102 Damage to concrete structures
5.1.2 Differential settlements
In structures which are statically indeterminate (such as a continuous
bridge girder on three or more supports), differential settlements can lead
to cracking as illustrated in Figure  5.5. The resulting crack patterns can
be compared with the crack patterns caused by direct loading. Indeed, a
differential settlement of a statically indeterminate structure will have the
same effect as an external load.
Consider as an example the first situation shown in Figure 5.5. Settlement
of the middle support can be compared with the situation of the entire
beam only supported by the two end supports and loaded by a vertical
downward force at the location of the middle support. The resulting crack
pattern is thus completely similar to the crack pattern of a beam in bending.
Figure 5.6 shows the possible effect of differential settlements in the case
of a concrete brick wall. The lines of damage are very similar to the crack
pattern in a continuous reinforced concrete wall loaded in shear.
5.2 PhysIcal actIons
5.2.1 frost damage
5.2.1.1 Mechanism
Concrete is a porous material containing pores with a multitude of
sizes  ranging from nanometre to millimetre (Neville and Brooks 2010).
Figure 5.4 Fatigue damage in a thin floor slab of a reinforced concrete bridge.