Concrete Cover to Eurocode 2: cmin, Δcdev and cnom

What this check does

Cover is the distance from the concrete surface to the nearest surface of reinforcement, including links and stirrups. It protects the steel from corrosion, lets the bars develop bond, and protects them in a fire. The value on the drawing is the nominal cover:

$$c_{nom} = c_{min} + \Delta c_{dev}$$

where:

• $c_{nom}$ - nominal cover, the value specified on drawings (mm)

• $c_{min}$ - minimum cover (mm)

• $\Delta c_{dev}$ - allowance in design for deviation during construction (mm)

The minimum cover

The minimum cover is the largest of three requirements:

$$c_{min} = \max\{\,c_{min,b};\; c_{min,dur} + \Delta c_{dur,\gamma} - \Delta c_{dur,st} - \Delta c_{dur,add};\; 10\ \text{mm}\,\}$$

where:

• $c_{min,b}$ - minimum cover for bond: the bar diameter for separated bars, plus 5 mm when the maximum aggregate size exceeds 32 mm (mm)

• $c_{min,dur}$ - minimum cover for environmental conditions, from Table 4.4N (mm)

• $\Delta c_{dur,\gamma}$ - additive safety element, recommended 0 (mm)

• $\Delta c_{dur,st}$ - reduction for stainless steel, recommended 0 (mm)

• $\Delta c_{dur,add}$ - reduction for additional protection such as coating, recommended 0 (mm)

The bond term applies to each bar on its own diameter. That is why the link, which sits outside the main bars, often ends up controlled by the main bar requirement.

Structural class and the durability value

Table 4.4N sets $c_{min,dur}$ by exposure class and structural class. The recommended structural class for a 50-year design life is S4. Table 4.3N moves it up or down: add 2 classes for a 100-year life; subtract 1 when the concrete strength is at least the class listed for that exposure (for example C30/37 in XC1); subtract 1 for slab geometry; subtract 1 with special quality control of concrete production. The class never goes below S1.

Durability values for structural class S4

The construction allowance

The recommended value of $\Delta c_{dev}$ is 10 mm. EN 1992-1-1 §4.4.1.3 allows a reduction to between 5 and 10 mm where fabrication is under a quality assurance system that includes measuring the cover, and to between 0 and 10 mm where a very accurate measuring device is used and non-conforming members are rejected. Check the values in your National Annex.

Surfaces cast against the ground

For concrete cast against prepared ground, including blinding, the recommended minimum nominal cover is 40 mm. For concrete cast directly against soil it is 75 mm.

Fire can govern

Cover for fire resistance is set by the axis distance in EN 1992-1-2. For columns, beams and slabs with long fire periods, that requirement is often larger than the durability value, so check both and use the larger.

Worked example

Member: internal beam in a heated building, exposure XC1, 50-year life, structural class S4. Main bars 25 mm, links 10 mm, maximum aggregate 20 mm.

The links are the outermost steel, so the cover is specified to them. Their own requirements:

$$c_{min,b,link} = 10\ \text{mm}, \qquad c_{min,dur} = 15\ \text{mm}$$

$$c_{min,link} = \max\{10;\; 15;\; 10\} = 15\ \text{mm}$$

The main bars need at least their own diameter, and they sit one link diameter further in:

$$c_{min,b,main} = 25\ \text{mm} \;\Rightarrow\; c_{link} \ge 25 - 10 = 15\ \text{mm}$$

Both conditions give 15 mm to the links. Adding the allowance:

$$c_{nom} = 15 + 10 = 25\ \text{mm}$$

Result: specify 25 mm nominal cover to the links, which gives 35 mm to the main bars. If a 90-minute fire resistance were required, the axis distance from EN 1992-1-2 would be checked next and could increase it.

Key points

• $c_{nom} = c_{min} + \Delta c_{dev}$, with $\Delta c_{dev} = 10$ mm unless quality control justifies less.

• $c_{min}$ is the largest of the bond, durability and 10 mm requirements.

• The bond requirement of the main bars often governs the cover to the links.

• Structural class S4 is the starting point for a 50-year life; Table 4.3N adjusts it.

• Check fire and ground-contact requirements separately.