Moisture movement through mortar joints
Figure 5a, c shows normalised moisture levels over time at surface and at depth for all the joints in all five walls. Granite is not present in Fig. 5a, c as very few differences were seen. Figure 5b, b’, d, d’ show the differences of moisture level from the measurement taken before rain (formula 5), which could be considered as a relatively dry state (t0). Figure 6 presents the results of drying test of mortar samples placed in the same sheltered area. Figure 7 shows the visual appearance of the back of the wall after spraying and 3.25 h of drying.
Drying is the transfer of liquid water of the porous building materials to the surrounding environment [11]. Drying is therefore expected to depend on both the external conditions and on the materials properties [16]. It is well established that drying occurs in two stages [48]. Stage I drying is defined by the transport of liquid water to the surface of the materials followed by evaporation [60]. 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) [60]. Stage II drying starts when transport of liquid water to the surface is no longer possible so that the rate of evaporation slows down [60]. 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 [11].
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.
Material comparison
In Fig. 5a, c, higher value means higher moisture level. It is clear that mortar joints from the different walls show different moisture level both after rain at absorption (t = 0 h) and during drying (t = 3 h to 144 h). As seen in Fig. 5a, mortar joints in wall 1 and especially wall 2 (which is made with mortar containing quicklime), show the lowest moisture level, whereas walls which have joints composed of mortars with wood ash (walls 3, 4, 5) show a higher moisture level. The same pattern is also seen in the drying curves of laboratory mortar samples (Fig. 6). Differences in absorption and drying of each wall can also be seen in Fig. 7 which shows the water egress through the back of the wall after spraying and 6 h of drying. Figure 8 compares the behaviours of pairs of individual walls (as explained in Table 1), using the same data set as presented in Fig. 5a, c.
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).
Gauging binder with quicklime appears to influence the pore structure of the mortar by reducing its capillarity and permeability as shown by Fig. 9, where capillary pores are in lower proportion in the mortar of wall 2. Most of the pores of wall 2 mortar in the small capillary range, below 1 µm. Only pores from 1 mm to 1 µm are practically relevant for capillary transport [7], which could explain the lower capillary coefficient (WACC) of the mortar in wall 2 (Table 2). The unimodal distribution of pores of wall 1 mortar could be explained by the higher water demand of the fresh NHL mix, which could have created larger pores [40].
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 [61], as shown by Fig. 9 for wall 4.
Comparisons between joints
Figure 7 already illustrated that for each test wall, the egress of moisture at the back of the wall was visually different depending on the joints and area of the wall. Figure 10 uses the same data set than Figs. 5a, c and 8 in order to compare over time, from absorption to evaporation, horizontal (beds) and vertical (perpends) joints, and all joints separately of walls 1 and 3.
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 [3]. 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 [62], 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 11 shows moisture level data for the absorption and desorption curves of each of the original test walls, using both the same data set than Fig. 5a, c and data for the same walls repointed. Surface corresponds to the repointed part of the wall. Figure 12 represents the percentage changes of moisture level between the original wall and the repointed wall at absorption during the simulated rain and over evaporation and drying, calculated with formula (6). For the same rainfall simulation followed by drying, differences in the amount of moisture level at absorption and during drying can be seen in mortar joints of all the test walls after repointing. Figure 11 shows that at both surface and depth, in each wall, the mortar joints follow a similar drying curve before and after repointing suggesting that the composition of the mortar is a major factor affecting the response of the joints.
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).
At depth, again apart from wall 2 and 5, mortar joints also show a lower moisture level after repointing (Figs. 6, 11b). The lower moisture level after repointing at depth could be explained both by the effect of repointing and by the mortar in the joints being older (broadly 18 months after construction), where the porosity and capillary capacity could have decreased. It could also show that repointing helps the wall dry quicker. The lower moisture level measured on most mortar joints (Fig. 12b) is also visible on Fig. 13 by visual assessment, where minimal moisture egress can be seen on each wall compared to Fig. 7.
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.