Protective in reinforced concrete building elements is called a concrete layer whose thickness is equal to the distance from the edge of the reinforcing frame to the surface of the monolith. Its minimum value is determined by regulatory documents and should provide reliable protection of the metal from corrosion in case of possible mechanical damage to the edge.
An improperly laid mesh or reinforcing cage entails a reduction in the thickness of the protection and the active effect of chemical and electrochemical corrosion. In the most difficult cases, the presence of bare steel reinforcement can entail a violation of the integrity of the reinforced concrete, and their subsequent destruction.
Exact adherence to the technology of mounting reinforcing elements allows you to:
- ensure reliable fastening of steel rods in the concrete body;
- evenly distribute the received loads throughout the monolithic structure;
- protect the metal from adverse external factors.
Therefore, the correct installation of reinforcement is one of the most important issues in the manufacture of reinforced concrete products and pouring monoliths at a construction site.
Dimensioning metrics
The standard thickness of the concrete protective layer for reinforcement is given in SNiP 52-01-2003. In this document, it is determined on the basis of the following initial data:
- grade and design diameter of the rods;
- type of reinforced concrete products;
- design mechanical loads;
- geometric dimensions of reinforced concrete elements;
- expected operating conditions.
It also says that the coating should correspond to the optimal standard value. Thin can not ensure safety, and too thick will lead to increased costs and loss of required strength.
Normative indicators
Building Norms and Rules (SNiP) determine the following conditions for the installation of a protective concrete layer for reinforcement in the foundation, which will provide:
- joint work of steel and concrete materials with a uniform distribution of loads;
- device joints of reinforcing elements without reducing the thickness of the coating;
- the possibility of anchoring parts;
- reliable metal protection against all types of corrosion;
- resistance to high temperatures.
The thickness of the concrete protection layer is taken into account the type of elements, grade and diameter of the reinforcement, the technical role of the reinforcing material.
In any situation, the coating thickness should not be less than 10 mm. In cases where a large fraction of crushed stone does not allow gaps of 10-20 mm, an increase in size to the required size is allowed.
For systems that do not have prestressing, the minimum covering layer, depending on operating conditions and the environment, is given in the table:
- in dry enclosed spaces - 20 mm;
- in indoor areas with high humidity - 25 mm;
- in the open air - 30 mm;
- in soil and on its surface - 40 mm.
For prefabricated reinforced concrete elements manufactured in the factory, these dimensions can be made smaller by 5 mm. However, in all cases, the thickness should not be less than the diameter of the reinforcement.
The technical guide for the design of reinforced concrete products provides additional conditions:
- for products from heavy concrete of M250 grade and higher, the layer thickness may be 5 mm less than the diameter of the metal rod;
- the same applies to all precast concrete products;
- for pre-stretched reinforcement, the maximum protective layer of concrete does not exceed 50 mm.
In this case, the pitch of the transverse reinforcing bars should not exceed the cross-sectional height of the finished concrete monolith, and for longitudinal ones - not less than 0.1 F, where F is the surface area of the element.
Depending on the type of construction product, the minimum concrete thickness is as follows:
- plates and walls up to 100 mm thick - 10 mm, all the rest - 15 mm;
- beams, lintels and edges of plates up to 250 mm - 15 mm, for thicker - 20 mm;
- columns and racks - 20 mm;
- prefabricated reinforced concrete for foundations - 30 mm;
- foundation monolith, in the presence of concrete preparation, - 35 mm, without preparation - 70 mm.
Cross distribution elements of all types of products are covered with a protection of 10-15 mm. The manufacturing conditions for concrete monoliths operating in an aggressive environment are determined by SP and SNiP II-A.5-73.
The minimum protective concrete layer for reinforcement is tested by non-destructive methods using special magnetic equipment.
The use of finished fixing parts
For quick and accurate installation of reinforcement inside the formwork, manufacturers of building materials produce inexpensive plastic clamps. You can see several types of such products. But, in fact, there are only two of them - vertical racks (supports, "high chairs") and round ("stars"). All other models are derivatives of these two types.
Vertical racks are used to install a reinforcing mesh or spatial structure in a position raised above the support. Their height and support recess may be different depending on the diameter of the reinforcement and the design height of the installation.
Round "stars" are worn with the help of a special latch-lock on the upper horizontal rows and vertical ones. The calculated radius prevents the rods from approaching the formwork and provides the necessary thickness of the protective layer. Available in various outer and inner diameters.
The use of plastic clips for mounting steel reinforcement allows you to:
- provide high accuracy of the thickness of the protective layer;
- reduce the time required to ensure the high quality of structures;
- reduce the cost of manufacturing reinforced concrete elements of buildings and structures.
The determining factor for use is the simple design of the clamps and their low cost.
Damage Repair
During the operation of reinforced concrete elements, cracks, chips and other defects that violate the integrity of the protective layer may appear on their surface. The reasons for such formations can be:
- structural loads exceeding the calculated value;
- ill-conceived use of special construction equipment;
- the construction of additional floors without changing the construction of the foundation;
- pressure of heaving and moving soils.
Violation of the rules and construction technologies almost always leads to damage. Restoring the integrity of the protection is possible, but will require additional costs.
A full range of repair work should include:
- reinforcement of concrete structure;
- installation of additional transverse elements;
- seal all existing cracks;
- restoration of broken and crumbled areas.
The works are carried out using concrete mixtures and high-grade cement mortar. For reinforcement, formwork is installed and reinforced concrete is added. With preliminary blocking of steel anchors into the old structure.
Recovery should not be done more than 2-3 times. In these cases, it is not required to repair individual elements, but a complete restoration of the building.
Brief conclusions
The presence of a protective concrete layer in the reinforced concrete is an important technological moment, which ensures the durability of the structure and its integrity. This is especially important in the construction of strip and slab foundations. It is not difficult to provide the necessary protection, but it is necessary to maintain the required thickness. To do this, you just need to comply with regulatory requirements and consider operating conditions.