What is concrete cover? Why is it important? (Precast)

What is concrete cover?

Cover refers to the distance between the outside of a concrete structure and the reinforcement. Perhaps see this from the diagram below:

This is a general overview. Now let us zoom in and see more detail.

The distance between the concrete and the reinforcement bars is called “cover”. This distance has to be a minimum distance – as specified by the structural engineer.

You need to have a minimum cover:

There needs to be a minimal distance between the reinforcement bar and the outside of the panel.

Why do you need this?

Reduces Corrosion

Having a decent amount of cover reduces the rate of the corrosion of those reinforcement bars. If you have only 5 mm of cover – if the bar is literally just below the surface of the concrete, then that reinforcement is going to corrode away very quickly – especially if you are close to the sea. This means that the concrete will lose its strength very quickly, and a catastrophic failure might be on the cards. That’s why it is very important that the concrete does indeed have some minimal cover.

To Improve the Structural Integrity of the Concrete:

If you have the reinforcement bar too close to the concrete, then the structural integrity of the structure will be somewhat compromised.

Fire Protection:

If at all there is a fire, you don’t want the reinforcement bars igniting. If it does then the fire is sure to blaze out of control. That’s another reason why it’s very important that the bars some minimal distance away from the surface of the concrete. That will better enable the structure to remain in tact if at all there is a fire.

A Response to a Reader’s Question:

A diagram attached with a question from a reader.

“(Q1) It appears that one of the lapped bar is bent while the other isn’t. Or is it just a drawing convention problem?”

(Q2) I don’t get why the lapped bars have to be positioned differently when placed on the top vs at the bottom in order to ensure that the concrete thickness will not reduce, as stated in the figure. My thought is that both positioning ways occupy the same volume.

The answer to this question is best understood by studying the below diagrams:

Caption: The engineer has specified that the minimum cover required for this concrete panel is 40mm. If we put the reinforcement bars one on top of the other, then the cover will be reduced (sometimes dramatically) – and this is not good! Minimum cover needs to be maintained. That’s why the bars should be position appropriately to ensure that minimum cover is guaranteed, or you could also purchase reinforcement that is bent a little bit.

Caption: This reinforcement configuration – with the reo slightly bent, ensures that the minimum cover (in this case 40mm) is maintained – as specified by the engineer.

The Answers to the Questions

(A1) When I draw reinforcement, I do not add a lot of the essential details which are assumed to be standard workshop practice. We are required to maintain a minimum cover. The diagram you have posted above is an example of what actually occurs in practice (but is almost never drawn that way). The reason it is bent is to ensure that the minimum cover requirements are not compromised.

(A2) That is absolutely correct – the panel’s thickness will not be reduced, but the thickness of the cover will change, depending on how one places the “reo” (reinforcement) rods.