Calculating nominal concrete cover is a fundamental step in concrete design to ensure durability and fire resistance of reinforced concrete elements. The nominal concrete cover ($c_{nom}$) is determined by combining the required minimum cover with an allowance for potential construction deviations.
The nominal concrete cover ($c{nom}$) is calculated by **adding to the minimum cover ($c{min}$) the allowance for deviation ($\Delta c_{dev}$)**.
The Formula
As specified in EN1992-1-1 4.4.1.3(1)P, the calculation follows a straightforward formula:
$c{nom} = c{min} + \Delta c_{dev}$
Let's break down the components of this formula:
- $c_{nom}$ (Nominal Cover): This is the target cover that should be specified in design drawings. It is the distance from the surface of the concrete to the nearest surface of the reinforcement.
- $c_{min}$ (Minimum Cover): This is the absolute minimum distance required between the surface of the concrete and the nearest reinforcement bar. This value is determined based on factors such as:
- Environmental exposure conditions (affecting durability)
- Required fire resistance
- Bar diameter
- Aggregate size
- $\Delta c_{dev}$ (Allowance for Deviation): This value accounts for inaccuracies that might occur during the construction process, such as tolerances in reinforcement placement or formwork position.
Allowance for Deviation ($\Delta c_{dev}$)
According to the reference based on EN1992-1-1 4.4.1.3(1)P, the allowance for deviation is specifically considered as $\Delta c_{dev}$ = 10.0 mm.
This standard value helps ensure that even with typical construction tolerances, the actual concrete cover is unlikely to fall below the required minimum cover ($c_{min}$).
Why is Nominal Cover Important?
Specifying the nominal cover ($c{nom}$) rather than just the minimum cover ($c{min}$) provides a practical target for contractors on site. It inherently includes the necessary buffer to account for typical construction variability, thereby increasing the probability that the actual cover achieved in the finished structure meets or exceeds the required minimum cover for durability, fire protection, and bond.
Example Calculation
Suppose the required minimum cover ($c_{min}$) for a specific concrete element, based on its exposure class and fire resistance requirements, is determined to be 25 mm.
Using the formula and the standard allowance for deviation:
$c{nom} = c{min} + \Delta c{dev}$
$c{nom} = 25 \text{ mm} + 10 \text{ mm}$
$c_{nom} = 35 \text{ mm}$
In this example, the nominal cover to be specified in the design documentation would be 35 mm.
By calculating and specifying the nominal cover using this method, designers incorporate a crucial safety margin against potential construction inaccuracies, contributing to the long-term performance and safety of the structure.
