Industrialised, Integrated, Intelligent sustainable Construction - I3con

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SUSTAINABLE CONSTRUCTION HANDBOOK 2 Relative humidity [%] 128 95 90 85 80 75 70 65 60 55 50 30-09-2007 05-10-2007 Highway bridge repair, SensoByg, Hedehusene 10-10-2007 15-10-2007 20-10-2007 E7E7E74C E7E7E759 E7E7E75A E7E7E75B E7E7E75C Figure 6. Diagram of relative humidity measured in the five sensors located right under the waterproofing membrane It is expected that in case of a leaking membrane the relative humidity in one of the sensors will increase abruptly, being an indication of water penetrating the asphalt. Such registrations will constitute a valuable decision support together with visual on site control. Monitoring will decrease the need for destructive tests on the asphalt and concrete in order to provide information on the condition of a waterproofing membrane. This philosophy is easily converted to other critical areas of civil structures such as prestressing tendons and drainage systems. Experiences with the highway bridge application clearly show that battery life issues are crucial since damages in such structures are often not expected until after a couple of decades. Therefore, new energy harvesting technologies are foreseen to influence the condition monitoring of civil structures. Furthermore, since the largest potential is lying within refurbishment and renovation works on existing structures, suitable post-installation techniques are to be developed. Moisture in buildings and indoor climate Indoor climate in buildings and houses is becoming more and more important as the requirements for buildings energy performance and air tightness are becoming stricter. Better energy performance means more insulation less window area and controlled ventilation systems. Therefore, it is important that indoor air quality is not impaired for health reasons. Wireless technologies are becoming more common in modern buildings. For instance in terms of fire alarms, electricity consumption readings, remote control of heating/cooling, etc. SensoByg will install wireless sensors in buildings in order to monitor the operation and maintenance phase. The following subjects are included in a full scale monitoring of two flats in Copenhagen: • Monitoring structural elements such as wooden roof structure or a basement where visual access is difficult to obtain. This would reduce the risk of rot and save expensive repair works. • Monitoring building elements in close contact with water such as bathrooms, sewage installations and the climate shield. Leaking water is the cause of damage in buildings and a built-in monitoring system would help minimise damage 25-10-2007 30-10-2007
HANDBOOK 2 SUSTAINABLE CONSTRUCTION • Monitoring indoor air temperature and relative humidity. A combination of temperature and relative humidity is used to evaluate whether there is a risk of fungi growth on the inner surfaces of the building. • Monitoring energy consumption for room heating. The building owner may use these data to alter the behaviour of the inhabitants so that energy is saved. Most of the above mentioned items are strongly correlated with measuring relative humidity and moisture. The moisture level in construction materials or in the indoor air is an excellent indicator for damage and unhealthy indoor climate. Another common denominator for these items is that moisture related damage generally develops slowly over time. Hence, it is difficult to detect unless you are recording data over time, which is impossible with conventional methods. Figure 7. Installation of wireless sensor in lightweight concrete wall next to water installation Therefore, the wireless RH sensors are ideal for such applications where continuous monitoring is needed. First of all it is important to determine the most suitable location of sensors in order to catch the most critical damage (Fig. 7). Secondly, the decision making needs tools so that the risk of damage or unhealthy indoor climate is assessed in a scientifically sound manner. Both these aspects are dealt with by experts in building materials and indoor climate. Applications in concrete production In connection with concrete castings there may be a need to monitor the concrete temperature in a structure for a number of reasons: • Firstly, it may be used to evaluate the maturity and strength of a concrete pour in order to decide time of formwork stripping and/or prestressing. The use of temperature registrations combined with age is converted into maturity, which again is related to the strength development and degree of hydration of concrete (Nielsen 2007). These principles are applicable to cast-in-place structures and precast concrete products. • Secondly, the temperature difference between the core of a casting and the surface should not exceed a certain limit due to the risk of early-age thermal cracking. This is especially a concern of massive cast-in-place structures where the heat of hydration escapes slowly. On-line monitoring of critical cross-sections could help eliminate the risk of early-age cracking and improve the durability of the structure. • Thirdly, temperature monitoring on the building site and on the formwork and reinforcement could be suitable to control active measures such as embedded cooling pipes or heating wires or it may help to warn against freezing conditions prior to casting. Generally it is a difficult task to establish a conventional temperature monitoring system by means of thermo couples due to the need for wiring and data loggers. Often the temperature readings are not gathered and investigated until weeks after the casting took place. This means that these readings 129
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Industrialised, Integrated, Intelligent sustainable Construction - I3con

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