- Research article
- Open Access
Drying response of lime-mortar joints in granite masonry after an intense rainfall and after repointing
© The Author(s) 2019
- Received: 5 December 2018
- Accepted: 25 May 2019
- Published: 10 June 2019
When rain impacts a building façade, it is essential that once it has entered, it leaves by evaporation to help the building dry out. Accumulation of moisture can lead to internal dampness, mould and decay of valuable masonry by salt weathering. In a solid masonry wall where the stone is of low permeability, such as granite which is found in many historic buildings, rain water mainly enters and leaves through mortar joints. If granite stone masonry needs repointing, the repair mortar must allow the overall masonry to dry out. This study evaluates the drying response of various lime-based repointing mortars mixes in small granite stone masonry constructions (test walls) subjected to a simulated intense short rain event and then left to dry. It determines the moisture movement through mortar joints, the influence of materials, joint types and workmanship, and whether repointing could mitigate moisture ingress and help masonry dry out. This study developed a novel experimental protocol which allowed comparison of the drying response of different mortar types in a low-porosity stone masonry system and the effect of repointing. Five test walls were built of Cornish granite with five different lime mortar mixes combining NHL 3.5 (St Astier) gauged with non-hydraulic quicklime (Shap), quartz and calcitic sand and biomass wood ash as additives. Simulated intense rain was sprayed on each wall over a 3.25 h spell. Drying was monitored over a week with a microwave moisture device (MOIST350B). Measurements were done at surface and depth on both mortar joints and granite units. Each wall was then repointed with the same mortar mix initially used when built and the same rain simulation was performed to evaluate differences repointing could make to the moisture dynamics. The importance of mortar in dealing with moisture movements in the test wall and absorbing moisture from the stones was demonstrated. Gauged binder and wood ash additives decreased the capillary absorption capacity of mortars while retaining a good drying rate. This study has also showed that after repointing water did not penetrate as deep under the same conditions. Therefore repointing reduces the threat of water ingress and shows that it could be a suitable conservation intervention to mitigate water ingress and accelerate drying.
- Non-destructive testing
- Test walls
- Hot lime
- Built heritage
Many historic and traditional solid masonry walls in England and other temperate maritime environments are exposed to high amounts of rainfall and wind-driven rain (driving rain) . When driving rain hits a building façade, even when a masonry wall is of considerable thickness (as often found in historic church towers) , it will enter the wall through mortar joints, cracks, and at the interface between stone and mortar . Driving rain can lead to moisture ingress through the masonry wall resulting in liquid water ingress and/or internal dampness . In addition, a lot of water can enter a wall from defects on the roof, parapets, and other elements that manage rainwater in building. Water is one of the main agents in the deterioration of building materials leading to chemical and physical weathering and surface erosion on brick, mortar and stone , biological growth , frost damage and salt efflorescence [4, 7] and damp internal conditions [4, 8]. An increasing frequency of rainfall has therefore been identified as one of the main threats of climate change to historic buildings .
It is therefore essential that once it has entered, moisture leaves masonry by evaporation to help it dry, if only partially [10, 11]. Most of the moisture movement occurs through mortar joints in a solid ashlar masonry wall. This is especially true for stone units of low permeability and low-porosity, such as granite, where rain water mainly enters and leaves through mortar joints [12–14]. During drying and evaporation, liquid water travels to the surface of building materials and can carry soluble salts that are present [10, 15]. Mortar in joints should absorb moisture from the surrounding masonry units, by having a stronger capillary force than the stones with coarser pores . If most of the evaporation occurs at the surface of the joints, then so will any salt crystallisation and efflorescence. This will minimise damage to the individual stones, such as disintegration. Climate change is expected to cause more driving rain and therefore which will increase the threats to masonry structure, mainly due to salt crystallisation [17, 18]. The role and performance of mortar in absorbing moisture is becoming even more important.
A significant number of historic buildings, especially churches, in southwest England are built with hard low permeable stones, such as granite . Buildings of low porosity stones often suffer from water ingress and high interior dampness if the mortar is also a dense and low-permeable material that does not encourage evaporation through external walls [2, 4, 14]. Weathered and aged pointing mortar or previous interventions that suffer from the use of inappropriate materials or bad workmanship during repair can also prevent moisture from leaving the building . The performance of the pointing mortar, and especially of repointing in the case of repair, is therefore critically important in reducing water penetration through the masonry and also to help it dry out [20, 21]. Careful repointing is thus a preventive measure against the ingress of water and internal dampness [4, 14]. It has also been shown that workmanship, which includes the comprehensive clearing of loose debris, careful compaction and filling , types of finishing  and protection of the new mortar, can significantly influence the water resistance of joints [23–25]. Rendering and grouting can be very effective at preventing moisture ingress and assisting drying, as the render would hold water before it enters into the masonry and grouting would fill in the voids that are pathways for liquid water . However, in many cases for listed buildings repointing is often the only possibility because it is less visually intrusive, a minimal intervention and less costly. When deciding on a repair mortar for historic masonry it is essential to consider the material properties, the workmanship and the masonry type, in relation to the building’s exposure and environmental conditions if it is to be effective [12, 27].
Research has been published on developing suitable mortars to use with granite. Mosquera et al. and O’Brien et al. argue that because the calcium in lime-based mortar reacts with sulphates from the environment and form salts that damage granite, cement with a high sand content should be used [28, 29]. However, as Hughes pointed out, the use of cement for historic buildings is usually inappropriate and not recommended; therefore appropriate lime mortars need to be developed . In addition the amount of sulphate in the atmosphere has drastically reduced since the 1990s. Hughes et al. have tested several compositions of mortar for repointing granite, which included using quicklime . Moreover, understanding the response of a masonry system to driving rain, comprising masonry units, mortar joints, but also a core filled with loose stones, rubble and mortar, is complex [30, 31]. Therefore some studies have used test walls to understand the effect of water ingress and compare conservation treatments  and to evaluate the response of a specific material to driving rain, mainly focusing on brick walls [25, 32] and stone . The bond of a range of pointing mortars with sandstone has also been evaluated with small masonry wall . However, little investigation has been done which compares different mortars designed specifically for dense and exposed stone masonry and their drying performance using test walls.
Therefore, suitable lime mortars for pointing and repointing low porosity stone masonry walls needs to be designed and tested to establish how effective they are at helping the drying process. Testing needs to develop a tailored and comprehensive way of assessing and validating these mortars. This study specifically looks at how various lime-based repointing mortar mixes in a small and dense stone masonry system (granite test walls) respond to drying and evaporation after an intense short rain event. It aims to determine whether moisture movement happens through mortar joints, the influence of materials, joint types and workmanship, and whether repointing could mitigate moisture ingress and help masonry dry out.
The experiment was designed to represent a low porosity masonry system, as often found in southwest England, exposed to high intensities of driving rain. For the purpose of enabling comparison between test walls and mortar joints the variables such as environmental variations, direct solar radiation and direct rainfall were minimised.
In southwest England, Devon especially, a significant number of the historic buildings and churches with their tall towers are made of a grey granite, such as Dartmoor granite . For this study, a light grey Cornish granite (Pipper and Sons) with an open porosity of 0.87 ± 0.05% and a density of 2.89 ± 0.05 g/cm3 was selected.
Lime mortar mixes and preparation
The composition of the lime mortar mixes was decided based on results from previous research looking at mortar for highly exposed areas  and impermeable dense stone . Natural hydraulic lime 3.5 (NHL) has been demonstrated to be suitable for conservation purposes [36–38], especially for exposed masonry as it has the ability to set in humid environmental conditions through hydration processes [39, 40]. Previous work  has shown the positive effects of using biomass wood ash in lime mortars for use in exposed buildings such as high open porosity and delayed capillarity saturation, which enables more water to be absorbed and a better ability to dry out. In recent years, experiments have been done to re-evaluate using quicklime, as it could be a more authentic method to make mortar and improve durability, adhesion and workability [41–43].
As Table 1 shows, the composition of the mortar in each wall varies by one variable so that pairs of walls can be compared to each other to assess the effect of one specific material. For example, wall 1 and wall 2 are paired to compare the effect of using a binder gauged with quicklime (pair A), wall 2 and wall 5 are paired to identified the effect of wood ash (pair B) and wall 3 and wall 4 to compare the use of different aggregates (pair C).
During mixing, no specific water:binder ratio was specified but rather water was added as necessary to obtain similar consistency in all mixes (± 10 mm by flow), based on the experience of the mason. Flow tests were carried out on each mix with a flow table (Matest) following BS EN 1015 5-3:1999 , to ensure a similar consistency was obtained (110–125 mm for building the walls and 107–119 mm for repointing).
Laboratory characterisation of mortar and granite samples
Five specimens of each mortar mix were made in the laboratory in prisms of 40 mm × 40 mm × 160 mm using polystyrene moulds. All specimens were demoulded after 5 days and kept for 90 days until testing in a ventilated environmental chamber (Sanyo-FE 300H/MP/R20) at 20 °C (± 3 °C) and 65% (± 5%) RH. Curing samples in moulds may impact some properties, due to water extraction , but in this case (low permeability of stones) this impact might be negligible.
Water vapour permeability
Optical microscopy of thin-sections impregnated in blue resin was performed on wall 1 (W1) and wall 3 (W3) mortars using a Olympus BX43 microscope at 10× magnification with transmitted light.
Experimental set-up of the test walls and repointing
The experiment was designed to represent a low porosity masonry unit of dressed stones, as often found in southwest England, exposed to high intensity driving rainfall. For the purpose of enabling comparison between test walls and mortar joints the variables were minimised by not having a rubble core and by larger joints than usually found in ashlar dressing.
WACC, open porosity and vapour permeability values of the mortar mixes applied in each wall
WACC (g/m2 min0.5) (n = 6)
Water vapour permeability (g m−1 day−1 kPa−1) (n = 4)
Open porosity (%) original mortar (n = 5)
Open porosity (%) repointed mortar (n = 5)
Rain simulation and evaporation monitoring
Prior to spraying, the sides of the walls were sealed with vapour proof membrane (DPC, Visqueen) and vapour proof double-sided mastic sealing tape (Pavatape 12, NBT) to force evaporation through the main faces (Fig. 1).
Each wall was sprayed simulating a short intense and extreme rain spell in Devon. An intense spell of 5 h or less in duration, likely to occur once every 5 years for a wall orientated southwest was chosen from the weather data from Chivenor (51°5′20″N, 4°8′49″W) [31, 54]. Rainfall was simulated using tap water from a cone low-flow nozzle with a uniform spray distribution. A stop-valve timer at a distance of 32 cm from the wall released 15 s of water every three minutes resulting in 10.32 L m−2 of simulated rain sprayed on the wall over a 3.25 h (Fig. 1). The timer was mounted on a tripod, so the height could be adjusted to be at the centre of the wall enabling an even distribution. In Fig. 1 the blue circle shows where the rain arrived on the face of the wall. The protective board in plywood were preventing water from touching the other face of the wall and a gutter was collecting the run-off water.
The microwave moisture device is a non-destructive and non-invasive technique, therefore suitable to use on historic buildings  and unaffected by the salt content of the building materials . The MOIST350B works by producing electromagnetic waves reflected by the material surface . The ratio of reflected and transmitted waves is called the reflection coefficient and is measured by the different reflection sensors of the device . By multiplying the reflection coefficient with a fixed known factor the moisture meter gives a direct reading in a unitless “Moisture Index” (MI) in arbitrary units . The MI is useful to assess relative change in moisture content over time but has not been calibrated against exact moisture content, therefore throughout the study it remains an indicator of moisture. In this study, two reflection sensors-heads were used to monitor the evaporation rate at near surface and depth: the R1M sensor, measuring up to 2–3 cm, in the 900–1400 MI range (referred here as “Surface”), and the PM sensor, measuring up to 20–30 cm in the range of 1300–2400 MI (referred here as “Depth”) . In dense stone like granite microwaves attenuates energy quickly, so it is considered that the reading from the PM sensor is up to 15 cm . Because they are in different arbitrary ranges of values, the data from surface and depth are not directly comparable.
Few studies have used the microwave moisture meter on both mortar joints and masonry units and especially not for comparison of different mortar properties. The microwave moisture meter was chosen for this study for its ease of use and data handling. It has also been proven to give accurate results when calibrated against gravimetric measurements of different building stones for a range of water contents . In addition, the device enables easy measurement within the thick masonry walls of historic buildings because it measures at different depths and access is only needed to one side of the wall.
Moisture movement through mortar joints
Drying is the transfer of liquid water of the porous building materials to the surrounding environment . Drying is therefore expected to depend on both the external conditions and on the materials properties . It is well established that drying occurs in two stages . Stage I drying is defined by the transport of liquid water to the surface of the materials followed by evaporation . Until all liquid water has travelled to the surface, evaporation at the surface occurs at a constant rate under constant conditions. Stage I drying is therefore highly influenced by the boundary external conditions (temperature, RH and air flow) . Stage II drying starts when transport of liquid water to the surface is no longer possible so that the rate of evaporation slows down . Stage II drying is characterised by water vapour diffusion mechanisms and therefore influenced by the microstructure of the materials [6, 11].
At absorption and during the first 24 h of evaporation, at surface (Fig. 5a) and depth (Fig. 5c), all mortar joints show a similar order of moisture level (wall 2 having the lowest and wall 4 the highest), and start drying at the same time. After 24 h, at depth, some test walls (specifically walls 3, 4, 5) experience a more abrupt drying (lower moisture level) than the other walls.
The first 24 h of drying seems to correspond to stage I drying. Due to the constant external conditions (17 ± 1 °C and 79 ± 2% RH), mortar joints throughout the wall behave similarly up to 24 h of drying at surface (Fig. 5a) and depth (Fig. 5c). Stage I drying is mainly determined by the environmental boundary conditions and it has been demonstrated that as the RH increases, the rate of drying decreases .
Figures 5b, d enable to see the capacity of absorption of the test walls from a ‘dry’ state (‘before-rain’) and the capacity of drying (as the curve goes back to a value close to the 0 line). As expected, mortar joints show higher absorption capacity (Fig. 5b, d) than the granite units (Fig. 5b’, d’), showing that with the low-permeable granite, the joints are the location of most moisture movement, especially at depth. Figure 7 illustrates the expected behaviour, in which most water travels through the mortar joints—although the edges of the granite show signs of having absorbed little water.
Figure 5b’ shows that the granite at surface is slightly affected by the rain events outside the sheltered area resulting in a more humid environment. Figure 5b’ also illustrates that for some walls (walls 2, 3, 5) the moisture level of the granite remains higher than at the start (‘before-rain’) throughout evaporation, whereas for walls 1 and 4 (red and blue lines), the moisture level of the granite quickly goes back to its initial value (represented by the 0 line) or even below. Although difficult to clearly identify this could give an indication of the action of mortar joints in absorbing moisture from the granite.
The walls in which mortar joints reach their 0 state or below show that the granite units also reach their initial value (Fig. 5b, walls 1 and 4), whereas walls in which mortar joints do not dry out as much (Fig. 5b, walls 2, 3 and 5), experience a granite unit that remains wetter (Fig. 5b’). Comparing Fig. 5a, b shows that mortar joints in wall 4 have likely absorbed moisture from the air before testing, therefore resulting in high moisture level (Fig. 5a) and capacity to dry below its starting point (0) (Fig. 5b). When stage II drying starts (after 24 h or later for some walls), the differences seen between each test wall may be explained in more details by material characteristics.
Figure 8a compares mortar joints made with NHL 3.5 (wall 1) and with gauged binder: NHL 3.5 and non-hydraulic quicklime (wall 2). At surface, mortars in wall 1 and 2 show similar drying patterns and little differences between their moisture level considering that wall 1 mortar has a higher MI level at t0. However, Fig. 5b has also shown that joints in wall 2 never reached their 0 state (‘before rain’) and wall 2 mortar show the slowest WACC and lowest water vapour permeability (Table 2) and. It means that in wall 2 capillary saturation is reached slower than for mortar in wall 1 and that water vapour travels slower through mortar joints. The mortar would be less capillary active to absorb moisture from the surrounding masonry units. Because less water is absorbed by the material, the drying slope is quicker than mortar joints in wall 1 that has absorbed more moisture.
At depth, mortar joints in wall 1 have a slower drying rate than the joints in wall 2 (Fig. 5a, d). This can also be observed in Fig. 7 (wall 1), where the joints exhibit higher moisture level than the ones in wall 2. Mortar in wall 1 has indeed the higher capillary absorption rate (Table 2).
Figure 8b allows comparison between mortar mixes made with and without wood ash (e.g. wall 5 versus wall 2). Mortar with wood ash has been shown to have pores that are predominantly in the small capillary range, which can be seen for W3, W4 and W5 in Fig. 9. The mortar containing wood ash keeps a high moisture level longer after rain has been sprayed (between 24 and 72 h after the rain event) (Fig. 8b). When mortar joints remain wetter at the surface for longer periods, it could also show the movement of moisture within the joint: the liquid water moves through the wall until it reaches the surface and evaporates. Mortar joints with wood ash also demonstrate a relatively sudden drying response: over 3 days the moisture level value returns to ‘before-rain’ conditions. This results in two clear phases of drying. At depth (Figs. 5d and 8b), the joints in wall 5 also remain wetter longer than those in wall 2, but reach a lower moisture level after drying for 7 days. Despite having a high absorption at depth, no ingress of moisture from the back of the wall could be seen for wall 5 (Fig. 7).
Little difference of moisture level throughout drying and evaporation is seen between the use of different aggregates comparing wall 3 and 4 (Fig. 8c). However, joints in wall 3, made with quartz sand, remain wetter longer as perhaps more moisture was absorbed. It is indeed the more porous and permeable of mortar mixes tested in laboratory (Table 2).
Comparing walls 3 and 4 with wall 5, which contains mixed aggregate, mortars made with one aggregate and with additives of wood ash, hold water longer (Fig. 8b, c). Calcitic aggregates have been shown to increase to proportion of pores bellow 1 µm and give higher porosity , as shown by Fig. 9 for wall 4.
Comparisons between joints
Vertical and horizontal mortar joints in wall 1 show differences in absorption (t = 0 h) and drying (from t = 3 h), especially at depth (Fig. 10a). Vertical joints show higher moisture level, which is probably linked to workmanship and the difference in the pressure applied during construction . Indeed, for wall 1, water egress at the back of the wall was especially visible at weak points present at the intersection between perpends and bed joints.
However, in all other test walls no significant differences are seen between perpends and beds, as joints in wall 3 illustrates (Fig. 10c). If there is little difference, as in walls 4 and 5, vertical joints show a higher moisture level, and horizontal joints dry quicker. Vereecken has shown that bed joints are a preferred pathway for moisture , which here is only visible for the drying.
Figure 10b, d show that within a same wall differences between each perpend and bed joint can be noted. For walls 1 and 3, b3 joint (Fig. 10e) is drier at both surface and depth (Fig. 10b, d). In wall 4, b1 is the driest, as Fig. 7 (wall 4) shows. In walls 3 and 2, p6 is the driest joint. The other perpends show equal moisture level in all other walls, apart in wall 5 where p1 is much wetter, as Fig. 7 (wall 5, bottom right joint) shows.
After repointing each test wall
Figure 12 clearly shows the differences between the original wall and the wall after it has been repointed. The moisture level right after the rain event (t = 0 h) is similar or lower for all mortar joints at surface and depth. More specifically, after drying for 24 h, at the surface of mortar joints (i.e. the repointed parts), all but wall 2 and wall 5 (green and pink curves) show a lower moisture level after repointing (Fig. 12a). In walls 4 and 5, the granite at surface, remains with a higher moisture level throughout the test, whereas joints at depth have a lower moisture level after repointing. This could show that for these walls, the moisture stayed mainly at the surface of the test walls. Walls 1 and 3 (blue and orange lines) have overall a lower moisture level after repointing, due perhaps to more run off, so less water entering the joints, as shown in scenario 1 explained in the discussion. Looking at the pore structure of the mortar in wall 1 and 3, a denser matrix with fewer pores is visible on the repointing mortar (Fig. 14a, b) in comparison to the original mortar in beds and perpends joints of wall 1 where more shrinkage cracks and larger pores are visible (Fig. 14c, d).
However, as shown by Fig. 12, mortar joints in wall 2 show the highest moisture absorption at the back of the wall, whereas it previously had the lowest WACC and MI level (Table 2, Fig. 7a). After repointing, mortar joints in each wall seems to behave slightly differently as to where the moisture moves and how the test wall dries out. Wall 2 has higher moisture level at surface until 48 h, and higher at depth after 48 h which perhaps shows that the evaporation occurred mainly through the back of the walls.
This study found that moisture movement in and out of a masonry test walls made of low-porous units clearly occurred though the mortar joints (Fig. 5). This was achieved by combining a simulation of an intense rainfall and monitoring of the drying response of different mortars near the surface and at depth over 7 days. This was undertaken in constant environmental conditions both after initial construction and after repointing.
Previous drying tests have shown that mortars dried under such high humidity (15 °C, 85% RH) show a longer stage I drying . This long stage I can also be seen in Fig. 6, where it last up until 73 h for laboratory samples under the same environmental conditions. A long stage I drying could be beneficial to allow the bulk of the masonry to fully dry out , which is here shown as the depth of the test wall dries out as much as the surface (Fig. 5c). It could also be argued that the effect of the increase of RH due to external conditions on walls 2 and 3 prevents the mortar joints from fully drying out (Fig. 5a, b). This shows that when mortar joints do not sufficiently dry to reach their initial state (‘before-rain’), the granite units remain slightly wetter at surface (Fig. 5b, d, walls 2, 3, 5), illustrating the action of mortar in absorbing moisture from the masonry.
The effect of workmanship
The study found no clear differences between beds and perpends joints. Differences noticed in joints within each test wall were mainly attributed to their location in the test walls and to workmanship. The joints at the bottom of the wall are wetter likely due to more water from run-off and the joints at the top of the wall are drying having better evaporation, more exposed to the air circulation and being less affected by run-off. Weaker points at the intersection between beds and perpends and on the bottom joint of the test wall, where the mortar could be less compacted, showed quicker moisture ingress (Figs. 7, 10). In comparing the different test wall types in combination with laboratory mortar samples, the study also showed that all materials that were added influenced the absorption and drying performance of the mortars (Fig. 8).
The importance of composition
The same drying pattern is seen between the drying curves of laboratory mortar samples (Fig. 6) and the evaporation of mortar joints (Fig. 5a) showing that differences in the behaviour of mortar joints of each wall can be attributed to the different mortar compositions and not to environmental factors. In addition, once repointed, on the surface of the walls similar evaporation pattern as the original mortar are seen. This shows again that differences are based on the composition of the mortar joints and materials specificity: the key variable is wood ash, the type of sand does not have a significant influence.
Gauged NHL with quicklime (mainly seen in wall 2) exhibited lower overall moisture levels, which could minimise water ingress while maintaining a quick drying rate. Most pores within the mortar of wall 2 are in the small capillary range, below 1 µm. Only pores from 1 mm to 1 µm are practically relevant for capillary transport , which could explain the lower capillary coefficient (WACC) of wall 2 mortar (Table 2). This could show that adding quicklime in mortar could help minimise water absorption and ingress through a masonry system.
Wood ash mortar holds moisture longer (Fig. 8b) and delayed the capillary absorption of mortar (Table 2). Previous research has shown the capacity of lime mortars made with wood ash to absorb high water content and to have an increased ability to dry out . The fact that joints remain wetter longer at surface and depth could prevent the accumulation of water which can lead to further subsequent water ingress. When joints have reached saturation and cannot absorb more water, if another rain event occurs before they have started to dry out the water would likely run off the façade.
NHL mortar (wall 1) had a quick absorption and a lower evaporation, which made this mortar less suitable to help a masonry dry out (Fig. 8a). Figure 9 indeed shows that the mortar mix used in wall 1 (W1) has the most proportion of pores in the capillary range and is therefore highly capillary active.
Mixed aggregates mortars (primarily walls 2 and 5), perhaps by their bimodal pore size distribution (Fig. 9) of pores in the capillary and vapour permeability range, seems to be good at mitigating water ingress while showing a good capacity to dry out (Fig. 5). Indeed, materials with a high proportion of capillary pores will dry more efficiently under the same conditions and will also show high evaporation because the capillary drying regime is helping to dry out the masonry .
From samples to test walls to real masonry
The methodology and experimental design developed in this study enabled a confident comparison of the mortar joints in the five test walls during absorption and over drying. By combining best-practice workmanship, specific mortar design and rain simulation this research has developed a novel method to test the suitability of mortar for granite masonry and repointing mortar. Each wall was built identically, by the same mason, received the same amount of simulated rain and was tested under constant environmental conditions. This enabled confidence in the measured moisture levels when comparing the walls. Although differences in moisture level of different joints within one wall could be attributed to workmanship, those differences were minimal and did not alter the overall comparison between each mortar joints and test walls. The environmental conditions during drying monitoring of the repointed test walls had lower RH (by 15%) which could partly explain the higher drying rate of the repointed walls. However, because the temperature was identical and the differences in RH low, the difference found after repointing can still be taken into account.
The single-layer test walls were a clear and straightforward design to assess the response of a masonry system to driving rain, on the façade, at surface and at the depth. However, it is important to remember that larger masonry structures often consists of two single stone layers (often ashlar dressed) and a core made of rubble stone, mortar and both through stone and voids that are an easy conduit for liquid water to travel through the depth of the wall toward the interior of the building. The benefit of looking at two depths suggests the data could be used for real traditional masonry wall with a rubble core. The depth would then correspond to the core of the wall.
The microwave meter allowed straightforward monitoring of the drying and evaporation of the different materials in the test walls: mortar and granite at surface and depth over time. The MI calculated from microwave moisture data allows comparison between granite and mortar. Other electromagnetic devices (such as ground penetrating radar) were used in this study to measure the moisture level of the different building materials (not presented here) and found similar results in the comparative behaviour of mortar joints and granite units . In addition, the drying curves found in laboratory experiments (Fig. 6) showed similar ranking of moisture level—wall 5 with the highest moisture content and walls 1 and 2 with the lowest. However, the drying in water content in g cm−2 recorded in the laboratory cannot be directly compared to the MI values without calibration and the drying curves of the mortar samples are different to the curves recorded with MI values. In addition, MI values from the R1M (surface) and the PM (depth) sensors cannot be directly compared, which could lead to some confusion when analysing the data.
Mechanical action of repointing and the suitability of the tested mortars for a dense masonry wall
To reduce deterioration and internal dampness, ingress of moisture through the wall should be limited so that moisture stays at or near the outer surface. Repointing is one remedial treatment against dampness. This study investigated whether repointing could help mitigate moisture ingress and if so, how.
This study has shown that when the test walls were repointed, the depth of the wall absorbed less water than before repointing immediately following the simulated rain event (Fig. 12). If the amount of moisture throughout the depth of the wall is minimised once the wall is repointed, it could show that repointing could reduce the threat of water ingress and mitigate water ingress. It also shows that repointing could support quicker rates of drying, as less moisture enters the depth of the wall.
Two explanations could be given as why these differences of moisture level are seen, despite each test wall having been repointed using the same mortar as originally used for each. Indeed, the mortar removed and the mortar replaced have the same composition and therefore similar properties.
Secondly, the greater ages of the original mortar left in the joints from initial construction (approximately 18 months since construction) could have decreased porosity and capillary capacity.
Based on different specific environmental conditions or different wall expositions or building features that have different requirements (e.g. chimney, pinnacles vs masonry) some tested mortar may be more suitable than others. Based on Figs. 12 and 13, and on material characteristics, once repointed, walls 1, 2 and 3 could correspond to scenarios 1 and walls 4 and 5 to scenarios 2. This has important implications for consideration of repointing under specific environments or on different wall expositions, and for the role of workmanship in contributing to the functional role of repointing.
The action of mortar in controlling moisture movement in the wall and absorbing moisture from the masonry was demonstrated.
Differences of evaporation between horizontal and vertical joints in the same test wall were mainly attributed to their location in the test walls and workmanship.
The selected composition of lime mortars, gauged with quicklime and wood ash additives gave mortar a decreased capillary absorption capacity and a good drying rate, making them suitable for use under very wet environmental conditions (i.e. mortars in walls 2 and 4), while mixed aggregate increased the capillary absorption.
This study has shown that when test walls were repointed, the depth of the wall absorbed less water than before repointing.
Repointing is therefore a suitable conservation solutions to mitigate the threat of water ingress and increase the drying capacity.
At the surface, repointed wall behaved differently, and two scenarios were identified, each suitable for slightly different environmental conditions or wall expositions:
Scenario 1, repointing mortar holds on to water when saturation is reached, so that no more liquid water can be absorbed into the mortar and vapour evaporation is favoured—suitable for under continuously wet environmental conditions, preventing water accumulation and ingress. Scenario 2, the repointing mortar has a high evaporation rate, facilitating its drying, so moisture stays mainly at the surface—suitable for mortar under many wetting and drying cycles as mortar that dries quickly and absorb slowly.
These findings have important practical implications when considering both the design of mortar for dense and low-porous masonry and of repointing, and when evaluating the performance and role of repointing mortar.
The authors would like to specifically thanks Bill Revie and Colin Burns for building the test walls to best-practice and providing expert advice. Specific thanks also go to the Facilities team at the School of Geography and the Environment, Alex Black, Joe Milkovic and Armpen Kristo, for their help preparing and monitoring the sheltered area for the test walls, and to Alison Henry, Elisabeth Laycock and Simon Cartlidge for their advice in designing the test walls.
This research was supported by the UK EPSRC grant for the Centre for Doctoral Training in Science Engineering in Arts, Heritage and Archaeology (SEAHA) (grant number EP/L016036/1) and by Historic England.
LF: Designed the experiments, contributing to building the test walls and making laboratory samples, collected all the data, analysed the data and wrote the paper. SAO: co-designed the experiment method, gave expert advice on the use of MOIST350B, created the script to read and extract the MOIST530B data, helped with the collection and analysis of the data and reviewed the paper. CW: provided funding for building the test walls and expert advice on building test walls for laboratory experiments. MOD: provided guidance of the suitable stone to use for the wall and advise of on-site issues with rainfall. HV: helped defined the aims and scope of the study and coordinated the different authors. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
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