Surface reinforcement and skin reinforcement

This article explains when the surface reinforcement should be added and the difference with the skin reinforcement. The Eurocode 2 (EN 1992-1-1: 2004) is used in this article. Particularly, the surface reinforcement is explained in annex J. However, we must emphasize that this annex of Eurocode 2 is informative.

First of all, we are going to clarify that the surface reinforcement and the skin reinforcement are not the same, although these two names are sometimes used interchangeably. The book "Designers' Guide to EN1992-1-1 and EN1992-1-2" determines the difference:

·         Skin reinforcement: Skin reinforcement to control cracking should normally be provided in beams over 1.0m in depth where the reinforcement is concentrated in a small portion of the depth. This reinforcement should be evenly distributed between the level of the tension steel and the neutral axis and be located within the links.

·         Surface reinforcement: Surface reinforcement may be required to resist spalling of the cover, e.g. arising from fire or where bundled bars or bars greater than 32mm are used. This reinforcement should consist of small-diameter, high-bond bars or wire mesh placed in the tension zone outside the links.

Surface reinforcement

According to section §J.1 (1) and (2) the surface reinforcement must be used when any of the following cases are met and depending on the case its function and required area will be different:

·         When the main reinforcement has a diameter greater than 32mm - It is used to prevent the concrete from spalling for example when there is fire.

·         When we use bundles of bars with an equivalent diameter greater than 32mm - It is also used to prevent the concrete from spalling

·         When the cover to reinforcement is greater than 70mm - It is used to ensure durability. It is not clear whether the concrete cover refers to the main reinforcement or the closest to surface reinforcement (generally the shear reinforcement). In slabs, for example, it would seem logical that it refers to the longitudinal reinforcement and not to the shear links. Reading the code seems that the distance to the links has to be used, which makes sense in the case of beams but not for the case of slabs.

As for the area to be provided, the Eurocode 2 reads as follows:

·         If the surface reinforcement is used because the diameter of the reinforcement (or bundle of bars) exceeds 32mm: At least, in each direction, should not be less than 0.01*Ac,ext. Ac,ext is defined as the area of the tensile concrete external to the links as shown in the following image. This minimum area can be modified in the national annex of each country:

Figure 1. Example of Surface reinforcement 

·         If the surface reinforcement is used for durability reasons, because the concrete cover is greater than 70mm, at least an amount equal to 0.005*Ac,ext must be placed in each direction

The longitudinal bars of the surface reinforcement may be taken into account as longitudinal bending reinforcement and the transverse bars as shear reinforcement provided that they meet the requirements for the arrangement and anchorage of these types of reinforcement.

For surface reinforcement, the concrete cover given in section §4.4.1.2 must also be respected.

Skin reinforcement

If we look for the word “skin” in Eurocode 2 (EN 1992-1-1: 2004) we see that it appears only in section §7.3.3 Control of cracking without direct calculation in subsection (3). In this section §7.3.3 (3) specifies the same as we have detailed before:

Skin reinforcement: Skin reinforcement to control cracking should normally be provided in beams over 1.0m in depth where the reinforcement is concentrated in a small portion of the depth. This reinforcement should be evenly distributed between the level of the tension steel and the neutral axis and be located within the links.

It also specifies that has to be calculated using the section §7.3.2 (2) Minimum reinforcement areas in the chapter §7.3 Crack control using the coefficient k = 0.5 and the steel stress equal to fyk. We will not go into detail in this article.

References

[1] Eurocode 2 (EN 1992-1-1:2004): Design of concrete structures. Part 1-1: General rules and rules for buildings

[2] Designers’ Guide to EN1992-1-1 and EN1992-1-2: Design of concrete structures. General rules and rules for buildings and structural fire design