Actions during hardening 71
reduction is called the chemical shrinkage (Koenders 1997, Bjøntegaard
1999, Tazawa 1999).
Due to the chemical shrinkage, the pore volume inside the hardening
cement paste will increase with increasing degree of hydration. When no
external water supply from the surrounding environment is provided,
the water content in the pore volume decreases, while the air content
increases. This is called self-desiccation. As a result of chemical shrinkage
and self-desiccation, the air pressure in the pores will decrease, and will
equilibrate with the capillary tension. The increasing capillary tension
will cause a macroscopic volume reduction of the hardening cementitious
material, which is referred to as autogenous shrinkage (Koenders 1997,
Bjøntegaard 1999, Tazawa 1999). In a more complete picture, it has to
be mentioned that autogenous shrinkage is influenced by several mechanisms: changes in the surface tension of the solid gel particles, disjoining pressure, and capillary tension (Kovler and Jensen 2007). Autogenous
shrinkage does not include any effects on the volume caused by external
mechanical loading, temperature variations, or loss or ingress of substances. Although autogenous shrinkage can be expressed as a percentage
of volume reduction, a more common way to quantify autogenous shrinkage is to express it as a one-dimensional length change, the autogenous
shrinkage strain.
Quite often, autogenous shrinkage is only considered after initial setting,
excluding volume changes generated in the fresh state of the cementitious
material. The reasoning behind this limitation is the fact that autogenous shrinkage is typically considered for the study of early-age cracking. Shrinkage stresses are only initiated once the material shows a certain
stiffness, which begins after percolation of the microstructure. As this
percolation coincides with the phenomenon of setting, the time of initial
setting is typically specified as the starting point of autogenous shrinkage. In a more general way, this starting point is referred to as ‘time zero’
(Miao et al. 2007). In order to determine the value of time zero, estimations
can be made based on the Vicat needle test, compressive strength results,
heat of hydration, or ultrasonic transmission measurements (Bentur 2003,
De Schutter 1996, De Schutter and Taerwe 1996, Robeyst 2008). For a
detailed discussion on this topic, reference is made to scientific literature.
The correspondence between chemical shrinkage and autogenous
shrinkage is illustrated in Figure 4.2. Immediately after water addition, the
hydration process evolves, introducing chemical shrinkage into the cementbased system. As soon as setting initiates, the effects of chemical shrinkage
and self-desiccation cause a macroscopic volume reduction of the hardening material, called autogenous shrinkage, which is much smaller than
the purely chemical shrinkage. The difference between chemical shrinkage
and autogenous shrinkage represents the volume of voids formed into the
system due to hydration.
reduction is called the chemical shrinkage (Koenders 1997, Bjøntegaard
1999, Tazawa 1999).
Due to the chemical shrinkage, the pore volume inside the hardening
cement paste will increase with increasing degree of hydration. When no
external water supply from the surrounding environment is provided,
the water content in the pore volume decreases, while the air content
increases. This is called self-desiccation. As a result of chemical shrinkage
and self-desiccation, the air pressure in the pores will decrease, and will
equilibrate with the capillary tension. The increasing capillary tension
will cause a macroscopic volume reduction of the hardening cementitious
material, which is referred to as autogenous shrinkage (Koenders 1997,
Bjøntegaard 1999, Tazawa 1999). In a more complete picture, it has to
be mentioned that autogenous shrinkage is influenced by several mechanisms: changes in the surface tension of the solid gel particles, disjoining pressure, and capillary tension (Kovler and Jensen 2007). Autogenous
shrinkage does not include any effects on the volume caused by external
mechanical loading, temperature variations, or loss or ingress of substances. Although autogenous shrinkage can be expressed as a percentage
of volume reduction, a more common way to quantify autogenous shrinkage is to express it as a one-dimensional length change, the autogenous
shrinkage strain.
Quite often, autogenous shrinkage is only considered after initial setting,
excluding volume changes generated in the fresh state of the cementitious
material. The reasoning behind this limitation is the fact that autogenous shrinkage is typically considered for the study of early-age cracking. Shrinkage stresses are only initiated once the material shows a certain
stiffness, which begins after percolation of the microstructure. As this
percolation coincides with the phenomenon of setting, the time of initial
setting is typically specified as the starting point of autogenous shrinkage. In a more general way, this starting point is referred to as ‘time zero’
(Miao et al. 2007). In order to determine the value of time zero, estimations
can be made based on the Vicat needle test, compressive strength results,
heat of hydration, or ultrasonic transmission measurements (Bentur 2003,
De Schutter 1996, De Schutter and Taerwe 1996, Robeyst 2008). For a
detailed discussion on this topic, reference is made to scientific literature.
The correspondence between chemical shrinkage and autogenous
shrinkage is illustrated in Figure 4.2. Immediately after water addition, the
hydration process evolves, introducing chemical shrinkage into the cementbased system. As soon as setting initiates, the effects of chemical shrinkage
and self-desiccation cause a macroscopic volume reduction of the hardening material, called autogenous shrinkage, which is much smaller than
the purely chemical shrinkage. The difference between chemical shrinkage
and autogenous shrinkage represents the volume of voids formed into the
system due to hydration.