technically costly.
What are the advantages of steel structures:
- high reliability (sometimes low-alloy steel with exceptionally high rates of rigidity and strength is used as reinforcement);
- resistance to huge loads, the ability to contain enormous pressure;
- electrical conductivity - this function is rarely used, however, with the help of it an experienced technician will be able to provide a concrete structure with high-quality heat for a long time;
- if welding is used in the connection of the steel frame, the strength and integrity of the entire structure does not change.
The individual disadvantages of steel, as a material for reinforcement:
- high thermal conductivity and, as a result, reinforced concrete foundations transmit heat more in buildings, which is not very good in residential premises at low external temperatures;
- susceptibility of the material to corrosion (this item is the largest “scourge” of large buildings, the developer can additionally treat steel from rust, but such methods are very economically disadvantageous, and the result is not always justified due to differences in loads and the influence of moisture);
- large total and specific gravity, which makes it difficult to install rolled steel without specialized equipment.
We will try to figure out what are the advantages and disadvantages of fiberglass reinforcement. So, the advantages:
- fiberglass is much lighter than steel analogues, therefore, it is easier to transport and easier to install (sometimes it does not require special equipment for laying);
- the absolute strength limits of fiberglass are not as great as those of steel structures, however, high specific strengths make this material suitable for installation in the foundations of relatively small buildings;
- not susceptible to corrosion (rust) makes fiberglass to some extent a unique material in the construction of buildings (the most durable steel elements often need additional processing to increase service life, fiberglass does not require these measures);
- if steel (metal) structures are by their nature excellent electrical conductors and cannot be used in the production of energy enterprises, then fiberglass is an excellent dielectric (that is, it does not conduct electrical charges well);
- fiberglass (or a bundle of fiberglass and a binder) was developed as a cheaper analogue to steel models, even regardless of the cross section, the price of fiberglass reinforcement is much lower than steel elements;
- low thermal conductivity makes fiberglass an indispensable material in the manufacture of foundations and floors to maintain a stable temperature inside the object;
- the design of some alternative types of fittings allows you to install them even under water, this is due to the high chemical resistance of the materials.
Of course, there are some drawbacks to using this material:
- fragility is in some way the hallmark of fiberglass, as already mentioned, in comparison with steel, the strength and stiffness indicators here are not so great, this repels many developers from using this material;
- without additional treatment with a protective coating, fiberglass reinforcement is extremely unstable to abrasion, wear (and, since the reinforcement is placed in concrete, it is impossible to avoid these processes under loads and high pressure);
- high thermal stability is considered one of the advantages of fiberglass, however, the binder in this case is extremely unstable and even dangerous (in the event of a fire, fiberglass rods can simply melt, so you can’t use this material in a foundation with potentially high temperature values), but this makes fiberglass completely safe to use in the construction of ordinary residential premises, small buildings;
- low values of elasticity (or the ability to bend) make fiberglass an indispensable material in the installation of some individual types of foundations with low pressure, however, again - this parameter is rather a minus for the foundations of buildings with high loads;
- poor resistance to certain types of alkalis, which can lead to the destruction of the rods;
- if welding can be used to connect steel, then fiberglass due to its chemical properties cannot be connected in this way (it is a problem or not - it is definitely difficult to solve, since even metal frames are more likely to be knitted than welded.
If you approach the varieties of reinforcement in more detail, then in the section it can be divided into round and square types. If we are talking about the square type, then it is used in construction much less often, it is applicable when installing angular supports and creating complex fence structures. The corners of the square type reinforcement can be either sharp or softened, and the side of the square varies from 5 to 200 millimeters depending on the loads, the type of foundation and the purpose of the building.
The round type of reinforcement is smooth and corrugated type. The first type is more versatile and is used in completely different areas of construction production, but the second type is common when installing the foundations, and this is understandable - the reinforcement with sequential corrugation is more adapted to heavy loads and fixes the foundation in the initial position even in case of overpressure.
The corrugated type can be divided into four varieties:
- the working type performs the function of fixing the foundation under external loads, and also take care to prevent the formation of chips and cracks in the foundation;
- the distribution type also performs the function of fixing, but of working reinforcing elements;
- the mounting type is more specific and necessary only at the stage of connection and fastening of the metal frame, it is necessary for the distribution of reinforcing bars in the correct position;
- the clamps, in fact, do not perform any function, except for a bundle of reinforcing parts as a whole, for subsequent placement in trenches and pouring concrete.
There is a classification of corrugated products and by type of profile: annular, sickle-shaped, mixed or combined. Each of these types is applicable in specific conditions of the load on the foundation.
The main parameter for choosing reinforcement for the foundation is its diameter or section. Such a value as the length or height of the reinforcement is rarely used in construction, these values are individual for each building and each technician has his own resources in the construction of the building. Not to mention the fact that some manufacturers ignore the generally accepted standards for the length of reinforcing bars and tend to produce their models. Foundation reinforcement is of two types: longitudinal and transverse. Cross-sections can vary greatly depending on the type of foundation and load.
Longitudinal reinforcement usually involves the use of ribbed reinforcing elements, for the reinforcement of the transverse type - smooth (section in this case is 6-14 mm) classes A-I - A-III.
If you are guided by normative codes of rules, you can determine the minimum diameter values of individual elements:
- longitudinal rods up to 3 meters - 10 millimeters;
- longitudinal from 3 and more meters - 12 millimeters;
- cross rods up to 80 centimeters high - 6 millimeters;
- cross rods from 80 and more centimeters - 8 millimeters.
As already noted, these are only the minimum permissible values for the foundation reinforcement and these values are permissible rather for the traditional type of reinforcement - for steel-type structures. In addition, do not forget that any issue in the construction of buildings, and especially in the construction of non-standard types of facilities with a previously unknown potential load, must be decided individually based on the rules of SNiP and GOST. Calculating the next value yourself is quite difficult, but this is also a recognized standard - the diameter of the iron frame should not be less than 0.1% of the cross section of the entire foundation (this is only the minimum percentage).
If we are talking about construction in areas with unstable soil (where the installation of brick, reinforced concrete or stone structures is unsafe because of their large total weight), then rods with a cross section of 14 mm or more are used. For smaller buildings, a conventional reinforcement cage is used, however, do not treat the process of laying the foundation connivingly even in this case - remember, even the largest diameter / section will not save the integrity of the foundation with an incorrect reinforcement scheme.
Of course, there are certain schemes for calculating the diameter of the rods, however, this is a "utopian" version of the calculation, since there is no single scheme that combines all the nuances of building individual buildings. Each building has its own unique characteristics.
Once again, it is worth making a reservation - there is no universal scheme for installing reinforcing elements of the foundation. The most accurate data and calculations that you can find are just individual sketches for individual and most often typical buildings. Based on these schemes, you risk the reliability of the entire foundation. Even the norms and rules of SNiP can not always be applied to the construction of a building. Therefore, only individual, general recommendations and subtleties on reinforcement can be distinguished.
We return to the longitudinal rods in the reinforcement (most often it is class AIII reinforcement). They should be placed in the upper and lower parts of the foundation (regardless of its type). This arrangement is understandable - the foundation will perceive most of the loads from above and below - from soil rocks and from the building itself. The developer has the full right to install additional tiers to strengthen the entire structure, but keep in mind that this method is applicable for large foundations of large thickness and should not violate the integrity of other reinforcement elements and the solidity of concrete itself. Without taking into account these recommendations, cracks and chips will gradually appear in the places of fastening / connection of the foundation.
Since the foundation of medium and large buildings usually exceeds a thickness of 15 centimeters, it is also necessary to install vertical / transverse reinforcement (smooth AI rods are already used here more often, their allowable diameter was mentioned earlier). The main purpose of the transverse reinforcement elements is to prevent the formation of foundation damage and to fix the working / longitudinal rods in the right position. Very often, cross-type reinforcement is used to produce frames / forms in which longitudinal elements are placed.
If we talk about laying a strip foundation (and we have already noticed that it is for this type of reinforcing elements that are most often applicable), then the distance between the longitudinal and transverse reinforcement elements can be calculated based on SNiP 52-01-2003.
If you follow these recommendations, then the minimum distance between the rods is determined by such parameters as:
- section of reinforcement or its diameter;
- concrete aggregate size;
- type of reinforced concrete element;
- placement of reinforced parts to the direction of concreting;
- concrete pouring method and its compression.
And, of course, the distance between the reinforcing bars themselves already in the bundle of the metal frame (if we are talking about a steel skeleton) should be not less than the diameter of the reinforcement itself - 25 or more millimeters. The distance between the longitudinal and transverse types of reinforcement has its own schematic requirements.
Longitudinal type: the distance is determined taking into account the variety of the reinforced concrete element itself (that is, the basis of which particular object is used for longitudinal reinforcement - column, wall, beam), typical values of the element. The distance should be no more than twice the height of the section of the object and be up to 400 mm (if the objects are of a linear mundane type - no more than 500). The limited values are understandable: the greater the distance between the transverse elements, the more loads are on the individual elements and concrete between them.
The step of the transverse reinforcement should not be less than half the height of the concrete element, but not be more than 30 cm. This is also understandable: the value is less when installed on problematic soils or at a high level of freezing, it will not have a significant effect on the strength of the foundation, the value is more possible, however, applicable to large buildings and structures.
Among other things, to install a strip foundation, do not forget that the reinforcing bars stand 5-8 cm above the concrete pouring level - for fixing and connecting the foundation itself.
How to calculate?
Some recommendations for the calculation of reinforcement have already been presented above. At this point, we will try to delve into the intricacies of the selection of fittings and will rely on more or less accurate data for the installation. Below will be described a method of self-calculation of reinforcing elements for the foundation of the strip type.
Self-calculation of reinforcement, subject to some recommendations, is quite simple. As already mentioned, corrugated rods are selected for horizontal foundation elements, and smooth for vertical ones. The very first question, in addition to measuring the required diameter of the reinforcement, is the calculation of the number of rods for your territory. This is an important point - it is necessary when purchasing or ordering materials and will allow you to draw up an accurate layout of the placement of reinforcing elements on paper - up to centimeters and millimeters. Remember one more simple thing - the larger the dimensions of the building or the load exerted on the foundation, the more reinforcing elements and the thicker the metal rods.
The consumption of the number of reinforcing elements per individual cubic meter of reinforced concrete structure is calculated based on the same parameters that are used to select the type of foundation. It is worth noting that few people are guided specifically by GOST in the construction of buildings, for this there are specially developed and narrowly targeted documents - GESN (State Elementary Estimated Norms) and FER (Federal Unit Prices). According to the GESN for at least 5 cubic meters of the foundation structure, at least one ton of metal frame should be used, while the latter should be evenly distributed over the foundation. FER is a collection of more accurate data, where the amount is calculated not only based on the area of the structure, but also on the presence of grooves, holes and other extras. elements in the design.
The required number of reinforcing bars for frames is calculated based on the following steps:
- measure the perimeter of your building / object (in meters), for the operation of which it is planned to lay the foundation;
- add the parameters of the walls under which the base will be located to the received data;
- the calculated parameters are multiplied by the number of longitudinal elements in the building;
- the resulting number (total base value) is multiplied by 0.5, the result will be the required amount of reinforcement in your area.
We advise you to add about 15% to the resulting number, during the laying of the strip foundation, just such an amount will be enough (taking into account the scraps and overlaps of reinforcing bars).
As already mentioned, the diameter of the iron frame should not be less than 0.1% of the cross section of the entire reinforced concrete base. The cross-sectional area of the base is calculated based on the multiplication of its width by height. A base width of 50 centimeters and a height of 150 centimeters form a cross-sectional area of 7500 square centimeters, which equals 7.5 cm of the reinforcement section.
When following the previously described recommendations, you can safely proceed to the next stage of the installation of reinforcing elements - installation or fastening, as well as related actions. For a beginner technician, creating a framework can seem like a useless and energy-consuming task. The main purpose of the frame under construction is to distribute the loads on individual reinforcing parts and fix the reinforcing elements in the primary position (if the load on one bar can lead to its displacement, then the load on the frame, which includes 4 corrugated rods, will be much less).
Recently, one can see the fastening of reinforcing metal rods through electric welding. This is a quick and natural process that does not violate the integrity of the frame. Welding is applicable at large depths of laying the foundation. But this type of fastening has its own minus - not all reinforcing elements are suitable for cooking them. If the rods are suitable, the letter “C” is present in their marking. This is a problem for the fiberglass frame and other reinforcing materials (less well-known like some types of polymers). In addition, if a power-based frame is used in the foundation, then the latter at the attachment points should have relative freedom of displacement. Welding limits these necessary processes.
Another way of attaching rods (both metal and composite) is wire knitting or strapping. It is used by technicians with a concrete slab height of not more than 60 centimeters. It involves only some types of technical wire. The wire is more plastic, it provides freedom of natural displacement, which is not the case with welding. But the wire is more susceptible to corrosion processes and do not forget that you need a quality wire to purchase - this is an additional cost.
The last and least common method of fastening is the use of plastic clamps, however, they are applicable only in individual projects of not especially large buildings. If you are going to knit the frame with your hands, then in this case it is recommended to use a special (knitting or screw) hook or ordinary pliers (in rare cases, a knitting gun is used). The rods should be connected at the place of their crossing, the wire diameter in this case should be at least 0.8 mm. In this case, knitting takes place immediately with two layers of wire. The total thickness of the wire already at the crossing can vary depending on the type of foundation and loads. The ends of the wire must be connected to each other at the final stage of fastening.
Depending on the type of foundation, the characteristics of the reinforcement may also change. If we talk about the foundation on bored piles, then ribbed reinforcement with a diameter of about 10 mm is used here. The number of rods in this case depends on the diameter of the pile itself (if a cross section of up to 20 centimeters is sufficient to use a metal frame with 4 rods). If we are talking about a monolithic tile foundation (one of the most resource-consuming types), then here the diameter of the reinforcement is from 10 to 16 mm, and the upper reinforcing belts should be placed so that the so-called grids of 20/20 cm are formed.
It is worth saying a few words about the protective layer of concrete - this is the distance that protects the reinforcing bars from the effects of the external environment and provides the entire structure with additional strength. The protective layer is a kind of cover that protects the overall structure from damage.
If you follow the recommendations of SNiP, then a protective layer is necessary for:
- creating favorable conditions for the joint functioning of concrete and reinforcing skeleton;
- proper strengthening and fixing of the frame;
- additional protection of steel from the negative effects of the external environment (temperature, deformation, corrosion effects).
According to the requirements, metal rods must be completely placed in concrete without protruding individual ends and parts, so that the installation of the protective layer is, to some extent, regulated by SNiP.
Do not be alarmed at the sight of our recommendations. Do not forget that the correct installation of the foundation without outside help is the result of more than one year of practice. It is better to make a mistake once, even following the indicated standards, and know how to do something the next time, than to constantly make mistakes, relying only on the advice of your acquaintances and friends.
Do not forget about the help of normative documents of SNiP and GOST, their initial study may seem difficult and incomprehensible to you, however, when you get used to installing reinforcement for the foundation, you will find these manuals useful and you can use them at home with a cup of tea or coffee. If any of the points is too complicated for you - do not hesitate to contact specialized support services, specialists will help you with accurate calculations and drawing up all the necessary schemes.
For how to quickly knit reinforcement for the foundation, see the next video.