74 Damage to concrete structures
4.2.2.4 Paste volume
Shrinkage occurs in the cement matrix, while the aggregates in mortar and
concrete do not show any shrinkage behaviour. As a result, as is the case for
drying shrinkage as well, autogenous shrinkage increases with increasing
paste volume in the concrete.
4.2.3 Mitigation
The most straightforward measure to reduce autogenous shrinkage is to
increase the water/cement ratio. This of course is not always possible, especially in cases where a (very) high concrete strength is required. Furthermore,
in high strength concrete, quite often silica fume is applied in order to refine
the pore structure and further increase the strength. Unfortunately, this
also further increases the autogenous shrinkage. Some reduction in autogenous shrinkage could be obtained by a good selection of the cement type,
avoiding a high C 3 A content, and by a well considered combination of mineral additions and super plasticizers.
In case of high strength concrete, special ‘internal curing’ measures
might be needed (Kovler and Jensen 2007). Internal curing is defined as
the incorporation of a component, which serves as curing agent, to the
concrete mixture. Internal curing can be classified into two categories:
internal water curing (or water entrainment) and internal sealing. Internal
water curing seems to be the most effective in order to mitigate autogenous
shrinkage. This method consists of the incorporation into the cementitious
system of a curing agent serving as an internal water reservoir. The water
is gradually released as the concrete dries out due to self-desiccation. Super
absorbent polymers (SAP) and water-saturated fine lightweight aggregates
(LWA) are mostly considered as internal water reservoirs.
The effect of internal curing (or any other method to reduce or mitigate
autogenous shrinkage) can be measured by means of specially developed
test methods. Autogenous shrinkage can be experimentally studied by
means of different methods, including volumetric and linear measurements
(Tazawa 1999, Loukili et al. 2000, Kovler and Jensen 2007).
4.2.4 example
As an example, the application of high performance concrete for a bridge
deck is considered, as discussed by Craeye et al. (2011). The effect of the
addition of SAP on the autogenous shrinkage of the high performance concrete, as measured by means of a vertical dilatometer for concrete, is shown
in Figure 4.3 (Craeye et al. 2011). A high performance reference concrete
(REF) was considered with the composition given in Table 4.1. The water/
cement ratio is 0.32. The water/binder ratio is 0.30. The super plasticizer
4.2.2.4 Paste volume
Shrinkage occurs in the cement matrix, while the aggregates in mortar and
concrete do not show any shrinkage behaviour. As a result, as is the case for
drying shrinkage as well, autogenous shrinkage increases with increasing
paste volume in the concrete.
4.2.3 Mitigation
The most straightforward measure to reduce autogenous shrinkage is to
increase the water/cement ratio. This of course is not always possible, especially in cases where a (very) high concrete strength is required. Furthermore,
in high strength concrete, quite often silica fume is applied in order to refine
the pore structure and further increase the strength. Unfortunately, this
also further increases the autogenous shrinkage. Some reduction in autogenous shrinkage could be obtained by a good selection of the cement type,
avoiding a high C 3 A content, and by a well considered combination of mineral additions and super plasticizers.
In case of high strength concrete, special ‘internal curing’ measures
might be needed (Kovler and Jensen 2007). Internal curing is defined as
the incorporation of a component, which serves as curing agent, to the
concrete mixture. Internal curing can be classified into two categories:
internal water curing (or water entrainment) and internal sealing. Internal
water curing seems to be the most effective in order to mitigate autogenous
shrinkage. This method consists of the incorporation into the cementitious
system of a curing agent serving as an internal water reservoir. The water
is gradually released as the concrete dries out due to self-desiccation. Super
absorbent polymers (SAP) and water-saturated fine lightweight aggregates
(LWA) are mostly considered as internal water reservoirs.
The effect of internal curing (or any other method to reduce or mitigate
autogenous shrinkage) can be measured by means of specially developed
test methods. Autogenous shrinkage can be experimentally studied by
means of different methods, including volumetric and linear measurements
(Tazawa 1999, Loukili et al. 2000, Kovler and Jensen 2007).
4.2.4 example
As an example, the application of high performance concrete for a bridge
deck is considered, as discussed by Craeye et al. (2011). The effect of the
addition of SAP on the autogenous shrinkage of the high performance concrete, as measured by means of a vertical dilatometer for concrete, is shown
in Figure 4.3 (Craeye et al. 2011). A high performance reference concrete
(REF) was considered with the composition given in Table 4.1. The water/
cement ratio is 0.32. The water/binder ratio is 0.30. The super plasticizer