Actions during hardening 73
the concrete normally does not have a very low water/cement ratio, so the
effect of autogenous shrinkage could be neglected in comparison with
the thermal effects.
4.2.2 Influencing parameters
Autogenous shrinkage strains of concrete can widely range from about
50 mm/m to about 500 mm/m or even larger. The main influencing factor
seems to be the water/cement ratio of the concrete, although other influencing factors can be mentioned.
4.2.2.1 Mineral composition of cement
As the chemical shrinkage is a driving force for the occurrence of autogenous shrinkage, it is clear that the mineral composition of the cement is
an important influencing factor. It is also clear that chemical and autogenous shrinkage will depend on the state of the hydration process with
increasing values for higher degrees of hydration. Tazawa (1999) clearly
showed that C 3 A and C 4 AF have a substantial influence on autogenous
shrinkage.
4.2.2.2 Mineral additions and chemical admixtures
Some mineral additions such as silica fume can increase the autogenous
shrinkage, while other mineral additions such as fly ash can lead to a
reduction (Tazawa 1999). The effect is dependent on the nature and the
fineness of the mineral addition possibly leading to swelling peaks and can
be further influenced by the presence of super plasticizers (Craeye et al.
2010).
4.2.2.3 Water/cement ratio
Autogenous shrinkage increases with decreasing water/cement ratio. The
effect of chemical shrinkage and self-desiccation is more pronounced in
cementitious systems with a low water content relative to the cement content.
This is totally different from drying shrinkage, which shows increasing
shrinkage values for increasing water/cement ratios (see Section 4.3). As a
result, autogenous shrinkage is a more important issue for structural elements made with high strength concrete, typically showing a low water/
cement ratio (0.35 or lower) and incorporation of silica fume, while it
is often neglected or implicitly incorporated in drying shrinkage models
for normal strength concrete with higher water/cement ratios (0.45 and
higher).
the concrete normally does not have a very low water/cement ratio, so the
effect of autogenous shrinkage could be neglected in comparison with
the thermal effects.
4.2.2 Influencing parameters
Autogenous shrinkage strains of concrete can widely range from about
50 mm/m to about 500 mm/m or even larger. The main influencing factor
seems to be the water/cement ratio of the concrete, although other influencing factors can be mentioned.
4.2.2.1 Mineral composition of cement
As the chemical shrinkage is a driving force for the occurrence of autogenous shrinkage, it is clear that the mineral composition of the cement is
an important influencing factor. It is also clear that chemical and autogenous shrinkage will depend on the state of the hydration process with
increasing values for higher degrees of hydration. Tazawa (1999) clearly
showed that C 3 A and C 4 AF have a substantial influence on autogenous
shrinkage.
4.2.2.2 Mineral additions and chemical admixtures
Some mineral additions such as silica fume can increase the autogenous
shrinkage, while other mineral additions such as fly ash can lead to a
reduction (Tazawa 1999). The effect is dependent on the nature and the
fineness of the mineral addition possibly leading to swelling peaks and can
be further influenced by the presence of super plasticizers (Craeye et al.
2010).
4.2.2.3 Water/cement ratio
Autogenous shrinkage increases with decreasing water/cement ratio. The
effect of chemical shrinkage and self-desiccation is more pronounced in
cementitious systems with a low water content relative to the cement content.
This is totally different from drying shrinkage, which shows increasing
shrinkage values for increasing water/cement ratios (see Section 4.3). As a
result, autogenous shrinkage is a more important issue for structural elements made with high strength concrete, typically showing a low water/
cement ratio (0.35 or lower) and incorporation of silica fume, while it
is often neglected or implicitly incorporated in drying shrinkage models
for normal strength concrete with higher water/cement ratios (0.45 and
higher).