Drying behaviour of masonry using quantitative infrared thermography

Non-destructive testing allows analysis of heritage masonry in-situ, but it can also be useful for testing composite masonry samples. Quantitative infrared thermography is used to monitor evaporation rates of mortars and substrates within small scale masonry system prisms. This enables comparison of the drying behaviour of system prisms incorporating different mortars. This can be used to support the selection of appropriate repair mortars to prolong the useful lifespan of historical buildings, and supports the adoption of sustainable retrofit strategies.


Introduction/Background
Masonry decay has been linked to moisture through several physical and chemical processes [1].Historical masonry provides a heritage asset to the community, yet it is often not well maintained.Using compatible materials that enable drying is vital for the durability of masonry, yet the moisture behaviour of mortar and substrate systems is not well documented.
Research on drying behaviour of masonry often follows methodologies based on gravimetric weighing to monitor moisture content of saturated samples as they dry.However, this method is not able to discern the drying behaviour of individual components of a sample, for example a stone unit or a mortar joint within a masonry system.Here, quantitative infrared thermography (QIRT) makes use of evaporative cooling of porous materials to find the evaporative thermal index, ETI [2]:  = ( −  )  ⁄ .
(1) which compares the surface temperature of a drying sample, Tt, to that of a dry reference sample of the same material, Td.This paper aims to show the suitability of QIRT to monitor drying rates of sandstone and to demonstrate its potential to compare evaporation rates of multiple surfaces within one mass.

Methodology
A reclaimed sandstone coping with open porosity 15% was cut into16 regular 50 x 50 x 20 mm (± 1 mm) prisms ensuring the natural bed plane was running parallel to the square face of the cut prisms.These were dried to constant mass, weighed and then submerged until constant mass.A FLIR E76 thermal camera, a precision balance and reference dry samples of stone were positioned inside a TAS HTCL environment chamber, which was preconditioned at 23˚C and 45% RH until conditions stabilised.Each prism's saturated weight was measured before sealing on 5 sides using aluminium tape.Each sample was placed on the balance inside the environment chamber where mass and thermographic images were taken at 5-minute intervals.
A preliminary test to determine the potential for QIRT to detect differences in drying behaviour between different materials within one mass saw the manufacture of one system prism of sandstone and mortar.A traditional lime mortar was thoroughly mixed by hand using quicklime powder and 0-2mm sharp graded quartz sand at a 1:3 volume ratio, with an addition of wood ash pozzolan at 10% of the quicklime mass.Two additional sandstone prisms were placed into a 50mm cubic mould, and the mortar pressed between the stones using a pointing tool to represent a masonry system with a 10mm mortar joint.This system prism was kept at 90% relative humidity (RH) for 72 hours, then removed from its mould and kept in lab conditions at 60% RH for 72 hours, before its weight was recorded.The system prism was then submerged in distilled water for 24 hours, saturated weight recorded and drying was monitored following the same methodology as the sandstone prisms.For each thermograph average surface temperature was measured for each drying component (stone/mortar) and for dry samples.

RILEM
where a and b are constants.For 16 sandstone samples the mean of the RE to ETI relationship a = 0.88 with standard deviation of mean 5.5%, average R 2 0.92 and average correlation coefficient 0.95.An example of RE calculated from ETI is compared to measured raw gravimetric RE data for a sandstone sample in Figure 1(a).The time series of gravimetric measured evaporation rate (RE) for the masonry system is plotted in Figure 1(b).Providing a similar relationship against time, the evaporation rate calculated by QIRT is shown in Figure 1(c), where the smoothed ETI series calculated using eqn.( 1) is shown for the mortar and for the average of both stones within the masonry system sample.A deviation in drying behaviour is detected by QIRT (Figure 1(c)), where mortar evaporation remains higher than stone.The knick-point described by BS EN 16322 [4] signifies the transition from phase one to phase two drying, when moisture content in pores drops below the threshold necessary for liquid capillary flow to the surface.At this moisture content evaporation drops dramatically allowing calculation of transition time from QIRT data by taking the minimum of the time derivative of ETI [2,5].For sandstone prisms the knick-point from QIRT differed from that of gravimetric method by 2.5%.

Discussion
The high correlation of ETI to RE for sandstone shows the efficacy of QIRT to estimate evaporation rate particularly through the transition phase as seen in Figure 1(a).Once constants (a and b) are determined for a saturated sample, evaporation rate RE can be estimated for dynamic wetting and drying of the same sample from ETI measurements through eqn.(3).
The ability of QIRT to determine evaporation rates of specific areas of a larger sample allows more detailed analysis of the moisture behaviour of composite samples as shown in Figure 1(c).Note, QIRT measures surface temperature and so only provides information about the effect of evaporative cooling on the outer surface of the material.

Conclusion
The outcomes of the present experimental work demonstrate how QIRT can be used to analyse the drying behaviour of components within a masonry system and to assess the influence of different mortars on the observed response.The methodology also accelerates data collection as many samples within the frame of the thermal camera can be analysed from one thermograph.Further work aims to link ETI measurements with moisture content of areas of the composite system and therefore work towards understanding the impact that different mortars might have on stone drying rates.
Commission 25-PEM [3] calculates evaporation rate, RE, as the change in sample mass dM, over change in time dt:  Corresponding author.luke.dickens@liverpool.ac.uk  The Author(s).This is an open access article distributed under the terms of the Creative Commons Attribution Licence (CC-BY) 4.0 https://creativecommons.org/licenses/by/4.0,which permits unrestricted use, distribution and reproduction in any medium, provided the original author and source are credited.DOI: 10.14293/ICMB230026  =  ( • ) ⁄ , (2) where A is evaporation area.For sandstone prisms a linear regression line describes the relationship of RE and ETI by:  =  •  + ,

Figure 1 .
Figure 1.(a) time series of RE of one sandstone prism measured by gravimetry and calculated from ETI measurements, (b) evaporation rate curves of saturated masonry system prisms calculated by gravimetry, (c) ETI from QIRT measurements of the same sample showing mortar and stone separately.