Actions during hardening 77
Although drying shrinkage can lead to a volume reduction, drying shrinkage is most commonly expressed as a linear drying shrinkage strain. For concrete, drying shrinkage strain values typically ranging between 200 mm/m
and 600 mm/m can be obtained, depending on several parameters.
As the concrete element loses water at the exposed surfaces, the drying process is controlled by a diffusion process, requiring water to diffuse from the inner parts of the concrete element to the exposed surfaces.
This diffusion process is quite slow so that drying of concrete is a timeconsuming process. Furthermore, the drying process of concrete elements
is not uniform, inducing internal moisture gradients, and leading to higher
shrinkage near the exposed surfaces and lower shrinkage in the core. This
non- uniform shrinkage leads to internal stresses due to internal restraint,
possibly leading to shrinkage cracks. The degree of self-restraint depends on
specimen size (Wittmann 2001). In most cases, however, drying shrinkage
cracking due to internal restraint is very limited. Internal microcracking
can occur, however, due to shrinkage restraint by non-shrinking aggregate particles (Bisschop 2002). For concrete structures, external restraint of
drying shrinkage deformations are more important, and can typically lead
to significant cracking, e.g. in concrete floors and linear concrete elements
(see examples in Section 4.3.4.).
Rewetting of (partially) dried concrete elements will lead to increase
in volume or swelling. However, part of the drying shrinkage will not be
recovered, which is called irreversible shrinkage (Figure 4.4). This probably is related to rearrangements in the CSH structure. When physically
adsorbed water is expelled, the distance between hydration products is
somewhat reduced. As a result, additional chemical connections can be
formed, which prevent full shrinkage recovery. The irreversible drying
Time
Irreversible shrinkage
Reversible shrinkage
Wetting
Drying
Shrinkage
Figure 4.4 Reversible and irreversible drying shrinkage.
Although drying shrinkage can lead to a volume reduction, drying shrinkage is most commonly expressed as a linear drying shrinkage strain. For concrete, drying shrinkage strain values typically ranging between 200 mm/m
and 600 mm/m can be obtained, depending on several parameters.
As the concrete element loses water at the exposed surfaces, the drying process is controlled by a diffusion process, requiring water to diffuse from the inner parts of the concrete element to the exposed surfaces.
This diffusion process is quite slow so that drying of concrete is a timeconsuming process. Furthermore, the drying process of concrete elements
is not uniform, inducing internal moisture gradients, and leading to higher
shrinkage near the exposed surfaces and lower shrinkage in the core. This
non- uniform shrinkage leads to internal stresses due to internal restraint,
possibly leading to shrinkage cracks. The degree of self-restraint depends on
specimen size (Wittmann 2001). In most cases, however, drying shrinkage
cracking due to internal restraint is very limited. Internal microcracking
can occur, however, due to shrinkage restraint by non-shrinking aggregate particles (Bisschop 2002). For concrete structures, external restraint of
drying shrinkage deformations are more important, and can typically lead
to significant cracking, e.g. in concrete floors and linear concrete elements
(see examples in Section 4.3.4.).
Rewetting of (partially) dried concrete elements will lead to increase
in volume or swelling. However, part of the drying shrinkage will not be
recovered, which is called irreversible shrinkage (Figure 4.4). This probably is related to rearrangements in the CSH structure. When physically
adsorbed water is expelled, the distance between hydration products is
somewhat reduced. As a result, additional chemical connections can be
formed, which prevent full shrinkage recovery. The irreversible drying
Time
Irreversible shrinkage
Reversible shrinkage
Wetting
Drying
Shrinkage
Figure 4.4 Reversible and irreversible drying shrinkage.