Actions during service 131
the concrete along micro cracks. On the one hand, the internal crystallisation process can lead to a weakening of the structural element, as in the
case of Figure 5.22 where internal crystallisation weakened some concrete
balconies near the Belgian coast. On the other hand, internal crystallisation
in micro cracks can cause some self-healing, restoring to some extent the
‘water tightness’ of the concrete.
In many cases, water movement on concrete structures can also lead
to vegetation or moss formation or to zones with different levels of gray
(darker and lighter zones) depending on the way dirt particles are washed
away, e.g. by the rain. This is only an esthetical problem without serious
harm to the concrete structure.
5.2.7 Pop-outs
Pop-outs can occur due to reactive aggregate particles in the concrete.
When the aggregate particle is positioned near the concrete surface, the
expansion can lead to pop-outs or small craters in the concrete surface.
Reference is made to Chapter 3, Section 3.4, where the effect of expansive
aggregate particles or other inclusions is described in more detail.
Figure 5.21 Formation of calcium deposits.