Durability and service life 15
to a strength reduction and to increased transport properties (higher permeability). Compaction of concrete on construction sites is traditionally
performed by means of vibration needles or pokers. In precast plants,
vibrating tables or formwork vibration can be applied as well as other techniques. Nowadays self-compacting or self-consolidating concrete (SCC)
is also available on the market, making compaction by external vibration obsolete (De Schutter et al. 2008). Modern SCC was developed in the
1980s in Japan, in order to overcome durability problems related to badly
vibrated traditional concrete. For a more detailed discussion on the intrinsic durability behaviour of SCC, reference is made to literature (De Schutter
and Audenaert 2007).
During casting, due attention has to be given to the cover thickness.
Positioning of formwork and reinforcement cages should be done with
utmost care in order to avoid too low values of the cover thickness.
Prescribed cover thickness values should be considered as minimum values
and should be carefully respected in practice by appropriate use of spacers. The cover thickness is very important in order to avoid reinforcement
corrosion due to carbonation or chloride penetration. The concrete within
the cover zone, the so-called ‘covercrete’, provides a barrier against carbon
dioxide and chloride ions. The higher the cover thickness, the longer it
takes before the carbonation depth or a critical chloride content reaches
the steel reinforcement (see Chapter 5). It is important to realize that the
time it takes for the carbonation front to reach the steel reinforcement is
not linearly proportional to the cover thickness. The carbonation depth is
typically more or less proportional to the square root of time with some
retardation at later age (Figure 1.9).
Time (year)
100
50
25
15
10
5
2
1
2
3
Prescribed cover thickness
½ Prescribed value
Carbonation Depth (cm)
√time scale
Figure 1.9 Effect of reduced cover thickness on service life in case of carbonation.
to a strength reduction and to increased transport properties (higher permeability). Compaction of concrete on construction sites is traditionally
performed by means of vibration needles or pokers. In precast plants,
vibrating tables or formwork vibration can be applied as well as other techniques. Nowadays self-compacting or self-consolidating concrete (SCC)
is also available on the market, making compaction by external vibration obsolete (De Schutter et al. 2008). Modern SCC was developed in the
1980s in Japan, in order to overcome durability problems related to badly
vibrated traditional concrete. For a more detailed discussion on the intrinsic durability behaviour of SCC, reference is made to literature (De Schutter
and Audenaert 2007).
During casting, due attention has to be given to the cover thickness.
Positioning of formwork and reinforcement cages should be done with
utmost care in order to avoid too low values of the cover thickness.
Prescribed cover thickness values should be considered as minimum values
and should be carefully respected in practice by appropriate use of spacers. The cover thickness is very important in order to avoid reinforcement
corrosion due to carbonation or chloride penetration. The concrete within
the cover zone, the so-called ‘covercrete’, provides a barrier against carbon
dioxide and chloride ions. The higher the cover thickness, the longer it
takes before the carbonation depth or a critical chloride content reaches
the steel reinforcement (see Chapter 5). It is important to realize that the
time it takes for the carbonation front to reach the steel reinforcement is
not linearly proportional to the cover thickness. The carbonation depth is
typically more or less proportional to the square root of time with some
retardation at later age (Figure 1.9).
Time (year)
100
50
25
15
10
5
2
1
2
3
Prescribed cover thickness
½ Prescribed value
Carbonation Depth (cm)
√time scale
Figure 1.9 Effect of reduced cover thickness on service life in case of carbonation.