Nonlinear Behavior of Concrete and Zero-Stress Temperature
Due to the exothermic reaction of the cement with the mixing water, hydration
heat develops during the setting period of the concrete. During hydration, the
temperature of the young concrete and the elastic modulus increases. At first,
compressive stresses developed due to the potential of volume increase with
restrained deformation, with a partial initial relaxation caused by the low initial
modulus of elasticity. After the maximum temperature is exceeded, a small decrease
in temperature will be sufficient to compensate the compressive stresses due to
relatively higher modulus of elasticity. The temperature leading to a stage of no stress
is called zero stress temperature (Aufleger et al., 2002; see Figure 5). A further
decrease in temperature generates tensile stresses that when exceeding the tensile
strength in concrete, will lead to thermal cracks. The higher the concrete’s zero stress
temperature and the faster the temperature decreases to an average ambient
temperature, the higher will be the resulting potential of thermal-induced cracking.
The cracks have to be divided into surface cracks and transversal cracks. Transversal
cracks are related to the whole cross section and require a relatively long period of
time because after increasing relatively rapidly the hydration heat flows off very
slowly. In general thermal induced stresses in concrete are caused by temperature
gradients and by temperature differences between the current concrete temperature
and the zero stress temperature. Depending on the spatial distribution and the
temporal development of the zero stress temperature and the actual concrete
temperature, thermal induced stresses can be strongly variable over the dam body.
Vertical zero stress temperature gradients, specifically caused by breaks, are as likely
as horizontal zero stress temperature gradients usually caused by the interaction
between concrete and ambient temperatures. In consequence, the zero stress
temperature will vary significantly over the dam body.
Chia sẻ với bạn bè của bạn: |