17:15 〜 18:30
[HTT15-P04] INFLUENCE OF POROSITY AND MINERALOGY ON INFRARED THERMOGRAPHY MEASUREMENTS
キーワード：infrared thermography, porosity, mineralogy, cooling rate index
Infrared thermography (IRT) is a new tool in development in the field of cultural heritage, due mainly to the fact of being a non-destructive and non-invasive technique. Infrared thermography gives information about the apparent temperature of an object, related to its composition, shape, isolated defects and also decay state. However, within a building, sometimes the images show different values for the same material. Thus, a deep study on the consistency, the repeatability, and the establishment of key values is needed to better understand the reliability of this technique and to improve the accuracy in the stone characterization.
Three kinds of studies were carried out to determine the reliability and the influence of heterogeneity in the infrared thermographic signal:
- Detection threshold of small variations in porosity. For this study, ten cubes of 4 cm size of Fontainebleau sandstone were selected, with pure quartzitic composition, homogeneous grain size and sample color. Differents degrees of grain compaction were observed, which gave enough porosity and variability of physical properties for this study. Porosity was obtained following the standard EN-1936 and capillary water absorption as stated in EN-1925. Ultrasound measurements were measured with a Pundit-PL200 with transducers of 250KhZ. After this characterization, the cooling rate index (CRI) for each sample was obtained by infrared thermography as follows. The samples were dried at a temperature of 70°C and then extracted at room temperature. The cooling from the time= 30 seconds up to 15 minutes and 30 seconds was monitored with an infrared thermographic camera FLIR A645 at a speed rate of 1 frame every 10 seconds. The data were analyzed with a software researchIR and the CRI calculated as:
Tf-To / tf - to
Where To is the average temperature of the measured face at the beginning of the monitoring (to=30 sec after extraction of the oven), Tf is the average temperature of the measured face at a time tf.
A relationship between CRI and the porosity of each stone was studied, as well as the thermal response of each face with its vectorial properties (capillary coefficient and ultrasounds velocity).
-Influence of induced thermal fissuration on the thermosignal. Four granitoids with differences in mineralogy and grain size were tested. The samples were slabs of 7 x 3,5 x 2 cm. One sample of each type was submitted to one high- temperature heating, that is 200°C, 400°C, 600°C, and 800°C and compared to the intact rock. The heating rate was 6°C/min to force the fissuration and the samples were kept at the set temperature for four hours, enough to get uniform temperature through the sample. The cooling down was carried out freely with a rate <1°C/min, to ensure that the damage was only produced by the fast heating. Porosity, capillary water uptake and ultrasounds velocity was determined as in the previous study, as well as CRI calculation by IRT monitoring.
-Influence of mineralogy on the thermosignal. Each mineral has its own thermal properties. Thus, a rock behaves differently against temperature variations depending on its mineralogy and texture. Ten granitoids with differences in mineralogy and crystal size were heated at 70°C and the CRI was measured by infrared thermography similarly to the other studies. A relationship between the mineral proportion, grain size and thermal properties of each stone were related to CRI results.
The results showed that only great variations of porosity are related to CRI values as occurred between the intact stone and the rocks heated at 600°C or 800°C. Lower temperatures as 200°C or 400°C produced a variation of the microstructural system, although the infrared thermography did not permit to differentiate it accurately. Mineralogy plays an important role in CRI values, although the grain size resulted more decisive than the mineral proportion.
Three kinds of studies were carried out to determine the reliability and the influence of heterogeneity in the infrared thermographic signal:
- Detection threshold of small variations in porosity. For this study, ten cubes of 4 cm size of Fontainebleau sandstone were selected, with pure quartzitic composition, homogeneous grain size and sample color. Differents degrees of grain compaction were observed, which gave enough porosity and variability of physical properties for this study. Porosity was obtained following the standard EN-1936 and capillary water absorption as stated in EN-1925. Ultrasound measurements were measured with a Pundit-PL200 with transducers of 250KhZ. After this characterization, the cooling rate index (CRI) for each sample was obtained by infrared thermography as follows. The samples were dried at a temperature of 70°C and then extracted at room temperature. The cooling from the time= 30 seconds up to 15 minutes and 30 seconds was monitored with an infrared thermographic camera FLIR A645 at a speed rate of 1 frame every 10 seconds. The data were analyzed with a software researchIR and the CRI calculated as:
Tf-To / tf - to
Where To is the average temperature of the measured face at the beginning of the monitoring (to=30 sec after extraction of the oven), Tf is the average temperature of the measured face at a time tf.
A relationship between CRI and the porosity of each stone was studied, as well as the thermal response of each face with its vectorial properties (capillary coefficient and ultrasounds velocity).
-Influence of induced thermal fissuration on the thermosignal. Four granitoids with differences in mineralogy and grain size were tested. The samples were slabs of 7 x 3,5 x 2 cm. One sample of each type was submitted to one high- temperature heating, that is 200°C, 400°C, 600°C, and 800°C and compared to the intact rock. The heating rate was 6°C/min to force the fissuration and the samples were kept at the set temperature for four hours, enough to get uniform temperature through the sample. The cooling down was carried out freely with a rate <1°C/min, to ensure that the damage was only produced by the fast heating. Porosity, capillary water uptake and ultrasounds velocity was determined as in the previous study, as well as CRI calculation by IRT monitoring.
-Influence of mineralogy on the thermosignal. Each mineral has its own thermal properties. Thus, a rock behaves differently against temperature variations depending on its mineralogy and texture. Ten granitoids with differences in mineralogy and crystal size were heated at 70°C and the CRI was measured by infrared thermography similarly to the other studies. A relationship between the mineral proportion, grain size and thermal properties of each stone were related to CRI results.
The results showed that only great variations of porosity are related to CRI values as occurred between the intact stone and the rocks heated at 600°C or 800°C. Lower temperatures as 200°C or 400°C produced a variation of the microstructural system, although the infrared thermography did not permit to differentiate it accurately. Mineralogy plays an important role in CRI values, although the grain size resulted more decisive than the mineral proportion.