Let's analyze in detail the natural wetting and drying cycle of a single brick - the fundamental building blocks of houses. The findings described here also apply to stones and other natural porous building materials.
For this experiment we used a single traditional brick. We have embedded a host of sensors inside it, monitoring the following parameters across multiple points:
- Surface temperature and relative humidity
- Depth temperature and relative humidity
- Ambient temperature and relative humidity
Readings have been taken and logged from each sensor with a professional Tektronix Keithley datalogger, every 2 seconds.
We have first taken a baseline readings from the dry brick. Then we have added 170 ml of water at the base of the brick to simulate rising damp. The amount of water was chosen to create a significant wetting effect but without saturating the brick. The water has risen up to about 80% up throughout the brick, it stagnated for a while, then during the next 7 days it gradually evaporated, the brick returning to its former dry condition.
The experiment has been performed in a steadily heated non-condensing indoor environment, with 40-45% RH and 17-18 °C temperature.
Here are some
- Hygrothermal: moisture and temperature related, and
- Electrical: current and voltage related
changes we have observed in damp masonry, with some comments and conclusions.
After the application of water at the base of the brick, humidity started rising immediately. This is first detected by the surface sensors (green and blue lines below), followed after a small delay by the embedded depth sensors (yellow, orange, red).
Even with partial wetting, without saturating the brick, surface sensors stabilize at just over 90% relative humidity (RH).
The three depth sensors installed at different heights peak one after each other, reaching 100% saturation within the first hour, as moisture rises higher and higher inside the brickwork.
During the dehydration phase something similar happens - only in reverse. The surface sensors (green, blue) start dropping first, followed after a long delay by the embedded depth sensors (red, orange, yellow).
Some conclusions we can draw from here:
- In this experiment it took about 6 hours for the water to rise 150 mm in the brick and reach the top embedded sensor (red), saturating it. However, if a brick is subject to a lot of water fast (saturated condition), the water can easily reach the same height much faster, in less than half an hour - confirmed by experiments,
- Mid Day 1 the brick reached a short "equilibrium" condition, denoted by the flat plateau of the top surface sensor (green). After that the dehydration started and the brick started drying.
- The dehydration first has been picked up by the surface sensors (green and blue), then by the embedded depth sensors.
- The depth "air-pocket" sensors (red, orange, yellow) reacted much slower than the surface sensors. It took another 2 days before they started registering any drop of the humidity, the upper sensor reacting first and the bottom one sensor reacting last. At this point the brick already looked completely dry, yet there was clearly some humidity left under the surface, which slowly evaporated during days 4-7.
- The surface humidity of even a moderately damp wall can easily reach over 90%. Similar measurements have confirmed that in case of real walls connected with the ground that draw up moisture from the soil, surface humidity stays permanently at 100%,
- At the end of the 7th day all humidity sensors - surface and embedded ones - have reached the RH level of the ambient, around 40%.
As you can see below, during the experiment the ambient temperature (purple dashed line) in the lab has been fairly constant, between 17 - 18 °C.
Once water has been applied at the base of the brick, the temperature started sharply dropping both on the surface and in depth.
During the first day the temperature difference between the base of the fabric and the ambient reached 3.5 °C; the base of the brick was 3.5 °C colder than the room temperature. (blue line = diff between the dashed purple and yellow lines).
The same applies to the surface of the brick, which due to the evaporation, became about 2 °C colder than the ambient room temperature (red line).
Once the evaporation started and the brick started to dry out, its temperature gradually increased.
Some further conclusions we can draw from here:
- Wet walls can lose significant heat, being several degrees colder than the environment. This cooling effect can be felt near large, full-scale old walls built on soil, which - due to the ongoing rise of moisture and subsequent evaporation - can often feel chilly and drafty.
- The surface temperature of a damp wall, despite heating, is also cooler than the ambient temperature, although not as cool as the core of the fabric.
- The surface temperature of a dry wall can easily be at or above ambient temperature - dry bricks can temporarily store the heat and thus level out short-term temperature variations.
In addition to hygrothermal effects, there is a significant amount of electrical phenomena occurring in damp brickwork. The research of these electrical aspects is still in its infancy and largely unexplored, with only a few research papers being published in this regard. This electrical aspect of damp masonry is our core research and here are some of our findings.
3. Electric Current Variations
There is a significant current flow in damp bricks.
By definition, electric current indicates a flow of energy. Just think about a small battery and an bulb. Once the bulb lights up, a current starts flowing between the 2 poles of the battery - a flow of energy between (+) to (–) .
Similar occurs in damp walls: the existence of a flowing current indicates the presence of electrical charges or voltage differences (battery effect). We are looking closely into the drivers of this and related phenomena, but regardless of underlying theory, the phenomenon exists and can be easily recreated.
The amount of electric current flowing in dry bricks - which are insulators - is just a few nano amperes (nA = billionth of an ampere). In damp brickwork things change significantly, which can be attributed to several factors:
- The liquid water inside the capillaries becomes conductive as water dilutes some of the salts from the bricks (which originate from clayey soil) - and saline water, as we know, is conductive.
- The water vapours: the floating "vapour cloud" above the liquid water inside the capillaries is partially conductive, thus it contributes to the movement and redistribution of electric charges inside the brick capillaries.
Once water is applied, large current variations can be measured in damp bricks. From a few nA (dry current), the current flow increases several hundred times (wet current); from 1-2 nA to over 1,000 nA (1 μA - micro amperes). Every so often the currents change direction, flowing up or down in the brickwork, moving electrical charges into and out of the bricks.
These currents are present in brickwork while some humidity exists, then they disappear completely once the brick dries out and becomes non-conductive again, as shown below.
4. Voltage Variations
Current variations in damp brickwork are also accompanied by voltage variations. The electric charge that moves up and down the brickwork creates voltage "accumulations" and variations.
For example, there is significant voltage movement at the beginning and towards the end of the wetting-drying cycle in rapport to the 0 axis, as shown below. The sign of the voltage - driven by the underlying current movement - also changes from time to time, from positive (+) to negative (–) and vice-versa.
Moreover, certain voltage patterns variations repeat itself at different parts of the brickwork at different times, indicating a movement of the electrical charge (electrical energy), or a charge-discharge effect of the fabric.
At the beginning of the wetting cycle several hundreds of millivolts (mV) voltage appears in damp walls from apparently nowhere. This voltage is present in the brickwork as long as moisture exists inside the wall fabric, then it disappears once the wall fabric dries out and becomes non-conductive.
At the end of the drying cycle a significant voltage movement occurs again, which to some extent mirrors the early wetting pattern - only in reverse, which is not surprising as dehydration is the reverse phenomenon of wetting. This voltage movement also takes place much slower because the dehydration (free evaporation of water) is a much slower process than the relatively fast capillary absorption.
Some more conclusions that we can draw from here:
- Dry brickwork is an insulator with no electrical activity present in it.
- There is significant electrical activity in damp brickwork. This occurs for multiple reasons, for e.g. water and humidity - due to the presence of salts in the material - make damp brickwork electrically conductive.
- There can be several hundred millivolt (mV) voltage and several hundred nano amperes (nA) current present in damp bricks at any given time, as long as there is humidity present inside the fabric.
- The polarity of currents and voltages change, and it is related to the intensity and movement of moisture.
- In dry brickwork electrical activity ceases or becomes insignificant.
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