What is a Cover Block in Construction? Types and Features

Cover blocks are definite-sized spacers that are used to lift and hold the reinforcement bars at the required cover distance in the R.C.C. structure. We know the importance of providing nominal cover for reinforced concrete structures. Hence, cover blocks are used to maintain the required cover between the reinforcement bars and the outer surface of the concrete surface to ensure durability and prevent corrosion.

The required cover is maintained throughout the structure with the use of these special stands or blocks called spacers. These must not be affected by corrosion and should be placed approximately every 1.00 m.

In this article, we will study the objectives of providing cover blocks, their types, and related features in detail.

What is the Purpose of Cover Blocks or Spacers in Concrete Construction?

Cover blocks are provided to ensure sufficient cover for reinforcement. Nominal Cover or Clear Cover is the clear distance between the outer surface of the concrete to the nearest reinforcement bar to the surface. This reinforcement bar can be either main reinforcement, stirrups, or links.

Cover blocks ensure proper cover throughout the structure which is necessary to:

• To maintain proper depth throughout the structure.
• To keep the rebars in place within the formwork, even during the concreting procedure.
• To let concrete flow underneath the rebar filling all spaces during the concreting process.
• To protect the reinforcement bars from environmental variables like weathering, corrosion, and erosion. 

Types of Cover Blocks For Concrete Construction

Cover blocks can be manufactured to provide the required cover, as in a 20 mm cover block or 15 mm cover block. These cover blocks can be made out of concrete, PVC, aluminium, etc. Let's discuss the types of cover blocks used in concrete construction in detail. 

1. Cement Mortar Cover Blocks (Single or Multiple Cover Spacers)
2. PVC Cover Block
3. Circular Mortar Cover Block with Lock Pin
4. Concrete Spacer Bars
5. Steel Chairs

1. Single or Multiple Cement Mortar Cover Blocks

If a cover block can accommodate a single cover, we call it as single cover block, Figure-2(a). If the cover block can accommodate one or more covers like 20mm, 25 mm, 40 mm, and 50 mm we call it a multiple concrete spacer or cover block, Figure-2(b).

Fig.2. Single or Multiple Cement Mortar Cover Blocks

Mortar Cover blocks can be square, circular type, or chair type as shown in figure-3 below. Circular cover spacers are mostly used as column spacers and they provide only a single cover. They are also available in chair type with a lock pin. 

Fig.3. Cover Spacers for Concrete Structures

Cement mortar covers can be made at the site or bought as per the site requirements. As it is also made from cement, it is no foreign material compared to the concrete poured over it. Hence, the cover block bonds properly with the concrete placed.

The mortar cover spacers can take different shapes as shown in the figure above. The tie wires are tied and lock them to the reinforcement bars. Concrete cover blocks, made from high-strength fiber-reinforced concrete, ensure better bonding with surrounding concrete and crucial protection to reinforcement bars.

2. PVC Cover Blocks

PVC cover blocks are made in different shapes and designs that would be suitable for different structures. It can be circular, square, chair and clip, and chair type. Each type, their usage, and possible cover sizes available are mentioned in the figure below. Circular cover spacers are also called as wheel spacers.

Fig.4. Types of PVC Cover Spacers for Concrete

PVC cover blocks, being foreign to concrete, lack a permanent bond and have a different thermal coefficient of expansion, potentially leading to issues. Unlike concrete cover blocks, they do not provide explicit protection against corrosion or fire, nor do they facilitate proper stress transfer to steel rebars.

Fig.5. Wheel Spacers for Reinforcement

 

If no alternatives exist, PVC cover blocks could be considered, but it's important to note that they may not offer the same level of performance. 

3. Concrete Spacer Bars

These are bar-type spacer bars used to provide cover for the R.C.C. structure. They can have square or trapezoidal cross sections with a large length compared to their width. They can be made out of steel or mortar and used accordingly as shown in figure 5 below. As shown the length can be straight, banana form, or curved in the case of cement spacer bars. 

Fig.6. Concrete Spacer Bars

4. Steel Spacers

Steel spacers are employed to maintain the proper spacing between the lower and upper reinforcement layers in floors, ceilings, and walls. They are particularly useful for positioning mats and steel bar reinforcements accurately. Additionally, their large support area minimizes the risk of tipping. Moreover, as they do not come into direct contact with the formwork, they prevent damage to the formwork surface and the formation of rust stains.

Fig.7. Steel Spacers

5. Chairs Spacers

Chair Spacers can be made out of Steel or Plastic.

Steel Chair Spacers

Prefabricated element, made out of thin steel rebar with plastic tipped legs to prevent corrosion of the support area between the rebar chair and the formwork. The position of the upper (negative) slab reinforcement, either placed over the support between two slabs or over the support between a slab and a balcony (cantilever slab), can be secured only with the use of special rebar chairs.

Fig.8. Steel Chair Spacers Provided Between the Reinforcement Layers/Not in Contact with the Formwork

As mentioned above, steel spacers or chairs are not used as spacers that directly come in contact with the formwork. So in that case we make use of mortar spacers or plastic chairs.

Plastic Chairs

Fig.9. Plastic Chair Spacers in Contact 

Points to Remember When Using Cover Blocks or Spacers on Construction Sites

Understanding cover blocks in concrete construction is crucial. These blocks, also known as concrete spacers, serve to achieve the required clear cover in concrete design and construction. Additionally, they ensure proper spacing and alignment of reinforcement in R.C.C. structures.

• Cover blocks are available in various types, materials, and styles tailored to different structural elements such as beams, columns, top and bottom reinforcement, etc.
• When using steel spacers or cover blocks, ensure they do not make direct contact with the formwork to prevent potential complications,  due to their potential to exacerbate corrosion issues. Instead, steel chairs positioned between layers of reinforcement are preferred, but not as nominal cover. To provide nominal cover we have the option of mortar cover blocks, spacers, and plastic spacers.
• By following these guidelines, you can optimize the use of cover blocks or spacers in construction projects, ensuring structural integrity and longevity.

Must Read

• What is Concrete Cover in Construction?
• Rebars Used in Construction