What is a Concrete Cover Block?

A concrete cover block also called a concrete spacer, is a type of rebar spacer. A rebar spacer is a device that secures the reinforcing steel or “rebar” in reinforced concrete structures as the rebar is assembled in place prior to the final concrete pour. The spacers are left in place for the pour to keep the reinforcing in place and become a part of the structure. The concrete cover block is a standard solution to fix the reinforcing steel. It has many advantages over the plastic spacer, which is gradually replacing plastic spacer.

The type and selection of cover blocks

Concrete Cover Block

Generally speaking, concrete, cement mortar, marble, granite, plastic blocks, brushed anti-rust paint rebar head, etc. can be used as cover blocks, these are also the materials that are used more frequently. 

1. Mortar cover blocks are preferable in combination with concrete, color, and luster. However, the block should generally be no lower than the concrete strength grade. The mortar cover block is defective in strength and is usually not used.  

2. Plastic cover blocks have the following advantages compared to traditional cover blocks.  

standardization, stereotyping, and versatility, with stable quality.

Can be adjusted by adjusting different placement angles to adjust the thickness of the cover, reducing the uneven thickness caused by human factors, easy to use.

Light weight, small volume, high strength, and corrosion resistance.

Stable support, not easy to fall off.  

3. Concrete is the ideal block material and is recommended for use.  However, it should improve the shape of the square block which was commonly used before. Under the premise of using mass production and strengthening supervision to ensure construction quality, I think it is the best choice at present.  

Experiments with different materials

Experimentconcrete cover blockmortar cover blockplastic cover blockconcrete cover blockmortar cover block
Accuracy of coverHighLowHighConsistent expansion coefficient without gapsPoor adhesion
Compression strength HighAverageLowCompletely integrated with concreteLow water resistance
Load strength  HighAverageLowLarge range of water penetration
Durability     HighLowAverage
Construction performanceHighAverageAverage

The effect and influence of concrete cover blocks

1. Maintenance of structural durability: If the cover in the concrete is too thin, it is easy to infiltrate moisture gas and water, which may cause the reinforcement to rust and expand, thus causing damage to the concrete and affecting the use and structural safety.  

2. To withstand the action of external forces. The cover has an anchoring force to the reinforcement, using the anchoring force between the concrete and the reinforcement, the two are closely combined and work together. When the cover is too thin or missing, it reduces its anchorage force and thus reduces the role of the structure to resist axial forces and bending moments. If the cover is too thick, the concrete surface is prone to cracking, which affects the force of the reinforcement in the concrete.  

3. From the positive cross-sectional bending bearing capacity calculation formula

M≤a l fcbx(h0一x/2)+fy’AS'(ho–a s’)一(σpO’一fpy’)A p'(ho–a p’)

It can be seen that when the cover layer of the face reinforcement is too thick leading to the reduction of the effective height h0 of the section, it will reduce the flexural bearing capacity of the section. It can be seen how important the cover thickness is to affect the structural bearing capacity.  

4. Fire protection layer. Concrete has a certain insulation effect. When encountering a fire concrete can protect the more easily softened reinforcement. If the thickness of the concrete cover is too thin, it is easy to crack in the high-temperature environment, resulting in overheating of the reinforcing steel to reduce the strength and thus damage the overall structure. Therefore, in the current quality acceptance work, the control of the thickness of the protective layer has been paid more and more attention. Generally speaking, it is stipulated that a physical inspection should be carried out during the acceptance process.

The use of the cover blocks

The use of cover blocks should pay attention to the following points. 

1. Different construction parts use different sizes of cover blocks. 

2. The number of cover blocks should meet the requirements, cover blocks of longitudinal and horizontal spacing control in about 100cm, and should not be too large and not too small.  

3. When the diameter of the reinforcement is small, the blocks should be appropriately encrypted.  

4. Concrete cover block generally should be a grade higher than the concrete strength grade, specifications can be 7cm × 7cm × concrete cover thickness.  If the height of the structure is greater than 1.5m, the concrete cover block specifications should be 10cm × 10cm × concrete cover thickness.

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