·Table of Contents
Compatibility assessment of building materials using Infrared Thermography
A. Moropoulou, N.P. Avdelidis, M. Koui
National Technical University of Athens, Department of Chemical Engineering, Section of Materials Science and Engineering,
9 Iroon Polytechniou St., Zografou 15780, Athens, Greece
In this work, infrared thermography is employed with the intention of evaluating the moisture distribution by capillary rise in reference porous material specimens in the laboratory, in an attempt to substantiate the investigation of real scale material systems in situ. The assessment of repair mortars, in lab and in situ on historic monuments in Greece, permits to evaluate the performance of conservation materials, regarding their compatibility to the porous building stones on historic masonries. Furthermore, the study of water evaporation transport phenomena in prototype simulating porous materials under stable environmental conditions (Relative Humidity & Temperature) by the use of infrared thermography provides essential information by becoming the key element in order to interpret the transport phenomena occurring at the masonry. It is assumed that infrared thermography by recording thermal images of the genuine surfaces under survey provides valuable information on the differential behaviour of the different materials on the masonry scale concerning the water saturation and evaporation phenomena, which trigger the weathering effects in porous materials. Consequently, infrared thermography can be exploited as a non-destructive technique, with the intention of assessing the performance of conservation interventions and materials, in compatibility to the authentic building materials on the level of the structures.
In previous works [1,2], it has been proved that water absorption by capillary rise and evaporation phenomena control the weathering effects in porous media and generate damages such as salt decay on heterogeneous systems like historic masonries. The study of vapour/moisture transport phenomena in porous building materials is vital, in order to understand the respiration behaviour and weathering of such materials. Incompatible capillary systems (heterogeneous material systems), such as historic masonries, have been found to go through discriminated weathering processes and rates, which generate reinforced damage to the weaker materials . Infrared thermography has been used to detect in situ differential weathering performance of building materials in real scale systems . The temperature variations detected by infrared thermography were up to now explained by the microstructural characteristic differences of the materials and the consequently reserved moisture contents, studied as equilibrium phenomena under controlled environmental (relative humidity and temperature) conditions . Nonetheless, the degree of the water capillary absorption, in addition to the evaporation rate, concerning the dynamic study of materials of the water/vapour transport phenomena in the porous systems is essential, for the explanation of the thermographs of real scale systems. In this work, the explanation of the in situ infrared thermographs is attempted by the study of moisture gradation and distribution, in real time, in porous building materials and simulating masonry prototypes in lab.
Experimental Procedure: Materials & Techniques
Infrared Thermography (Avio Tvs-2000 MkII LW, 8-12mm) was used, for the evaluation of the moisture distribution by capillary rise (absorption, evaporation) in reference samples of porous materials in the laboratory, in order to validate the examination of a real scale material system in situ. Infrared Thermography detects the radiation that a material emits and can render the image of the surface area in colours, in relation to a temperature scale, providing thermal maps.
Capillary rise tests for the determination of the water absorption percentages and coefficients were performed in reference samples in a normalised sand bed apparatus. The water absorption coefficient was calculated as:
where : CA: (g*cm-2*s-1/2), according to standard NORMAL 11/85 is :
DB=B-B0: the length of the asymptotic value where the DB difference between the initial and the final weight of the sample is less than 1% (g)
S: the surface area of the sample contact in water (cm2)
t: the time of the absorption resultant from the asymptotic length of DB and the tangential extrapolation of the linear part of the curve (s).
Along with the capillary rise tests, the behaviour of these samples and the simulating prototypes during evaporation was studied .
The material samples, the reference porous stone, the three basic categories of repair mortars and the simulating prototypes, were examined in the laboratory (Table I). A historic masonry, from the Venetian Fortifications in Heraklion, Crete was also examined.
||Description of the Samples
||Cement Mortar: C/S 1:3
|| Hydraulic Lime Mortar: HL/S 1:3
|| Lime Mortar: L/S 1:2
|| Porous Stone
|| Masonry Prototype: porous stone (ST1) & cement mortar (CS1)
|SP2 || Masonry Prototype: porous stone (ST1) & hydraulic lime mortar (HLS1)
|| Masonry Prototype: porous stone (ST1) & lime mortar (LS1)
|Table 1 :|
Results & Discussion
The results obtained from the capillary rise tests regarding the water capillary absorption and evaporation rate allow for the interpretation of the thermographs in lab and in situ. In particular, samples that present high water absorption percentages, exhibit considerable temperature reductions, while samples with low water absorption percentages offer small temperature differences. In Table II, the water absorption coefficients of stone, hydraulic lime, lime and cement mortars, present a sequence of declining values. This sequence interprets the thermographs of the simulating prototypes, since the water absorption gradation is more acute in the opposite sequence SP1>SP2>SP3. The intensified weathering of the porous stone depends on the anisotropy of the water gradation and distribution within it, due to the diversified water/vapour transport at the interface with the joint mortar, as observed in figure 1, left column. Hence, the porous stone (ST1) is compatible to the hydraulic lime mortar (HLS1) and to the lime mortar (LS1) and incompatible to the cement mortar (CS1). Environmental conditions like temperature and relative humidity for a given aeration to the surface determine the evaporation rate of an impregnated building material . This explains the evaporation results of the simulating prototypes (Figure 2), where high temperature and low relative humidity percentage, (40°
C & 60%), accelerate the evaporation rate, which is in agreement with the thermographs (Figure 1, right column). The evaporation behaviour of the stone in the prototypes accounts for the intensified weathering under evaporation conditions in the same relation to repair mortars as in the case of water absorption. This explains the thermograph of the Venetian Fortifications in Heraklion after in situ investigation (Figure 3), where sampling in the hot and dry areas evidenced cement repair mortars, whereas in the colder areas building stones and traditional lime mortars are evidenced . It is deduced that Infrared Thermography by recording thermal maps of the real surfaces under study provides information on the differential behaviour of the various materials on the masonry scale regarding the water impregnation and evaporation phenomena, which control the weathering effects in porous media. Hence, infrared thermography might be used as a non-destructive technique to evaluate the performance of conservation interventions and materials, in compatibility to the original materials on the level of the structures.
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