In Situ, Non-Destructive Testing for Evaluating the Role of Pointing Mortar in Preventive Conservation Strategies. A Case-Study on Reigate Stone at the Wardrobe Tower, Tower of London
- How can NDT best be used to determine the effect of pointing mortar on surrounding masonry?
- Is there a discernible difference between the performance of different pointing mortar recipes used in Reigate Stone conservation?
- Does pointing mortar constitute a measurable control on the decay of the Wardrobe Tower?
1.1. Pointing Mortars in Building Conservation
1.2. Non-Destructive Testing (NDT)
2. Site and Methods
2.1. Wardrobe Tower
2.1.1. Construction and Conservation
2.1.2. Present Condition
- The entire western flank of south facing ashlar masonry. The upper part of this area was more extensively repointed in 2017; the lower part was left with extant pointing mortar, believed to date to the 1995 work.
- An area of ashlar interspaced with brick courses at the upper eastern flank of north facing masonry. This area was almost entirely repointed in 2017, and it includes the area repointed with NHL.
- A large area of ashlar and rubble interspaced with brick courses and other stone types, covering the upper half of the central and western parts of north facing masonry. This area was only partially repointed in 2017 and contains a patchwork of older pointing mortars.
2.1.3. Material Composition
2.2.1. Field Surveys
- Type and condition of pointing mortar. Condition was initially determined based on bond, width and profile. Type was defined as one of three categories:
- New lime putty mortar used in April 2017 conservation.
- New NHL applied to test area in April 2017.
- Old pointing mortar. Treated as one group. In situ differentiation of pointing mortars was not always possible due to discolouration. Two historic mortars analysed in previous section likely to be representative of joints surveyed.
- Contextual factors, including aspect, height and adjacent masonry.
2.2.2. Non-Destructive Testing (NDT)
3. Results and Discussion
3.2. Capacitance Meter Measurements
3.2.1. Local Distribution
3.2.2. General Distribution
3.3. Surface Hardness Measurements
3.3.1. General Distribution
- Date of survey. SH data from the November 2017, August 2018 and January 2019 surveys were included to be representative of different wetting and drying scenarios.
- Aspect. The data were grouped into north and south, to evaluate the role of exposure across the different wetting and drying scenarios.
- Mortar. The data were grouped into all measurements taken along edges adjoining specific mortar types (i.e., putty, NHL or old mortar) or at the centres of all blocks.
- Block. The data were grouped into all measurements taken on a single block.
3.3.2. Local Distribution
3.4. Material Context
Conflicts of Interest
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|Survey Date||Summary of Environmental Conditions||Conditions in Preceding 2-Week Period||Most Recent Rainfall and Wind Direction before Survey||Drying Conditions during Survey|
|Prolonged wetting and poor drying||Relative humidity (RH) above 70%, temperature around 10 °C. Repeated rainfall.||Series of heavy spells on the 10.11 and 11.11, in which a total of 18 mm fell (peaking at 8 mm in one hour). Light winds.||Temp and RH stable. Average solar radiation was 100–150 W/m2. Light rainfall.|
|Prolonged wetting and good drying||RH above 70%, temperatures around 10 °C. Prolonged rainfall.||Heavy, wind driven spell in which 5 mm fell in 5 h on 2.05. Wind direction varying from S to NWW at speeds of up to 5 m/s.||RH below 70%, daytime temperature rose to 20 °C, with average solar radiation of 500–600 W/m2. No rainfall.|
|Rapid, heavy wetting and good drying||RH under 80%, daytime temperatures between 20 °C and 30 °C. No rainfall before 09.08.||Heavy, wind-driven rain on 9.08 and 10.08, with 25 mm falling in a series of spells with SSW wind direction (peaking at 8.5 mm in 20 min with wind speeds of 6.5 m/s).||Daytime temperature 18 °C with average solar radiation of 250–300 W/m2. No rainfall.|
|Mild wetting and poor drying||RH mostly above 70%, temperature below 10 °C. Little rainfall.||2 mm falling on 16th and 17th January in a series of light spells.||Temp and RH stable. Average solar radiation was 150–250 W/m2.|
|Recent, mild wetting||Variable RH, temperatures between 15 and 20 °C. Occasional light showers.||Several light rain spells on the first two days of the survey, with varying wind direction and speed up to 5 m/s.|
|Survey||Areas||Used for||Total Blocks||Total Number||Stone n||Mortar Number|
|Nov 17||S.1; N.1; NW.1||Calibration||18||122||90||10||8||14|
|May 18||S.1; N.1; N.2; N.3; NW.1||Calibration; Spatial distribution (micro)||12||355||180||21||74||80|
|Aug 18||S.1; S.2; N.1; NW.1||Calibration; Spatial distribution (micro and macro)||51||656||447||51||87||71|
|Jan 19||S.1; S.2; N.1; N.2; N.3; NW.1||Calibration; Spatial distribution (micro and macro)||55||555||507||36||57||90|
|Oct 19||S.2||Spatial distribution (macro)||30||132||132||n.a.|
|Survey||Areas||Used for||Total Blocks||Total Number||Stone n||Mortar n|
|By Type||By Aspect||NHL||Put||Old|
|Nov 17||S.1; N.1; NW.1||Calibration; ANOVA||18||710||90||70||135||155||280||170||50||85||125|
|May 18||N.1; N.2; N.3||Calibration||10||494||110||n.a.||224||160||494||n.a.||n.a.|
|Aug 18||S.1; N.1; NW.1||ANOVA; Spatial distribution||21||1410||174||162||264||270||570||300||126||228||186|
|Jan 19||S.1; N.1; N.2; N.3; NW.1||ANOVA; Spatial distribution||22||1009||129||123||173||221||436||210||72||112||179|
|Oct 19||Not used||n.a.|
|All Dates||November 2017||August 2018||January 2019|
|Df||F Value||Pr (>F)||Df||F Value||Pr (>F)||Df||F Value||Pr (>F)||Df||F Value||Pr (>F)|
|Date||2||46.32||<2 × 10−16||-||-||-||-||-||-||-||-||-|
|Aspect||1||40.43||2.67 × 10−10||1||14||0.0002||1||30.34||5.50 × 10−8||1||9.18||0.0026|
|Block *||-||-||-||16||4.65||1.38 × 10−8 **||16||6.1||1.50 × 10−12||-||-||-|
|Mortar:Block *||-||-||-||24||2.26||0.0007 **||24||2.7||2.92 × 10−5||-||-||-|
|Material||Total Number||Average (HLD)||StdDev (HLD)|
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Michette, M.; Viles, H.; Vlachou, C.; Angus, I. In Situ, Non-Destructive Testing for Evaluating the Role of Pointing Mortar in Preventive Conservation Strategies. A Case-Study on Reigate Stone at the Wardrobe Tower, Tower of London. Minerals 2021, 11, 345. https://doi.org/10.3390/min11040345
Michette M, Viles H, Vlachou C, Angus I. In Situ, Non-Destructive Testing for Evaluating the Role of Pointing Mortar in Preventive Conservation Strategies. A Case-Study on Reigate Stone at the Wardrobe Tower, Tower of London. Minerals. 2021; 11(4):345. https://doi.org/10.3390/min11040345Chicago/Turabian Style
Michette, Martin, Heather Viles, Constantina Vlachou, and Ian Angus. 2021. "In Situ, Non-Destructive Testing for Evaluating the Role of Pointing Mortar in Preventive Conservation Strategies. A Case-Study on Reigate Stone at the Wardrobe Tower, Tower of London" Minerals 11, no. 4: 345. https://doi.org/10.3390/min11040345