Durability and service life 17
factor, shown in the vertical axis, refers to the proportional increase in
cover thickness needed to compensate for the reduced concrete quality
as indicated by the increasing water/cement ratio. A lower concrete quality (due to increased water/cement ratio or improper curing) will quickly
lead to a significantly higher required cover thickness in order to reach the
target service life.
A second consequence of bad curing can be the formation of plastic or
drying shrinkage cracks at early age. As the loss of water is accompanied
by a volume reduction of the concrete, shrinkage cracks could occur in
the drying concrete surface. As a result, the hardened concrete element
will contain some small shrinkage cracks, which will further influence the
transport properties. Through the cracks, aggressive liquids and gasses
can easily penetrate the concrete and reach the reinforcement much faster
than expected. As an example, the effect of small shrinkage cracks on the
penetration of chloride ions is schematically illustrated in Figure 1.11, as
experimentally obtained on laboratory samples with crack width 0.3 mm
and crack depth 20 mm exposed to chloride migration tests. Due to this
effect, chloride ions can very quickly initiate corrosion of the reinforcing
bars. The penetration of carbon dioxide is somewhat less sensitive to the
existence of very small cracks below 0.3 mm. Although the above explanation is linked to the occurrence of shrinkage cracks due to lack of curing,
a similar influence is obtained in case of early age thermal cracking due to
heat of hydration (see also Chapter 4).
1.5 PractIcal DurabIlIty aPProach
1.5.1 typical code provisions
Because the previously discussed parameters water/cement ratio and
cement content have been considered important durability parameters for
a long time, it is no surprise to see that typical code provisions related to
durability of concrete are often based on them. In Europe, the standard
EN 206-1 ‘Concrete—Part 1: Specification, performance, production and
conformity’ departs from the notion of exposure classes, designated with a
capital letter X, followed by another letter depending on the specific degradation mechanism to be considered: C for carbonation, D for de-icing salts,
S for sea water, F for frost, and A for chemically aggressive environment.
To this letter combination, a number is added, which in most cases is linked
to specific humidity conditions. In total, 18 exposure classes have been
defined, as listed in Table 1.2.
Depending on the environment, several degradation mechanisms can
occur in parallel. Therefore, it is necessary to select all relevant exposure
classes for the considered application. Finally, the concrete composition
will have to be designed considering the most severe exposure class.
factor, shown in the vertical axis, refers to the proportional increase in
cover thickness needed to compensate for the reduced concrete quality
as indicated by the increasing water/cement ratio. A lower concrete quality (due to increased water/cement ratio or improper curing) will quickly
lead to a significantly higher required cover thickness in order to reach the
target service life.
A second consequence of bad curing can be the formation of plastic or
drying shrinkage cracks at early age. As the loss of water is accompanied
by a volume reduction of the concrete, shrinkage cracks could occur in
the drying concrete surface. As a result, the hardened concrete element
will contain some small shrinkage cracks, which will further influence the
transport properties. Through the cracks, aggressive liquids and gasses
can easily penetrate the concrete and reach the reinforcement much faster
than expected. As an example, the effect of small shrinkage cracks on the
penetration of chloride ions is schematically illustrated in Figure 1.11, as
experimentally obtained on laboratory samples with crack width 0.3 mm
and crack depth 20 mm exposed to chloride migration tests. Due to this
effect, chloride ions can very quickly initiate corrosion of the reinforcing
bars. The penetration of carbon dioxide is somewhat less sensitive to the
existence of very small cracks below 0.3 mm. Although the above explanation is linked to the occurrence of shrinkage cracks due to lack of curing,
a similar influence is obtained in case of early age thermal cracking due to
heat of hydration (see also Chapter 4).
1.5 PractIcal DurabIlIty aPProach
1.5.1 typical code provisions
Because the previously discussed parameters water/cement ratio and
cement content have been considered important durability parameters for
a long time, it is no surprise to see that typical code provisions related to
durability of concrete are often based on them. In Europe, the standard
EN 206-1 ‘Concrete—Part 1: Specification, performance, production and
conformity’ departs from the notion of exposure classes, designated with a
capital letter X, followed by another letter depending on the specific degradation mechanism to be considered: C for carbonation, D for de-icing salts,
S for sea water, F for frost, and A for chemically aggressive environment.
To this letter combination, a number is added, which in most cases is linked
to specific humidity conditions. In total, 18 exposure classes have been
defined, as listed in Table 1.2.
Depending on the environment, several degradation mechanisms can
occur in parallel. Therefore, it is necessary to select all relevant exposure
classes for the considered application. Finally, the concrete composition
will have to be designed considering the most severe exposure class.