Historically, in situ monitoring of moisture movement in existing walls has been a challenge. Even today, few reliable, validated methods exist which are non-invasive or destructive. The task is made far more difficult for earthen materials (rammed earth, adobe, etc.) for which destructive techniques can be especially damaging, and feasible, nondestructive methodologies are generally absent from the discussion. Given the contemporary, as well as future context of intensifying, global climatic phenomena, particularly the evolving severity and frequency of precipitation patterns, earthen constructions remain among the most vulnerable. This investigation reviews current strategies for moisture monitoring in adobe walls and reviews the implementation of new methods for doing so, namely the use of passive radio frequency identification (RFID) technology as a nondestructive, early indicator of moisture ingress. Following the four-tiered approach established by the National Park Service in its Climate Change Response Strategy (1), this research not only aims to develop a new monitoring methodology for vulnerability assessment, but to contribute to the dialogue among preservation scientists and site managers by sharing the methodology and its results. The product of this investigation will have broad applicability to other climate sensitive materials and construction types, both new and old.
The use of RFID tags to measure the moisture content of their surrounding environment is based on the fundamental principle that water, with a high dielectric constant between 70 and 80, changes the effective dielectric constant of the tag when wet (2). This results in a change in the tag antenna’s resonant frequency due to ohmic losses in the antenna’s near-field15 and a reduced backscatter signal strength to the receiver (requires more power to transmit, as some has been absorbed by the surrounding moisture). These relative changes in the tags electrical properties can then be used to infer the properties of its surrounding material, or rather changes to these properties over time. The benefits of this method include the ability to transmit data without requiring optical line-of-site or physical contact with the medium being measured. This latter point is the most significant advantage of this technique when applied to the issue of moisture detection in adobe walls for the reasons mentioned above.
Several tests were conducted to determine the operational thresholds of using RFID for moisture monitoring in adobe walls. Evaluated were the behavior of multiple tags across varying distances in open air to determine the attenuation curve for loss in signal strength and change in frequency, the results of which could be factored out later in embedded situations; a single tag in an isolated container exposed to increasing (and measured) amounts of moisture (see figure of the results below); and a final (pending) test to test various sensors embedded within the mortar beds of a mock-up adobe brick wall. A more detailed project background, full methodology and results can be found here.
One of the great strengths of this technique is the one that most sets it apart from current methods, and that is the low-cost, sacrificial, and passive nature of the embedded elements. In other words, the RFID tags are ‘install and forget’ elements. This being said, this system is only as effective as its implementation. Pending actual in situ installation, it can be logically hypothesized that this technique is particularly well-suited to overcoming the enduring dilemma in preservation of long-term monitoring of historic structures, and that is the installation of the system—not necessarily the system itself. Given that the embedded component of this system is passive, it lends itself to being easily dovetailed with, hopefully cyclical, repair and maintenance events, such as wall-top capping or shelter coats.
Furthermore, this technique leverages the dual-purpose nature of the RFID tags as both a databank and a ‘sensor.’ The sensor functionality is actually a byproduct of the former category in that by monitoring the relative changes in RSSI and frequency of a particular tag while attempting to read its stored information, inferences can be made about the moisture content of the tag’s surroundings. Simultaneously, the tag has ability to store static information. In addition to its fixed 64bit tag id (TID), and 128bit electronic product code (EPC), each tag can store up to 144bits of user memory. This latter category can be used to store up to 18 ASCII encoded (8-bits per character) characters: the last 18 moisture readings (or RSSI values), the last repair (date and name), a GIS coordinate to be integrated into a GIS database, the last recorded weather conditions (T/RH) from a nearby weather station, the values of the previously read adjacent tag (in a linked-list implementation23), and so forth.
Although challenges lie ahead, particularly the development of a reliable system to autonomously record data from a network of tags on a regular basis, this technique demonstrates great promise as a novel alternative to nondestructive monitoring of historic adobe walls. And with the development of the infrastructure for this technology will evolve other, new applications. For instance, it is known that SmarTrac is developing a hybrid moisture and temperature sensor, which could be used in tandem with surface infrared thermography as a way to map surface temperature with validated subsurface moisture and temperature readings.
- (1) National Park Service. “Climate Change Response Strategy.” United States Department of the Interior, 2013. List item
- (2) J. B. Ong et al., “A Wireless, Passive Embedded Sensor for Real-Time Monitoring of Water Content in Civil Engineering Materials,” IEEE Sensors Journal 8, no. 12 (2008).