Actions during hardening 83
early-age thermal cracking (De Schutter 1996). The problem of early-age
thermal cracking in massive concrete structures is not new. Early examples
mentioning severe damage ascribed to thermal stresses occur in the literature such as the St. Francis dam in California in 1928. In more recent times,
early-age thermal cracking has been reported in more slender structural elements made with high performance concrete containing very high cement
contents.
Early-age thermal cracking can be subdivided in two categories: cracking
due to internal restraint, and cracking due to external restraint. Although
both categories typically occur simultaneously, they will be explained separately hereafter.
4.4.1.1 Early-age thermal cracking due to internal restraint
The main driving force for early-age thermal cracking in hardening concrete elements is the heat of hydration. Hydration of cement is an exothermic process. The produced hydration heat, depending on the type and
amount of cement, causes a temperature rise within the concrete. As the
heat conduction in concrete is relatively low (much lower than in steel), and
as heat is exchanged with the environment at the exposed surfaces, the core
of the concrete element will show a higher temperature than the surface
zone. The environmental temperature as well as the casting temperature of
the fresh concrete will have an important influence on the observed difference. Furthermore, the temperature gradient between core and surface will
be further enlarged due to the fact that the hydration process is accelerated
at higher temperatures.
In the case where the concrete core and surface are able to deform freely,
different thermal deformations (expansions) would be obtained, as shown
in Figure 4.8. However, as the concrete element is one solid block, free
deformations of the different zones are not possible. The larger free thermal
expansion of the element core will be restrained by the smaller free thermal
expansion of the concrete surface zone. As a result, an equilibrium deformation is obtained, inducing thermal compressive stresses in the core and
tensile stresses in the surface zone. The magnitude of the thermal stresses
at this stage heavily depends on the Young’s modulus and on the creep
and relaxation behaviour of the hardening concrete. Thermal cracks can
be formed in the surface zone when the tensile strength of the hardening
concrete is not sufficient to withstand the thermal tensile stresses. As the
Young’s modulus of hardening concrete is developing relatively faster than
the strength, a high cracking risk can occur.
At a later stage within the hardening process, the hydration process will
slow down, and the concrete element will start to cool down. As the core
of the element will have to cool down more than the surface zone, a stress
inversion will be obtained, as shown in Figure 4.8. Tensile stresses will
early-age thermal cracking (De Schutter 1996). The problem of early-age
thermal cracking in massive concrete structures is not new. Early examples
mentioning severe damage ascribed to thermal stresses occur in the literature such as the St. Francis dam in California in 1928. In more recent times,
early-age thermal cracking has been reported in more slender structural elements made with high performance concrete containing very high cement
contents.
Early-age thermal cracking can be subdivided in two categories: cracking
due to internal restraint, and cracking due to external restraint. Although
both categories typically occur simultaneously, they will be explained separately hereafter.
4.4.1.1 Early-age thermal cracking due to internal restraint
The main driving force for early-age thermal cracking in hardening concrete elements is the heat of hydration. Hydration of cement is an exothermic process. The produced hydration heat, depending on the type and
amount of cement, causes a temperature rise within the concrete. As the
heat conduction in concrete is relatively low (much lower than in steel), and
as heat is exchanged with the environment at the exposed surfaces, the core
of the concrete element will show a higher temperature than the surface
zone. The environmental temperature as well as the casting temperature of
the fresh concrete will have an important influence on the observed difference. Furthermore, the temperature gradient between core and surface will
be further enlarged due to the fact that the hydration process is accelerated
at higher temperatures.
In the case where the concrete core and surface are able to deform freely,
different thermal deformations (expansions) would be obtained, as shown
in Figure 4.8. However, as the concrete element is one solid block, free
deformations of the different zones are not possible. The larger free thermal
expansion of the element core will be restrained by the smaller free thermal
expansion of the concrete surface zone. As a result, an equilibrium deformation is obtained, inducing thermal compressive stresses in the core and
tensile stresses in the surface zone. The magnitude of the thermal stresses
at this stage heavily depends on the Young’s modulus and on the creep
and relaxation behaviour of the hardening concrete. Thermal cracks can
be formed in the surface zone when the tensile strength of the hardening
concrete is not sufficient to withstand the thermal tensile stresses. As the
Young’s modulus of hardening concrete is developing relatively faster than
the strength, a high cracking risk can occur.
At a later stage within the hardening process, the hydration process will
slow down, and the concrete element will start to cool down. As the core
of the element will have to cool down more than the surface zone, a stress
inversion will be obtained, as shown in Figure 4.8. Tensile stresses will