Durability and service life 11
parameters like cement content (C), water content (W), and their mass ratio
called the water/cement ratio (W/C) are key parameters for the evaluation
of the durability of concrete. The role of these parameters will be explained
in the following paragraphs. Some critical remarks however will also be
given further in this chapter.
1.4.2 Water/cement ratio
Considering that no aggressive substances are mixed within the concrete
itself, the evolution of degradation mechanisms is largely dependent on the
penetrability of the concrete for aggressive substances coming from the environment. In other words, the permeability of the concrete will play an important role. For cement pastes with similar degrees of hydration, a higher water/
cement ratio will lead to a higher porosity and a higher permeability. While
porosity could be considered to linearly depend on the water/cement ratio,
permeability evolves in a highly non-linear way, as shown in Figure 1.8.
For lower water/cement ratios (0.3 to 0.5), the inclination of the curve
is significantly lower than for higher water/cement ratios (0.6 to 0.7).
A decrease of the water/cement ratio from 0.7 to 0.3 decreases the permeability of the paste by at least two orders of magnitude. This drastic
influence of the water/cement ratio on permeability can also be found for
concrete. For a water/cement ratio of 0.75, a typical concrete permeability
is in the order of 10 −10 m/s, while it is 10 −11 to 10 −12 m/s for a water/cement
ratio of 0.45 (Neville 1995).
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0
20
40
60
80
100
120
140
Water/Cement Ratio (–)
Permeability (10
–14
m/s)
Figure 1.8 Relation between permeability and water/cement ratio of cement paste.
parameters like cement content (C), water content (W), and their mass ratio
called the water/cement ratio (W/C) are key parameters for the evaluation
of the durability of concrete. The role of these parameters will be explained
in the following paragraphs. Some critical remarks however will also be
given further in this chapter.
1.4.2 Water/cement ratio
Considering that no aggressive substances are mixed within the concrete
itself, the evolution of degradation mechanisms is largely dependent on the
penetrability of the concrete for aggressive substances coming from the environment. In other words, the permeability of the concrete will play an important role. For cement pastes with similar degrees of hydration, a higher water/
cement ratio will lead to a higher porosity and a higher permeability. While
porosity could be considered to linearly depend on the water/cement ratio,
permeability evolves in a highly non-linear way, as shown in Figure 1.8.
For lower water/cement ratios (0.3 to 0.5), the inclination of the curve
is significantly lower than for higher water/cement ratios (0.6 to 0.7).
A decrease of the water/cement ratio from 0.7 to 0.3 decreases the permeability of the paste by at least two orders of magnitude. This drastic
influence of the water/cement ratio on permeability can also be found for
concrete. For a water/cement ratio of 0.75, a typical concrete permeability
is in the order of 10 −10 m/s, while it is 10 −11 to 10 −12 m/s for a water/cement
ratio of 0.45 (Neville 1995).
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0
20
40
60
80
100
120
140
Water/Cement Ratio (–)
Permeability (10
–14
m/s)
Figure 1.8 Relation between permeability and water/cement ratio of cement paste.