Micro-environmental Monitoring of Temperature & Moisture Changes in Building Stones Using Embedded Electrical Sensors

The monitoring of temperature and moisture changes in response to different micro-environment of building stones is essential to understand the material behaviour and the degradation mechanisms. From a practical point of view, having a continuous and detailed understanding of micro-environmental changes in building stones helps to assist in their maintenance and repair strategies. Temperature within the stone is usually monitored by means of thermistors, whereas wide ranges of techniques are available for monitoring the moisture. In the case of concrete an electrical resistance method has previously been used as an inexpensive tool for monitoring moisture changes. This paper describes the adaptation of this technique and describes its further development for monitoring moisture movement in building stones. In this study a block of limestone was subjected to intermittent infrared radiation with programmed cycles of ambient temperature, rainfall and wind conditions in an automated climatic chamber. The temperature and moisture changes at different depths within the stone were monitored by means of bead thermistors and electrical resistance sensors. This experiment has helped to understand the thermal conductivity and moisture transport from surface into deeper parts of the stone at different simulated extreme climatic conditions. Results indicated that variations in external ambient conditions could substantially affect the moisture transport and temperature profile within the micro-environment of building stones and hence they could have a significant impact on stone decay. INTRODUCTION Civil infrastructure is considered to be the most expensive asset of any country. Maintaining this infrastructure for the intended service life is a major issue, especially if it is deteriorating due to changes in external environmental conditions. Most concrete and masonry structures are prone to chemical attack in aggressive environments experienced in urban settings (Hall and Hoff, 2002). The deterioration of porous building materials usually involves movement of aggressive gases and/or liquids from the surrounding environment into the near surface zone of the porous materials, followed by physical and/or chemical changes in their internal structure (Basheer and Nolan, 2001). The movement of these aggressive substances occurs due to differentials in humidity, ionic concentration, pressure and temperature within their microstructure. Despite advanced non-destructive methods currently used for assessing the deterioration processes and their extent, the fate and extent of inner contamination of building materials remains largely unaccounted for by such methods. Therefore, continuous health monitoring of these building materials is inevitable, not only from the standpoint of economic planning and maintenance, but also on cultural, technical and scientific grounds (Mauricio et al, 2005). Moisture plays a dominant role in weathering and decay processes of building stones. Almost all the weathering processes, physical, chemical and biological processes, happen in presence of moisture. Normally moisture induced weathering in building stone occurs through various external sources, like ingress of acid rain water, capillary rise of ground water with dissolved salts through ineffective damp course, flooded water, and water leakages from pipes and roofs. In most of the cases it is the cyclic process of ingress-egress thrived by absorption and evaporation of moisture that creates problems in building stones; these are, in turn, dependent on the intrinsic properties inherent to the material, like internal pore structure of the stone. Although these cyclic processes depend on the intrinsic properties of the material of stone, they are mainly caused by sudden changes in external environmental conditions, like relative humidity, temperature, wind and orientation of stone facade to these parameters (Smith et al, 2003). As mentioned earlier, moving-water inside stone contains contaminants originating from atmosphere and soil beneath the structure. During the drying phase as the evaporation of moisture increases from the surface the salt concentration increases and crystallisation of salt takes place at the near surface region of the building stone, as shown in Fig. 1. The salt crystallisation within the pores of building stone is an important cause of decay of stone minerals at the surface and in some cases can lead to catastrophic decay due to combination of several deterioration processes, as depicted in Fig. 2. The position of salt crystallisation depends to some extent on the nature of salts, texture of material, porosity at near surface region and condition of evaporation (Amoroso and Fassina, 1983). Evaporation from surface of porous stone depends on the micro-environmental conditions and the evaporation rate increases with increase in ambient temperature, decrease in relative humidity and increase in wind speed on the surface of stone (Hall and Hoff, 2002). By developing a monitoring system for building stone, decay induced by salt crystallisation process can be understood by knowing the time of arrival of moisture, extent of moisture penetration along depth and by studying the conditions favourable for evaporation of moisture. The above essential need to develop an efficient monitoring system has provided a wide range of techniques for monitoring temperature and moisture in porous media. Temperature within the stone is usually monitored by means of thermistors. Electrical resistance probes were used in this study to monitor moisture arrival time and moisture penetration depth in building stones. ELECTRICAL RESISTANCE BASED (ER) SENSORS Electrical resistance based sensors are most popularly used in measuring moisture ingress in building materials in terms of their electrical resistance or dielectric properties, which vary with moisture content in addition to the effect of temperature. The relationship of electrical resistance with moisture content of Fig. 1 Salt crystallisation on surface from capillary rise water Fig. 2 Rapid catastrophic decay of limestone building stone was first described by Knowler (1927). He also explained that unlike in DC based measurements, AC measurements have negligible disturbance caused by the polarisation of electrodes. In recent years considerable volume of research has been carried out on developing electrical resistance based sensors for monitoring moisture changes in building materials. An electrical resistance array system (covercrete array) developed by McCarter et al at Heriot-Watt University, Edinburgh (McCarter et al, 1992, 1995) is one amongst these methods, which is used to monitor moisture and ionic movement in cover zone of concretes. The covercrete electrode array comprises 10 electrode-pairs mounted on a small perspex former at different depths, sleeved with heat shrink tubes so as to expose a length of 5mm at the tip (Fig. 3). The pairs of electrodes are mounted parallel to the suction surface enabling the electrical properties of the material (resistance in this instance) to be obtained at 10 discrete points from the surface (up to a depth of 50mm). Thermistors are also mounted on the Perspex former thereby enabling temperature profiles to be obtained. According to McCarter et al (1995), the measurements across the pairs of electrodes could be presented in the following four ways: i) Variation of as-measured conductivity (in Siemens/m, S/m) as a function of time, t, for each electrode position on the sensor, where conductivity is the inverse of the as measured resistivity; ii) Variation of conductivity measurements obtained in i) which have been standardised to the reference temperature (20°C) thereby allowing changes in conductivity due to temperature to be minimised. This is particularly important for site measurements; iii) Variation of normalised conductivity using the values in ii) above where the normalised conductivity, NC, is defined as: