International Symposium (NDT-CE 2003)Non-Destructive Testing in Civil Engineering 2003
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APPLICATION OF THERMOGRAPHY AND ULTRA-SOUNDS FOR WALL ANOMALIES DIAGNOSIS A LABORATORY RESEARCH STUDYCARLOS PINA SANTOS, National Laboratory of Civil Engineering, Lisbon, Portugal
LUIS MATIAS, National Laboratory of Civil Engineering, Lisbon, Portugal
ANA CRISTIAN MAGALHÃES, National Laboratory of Civil Engineering, Lisbon, Portugal
MARIA ROSARIO VEIGA, National Laboratory of Civil Engineering, Lisbon, Portugal
The presence of moisture (liquid water or water vapour) and the occurrence of detachments of renders are the cause of performance degradation of building walls and of the reduction of their durability.
Moisture and detachment detection is usually done by destructive and non-destructive methods. Thermography and ultra-sound techniques can reveal as interesting non-destructive methods in the detection of the location and evaluation of the detachments extent and of the moisture distribution in building elements. Nevertheless several factors may difficult their use in field applications.
In order to obtain basic information on the potentialities of thermography and ultra-sound techniques in this field a research study is being developed at the National Laboratory of Civil Engineering (LNEC).
Under controlled laboratory conditions a hollow lightweight concrete block wallette was submitted to weathering cycles (temperature and driving rain). The surface degradation was assessed with thermographic analysis and ultra-sound measurements.
Moisture and detachment detection in building elements is usually done by destructive and non-destructive methods . Thermography can reveal an interesting non-destructive method in the detection and evaluation of moisture distribution in building elements, although several factors may difficult its use in field applications. Thermography and ultra-sound techniques may also be applied to the detection of detachments in multilayered building elements, namely rendered walls.
A research study is presently being carried out at LNEC´s Building Department in order to obtain essential information on the potentialities of both techniques in these domains. The scope of ongoing first phase of the research study includes laboratory-controlled experiments. Subsequent field applications are envisaged for the second stage of the study.
Some previous laboratory studies have been performed at LNEC, using termographic  and ultrasound techniques  on small size test specimens, with the aim of exploring their potentialities for diagnosis.
Following these initial studies a full size hollow lightweight concrete block test wall was used to assess the laboratory application of both non-destructive tecnhiques (thermography and ultra-sounds) to the detection of moisture and render detachments. Tests performed and main results of this study are described hereafter.
2 Test specimen
A lightweight concrete hollow block wall (fig. 1) with overall facial dimensions of 3,6 m x 2,2 m and 0,34 m thick was used. Both faces were rendered with traditional lime/cement mortar. Due to previous weathering tests (heat/cold, driving rain and freeze/thaw cicles)  one of the major faces of this wallette suffered several degradations, namely cracking and localized render detachments. After the weathering tests some mortar (fig. 1, bottom) and a concrete block (fig. 1, left side) were removed to inspect mortar joints. After inspection mortar render was repaired (fig. 1).
Thermal images (thermograms) were acquired and recorded using infrared thermographic equipment (fig. 2) with a spectral sensitivity range of 3 m m to 5,4 m m and 0,1° C minimum resolvable temperature difference (MRTD).
Ultra-sound equipment (fig. 3) with one pair of standard probes, one generator of electric pulses, an amplifier, an electronic circuit of time measure. The measure range of transmission time of this test instrument is 0,1 to 999,9 ms and the standard probes resonance frequency is 40-50 kHz.
Temperature and relative humidity was measured in three selected points of the test specimen using mini-probes with a diameter of 5 mm and a length of 50 mm. The probes were inserted in 6 mm diameter and 50 mm deep holes drilled on the vertical axis of the non-exposed face of the test wall specimen.
Weathering (vd. 2), driving rain and convection heating (vd. 4.1, 4.2) of one of the test wall surfaces was achieved through the use of a programmable large size climatic chamber.
4 Test procedure and results
4.1 Moisture detection
After the end of the wetting period the wall was allowed to dry naturally under normal laboratory conditions. Thermal images (thermograms) were obtained both during wetting and subsequent drying period.
As expected, due to evaporative cooling and as long as moisture was visible on the faces of the wall, thermograms clearly revealed its presence (fig. 4).
On the non-exposed surface temperature differences between wet and dry surface areas were around 1,5 °C. As drying progressed surface moisture stains dissipated. Four days after the end of the wetting phase moisture was hardly visible however thermal heterogeneity due to evaporation still was thermographicaly detected for another six days (fig. 5).
Long after last thermograms were obtained mini-probe measurements of relative humidity under this surface were steadily above 95% RH revealing that the wall still had a relatively high moisture content.
The test wall surface subjected to driving rain exposure was completely moisted and therefore in the initial drying phase evaporation rate and temperature were significantly uniform (surface temperature was 3 °C lower than ambient temperature). Gradually the surface kept drying and after seven days moist areas were not visible anymore on this surface. However thermographic images revealed the presence of heterogeneous moisture distribution (fig. 6) up to ten days after the end of the wetting phase.
In a thermally stable environment (such as the one in this experiment) moisture can only be detected by the rate of evaporative cooling on the surface. The presence of a significant amount of liquid water (initial drying phase) provides a high rate of surface evaporation and consequently sensible temperature differences. As surface water content decreases surface temperature heterogeneity attenuates and thermographic qualitative assessment of moisture distribution becomes more and more difficult.
The second phase of the experiment intended to take advantage of the higher thermal capacity and conductivity of the moisture still retained in the test wall. After ten days of drying the previously wetted test wall surface was heated through an imposed convective heat flux. Warm air (25 °C) was blown onto the wall surface for 16 hours and new thermograms were obtained.
Although surface thermal heterogeneities were still detected they could hardly be attributed to moisture presence (vd. 4.2). Cooler stains shown in figure 7 have the same pattern and location as those found in detached areas (vd. 4.2.1). It is not reasonable to admit that water could still be trapped in detached areas.
4.2 Detachment detection
A detached render creates a thin air layer that introduces an additional thermal resistance. Compared with the thermal resistance of the wall, the one associated with the air space behind the detached render is too small to consider the interest of a steady state heat transfer setup. Unsteady conditions had to be created in order to create transient detectable thermal heterogeneities.
As in the previous experiment, the degraded wall surface was heated through an imposed convective heat flux. Hot air
A clear identification of localised detached areas was possible up to 15 minutes after surface heating ended (fig. 8). Although 70 °C is a too high temperature, under field and common circumstances, the lower temperature level (25 °C) created in the moisture detection experiment (vd. 4.1) has also revealed the detachments location.
This points out to the practical interest in exploring the use of incident solar radiation as a surface heating source for field evaluation of detachment occurrences. Obviously this approach requires sun irradiated external surfaces and a careful choice of the best time and conditions to perform the thermographic analysis.
4.2.2 Ultra-sound evaluation
Several measurement profiles were defined on the test wall surface (fig. 10). In each profile the ultra-sound emitter probe was fixed in a given position and the receiver probe was moved along the profile line. Table 1 summarizes results of measurements done. Complementarily, the ultra-sound indirect method was also applied on the opposite wall surface. This surface had not been directly exposed to weathering cycles but showed some evidence of degradation, namely cracking. The results of these measurements are also reported in table 1.
Table 1 shows that, in general, lower measured ultra-sound transmission speeds were obtained on the directly weathered test wall surface, as a possible consequence of render cracking and detachments.
Although it seems possible to conclude that lower transmission speeds are an evidence of the degradation of the directly weathered surface, they probably are rather a consequence of generalized macro and micro cracking of the mortar render then of detachments. As a matter of fact a clear relation was not found between lower transmission speeds and eventually detached areas detected by thermografic evaluation (vd. 4.2.1). This hypothesis is supported by the fact that when using an indirect method (fig. 9), ultrasound transmission measurement may eventually not be affected by deeper located anomalies of the render (detachments probably occur in the interface between the render layer and the concrete blocks substrate). It must also be recalled that even micro cracking can affect ultra-sound waves propagation and decrease corresponding transmission speed (longer propagation paths).
5 Conclusions and future developments
Under controlled laboratory conditions thermography has proven to be a helpful non-destructive method to detect the presence and distribution of moisture and render detachments in building walls. Nevertheless, relatively low water contents reduce significantly the ability to detect their presence.
The ongoing research study will continue to analyse the possibilities and limits of this technique.
Next experiments will be conducted both under laboratory controlled conditions and field conditions. In situ tests will focus on the potential of using solar radiation as a natural and uniform source of heating sunlit wall external surfaces.
In what concerns ultra-sound techniques the results of the present tests point out to the need of an increase of the measurement density (more points and more closely spaced), complemented with a previous identification of probable degraded areas. Nevertheless it is questionable if an indirect method, such as the one used, is adequate to the detection of render detachments, unless they are located close to the external surface.
In the scope of the ongoing research project the combination of the two non-destructive techniques will provide a sounder basis for the assessment of the capabilities of the use of ultra-sound measurements as a non-destructive render degradation diagnosis method.