On the concrete cover of fire-exposed reinforcement
What is the minimum concrete cover required?
When choosing the concrete cover for reinforced concrete members (e.g. reinforced concrete columns) designers have different goals in mind. For permanent structures, sufficient concrete cover must be selected so that the reinforcement is protected from corrosion by the surrounding concrete. This in turn depends on the respective exposure class of the component. From the point of view of constructive fire protection, the highest possible concrete cover initially appears to be advantageous because the concrete has a low thermal conductivity (in contrast to structural steel) and thus the heating and material degradation of the reinforcing steel is delayed. On the other hand, the inner lever arm of the reinforcement is reduced, so that the moment load-bearing capacity of the column is reduced at room temperatures, which is unfavourable from a static point of view.
Figure 1 shows that the choice of concrete cover can therefore only ever represent a compromise between these opposing goals.
When does the question of sufficient concrete cover arise?
Components that are dimensioned in accordance with the currently applicable normative rules of course usually meet the requirements for adequate concrete cover. Unless there are errors in the construction, e.g. the reinforcement bars of reinforced concrete columns are placed in a wrong position during construction.
With existing buildings, however, the question of a sufficient concrete cover often arises and occurs in particular during renovations. For example ceilings were often made of non-ribbed structural steel at the beginning of the 20th century and, from today's point of view, insufficient concrete cover. In addition, the material costs in relation to the wages were significantly higher at the time, which meant that the reinforcement was rather highly utilized. With the help of a fire design we can show renovation options for such structures.
What concrete cover is required for structural fire protection?
Unfortunately, there is no general answer to this question either. For example, the following parameters the required concrete cover of a fire-stressed reinforced concrete column.
- Ratio of surface exposed to fire to the volume of a component (ratio A/V);
- Type of fire exposure (one-, three- or all-sided);
- Load utilization of the fire-exposed member;
- Duration of fire exposure or required fire resistance.
How we can help you
For the reasons given above, the optimal concrete cover depends on a wide variety of influences. It can therefore be economically interesting to carry out a more detailed examination of a reinforced concrete column with a so-called fire design. We can also help you with execution errors in the reinforcement.
We determine the exact fire resistance of a component for you. In doing so, favourable factors can be taken into account that increase the fire resistance period of a member, e.g. the actual load utilization in the exceptional fire load case, the concrete moisture and possibly also natural fire curves . This means that any existing load-bearing capacity reserves can be exhausted in the event of a fire.
What are your advantages?
Our primary goal is to avoid expensive structural renovation. If this succeeds, high costs can be saved.
Our initial consultation for your project
The best way to clarify your specific task is to give us a call. Or you can send us an e-mail with a few key data. On this basis, we can discuss the general conditions and make you an attractive offer.