76 Damage to concrete structures
In order to mitigate the autogenous shrinkage and to prevent early-age
cracking, internal curing can be applied by means of SAP. The applied SAP
has a water absorption capacity of 45 g/g after 5 minutes (the approximate mixing time). Based on this absorption level, the required SAP content is estimated, aiming for an amount of internal curing water equal to
50 kg/m 3 (SAP50), 70 kg/m 3 (SAP70), and 90 kg/m 3 (SAP90). This leads to
a corresponding SAP amount of respectively 1.11 kg/m 3 , 1.56 kg/m 3 and
2.00 kg/m 3 , as shown in Table 4.1. The extra (internal) curing water has to
be added to the concrete during mixing. In order to obtain 1 m³ of concrete,
the sand content is reduced accordingly.
From Figure 4.3, it is clear that a significant reduction of the autogenous
shrinkage is obtained, although full mitigation is not reached. In parallel however, other aspects have to be studied, as the addition of SAP and
internal curing water will also influence mechanical and thermal properties. More details can be found in Craeye et al. (2011). As a conclusion,
however, it is found that due to the internal curing and its significant effect
on autogenous shrinkage strains, early age cracking within the high performance bridge deck can be prevented.
4.3 DryIng shrInKage
4.3.1 Mechanism
Cement paste typically contains different types of water: chemically bound,
physically adsorbed, and free water (Hansen 1986). The chemically bound
water has become part of the hydration products, as a result of the chemical reactions between cement and water. Physically adsorbed water can be
found in the gel pores, adsorbed at the surface of the hydration products.
Free water is present in the capillary pores. Physically adsorbed water and
free water are also referred to as evaporable water, which can be removed
from the cement paste by drying. The amount of evaporable water is
strongly dependent on the water/cement ratio and the degree of hydration.
Drying shrinkage of concrete occurs when evaporable water is removed
to a non-saturated environment. First, the free capillary water is removed,
leading to a volume reduction of the concrete. When the capillary pores are
empty, the physically bound water will also be slowly expelled, significantly
increasing the volume reduction. Although from a phenomenological point
of view the origin of drying shrinkage is straightforward (loss of water to
a non-saturated environment), the contributing fundamental mechanisms
at nano scale are not fully clear. Different mechanisms seem to contribute: capillary stress, disjoining pressure, movement of interlayer water, and
changes in free surface energy (Mindess and Young 1981, Hansen 1987,
Bisschop 2002).
In order to mitigate the autogenous shrinkage and to prevent early-age
cracking, internal curing can be applied by means of SAP. The applied SAP
has a water absorption capacity of 45 g/g after 5 minutes (the approximate mixing time). Based on this absorption level, the required SAP content is estimated, aiming for an amount of internal curing water equal to
50 kg/m 3 (SAP50), 70 kg/m 3 (SAP70), and 90 kg/m 3 (SAP90). This leads to
a corresponding SAP amount of respectively 1.11 kg/m 3 , 1.56 kg/m 3 and
2.00 kg/m 3 , as shown in Table 4.1. The extra (internal) curing water has to
be added to the concrete during mixing. In order to obtain 1 m³ of concrete,
the sand content is reduced accordingly.
From Figure 4.3, it is clear that a significant reduction of the autogenous
shrinkage is obtained, although full mitigation is not reached. In parallel however, other aspects have to be studied, as the addition of SAP and
internal curing water will also influence mechanical and thermal properties. More details can be found in Craeye et al. (2011). As a conclusion,
however, it is found that due to the internal curing and its significant effect
on autogenous shrinkage strains, early age cracking within the high performance bridge deck can be prevented.
4.3 DryIng shrInKage
4.3.1 Mechanism
Cement paste typically contains different types of water: chemically bound,
physically adsorbed, and free water (Hansen 1986). The chemically bound
water has become part of the hydration products, as a result of the chemical reactions between cement and water. Physically adsorbed water can be
found in the gel pores, adsorbed at the surface of the hydration products.
Free water is present in the capillary pores. Physically adsorbed water and
free water are also referred to as evaporable water, which can be removed
from the cement paste by drying. The amount of evaporable water is
strongly dependent on the water/cement ratio and the degree of hydration.
Drying shrinkage of concrete occurs when evaporable water is removed
to a non-saturated environment. First, the free capillary water is removed,
leading to a volume reduction of the concrete. When the capillary pores are
empty, the physically bound water will also be slowly expelled, significantly
increasing the volume reduction. Although from a phenomenological point
of view the origin of drying shrinkage is straightforward (loss of water to
a non-saturated environment), the contributing fundamental mechanisms
at nano scale are not fully clear. Different mechanisms seem to contribute: capillary stress, disjoining pressure, movement of interlayer water, and
changes in free surface energy (Mindess and Young 1981, Hansen 1987,
Bisschop 2002).