In construction, whether in bridges, piers, or residential houses, we use concrete for vertical or horizontal elements, and huge volumes of concrete are used especially in the construction of a high-rise building. When you talked about concrete then the most important part is the concrete cover would come in, the other important material involved is “steel reinforcement.”
When these two important materials bend together, then you need to start checking the concrete cover for the various concrete elements to be cast in your project.
A concrete cover contributes to the integrity and soundness of the building. If not done well, it may also cause failure to the building and reduce the durability of the structure. You must observe concrete covers when constructing the pile foundations and/ or footings to the ground beams and slab-on-grade, because these types of concrete elements are exposed to weathering, and are the most likely to experience uplift pressure from the water below ground.
Of particular interest here are, those areas where projects are located by the sea, like the Burj Al Arab in Dubai, The Golden Gate Bridge, The Sydney Opera House in Australia. Projects like this are built near the seashore where the water table is almost level with the ground. There are many more that we did not mention here— you name it. —but most probably those projects have all applied the highest quality standards, especially to the concrete cover, which we are talking about.
It is important for you to know and remember the concrete cover that you should use in your project. Here are the different concrete covers that we used, and you can use these too.
1. Concrete Covers
|Concrete Element||Formed Exposed to the Weather||Formed not Exposed to the Weather||Not Formed Cast Against the Ground|
Raft/ Pile Cap
|Tie Beam||40mm TOP|
|Slabs on Ground||40mm TOP|
|Suspended Slab||40mm||25mm TOP, BOTTOM & SIDES||-
2. Avoid Corrosion on Steel Reinforcement
The first reason why we need a concrete cover is to avoid corrosion. During its tenure inside the concrete, there should be no water that can penetrate the concrete and reach the steel reinforcement or rebar. The steel should be secure and free from any moisture intake and oxygen that can cause corrosion. because corrosion can lead to the failure of the structure and shorten its lifespan. Concrete buildings could have a lifespan of 50 to 100 years before they can be unusable, and failure to follow requirements for the concrete cover may reduce the lifespan of your building.
3. The Steps to Maintain a Concrete Cover
When you inspect a footing, raft foundation, slab-on-grade, or whatever element for which you are pouring concrete, make sure that the concrete cover is maintained. You may want to use ready-made concrete spacers, to directly placed on the bottom or side of the steel reinforcement, as that is what a municipality engineer may commonly suggest. A ready-made spacer is a good idea for your concrete cover. It will be available in two types: the plastic spacer and the concrete spacer.
A plastic spacer is preferable to use on the side forms, while a concrete spacer is preferable for the bottom spacer.
4. Why Have a 75mm Concrete Cover at the Bottom of the Footing?
You always see a 75mm cover on the bottom of the footing, mat foundation, raft foundation, and piles, so this might be your question. From a practical view, we need to thicken the cover over that area of the foundation because that area is subject to high uplift pressure. There is always uplifting water pressure, where the water tries to lift up the foundation, especially when the project has a deep foundation. The deeper the foundation, the higher the uplift pressure.
This is no different from a man wearing a thicker shoe, it prevents a nail from reaching the sole of his foot, causing it to create a wound. It is the same with a footing with a thicker bottom cover.
Also, we cannot prevent a crack in the concrete in that part of the footing, so by having a 75mm thickness of the cover, it accommodates the crack width.
So, in order to prevent the water from reaching the bottom steel reinforcement, we have to put a thicker concrete cover, such as 75mm.
A quality engineer should know how to inspect a footing prior to pouring concrete. You should not overlook this part of inspection because the foundation plays a vital role in carrying all the loads from the superstructure.
5. Why Have a 25mm cover on the Top of the Concrete Slab?
Again, this might be your question. This portion of the slab is different from the bottom part, which we described in the previous subtopic. There is no uplift pressure on the top, and there is no water pressure in a downward action either, so, we can greatly reduce the cover there.
To be clear, the actual cover after completing the floor finishes is 118mm, which is much thicker than the cover of the bottom of the foundation.
6. Why Have a 50mm Cover on an Exposed Column?
A column exposed to the weather often experiences water pressure and oxidation, which are the most common causes of steel reinforcement failure. If the steel is undergoing oxidation, the column would definitely last only a few years after construction.
The same goes for water. When rainwater penetrates the concrete core, the steel reinforcement will react and again, oxidation comes, which will produce corrosion.
7. Where DO the Dimensions of a Concrete Cover Start From?
It is sometimes confusing knowing where the covers start from. Is it from the center of the main bar or the center of the stirrups? Now, here is the truth. The cover should start from the outer face of the stirrups to the side of the forms.
So, when you check any kind of concrete element for which you will pour concrete, just remember this post. You can still come back here if you forget.
The concrete cover has an important role in the construction of any concrete structures. Don’t take it for granted because if you, do your project will soon become a failure. As a quality engineer, you have to always inspect, watch and observe what is happening in an ongoing project— especially this case.
What are the most common concrete covers you use? Do you use the same concrete cover as stated above? Share your thoughts in the comment box below.
What is Concrete Cover?
A material composed of concrete (same strength as of the concrete element where it is embedded) that has a variety of dimensions depending on the concrete element where it place. The dimensions ranges from 25mm to 75mm.
Why Have a 75mm Concrete Cover at the Bottom of the Footing?
From a practical view, we need to thicken the cover over that area of the foundation because that area is subject to high uplift pressure. There is always uplifting water pressure, where the water tries to lift up the foundation, especially when the project has a deep foundation.
What is the strength of Concrete Cover?
The strength of concrete cover shall be similar to the concrete where it will be placed. Say, for example, the strength of the mat foundation is 40mpa then, therefore, the strength of concrete cover is 40mpa.
Sir can you make a post related to Aluminium shuttering(Mivan).in my current project we are using Aluminium shuttering (shear wall technology).
Hi Noel, thank you for this information. However, i have questions on given requirements in the codes and construction notes.
My question are:
1. In the code and notes, the concrete cover indicated always was in minimum. Example: minimum concrete cover of beam is 40mm but no maximum. So if i take it literally, it is also acceptable to make the concrete cover more than 40mm. Is there an acceptable range for the concrete cover? Is there a reference on this sir?
2. You made mentioned sir that the concrete cover of slab is 25mm only since “There is no uplift pressure on the top, and there is no water pressure in a downward action either, so, we can greatly reduce the cover there”. Is the 25mm also applicable to beam, column and wall specifically located inside the building since they are not subject to any water pressure or oxidation ? Why or Why not?