First, we clear the space where high-quality formwork will be placed. It would be best to use wooden frame elements for it, since several transverse rods will be driven into the walls. We use boards up to 2 centimeters thick or OSB sheets with a thickness of 10 mm or more. The optimal thickness is 15 millimeters to make fewer supports. Let's look at the detailed instructions on how to assemble it.
Step 1 put up wooden boards.
Place wooden panels or OSB sheets opposite each other and fasten them on top with several transverse planks, insert several wooden blocks 30 centimeters long inside between the sheets and screw them with screws to each side from the outside.
Step 2 install oblique supports every 50 centimeters in a checkerboard pattern.
They can be made from wooden blocks of 50x50 millimeters, where the bottom is left with an even cut, and the top is cut at an angle of 45 degrees and screwed to the wall with a screw.
Step 3 Make 12 mm holes with an electric drill at 30, 70 and 120 centimeters from the floor.
Do it every 2 meters along the entire perimeter of the sheathing so that you can freely drive cross-sectional reinforcement there.
Now the work on creating the sheathing can be considered complete. Use a level while working.
The corners of the formwork should be as even as possible, since they will be reinforced the most - distortions must not be allowed.
Cutting the reinforcement and choosing a frame diagram
First, let's decide which scheme is suitable for the home. There are several options. The most common types: mesh (equilateral cells around the entire perimeter), parallel formation, parallel laying with corner reinforcements and chaotic throwing of metal (a typical look for beginners). We will choose the simplest and most effective method - strengthening the corners while laying longitudinal reinforcement in parallel, since during subsidence it is the pressure on the fracture that is dangerous, and the crossbars play the role of protection against soil pressure from the sides. Let's look at how to do it.
Step 1 hammer the verticals in the corners.
We hammer in verticals in the corners and every 120 centimeters around the entire perimeter. We take reinforcement 8-10 mm thick (you don’t need it too strong, it’s only for tying) and hammer 2 pieces into each corner. The distance between them is 20 centimeters and 5 centimeters from each wall of the sheathing. That is, as much as possible in the center.
Step 2 We insert 30 centimeter long twigs into the pre-made holes.
The thickness of the reinforcement is also approximately 10 millimeters - it will not have any tensile strength here. We immediately fill in the 3 rows that we drilled using an electric drill. It is better to use durable chrome steel that does not corrode, but the cost of the product will be 2-3 times higher.
Step 3 lay parallel rods.
Here it is necessary to think about strength, because the longitudinal parallel reinforcement will take the entire “impact” of the structure as a result of subsidence. The diameter of the rod should be 14-16 millimeters. Too large a diameter is also not needed, since we need to create a flexible base, and not a strong and massive one - this is the role of concrete.
Step 4 the reinforcement frame for the foundation is fastened into one system.
It is necessary to take a well-knitted wire, preferably copper, with which you can wrap all the connections without much effort. It is important to understand that it is not necessary to tighten it, weld it together and perform other operations to strengthen the joints. They do not play any role; they simply tie the reinforcement together so that it lies in its place and does not move when concrete is poured. Tying reinforcement for the foundation may not be necessary at all if everything is poured carefully and the elements are not moved from their place. But to be sure, it would be better to fasten everything together.
Now we have fully covered how to knit reinforcement for the foundation, but there is another important point: reinforcing the corners. If the room has a large mass (a two-story house or more), then the heaviest load falls on the corners. It is necessary to increase the number of parallel reinforcements precisely at the corners (at a distance of 2 meters) by 2 times. And make oblique connections at an angle of 45 degrees between three tiers of reinforcement (30, 70 and 120 centimeters from the floor).
When making such a structure, a novice craftsman can make many mistakes, which will lead to serious problems with the future structure or expensive foundation repairs. Now we will look at how to avoid the most “popular” mistakes and make a high-quality foundation.
- Do not place reinforcement too close to the formwork walls. According to SNIP, foundation reinforcement is carried out in such a way that the metal is buried at least 5 centimeters into the concrete on all sides of the structure.
- Use cement grade M 400 and higher, proportion 3:1 with sand, not weaker. Concrete that is too loose will be exposed to moisture and building pressure. It may crumble at the edges or crack, despite high-quality reinforcement.
Fittings
This is a traditional material for creating the frame of foundation structures. It is used in strip foundations, pile foundations, column foundations and prefabricated reinforced concrete foundations. Class AI reinforcement is used to create a reinforcement frame in the foundations for “light” houses: frame, wooden, less often for houses made of foam concrete and aerated concrete, and other lightweight structures. This is due to the fact that AI has a circular cross-section in cross-section and a smooth surface. Because of this, its adhesion to concrete decreases. The diameter of the reinforcement used is 6 mm and above. Calculated at the house design stage.
Class AIII fittings are made of alloyed and high-quality steel with increased strength characteristics. It is ribbed in cross section. The surface pattern can be of several types: ring, crescent or mixed. Reinforcement with a ring profile is designed for massive structures due to its high adhesion to concrete. Reinforcement with a crescent pattern can be used in structures subject to tensile loads. For tensile loads, reinforcement with a larger diameter is used.
The diameter of the reinforcement is calculated at the design stage. For “light” houses with relatively low weight, reinforcement with a diameter of 8-10 mm, or mounting 6 mm, is allowed. In houses with heavy walls, reinforcement with a diameter of 12-14 mm is used to reinforce the foundations.
Non-carbon steel reinforcement can be welded when preparing the foundation frame. Welding of carbon steel reinforcement is not recommended. The weld area will be fragile. It is better to knit the reinforcement with tying wire.
The fittings are produced in coils and in the form of rods. Reinforcement with a diameter of 6-10 mm is supplied in coils. Large diameter fittings are supplied in rods. Standard rod lengths: 6 m, 9 m, 11.7 m. The maximum diameter of the manufactured reinforcement is 32 mm.
Composite reinforcement
Its main advantage is its significant corrosion resistance and inertness to any aggressive environment. Such characteristics make it possible to reduce the diameter of the reinforcement when reinforcing foundations without reducing the strength of the structure as a whole. The use of reinforcement will make it possible to reduce the protective layers of concrete. The calculated and expected durability is assumed to be about 75 years. Based on materials, composite reinforcement is divided into: glass, basalt, and Kevlar.
The advantages of composite reinforcement: strength is 1.5 times higher than steel, is not subject to corrosion and is 3.0-3.5 times lighter than steel. It has dielectric properties and is radiotransparent. Frost-resistant. The coefficient of thermal expansion (CTE) of composite reinforcement is equal to the CTE of concrete. With a lot of advantages, there is one technical drawback - low heat resistance.
Composite reinforcement is more expensive than conventional reinforcement. But there will be a savings item on the diameter of the reinforcement used. To reduce costs, you can use a rubble concrete foundation, when coarse aggregate is added when pouring concrete: broken brick and rubble stone (savage), crushed stone and gravel.
Fiber concrete
A material that is gaining popularity when pouring foundations is reinforced concrete. The properties of concrete depend on the type of fibers. Fiber-reinforced concrete is used for pouring strip foundations and for complex reinforcement to improve the quality of the foundation.
To reinforce concrete, different types of fiber are added. Fiber is a waste product from the production of nails and cutting of steel wire for steel fiber. Basalt fiber is made from basalt fibers. Similarly, glass fiber and polymer fibers, in particular polypropylene, can be added to concrete. Textiles are added.
Adding fiber increases impact resistance by a fantastic 500% and abrasion resistance by 50%. In this case, 900 grams of propylene fiber per 1 cubic meter will be sufficient. m of concrete, or 20-50 kg of steel fiber per 1 cubic meter. m.
These additives are designed to perform the function of reinforcement in concrete, increasing crack resistance and resistance to deformation. Such qualities as frost resistance and water resistance are improved. Adding fiber to concrete lightens the weight of structures made from it. Along with fibers of various natures, binders and modifying additives are added to fiber-reinforced concrete. All together gives concrete completely new properties, which can significantly reduce construction time and save material consumption.
Personal production of a reinforced concrete foundation is the most important of all stages of construction. The required rigidity and strength are provided by embedded reinforcement, so today we will eliminate the gaps in understanding the functions of reinforcement and explain the methodology for calculating reinforcement for the foundation.
How does foundation reinforcement work?
Concrete has excellent compressive strength. This means that if a concrete block is placed under a press, it will only begin to collapse under very high pressure.
The realities of operating reinforced concrete products are such that it is impossible to accurately predict what forces will act at a single point in the array. This is because the configuration of a concrete product does not mean as much as the physical and mechanical characteristics of the base on which this product is installed. And they are almost always unpredictable.
The load in concrete is distributed unevenly. The maximum tension occurs at the fulcrum, and the rule of leverage always applies - the force increases in proportion to the leverage. If you hang a concrete beam from both edges, the impact on the center will directly depend on the length of the beam.
Scheme of beam operation in bending: a - concrete beam; b - reinforced concrete beam; 1 - fittings
Also interesting is the nature and direction of deformations at different points. When bending, one side will compress, but this, as we found out, does not promise big trouble. It is much worse that on the back side of the product the concrete will stretch, which, with low elasticity, will result in a crack and breakage.
The main task of reinforcement is to prevent concrete from stretching. This is achieved due to friction forces, which transfer the load from the concrete layer to embedded elements, which have an elastic modulus much higher than that of concrete. And, of course, the reinforcement should be distributed as evenly as possible so that each individual section of the structure does not have weak points with poor ligation. Otherwise, reinforcement loses all meaning.
How to strengthen the foundation
There are two types of fittings. Working reinforcement performs the direct function of reinforcement - it takes on the load in the applied plane. Structural reinforcement serves to organize the lines of working reinforcement in the concrete layer and obtain additional connections, if necessary.
Hot-rolled bars of periodic or smooth profile according to GOST 5781-82 are traditionally used as working reinforcement. Steel reinforcement can be welded or non-welded, depending on the thermomechanical reinforcement and the area of use.
For the foundation, it is advisable to use a periodic profile as working reinforcement, which has the highest adhesion to the surrounding mass. Auxiliary reinforcement, on the contrary, is performed with smooth rods, although this is not a categorical rule.
The material is also important; the steel grade determines the class of the reinforcement. The classes A400-A600 are most in demand for private developers: they are most widely used on construction bases and do not require special joining means: the entire frame is assembled with viscous material. Composite reinforcement (GOST 31938) made of plastic reinforced with carbon and fiberglass is increasingly used. Such reinforcement is much lighter than steel and is absolutely not subject to corrosion, but how important this is within the framework of a particular project is up to you to decide.
Basic reinforcement parameters
In each specific calculation there are a number of key values described in the manual for SNiP 2.03.01:
- Reinforcement packing density (reinforcement coefficient). It is determined from the cross section of the product as the ratio of the sum of the sections of the reinforcing bars to the section of the concrete mass. The minimum established by the standards is 0.05%, although the coefficient can increase as the ratio of the length of the segment to its height increases, up to 0.25%.
- Thickness of rods. For a segment length of more than 3 meters, reinforcement with a diameter of at least 12 mm is used, for more than 6 meters - over 14 mm, and for a length of 10 meters - 16 mm or more.
- Reinforcement distribution. If the foundation is about a meter deep, then which edge should be strengthened against tension: the top or the bottom? What is better - a small number of thick rods or many lines of thin reinforcement? In practice, all working reinforcement is often placed at one face, divided into as many rods as possible that do not interfere with the pouring of concrete. Then the same belt is duplicated at the opposite edge.
- The reliability coefficient (re-reinforcement) is a concept that directly follows from the previous paragraph. The strength of the foundation can be deliberately increased by 2 or 3 times in case of unforeseen changes in the geomorphology of the region or in the absence of a completed project at the time of construction.
The latter should be classified as an exception, but in practice almost half of individual housing construction projects are built this way. The problem is that without comprehensive design data, you do not have the opportunity to accurately determine the weight of the building, determine from it a sufficient area and depth corresponding to the supporting capacity of the soil, then, using standard proportions, calculate the linear characteristics of the foundation, and from them derive optimal methods for strengthening its structure , adequate to the design load.
Reinforcement configuration for NZLF, tape and slab
Strip foundations lying above the freezing depth are reinforced with a rectangular frame. An unlimited number of reinforcement lines can be located between the external ribs, between which the standard clearance must be maintained. As a rule, such frames consist of separately connected modules, the length of which is convenient for transportation and installation. Structural reinforcement here is represented by U-shaped or closed clamps encircling the working reinforcement bars every 0.6-1.1 meters.
Reinforcement of the straight section of the strip foundation: 1 - working longitudinal reinforcement; 2 - structural reinforcement (clamps)
Recessed foundations are strengthened like a strip - with a frame. The reinforcement lines, as mentioned, are duplicated and concentrated at the top and bottom edges. Additionally, intermediate lines can be laid to compensate for pressure forces and soil heaving, if required by the project. The reinforcement is connected to each other by vertical rods. This reinforcement looks structural, but it also performs a working function, significantly preventing torsional and lateral pressure deformations.
The slab is reinforced in the most simple way: two reinforcing mesh, each can consist of several layers. The meshes are spread to the upper and lower planes in accordance with the standard protective layer. The parameters of the reinforcing mesh are tabular; the rod and cell are calculated depending on the dimensions of the slab. As for the stiffening ribs under the slab, they are formed like the MZLF frames, and then fastened to the slab mesh with vertical rods of structural reinforcement.
Knitting, installation and control
With linear sections everything is simple, but the foundation has turns and intersections. On them, the lines of converging frames are connected by bent embedded elements made of reinforcement of the same section. The edges are installed with an overlap of 40 to almost 100 nominal diameters. It is a fairly common practice to strengthen foundation corners with 12x150x150 mm reinforcement mesh, especially on soft soils and in earthquake-prone regions.
Reinforcement of junctions and corners of strip foundations: 1 - working longitudinal reinforcement; 2 - transverse reinforcement; 3 - vertical reinforcement; 4 - L-shaped clamps
We have already described the advantages of tying reinforcement before welding and strongly recommend using only this method, unless we are talking about special-purpose foundations.
Each subsequent frame segment is installed on spacer pads or rings that prevent damage to the protective layers. The rods at the ends are tied with a standard overlap, 2-3 wire clamps at each joint.
As a result, the reinforcing frame must be formed in such a way that people can easily move around it. Before pouring, the frame is carefully checked for bond strength. If, when pouring concrete, the ligations of the lines diverge, this is fraught with complete rejection of the entire structure. Therefore, during pouring and shrinkage, special attention must be paid to the position and integrity of the reinforcement connections.
The foundation is the foundation of the building, so it must be reliable enough to support the weight of the structure. It can be different and is chosen based on the characteristics of the soil and other conditions. Reinforcing the foundation makes it more reliable and increases the service life of the entire structure. Before you start creating the foundation for your house, you need to correctly calculate the reinforcement. For this purpose, a foundation reinforcement scheme is drawn up.
How to position the fittings correctly
The main load acting on the foundation is supported by longitudinal reinforcement located in the lower and upper parts of the foundation. If the height of the base exceeds one and a half meters, smooth reinforcement rods are used, the diameter of which can be from 6 to 8 mm.
Longitudinal bars reduce the likelihood of cracks in concrete. In order for the frame to fulfill its functions, building codes must be taken into account during its creation.
According to SNiP, the distance between the longitudinal rods should be calculated based on the type of structure. It is worth noting that this distance should not be more than 400 mm. If these rules are neglected, the foundation may not be strong enough to support the weight of the structure.
Rules for reinforcing strip foundations
In order to correctly reinforce the foundation with your own hands, it is worth considering the main mistakes of beginners and learn about some recommendations that must be followed during work. This may affect the quality of the base and its service life. During operation, the following recommendations must be observed:
- If a strip foundation is created for a 1-2-story house, rods with a diameter of 10 to 24 mm are used. A smaller diameter is unacceptable, since when installing thin rods the foundation may be deformed.
- It is prohibited to connect rods by welding, since such a connection overheats the metal, causing it to become less tensile strength. The connection must be made using wire. Even a person who has never done such work before can make the connection.
- If the soil has a uniform density throughout the entire area, you can use reinforcement with a diameter of 10-14 mm. If the density is not uniform, rods with a thickness of 16 to 24 mm are used.
- You should not choose smooth reinforcement for the foundation, as the adhesion of the rods to the concrete will be worse. Smooth reinforcement can only be used as transverse elements, since they are subject to less load.
- Longitudinal reinforcement should be located no closer than 5 cm from the formwork. If this rule is not followed, the concrete may begin to crumble and the rods may begin to rust.
- The distance between the transverse elements of the reinforcement frame should be in the range from 25 to 45 cm. Violation of this rule may result in the base being less strong.
- Reinforcement of corners differs from the process of laying reinforcement in a trench.
- Longitudinal rods should be laid every 40 cm of the base height.
By following the strip foundation reinforcement technology, you can complete all the work yourself.
Installation of formwork
Properly created formwork helps to save concrete mortar, as well as simplify the reinforcement process. Such a frame is created in several stages:
- Selection of material for formwork. If the height of the foundation is small, materials such as plywood, fiberboard and OSB boards are used. But the selected material must have sufficient strength to withstand the load created by the concrete mixture.
- Creating a foundation. After digging the trench, it is necessary to create a sand cushion and a base for the foundation. After backfilling and compacting 15 cm of sand, 4-5 cm of concrete is poured. This is necessary to level the surface. It is worth remembering that at this stage you need to think about where the communications will be located. If this is not taken into account, holes will have to be created in the finished structure, which can lead to its deformation.
- Strengthening the formwork. At this stage, the pegs and spacers that are necessary to secure the structure are installed. If such work is not carried out, the formwork may become deformed during concrete pouring.
The formwork for the foundation must be rigid and without gaps so that the mixture does not flow out through the cracks during pouring.
Important! Many people lubricate the formwork from the inside with technical oil or waste, as this makes it easier to peel it off from the concrete after it has hardened.
Strip foundation reinforcement
If the base height is more than 150 mm, transverse and vertical reinforcement is installed, the diameter of which can be 6-8 mm. It can be either metal or fiberglass. According to the rules, the distance between the longitudinal reinforcement rods should not be more than 400 mm. There should be a distance of no more than 300 mm between transverse reinforcement.
It is worth remembering that reinforcing elements should not be connected by welding, as this leads to a decrease in the strength of the frame. That is why knitting wire is used during the creation of the frame. When installing the reinforcement, it is important to ensure that the metal does not come into contact with the ground, as this can lead to rust.
When carrying out such work, special attention must be paid to the reinforcement of the corners of the foundation, since they are subject to a large load. There should not be simple reinforcement crosshairs in these places. In order for the corners to withstand the load, it is necessary to strengthen them and secure them with additional clamps.
At the same time, it is important to connect all the elements of the structure in such a way that it is monolithic. It is worth remembering that saving material and ignoring the rules that corners need to be strengthened can lead to chips or cracks in the foundation. As a result, the base will begin to deform.
Reinforcement of the strip foundation occurs as follows:
- First, wooden formwork is installed.
- After this, a sand cushion is created, the height of which should be about 15 cm. After backfilling, it is important to spill the sand with water and compact it thoroughly using a vibrating plate. Thanks to this, the foundation will not settle under the building.
- Then the base is poured in the form of a tape about 10 cm thick.
- At the next stage, a reinforced frame is created by connecting longitudinal and transverse rods.
Reinforcement of a columnar foundation
A columnar base is usually created during the construction of a wooden house. It is also used to create fences. Such structures are light in weight and resistant to the negative influences of the soil.
After creating a hole in the soil, a sand cushion is filled in and formwork is created for future pillars. At the next stage, a reinforcement frame is created from 4 rods arranged vertically and several horizontal elements.
The column length is calculated based on data on soil freezing in a certain region. The concrete base should be below the level to which the soil freezes. Most often, pillars are created with sides of 25x25 cm.
The number of supports created depends on the dimensions of the structure. Typically the columns are placed at a distance of approximately 1.5-2 meters. Reinforcement of a columnar foundation is made with metal rods with a diameter of 10-12 mm. In this case, the rods are cut into pieces 35-45 cm long.
From the described elements, a lattice is created with cells of 10x10 or 15x15 cm. They are placed on stands mounted on a sand cushion. Instead of supports for a reinforced column, you can use bricks.
Pile foundation reinforcement
This type of foundation for a house is usually chosen when creating a building on soft soil or with a high groundwater level. Also, a pile foundation is installed if there are large differences in relief on the site. When creating such a structure, the piles are usually connected with a grillage, which is made of concrete with metal reinforcement.
Before starting work, it is necessary to determine the optimal depth of the piles. To do this, you need to determine the composition of the soil. It is important to take into account the weight of the future structure and payloads.
To create the type of foundation described, bored or screw piles are usually used. But regardless of the choice, they must be reinforced to make the structure more reliable. Most often, the pile foundation is strengthened with reinforcement with a diameter of 10-12 mm. Reinforcement of a pile-grillage foundation must be done only after drawing up a diagram on which the cell sizes and other parameters of the metal frame will be indicated.
Reinforcement of slab base
The slab foundation is a monolithic foundation, which is created on a sand cushion. This type of foundation is chosen if the house is built on soils prone to heaving and mobility.
It is worth remembering that creating such a foundation requires a fairly large amount of money. The main advantage of this type of foundation is that it can withstand heavy loads and withstand ground movements. In this case, such a foundation can serve as a subfloor.
Before laying such a foundation, part of the soil is removed and the site is leveled. After this, a sand and gravel cushion is poured. The next stage is the installation of wooden formwork. It is important to take into account the load on the structure and create spacers.
A foundation is a structural element of a building that transfers its loads to the ground. The building itself, the foundation and the soil are a single system, influenced by natural and anthropogenic environmental factors, which create additional loads on the foundation. These are loads from soil movement, snow weight, wind pressure, as well as loads arising during the operation of the house or during construction work.
Common types of foundations
In the practice of suburban low-rise construction, the following types of reinforced concrete foundations are most often used: pile, pile-grillage (a monolithic reinforced concrete frame or a monolithic reinforced concrete slab can act as a grillage), buried or shallow strip foundation, monolithic slab (flat or ribbed).
The design of the foundation must ensure uniform distribution of loads on the underlying soils and guarantee minimal changes in the position of the foundation and the entire architectural structure when the properties of the soil at the building site change. The cause of such changes may be natural factors - drying out or watering, freezing or soldering of the soil. The most dangerous to the integrity of reinforced concrete foundations are local movements of soils or changes in their properties, resulting in uneven loads on the structure.
Steel and concrete
The resistance of concrete to compression is 50 times higher than to tension. To increase the resistance of concrete structures to fracture, shear or tensile loads, it was invented to enhance structural strength through the use of steel (later composite) reinforcement. Steel can elongate without breaking under a tensile load of 4 to 25 mm, and unreinforced concrete loses its integrity when stretched by only 0.2-0.4 mm. Reinforced concrete (concrete reinforced with steel rods) can withstand a range of loads in both compression and tension.
Project and following the rules
In order for the foundation to have the necessary characteristics that ensure its integrity, reinforcement must be performed according to certain rules. Unfortunately, when building a house on your own or when building a house by a team of shabashniks (who build houses without the design and supervision of an architect), reinforced concrete foundations are often insufficiently or incorrectly reinforced. It is not surprising that on construction forums on the Internet there are constantly questions about cracked reinforced concrete foundations, and some homeowners are generally convinced that the concrete foundation “must burst” sooner or later.
It is difficult to talk about all the norms and rules for reinforcing reinforced concrete foundations in one article. Let us focus on common reinforcement errors that can lead to undesirable and even dangerous consequences.
Not all fittings are made of metal
From books for summer residents of the Soviet period, when the country had known difficulties in purchasing any products other than the printed works of V.I. Lenin, many got the idea that concrete can be reinforced with any iron objects - pipes, bed parts, fence nets. However, not all such products have the required characteristics to adequately withstand tensile loads, and do not protect concrete from deformation and cracking. Thus, the popular reinforcement of a concrete foundation with railway rails is not recommended due to poor adhesion of concrete to a smooth metal surface. And the inclusion of aluminum products in concrete as reinforcement generally leads to chemical reactions that destroy concrete.
Types of fittings
For working reinforcement of reinforced concrete foundations, modern periodic profile reinforcement of weldable class A500C should be used (the letter C means that such reinforcement can be connected by welding). When using outdated reinforcement class A-III (A400), costs will increase by approximately 10%, since reinforcement will require more reinforcement due to its lower tensile yield strength. Such reinforcement will have to be connected along the length not by welding, but by direct anchoring (fastening the reinforcement in concrete), that is, by overlapping the rods by an amount equal to at least 50 diameters of the reinforcement. Connecting non-weldable class reinforcement (without the letter C) by welding will lead to a local weakening of the metal structure, possible fracture and rupture of concrete under load. The reinforcement should be ribbed for better adhesion to concrete. Smooth reinforcement is used only for auxiliary transverse reinforcement.
Diameter of reinforcement bars for reinforced concrete foundations
The minimum permissible diameter of reinforcement in concrete foundation elements up to 3 m long is 10 mm, and over 3 m - 12 mm. In bored piles, the minimum reinforcement diameter is 12 mm. Longitudinal working reinforcement must be made of rods of the same diameter. If rods of different diameters are used, then rods of larger diameter should be placed at the bottom of the foundation strip - in the tension zone.
The total number of longitudinal reinforcement rods and their diameter depend on the cross-sectional area of the grillage or foundation strip. The total cross-sectional area of the working reinforcement rods must be at least 0% of the cross-sectional area of the foundation strip or grillage.
For the manufacture of transversely bending elements (clamps) in foundation frames with a height of up to 70 cm, reinforcement with a diameter of at least 6 mm is used, and for a foundation section height of more than 80 cm, at least 8 mm is used. In general cases, the installation step of transverse reinforcement (clamps) should not exceed 50 cm. When the foundation height is more than 70 cm, additional structural reinforcement rods are required at the side faces, which can withstand additional loads - such as shrinkage and expansion - as the concrete gains strength and temperature expansion.
Location of reinforcement bars and protective layer of concrete
The working reinforcement bars must be located as close as possible to the edges of the structure in order to ensure the maximum value of the reinforced section of the foundation, but at the same time, the concrete layer that protects the reinforcement from corrosion should not be less than certain values.
In general cases, longitudinal working reinforcement in concrete should be located no closer than 70 mm to the edges that are in constant contact with the ground. But if this is the base of the foundation, which has a concrete preparation, then the protective layer of concrete can be halved - up to 35 mm.
A common mistake is the uneven location of the working reinforcement, which leads to a variable value of the reinforced section of the foundation. According to the standards, deviations from the position of reinforcement bars should not exceed 10 mm.
Surface of steel reinforcement
The condition of the surface of the reinforcement ensures the quality of adhesion between metal and concrete. It should be free of any “intermediate” layers - dirt, loose rust, ice and snow. The fittings cannot be painted. Only a special epoxy coating is acceptable, which, although it reduces the adhesion of concrete, slows down metal corrosion.
But the strange, at first glance, habit of some builders of pouring water on steel reinforcement several days before laying it so that it rusts and “concrete sticks to it more strongly” is not a hack or a mistake. For example, in the official comments to the American Code of Practice for Structural Concrete ACI-318-08, paragraph R7.4 states: “Ordinary surface non-flaking rust increases the adhesion force of the reinforcement to the concrete. The rusty surface adheres better to the cement gel in the concrete. But the flaking rust needs to be removed.”
Bending steel reinforcement
In many cases, steel reinforcement will have to be bent to anchor the reinforcement bars, to properly reinforce the corners and junctions of strip foundations and grillage frames. Class A-III reinforcement can be cold bent without loss of strength at an angle of up to 90 degrees. The bending diameter must be at least 6 diameters of the reinforcement.
Connection of reinforcement bars
Why is it necessary to correctly connect the reinforcement in the foundation? First of all, the connection of the reinforcement ensures the transfer of design forces from one joined rod to another. Modern requirements for maintaining structural integrity require the presence of at least two continuous reinforcement contours in those areas that are subject to tensile loads.
The easiest way to connect welded steel reinforcement. It is welded with an overlap of at least 10 diameters of the reinforcement bar. But when connecting non-weldable reinforcement with an overlap (direct anchoring), many mistakes are usually made. Firstly, the overlap length of the reinforcement must be at least 50 times the diameter of the reinforcement. Secondly, connecting reinforcement without welding, with an overlap, does not at all mean physical contact of the reinforcement rods: the rods should not touch each other so that the concrete mixture, when laying, can “encompass” the connected reinforcement rods from all sides and fix them. The distance between the overlapping rods of the working reinforcement must be at least 25 mm and no more than 8 of its diameters.
Reinforcement of corners and junctions
The desire to reduce labor costs or misunderstanding of individual publications leads to errors in reinforcing foundation zones with the highest stress concentration - corners and abutments. In folk construction mythology, an unacceptable form of reinforcing corners and junctions with the help of simple crosshairs of the ends of the reinforcement twisted with knitting wire was born and firmly established. This type of reinforcement is fraught with spalling of the foundation layers along the width and the formation of cracks in the corners, since a simple intersection of the reinforcement with a “crosshair” is not a connection (anchoring), but actually represents a rupture of the reinforcement. In this case, the tape or grillage loses its solidity, turning into a structure of separate reinforced concrete beams, uniform in appearance, but not structurally, since the transfer of forces from rod to rod does not occur in this case. Correct reinforcement of corners and junctions is a system of anchoring reinforcement bars by bending or using anchoring with U-shaped reinforcing elements (clamps), the length of which must be at least twice the width of the tape or foundation grillage (clause 10-4.5 SP 63.13330.2012 “Concrete and reinforced concrete structures").
Contact the specialists
Inattention to the design and construction of the foundation, driven by the understandable internal motive of the developer or workers to do it “cheaper and faster,” most often leads to problems in the future. As a rule, they are associated with expensive repairs or restoration of foundations that have lost their integrity and damaged houses. Lack of competence, haste and savings during construction sometimes lead to irreparable damage to the building and, as a consequence, to the loss of all the money and time invested in building the house. I hope that a short review of reinforcement errors will serve as a reason for the future developer to turn to specialists, or at least to SNiPs and codes of rules (SP), which should be the basis for any construction, even if everyone around is guided by “how the neighbor did it.”
