What types of screws for concrete are there and how to choose them
Concrete is an extremely hard and dense material, so the fasteners for it must be special. Most often, to attach various structures to a concrete base, anchors and screws with dowels are used, for which holes are drilled. But there are also special self-tapping screws that do not require the installation of dowels.
Before purchasing hardware, we suggest you find out what criteria to choose them by, as well as how to screw a self-tapping screw into concrete - what tools you will need for this.
Material of manufacture
These screws are made from brass, stainless or carbon steel. The first two types are non-corrosive and can be used in any operating conditions: inside and outside, in dry and wet rooms. But brass is an expensive material and not too rigid; fasteners made from it cannot bear a large load, so it is used very rarely.
Alloy stainless steel screw Source allegroimg.com
Products made of carbon steel are the most in demand, as they have an acceptable cost and fairly high strength. However, without an outer coating, they can rust, especially if used in conditions that cause metal corrosion. First of all, this is high air humidity and direct contact with water.
Such hardware is selected by color, which indicates the type of coating.
- Silver products are galvanized, a layer of zinc is applied to them using a galvanic method, and is a reliable protection against corrosion. This is a universal self-tapping screw for concrete, as it can be used in any conditions.
Galvanized screws for concrete Source 24aul.ru
- The golden color indicates processing with copper or brass and the possibility of using such fasteners only for interior work.
Note! Sometimes galvanized hardware also has a yellow tint; check this point with the seller.
- Black and gray hardware acquire this color due to an oxide or phosphate film (oxidized and phosphated self-tapping screws). Such a coating can protect them from rust only when used in indoor areas with dry air.
Screws made of oxidized carbon steel Source der-schweighofer.de
See also: Catalog of companies that specialize in engineering systems and related work
Type of thread
Several types of self-tapping screws are suitable for installation in concrete, differing from each other in the type of thread.
- Universal - with frequent and low threads.
Can be used with or without dowels.
- Pins - with variable thread height and the presence of notches on its edges.
These are self-tapping screws and do not require expansion dowels for installation. But you need to know, when buying such concrete screws, how to tighten them correctly in order to obtain a reliable fixation. A narrow channel is drilled under them, which should be several millimeters longer than the hardware itself.
The dowels have different lengths, but the same diameter 7.5 mm Source fastening.rf
- Herringbone self-tapping screws with inclined threads in the form of cones located one behind the other.
Without dowels they can only be used for fastening lightweight structures. Mostly they come complete with a dowel, into which they are not screwed, but driven in with a hammer.
Herringbone threaded fasteners Source dek.cz
reference Information
To calculate the load, it is recommended to familiarize yourself with the tabular data:
| Load, kg | Fastener size |
| Until 1.00 | 3*16 |
| Until 5.00 | 3*20 |
| Until 10.00 | 4*30 |
| Until 20.00 | 4*70 |
| Until 30.00 | 5*50 |
| Until 50.00 | 6*60 |
| Until 75.00 | 8*80 |
| Up to 100.0 | 10*100 |
Standard dimensions of self-tapping screws for concrete with pre-drilling are indicated in the table data:
| Concrete screw - dimensions | |||||||
| D internal, mm | 7.5*182 | 7.5*152 | 7.5*132 | 7.5*112 | 7.5*92 | 7.5*72 | 7.5*52 |
| 7.35-7.65 | |||||||
| D outer, mm | 6.30-6.70 | ||||||
| Thread pitch, mm | 2.55-2.75 | ||||||
| D heads, mm | 10.82-11.80 | ||||||
| Head height, mm | 2.80-3.20 | ||||||
| Slot type and depth, mm | TORX 2.30-2.70 | ||||||
| Length, mm | 180*184 | 150*154 | 130*134 | 110*114 | 90*94 | 70*74 | 50*54 |
| Tightening torque, kN*m | 20.00 | ||||||
| Min. pulling force, KN | 2.50 | ||||||
| Drill D, mm | 6.00 | ||||||
When working with a drill, it heats up, which requires regular wetting with water.
Anchoring and drilling depth:
| Material | Min anchoring depth, mm | Min drilling depth in wall structure, mm |
| Lightweight concrete | 60.00 | 75.00 |
| Heavy concrete | 30.00 | 48.00 |
Features of installing fasteners in concrete
There are two options for how to screw a self-tapping screw into concrete.
- With installation of a plastic dowel.
- Without installing a dowel - only suitable for dowels with periodic threads and notches.
In any case, installation begins with drilling an installation hole in the concrete structure.
With dowel
A dowel is a plastic sleeve with a ribbed outer surface. The ribs act as spacers and provide rigid fixation of the fasteners in the concrete body. It is indispensable when working with porous and cellular materials, as well as when the screw is subject to a high load or “works” under vibration conditions.
The channel for installing the sleeve is drilled with a hammer drill, impact drill or screwdriver.
- A rotary hammer or drill is used for drilling hard and dense foundations.
- For cellular concrete, a screwdriver is better suited.
The diameter of the hole must match the outer diameter of the dowel; it dictates the choice of a suitable drill or drill. And the depth of the channel is made 4-5 mm greater than the length of the plastic insert.
Installation diagram for fastening with dowel Source krepometr-tula.ru
Requirements for the location of screws
Before installing the screws, mark the locations for their installation. Do this by following these rules:
- the distance from the edge to the first fastening point is greater than two screw lengths;
- the hole is deeper than the length of the screw;
- in bases made of porous concrete, dowels (screws) with a length of 60 mm are used, in bases made of dense concrete – from 40 mm.
- anchors are located at a distance of 120 mm from one another in cellular concrete or 150 mm in dense concrete.
- The hardware is screwed in so that cold bridges do not form between them.
