[HTT22-P05] Use of strain gages in the control of frost action
キーワード：frost weathering, unidirectional freezing, strain gages, limestone
Frost action is one of the major weathering factors of building stones. Freezing of water inside the porous network causes dilation due to the volume change of water (9%) and the movements of liquid water due to cryosuction. On a monument, the particularity is that frost enters the stone only through the exposed façade. The surrounding blocks and the milder indoor conditions protects the other faces. To assess the effect of freeze-thaw on stones, standard tests are commonly used or adapted in terms of temperature or time of freezing. All these tests, even the adapted ones, consist in freezing the samples after prior immersion in water and thawing them in room temperature water. All the stone faces are exposed to damage, though this does not reproduce the real monument conditions.
These standardized tests assess the decay produced by the ice by means of loss of material (visible or by weight measurements) and tensile strength decrease. In both cases, if the test itself is not destructive enough to produce material detachment, these evaluation methods are irrelevant. In addition, this kind of tests cannot be performed out of the laboratory and in real monuments with immovable materials.
In this study we tested an alternative solution to these constraints with a non destructive setting that allowed to produce freezing on only one face of the stone and the use of strain gages attached along the stone to assess microscopical structural damages. This setting was tested on a common building Lutetian limestone.
Two kinds of water saturation were tested: i) partial saturation obtained by continuous capillary water supply, and ii) partial saturation obtained by a previous total immersion during 48 hours. In the first one, the capillary fringe reached 6cm height of the sample and a differential damage produced between the bottom and the top of the sample was expected. In the second one a homogeneous damage along the sample was expected.
To produce the freezing, a cooling plate was placed vertically in contact to one vertical face of the prismatic sample. The rest of the faces were sealed with a thermal insulator to avoid evaporation and cooling.
To assess frost action strain gauges were set up at different heights of the sample with different orientations from the direction of freezing penetration. To control the freezing penetration inside the sample, thermocouples were placed at different depths within the stone specimen. Even this was a partially destructive method, it was necessary to control if freezing was produced within the stone and at which moment, and then to establish the best experimental protocol. Previous tests showed that the cooling plate had to be set up at -15°C in order to reach temperature below 0°C inside the sample.
Five cycles of continuous freezing were applied. Cycles were divided in 12 hours of freezing at -15°C and 12 hours of thawing at +10°C. During the whole time of the two tests, the sample was water supplied by capillary absorption from a bottom water tank.
For both tests, the sample could be divided in three parts according to its dilation behavior: the capillary zone (up to 6 cm), the fringe zone (6 cm) and the upper zone not soaked.
Results showed the deformation perpendicular to the freezing direction was negligible except for the fringe zone where it could reach 6.10-4. The deformation parallel to the freezing direction showed an expansion during freezing and a contraction during thawing whose intensity was correlated to saturation. Only at 6 cm this deformation was close to zero. A small residual deformation was recorded only with the total saturation.
The use of gauges small enough (2mm) to produce the less damage possible even when removed after the test, could be applied in real stone monuments. In addition, they measured the damage produced on the surface not only by ice but also by temperature changes or liquid water movements. The gauges allowed measuring the immediate damage before it reached the inside of the stone and before a visual degradation. This original test also showed the importance of the capillary fringe in the frost action on building stones.
These standardized tests assess the decay produced by the ice by means of loss of material (visible or by weight measurements) and tensile strength decrease. In both cases, if the test itself is not destructive enough to produce material detachment, these evaluation methods are irrelevant. In addition, this kind of tests cannot be performed out of the laboratory and in real monuments with immovable materials.
In this study we tested an alternative solution to these constraints with a non destructive setting that allowed to produce freezing on only one face of the stone and the use of strain gages attached along the stone to assess microscopical structural damages. This setting was tested on a common building Lutetian limestone.
Two kinds of water saturation were tested: i) partial saturation obtained by continuous capillary water supply, and ii) partial saturation obtained by a previous total immersion during 48 hours. In the first one, the capillary fringe reached 6cm height of the sample and a differential damage produced between the bottom and the top of the sample was expected. In the second one a homogeneous damage along the sample was expected.
To produce the freezing, a cooling plate was placed vertically in contact to one vertical face of the prismatic sample. The rest of the faces were sealed with a thermal insulator to avoid evaporation and cooling.
To assess frost action strain gauges were set up at different heights of the sample with different orientations from the direction of freezing penetration. To control the freezing penetration inside the sample, thermocouples were placed at different depths within the stone specimen. Even this was a partially destructive method, it was necessary to control if freezing was produced within the stone and at which moment, and then to establish the best experimental protocol. Previous tests showed that the cooling plate had to be set up at -15°C in order to reach temperature below 0°C inside the sample.
Five cycles of continuous freezing were applied. Cycles were divided in 12 hours of freezing at -15°C and 12 hours of thawing at +10°C. During the whole time of the two tests, the sample was water supplied by capillary absorption from a bottom water tank.
For both tests, the sample could be divided in three parts according to its dilation behavior: the capillary zone (up to 6 cm), the fringe zone (6 cm) and the upper zone not soaked.
Results showed the deformation perpendicular to the freezing direction was negligible except for the fringe zone where it could reach 6.10-4. The deformation parallel to the freezing direction showed an expansion during freezing and a contraction during thawing whose intensity was correlated to saturation. Only at 6 cm this deformation was close to zero. A small residual deformation was recorded only with the total saturation.
The use of gauges small enough (2mm) to produce the less damage possible even when removed after the test, could be applied in real stone monuments. In addition, they measured the damage produced on the surface not only by ice but also by temperature changes or liquid water movements. The gauges allowed measuring the immediate damage before it reached the inside of the stone and before a visual degradation. This original test also showed the importance of the capillary fringe in the frost action on building stones.