What Is a Concrete Cover?

The concrete cover is used to protect the reinforcement.

Concrete cover

The concrete protective layer refers to the concrete layer between the outer edge of the outermost steel bar and the concrete surface in the concrete structural members, which is referred to as the protective layer. (This is a modification of the previous version of the new specification in 2010) ^[1]

(1)

The greater the thickness of the concrete protective layer, the better the anchoring performance, durability, and fire resistance of the reinforced members of the component. However, an excessively large thickness of the protective layer will cause the width of the cracks generated by the component to be too large, which will affect its performance (such as damage to the decoration layer on the surface of the component, and excessively wide crack widths will cause panic, etc.). Large protective layer thickness can also cause economic waste. therefore,"

"Specifications" GB50010-2010

Article 9.2.2 Prefabricated components produced in factories in a first-class environment, when the concrete strength level is not lower than C20, the thickness of the protective layer can be reduced by 5mm according to the provisions in Table 9.2.1 of this code, but the protection of prestressed steel bars The thickness of the layer should not be less than 15mm; when the prefabricated components are in the second-class environment and produced by the factory, the protective layer thickness can be taken according to the first-class environmental values in Table 9.2.1 of this specification when effective protection measures are taken on the surface.

The thickness of the protective layer at the ends of the precast reinforced concrete flexural members should not be less than 10mm; the thickness of the protective layer of the main rib of the precast ribbed plate should be taken according to the value of the beam.

Article 9.2.3 The thickness of the protective layer of the reinforcing bars distributed in slabs, walls and shells shall not be less than the corresponding value minus 10mm in Table 9.2.1 of this code, and shall not be less than 10mm; the protective layer of beams, columns, stirrups and structural steel bars The thickness should not be less than 15mm.

Article 9.2.4 When the thickness of the concrete protective layer of longitudinally-stressed steel bars in beams and columns is greater than 40mm, effective protective crack prevention measures shall be adopted for the protective layer.

The cantilever board in the second and third environment should be protected effectively.

Article 9.2.5 For buildings with fire protection requirements, the thickness of the concrete protective layer shall still meet the requirements of the relevant national standards.

The thickness of the concrete protective layer of the buildings in the fourth and fifth environment should still meet the requirements of the relevant national standards.

Article 10.1.2 The national standard GB50204-2015 "Specifications for the Acceptance of Construction Quality of Concrete Structure Engineering" stipulates that structural entities must be inspected for important parts, mainly inspecting the strength of concrete and the thickness of the protective layer of steel bars. Reinforcement protective layer thickness inspection requires the use of non-damaged (with advanced reinforced protective layer thickness measuring instrument) or locally damaged important components, especially cantilever beams and slab components, as well as parts that are prone to steel bar displacement and exposed tendons. Method inspection. At this time, the allowable deviation of the thickness of the longitudinally-stressed reinforcing steel protective layer is -7 ~ + 10mm for beam members and -5 ~ + 8mm for plate members. The qualified point rate of the steel bar thickness inspection is 90% and above. When the qualified point rate is less than 90%, but not less than 80%, the same number of components can be taken for inspection. Only when the qualified point rate calculated by the sum of two samplings is 90% or more can it be judged as qualified. And the maximum deviation of the unqualified points in each sampling result should not be greater than 1.5 times the allowable deviation.

"Specifications" GB50010-2010

Article 8.2.1 The concrete of ordinary steel bars and prestressed steel bars in members shall meet the following requirements.

1 The thickness of the protective layer of the reinforced bar in the component should not be less than the diameter d of the bar.

2 For concrete structures with a design life of 50 years, the thickness of the protective layer of the outermost steel bar shall meet the requirements of Table 8.2.1.

Table 8.2.1 Minimum thickness of concrete protective layer c (mm)

Environmental grade	Siding shell	Beam
One	15	20
Two a	20	25
Two b	25	35
Three a	30	40
Three b	40	50