120 Damage to concrete structures
nickel steel and the outer tank is made of reinforced or prestressed concrete.
However, the inner tank itself could also be constructed of prestressed concrete (The Concrete Society 1981).
The properties of concrete in a temperature range between 20°C and
cryogenic temperature depend on the moisture content. For moist or saturated concrete, the crystalline ice structure below 0°C is of importance
(Waagaard 1981). At normal air pressure, the stable phase of ice is called
ice I, with two variants: hexagonal ice I h (formed between 0°C and −115°C)
and cubic ice I c (formed below −115°C). At even lower temperatures around
−200°C, orthorhombic ice is formed. The density and the mechanical properties of ice depend on the crystalline ice structure, which, in turn, influences the cryogenic concrete behaviour.
Wiedemann (1982) studied the cryogenic behaviour of saturated concrete. While considering a cycle of cooling and reheating, he distinguished
different temperature ranges as illustrated in Figure 5.15. Between 0°C and
−20°C, the water in the larger pores freezes and becomes ice, pushing excess
water to partially filled pores or to air voids. The ice, which first completely
filled the larger pores will, while further cooling down, contract more than
the cement matrix. Because of thermodynamic non-equilibrium, water will
now move from smaller pores toward the ice in the larger pores and freeze
to ice. In this temperature range, the concrete contracts,while compressive
strength and ultimate strain increase with decreasing temperature because
ice-filled pores are stronger than water-filled pores.
When further cooling down within the range of −20°C to −60°C, the
coarser pores are totally filled with ice. This will obstruct the movement
of water from the smaller pores, which will lead to stress formation in the
concrete, possibly leading to cracks. The concrete will expand and the rise
of the compressive strength while cooling down is partly counteracted by
internal crack formation. The ultimate strain reaches a peak value.
Between −60°C and −90°C, the ice in the pores contracts more than the
cement matrix, relaxing internal stresses in the concrete. Furthermore, ice
filling the pores contributes to a further increase in strength while cooling down. Even in the very small pores, water is now freezing. Thanks to
existing internal cracks, ample space is available and no additional stresses
are initiated. The higher thermal contraction of the matrix in comparison
with the aggregates, caused by a difference in coefficient of thermal dilation, results in a prestress of the interfacial transition zone (ITZ). Due to
this internal prestressing effect, crack propagation starting at the ITZ is
mitigated, yielding a further increase of the compressive strength of the
concrete.
Further cooling between −90°C and −170°C leads to a relatively higher
volume reduction of the ice in comparison with the reduction in pore volume due to contraction of the concrete. The increase in compressive strength
will be less pronounced, while the ultimate strain shows a decrease. Within
nickel steel and the outer tank is made of reinforced or prestressed concrete.
However, the inner tank itself could also be constructed of prestressed concrete (The Concrete Society 1981).
The properties of concrete in a temperature range between 20°C and
cryogenic temperature depend on the moisture content. For moist or saturated concrete, the crystalline ice structure below 0°C is of importance
(Waagaard 1981). At normal air pressure, the stable phase of ice is called
ice I, with two variants: hexagonal ice I h (formed between 0°C and −115°C)
and cubic ice I c (formed below −115°C). At even lower temperatures around
−200°C, orthorhombic ice is formed. The density and the mechanical properties of ice depend on the crystalline ice structure, which, in turn, influences the cryogenic concrete behaviour.
Wiedemann (1982) studied the cryogenic behaviour of saturated concrete. While considering a cycle of cooling and reheating, he distinguished
different temperature ranges as illustrated in Figure 5.15. Between 0°C and
−20°C, the water in the larger pores freezes and becomes ice, pushing excess
water to partially filled pores or to air voids. The ice, which first completely
filled the larger pores will, while further cooling down, contract more than
the cement matrix. Because of thermodynamic non-equilibrium, water will
now move from smaller pores toward the ice in the larger pores and freeze
to ice. In this temperature range, the concrete contracts,while compressive
strength and ultimate strain increase with decreasing temperature because
ice-filled pores are stronger than water-filled pores.
When further cooling down within the range of −20°C to −60°C, the
coarser pores are totally filled with ice. This will obstruct the movement
of water from the smaller pores, which will lead to stress formation in the
concrete, possibly leading to cracks. The concrete will expand and the rise
of the compressive strength while cooling down is partly counteracted by
internal crack formation. The ultimate strain reaches a peak value.
Between −60°C and −90°C, the ice in the pores contracts more than the
cement matrix, relaxing internal stresses in the concrete. Furthermore, ice
filling the pores contributes to a further increase in strength while cooling down. Even in the very small pores, water is now freezing. Thanks to
existing internal cracks, ample space is available and no additional stresses
are initiated. The higher thermal contraction of the matrix in comparison
with the aggregates, caused by a difference in coefficient of thermal dilation, results in a prestress of the interfacial transition zone (ITZ). Due to
this internal prestressing effect, crack propagation starting at the ITZ is
mitigated, yielding a further increase of the compressive strength of the
concrete.
Further cooling between −90°C and −170°C leads to a relatively higher
volume reduction of the ice in comparison with the reduction in pore volume due to contraction of the concrete. The increase in compressive strength
will be less pronounced, while the ultimate strain shows a decrease. Within