Anchors for concrete are special fasteners necessary for reliable fixation of various objects and elements on concrete structures. Fastening elements are used where the strength of the walls does not allow the use of screws or nails. Usually, equipment and furniture are attached to walls in this way; they are also relevant for installing doors and windows.
Externally, a concrete anchor looks like a bolt. It is made of galvanized or stainless steel in the shape of a cylinder with a cone-shaped extension at the end of the fastening. The spacer part opens into the concrete after installation and ensures maximum density between the elements and guarantees reliability.
The concrete anchor is installed in a hole of the required size. The high holding capacity of the anchor is ensured by the force of friction, gluing using a special composition, and the interaction of the thrust element with the inner walls of the hole.
Working principle and application
Concrete is a porous material with a heterogeneous structure. And in the places of fastening, various forces appear - twisting, bending, shearing, shearing, compression, tearing. The concrete anchor takes over them, distributing them together with the supporting structure.
Basic principles of operation of concrete anchors:
- At the moment of interaction between the base material and the anchor, a frictional force appears - the expansion is carried out with dowels and metal collets.
- When, at the depth of anchoring, the material provides resistance to fracture or crushing - due to collet bushings on the fasteners, the curved shape of the rod, and expansion.
- Loads at the point of contact between the base and the rod are compensated by tangential stresses during embedding or gluing - this is how smooth, adhesive anchors work.
Anchors for concrete can be of different designs, types, sizes. They are made from special steel according to GOST and coated with a layer of anti-corrosion agent. The rod can be 6-20 millimeters in diameter and up to 220 millimeters long.
Any anchor includes the following parts:
- The bolt itself
- Cone with a layer of thread inside
- Bushing with special cutouts
Anchors perform a constructive or load-bearing function. The load-bearing function is implemented in cases of connecting floor slabs, beams, columns, balcony consoles, landings and flights, finishing and wall panels, engineering equipment, communications, hoods, ceiling lamps, etc. Anchors are also used for installing joists on concrete or hollow-core floors. They are used to attach electrical equipment and hanging furniture to walls.
Structural fasteners are used to counteract the displacement of parts of the assembly if their stability is guaranteed by their own weight; anchors are also relevant when straightening in construction.
Types and types of anchors and methods of their fastening
Anchors for concrete can be very different, using one or more acting forces - emphasis, intermolecular bonding during the gluing process, friction, bending, compression, etc. Depending on their purpose, concrete anchors can be frame, ceiling, foundation or universal. In shape - curved and straight, with a prefabricated or solid structure.
The surface of the anchor can be corrugated or smooth. There are different types of fasteners depending on the contact area - those that are used for porous materials and those that are used for dense ones. According to the installation method, anchor fasteners for concrete may involve through connection, gluing, hammering, screwing, etc.
The material from which the hardware is made is of great importance. If it is steel of strength class 6.8 or higher, coated with an anti-corrosion coating, then the fastening will be as strong as possible. But brass will not withstand serious vertical loads.
Spacers
Expansion anchors for concrete are usually used with nuts, here the working force is the friction force. They are a small threaded pin with a cone-shaped tip and a sleeve. When the fastener is screwed into the base, it wedges and firmly holds the fastener in the monolith.
The main advantages of such anchors: the ability to perform through installation, lightness and simplicity of installation, excellent load-bearing capacity. Wedge anchors are chosen mainly for dense concrete; they sink deeply and cannot be used a second time.
If the anchor bolt has a hook (bracket) installed at the end instead of a nut, the mount is suitable for attachments. Wedge anchors come in sleeve and sleeve types. Such a bolt looks like a bushing with special cutouts; at the end it can be made with a stud with a conical extension or a wedging nut. Used for medium and high density substrates. A small contact area, even with a minimum diameter of the product, makes it possible to withstand significant loads.
The anchor bolt with nut can be electrogalvanized (KA), hot-dip galvanized (HAK), and also have acid-resistant properties (KAN). This also includes screw anchors made of stainless steel (RAR).
Drivers
Drive-in concrete anchors are chosen for dense concrete. These are short bolts with an internal cone and metric threads. The fastener wedges the base when a pin or bolt is screwed into the concrete; the fastener does not protrude onto the surface.
Installing anchors in concrete using this type of bolts is relevant when installing ducts, ceiling air ducts, and installing engineering equipment. Before hammering an anchor into a concrete wall, drill a corresponding hole. Then they hammer in the hardware, use a punch-rim to wedge it, and screw in a threaded rod of the required length.
This type of anchor includes a dowel-nail and a ceiling anchor. It is convenient to attach suspensions, slats, and Armstrong ceilings to them. Fasteners are often used as fireproof or vandal-proof, since the bolt is non-removable.
Frame
Used for fastening door and window frames. The sleeve is made with a cut along its length; a small wedging nut, during the tightening process, presses the structure well against the opening, bringing it to the desired position. To protect against displacement and rotation around its own axis, there are stops at the top of the sleeve.
Self-tapping screws for concrete
Here, the reliability of the connection is ensured by threads running along the entire length of the bolt. During the process of screwing into the concrete base, a powerful resistance to tearing or shearing of the product is created. Therefore, when using this type of fastener, a very high load-bearing capacity is observed. The self-tapping screw can easily take up to 100 kilograms of load, is very reliable in operation and easy to install.
Expanding
In everyday life, this type of anchor is called a “butterfly” (aka Molly bolt). Used for mounting cornices, shelves, lamps, paintings and other elements in hollow structures with low load-bearing capacity. The collet on the screw or bolt is made in the form of a wedging skirt, which, in the process of screwing the rod into the base, rests against the inside of the base cavity. On the outside of the collet, special spikes are embedded in the concrete, which prevent the bolt from moving or turning during installation.
Chemical anchor
The anchor bolt for concrete is made in the format of a semi-liquid mass that quickly hardens. It is with this mass that the bolt or pin is glued into the monolith. Thanks to the composition, it is possible to ensure a reliable connection between the base and the bolt without local or point stresses, but with a uniform distribution of the load along the entire length of the fastener.
Chemical anchors for concrete do not provoke internal stress at all, significantly reducing the risk of destruction and deformation.
Features of fasteners for concrete walls
A dowel for concrete is different from a brick one. Therefore, in order to avoid mistakes when installing it, you need to become familiar with its features.
In practice, it is not very rational to use concrete dowels on brickwork, especially if the wall is made of hollow material. In this case, fasteners of a special design and installation specifications are used.
The differences between the dowel for concrete and the option for brick are presented in the table:
| For brick | For concrete |
| Bushing for brick dowel This is a metal or plastic bushing, the design of which has a double thrust mechanism. This guarantees the strength of the fastening - in any case, one of the spacers hits and is fixed on a solid base. The rod that plays the role of a spacer element is a threaded stud of the appropriate diameter and other anchor fasteners, which depends on the load on them. |
Such a dowel will never create a reliable fastening in a hollow material, no matter how long it is. During installation, the fasteners are driven into the hole, which speeds up the work. When using this technology in loose materials, the mounting hole is destroyed and high-quality fastening is not created. |
Dimensions and characteristics of anchor bolts
Before deciding which anchor is best for concrete in performing specific tasks, it is necessary to consider not only the variety of designs, but also the sizes, because fasteners can be of different lengths and diameters.
Classification of anchors by size:
- Small – cross-section up to 8 millimeters/length up to 55
- Medium anchors – up to 12/120 millimeters
- Large - with maximum dimensions of diameter 24 and length 220 millimeters
An anchor bolt for concrete can have different sizes: the characteristics are determined by three indicators. So, if we decipher the marking M8 10/60-115, then it will be: the diameter of the thread of the anchor M8, the outer diameter 10 (this is the diameter with which the hole for fastening is drilled, this is the diameter of the drill), the length of the anchor is 115 millimeters, the approximate thickness of the element that it is planned to strengthen - 60 millimeters.
All technical characteristics of anchors are indicated in the table, these can be weight, maximum torque and permissible bending moment, minimum pull-out force, optimal loads, distance between axes and to the edge, etc. In accordance with all these parameters, suitable products are selected and high-quality fastening to concrete with anchors is performed.
Installation instructions
Before fixing the anchor bolt in concrete, you need to prepare the tools. To complete the work you will need: a wrench, a construction vacuum cleaner, a drill with a hammer function or a rotary hammer, a concrete drill, a hammer.
Drilling
First, the base is carefully marked for future drilling locations. Here you need to do everything as accurately as possible, since after installation it is not possible to remove the anchor bolt from the concrete - the fixation is very strong. Next, according to the markings, holes of the required length are drilled in the concrete (measured along the spacer part of the bolt), installing a limiter on the drill.
The distance from the edge of the wall should be at least 2.5 times the depth of the hole. Next, the holes need to be cleaned with a vacuum cleaner, compressed air from a can, or a rubber blower. If you cannot remove the crumbs without leaving a residue, you can deepen the hole by 1-2 centimeters.
Installation of anchor with nut
The spacer part of the bolt is inserted into the base and driven in with a hammer until it is completely immersed in the concrete (so that the edges are flush with the wall). Then a threaded rod or bolt is inserted, the nut is tightened, twisting as much as necessary so that further turning is impossible.
It is advisable to learn in advance how to use a specific type of fastener. Manufacturers may set the maximum torque, in which case a torque wrench will be needed. Tightening all the way when working with some substrates (aerated concrete, foam concrete) can cause deformation and destruction of the material.
Chemical anchor installation
To install chemical fasteners in concrete, you need a slightly modified set of tools: you will need a hammer drill, a wrench, a construction vacuum cleaner, as well as threaded rods, glue or capsules, and a mounting gun. For a large number of fasteners, glue is chosen; the choice of capsules (ampoules) is relevant where small volumes of work are planned.
First, mark the base, drill and clean the holes. Before pouring the adhesive solution into the concrete, it is advisable to screw in a mesh sleeve, thanks to which the chemical substance will be retained inside. Then you need to fill the holes 2/3 with glue (using a mounting gun) or install the capsules, insert the pin, gradually screwing it in to evenly distribute the adhesive. Now you need to wait until the glue dries and only then tighten the nut with a wrench.
Varieties
Mechanical anchor bolts for concrete can be divided into the following types:
- Spacer. This sleeve anchor is one of the most common types, since during installation it only requires compliance with the diameter of the hole. It makes a high-quality connection due to the expansion of the conical element when the bolt moves and is suitable for structures made of dense and solid material. This element with spacers is suitable for working with concrete and stone. The expansion anchor can be: with a ring, with a nut, with a washer, with a hook.
- Driven. A reliable anchor, however, requires precise calculation of the depth and diameter during the drilling process. During installation, the rod will need to be driven in so that the element expands. The base into which the hammer anchor will be driven must be solid. Suitable for working with brick, stone and concrete. The drive-in anchor is made from brass and steel.
- Klinova. At the end of the sleeve there is a wedge that pushes its edges apart during the driving process. Suitable for dense materials such as brick, concrete and stone.
- Anchor with hook or ring. This type of fastening is used when distributing additional external loads and fixing various objects. They are used in fastening cables and various suspended structures. They have less reliable strength, but are convenient for home construction work.
There are many different bolts
In addition to concrete and brick, builders often use anchors when working with timber if it is necessary to attach wood to concrete. Then the anchor elements are driven into the concrete through holes in the wood. The modern construction market offers special anchors for hollow-core floor slabs. During installation of the element, it opens special petals that create an emphasis in the material. These bolts are often used in suspended ceiling applications.
The load on the driven anchor is reduced if there are cracks in the concrete. For gas silicate blocks, a chemical anchor is used, the special glue of which will penetrate the porous structure of the material.
Pros and cons of use
The main advantages of using anchor bolts for concrete are maximum reliability and strength of structures. Anchors can withstand enormous loads and guarantee high-quality fastening in concrete with characteristics that cannot be achieved using other types of fasteners.
The main advantages of concrete anchors:
- Resistance to deformation and corrosion
- Lightness, simplicity and high speed of installation
- Resistance to dynamic and static loads of various types
- Large selection of anchors by length, diameter, type of fastening, load, function, shape, material, etc.
Selecting an anchor bolt for a particular task is not difficult. There are no obvious disadvantages to this type of fastener. Unless you need to remember some features: the importance of accurate markings, the correct selection of drills for concrete and determining the depth of the hole, the need to use a certain set of tools.
Main characteristics of anchors
The anchor is a steel rod, ranging in size from 45 mm to 200 mm. A characteristic design feature is provided by a sleeve with longitudinal slots and a nut.
In the product name, the first number indicates the diameter of the suitable hole, the second – the length of the fastener itself, and the third – the thread size.
When choosing a reliable connection, you need to pay attention to the following:
- model and location of the structure;
- terms of Use;
- fastening type;
- options;
- results of load calculation (exceeding 25% of the load specified in GOST is not permissible).
For foam concrete
Foam concrete is used today in many areas due to its poor flammability, low density, excellent thermal insulation properties and resistance to temperature changes of the material. The structure of the foam block consists of a large number of frozen bubbles. Screws, dowels, and self-tapping screws can be used to fasten the material. The highest level of reliability of foam block fastening is provided by plastic and chemical anchors.
Features of concrete anchors
As mentioned above, the mechanical anchor is held in the hole due to the powerful wedging of its spacer parts. This suggests a conclusion: concrete for such fastening must be dense and durable, without voids, shells and cavities. Therefore, mechanical anchors should not be chosen for lightweight, porous concrete, such as foam concrete, sawdust concrete, aerated concrete, which have become widespread in the construction of modern energy-efficient houses.
Reliable fastenings to “weak” materials are made with chemical anchors.
How to install an anchor bolt in concrete
The whole process is quite simple, the main thing is to take the measurements correctly and implement everything as accurately as possible. But there are several nuances that need to be taken into account.
Practical tips for installing anchors in concrete:
- The strength of the fastener is influenced not only by the anchor, but also by the quality of the base material (concrete in this case), proper preparation for installation, taking into account the material of the attached structure and the design of the bolt itself.
- If a layer of finish is applied to the wall, the hole must be deeper because the finish is usually less durable than concrete.
- It is advisable to take a drill 0.5 millimeters thinner than the diameter of the future hole.
- When working with a hammer drill, it is best to use a tip made of a special carbide material.
- Read the instructions carefully - the packaging with anchors should indicate the permissible force and the maximum number of revolutions.
- When screwing the anchor into foam concrete or aerated concrete, under no circumstances twist it all the way, as the material may collapse.
Eternal fastener: concrete dowel
A dowel is a spacer element made of polypropylene or nylon, which is similar in function and performance to an anchor. It cannot be used independently; it is fixed in concrete with special nails or self-tapping screws. To strengthen the fixation, it is often performed with spikes or antennae.
This type of fastening is used where there are not too large loads on the concrete - fastening shelves, TV hooks, lighting fixtures. The dowels are driven into the concrete wall with a hammer, and the screws are screwed in with a screwdriver or screwdriver. You will also need a regular hammer to drive in nails.
You can also find dowel nails designed for use with a mechanical gun. The fasteners in concrete are made of metal; in appearance, they somewhat resemble a bullet. Relevant where you need to hang a large number of small items.
Concrete dowel: fast but durable
Pins are concrete screws that are most often used to install fasteners near the edges of a concrete wall. The fastener does not require additional fixation; it is screwed into the hole, where a little epoxy glue is first poured. There is no need to drill holes - the dowel can be screwed directly into the concrete wall. If you still need to make a hole in concrete, then it is better to choose a drill (a hammer drill can damage the wall).
They screw it in with a screwdriver - and after that it is no longer possible to unscrew the dowel back. Therefore, preliminary marking must be done especially carefully.
