Civil Engineering Reference
In-Depth Information
reinforced concrete set out acceptable limits on the width of cracks and give formulae
for calculating their width and spacing for bending members. However the designer
should, while respecting the codes, not assume that they are providing him with a
reliable and adequate basis for guiding him in his quest to build a good structure.
The cracking of concrete is only relevant to the SLS, that is at working load.
Consequently, although the proportions of reinforced concrete structures are defi ned
principally at the ULS, the designer must understand their behaviour at working
load.
3.5.2 Cracking due to bending of normally proportioned beams
As concrete cannot follow the extension of the steel reinforcement stressed in tension,
a bending member will inevitably crack on its tension face. Most codes of practice are
based on the interpretation of tests. These tests give reasonably reliable results for the
spacing and width of cracks in areas where the bending moments are changing slowly,
such as the bottom fi bre of beams subjected to sagging bending moments. However,
they are unlikely to give accurate predictions where bending moments are changing
rapidly, such as over intermediate supports.
There are instances where the designer would be advised to be more cautious than
permitted by the codes. One particular area is the cracking of structures that have a
very high proportion of dead load and where the steel stresses under these loads are
above 250 MPa. Examples of such structures are the roofs of cut-and-cover tunnels,
and footbridges. Cracks that are permanently open tend to become highlighted as the
concrete weathers, severely compromising its appearance.
3.5.3 Effect of cracking on durability
Codes of practice recommend limiting the width of cracks in concrete in order to
protect the steel reinforcement from corrosion, with the acceptable width of cracks
depending on the aggressiveness of the environment. There has been confl icting
experimental evidence on the deleterious effect of cracks on the durability of reinforced
and prestressed concrete. However, the designer should err on the side of caution, and
of common sense.
It seems most probable that cracks that are permanently open will reduce the
protection of the steel reinforcement. The deleterious effect of cracks that only open
under the effect of live loads will depend on the frequency of application of the live
loads. For highway bridges for instance, which are designed to resist exceptional loads
that may only occur at rare intervals in the life of the bridge, cracks that open under
these loads are most unlikely to be damaging. At the other extreme, bridges carrying
mass transit railways, where the design live load may occur every few minutes round
the clock should be treated more conservatively.
Bending cracks are associated with a compressed layer of concrete, and hence do
not allow water to penetrate through the member. On the other hand, cracks due to
the restrained heat of hydration cooling (
3.6
) penetrate through the member and will
allow water leakage, Figure 3.7. Such through cracks, even when very fi ne, are likely
to be more deleterious than bending cracks for the durability of a concrete structure
that retains water such as a reservoir, or that excludes water such as a tunnel.
