Industrialised, Integrated, Intelligent sustainable Construction - I3con

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SUSTAINABLE CONSTRUCTION HANDBOOK 2 cannot be used for real time decision making. Wireless temperature sensors would make on-line assessment of the temperature conditions possible in a real time environment and the building site engineers could use it to make actual decisions. For precast concrete products the elements are moved around from the casting facility to temporary storage, to the building site and finally to its final position in a building. Hence, precast concrete is often subject to various curing conditions and the need for continuous monitoring may be helpful for certain products. Moisture monitoring in concrete. Another important aspect of concrete production is the moisture content. During hydration the cement reacts chemically with the mixing water but even after complete hydration there is still say 110 litres of water per m 3 in the concrete pore system (Grasley et al. 2006). If the concrete is applied into a slab on grade and the floor material is a moisture sensitive material, e.g. wood or glued plastic flooring the relative humidity in the concrete should not exceed say 85 %- RH. Otherwise the glue looses its adherence and the basis for growth of fungi is there, increasing the risk of poor and unhealthy indoor climate. For conventional concrete for houses this means, that there is an excess amount of water of approximately 30 litres/m 3 , that needs drying out before flooring is applied. During the cold season concrete drying is a very slow and lengthy process, requiring a lot of energy for heating and ventilation. Drying out of concrete will often require 2-3 months with controlled drying climate, i.e. moderate temperatures and fairly dry air. It is a big challenge to plan this process into a tight construction schedule. Furthermore, a building site is always subject to rain and other sources of water during construction that may hinder the drying out process further (Figure 8). Figure 8. Ponds of rain water on a concrete slab delay the drying out of the concrete In Denmark the floor contractor is obliged to document that the moisture, level in terms of relative humidity in the concrete pores, is below the threshold value before the flooring is applied. Conventional measuring of relative humidity in a floor slab is often destructive and time consuming as described below: 130 • A RH sensor is placed in a drilled hole so that the relative humidity is registered in half thickness of the slab. The sensor should be left in the hole for at least a day in order to obtain moisture equilibrium. Then the operator has to return the next day to take a reading. At a building site it is not safe to leave expensive and delicate equipment such as a RH sensor in place for several hours. Also the readings may be sensitive to temperature variations.
HANDBOOK 2 SUSTAINABLE CONSTRUCTION • Alternatively a sample of concrete is taken from the slab in half thickness depth. This sample is then sealed in plastic bags and taken to a laboratory where it is brought in temperature equilibrium and then in contact with a RH sensor. Both methods are destructive and they need special equipment such as hammer drill. It is not normal to take more than a single sample from even a large concrete floor and the variation in moisture content is not considered. Another restriction on moisture measuring is the presence of cast-in heating pipes, which makes destructive sampling difficult. Furthermore, the methods require skilled persons to condition and handle the sample. This person is never the same as the one who is running the building site. A number of capacitive surface probes exist, being able to assess the moisture level in an easy manner. However, these methods are not suitable for determining the moisture content below a depth of a few mm. Wireless RH sensors (similar to those in Figures 2 and 3) are placed in a certain area of newly cast concrete makes the contractor able to monitor the drying process continuously and make decisions on when to bring in extra heating, ventilation or air driers to the building site. He would also be able to plan his work better and to control the interfaces between the different labor groups on the site. At the end of the day the floor quality would be better and the need for expensive repair work is reduced or avoided. SensoByg is demonstrating these applications on full scale building sites and furthermore, a forecast tool is developed so that the contractor may use the RH readings to predict the drying process based on certain assumptions. It is obvious that the installation of such embedded sensors should be carried out with high precision. Otherwise the measurements become subject to large variations and therefore useless. Another important aspect is the disturbance of the moisture profile by the sensor unit itself. Finite element calculations have been carried out on one-dimensional drying of a concrete slab (Nilsson & Fredlund 2009), showing that the orientation of the sensor unit has significant impact on the moisture profile. If the sensor unit is “blocking” the natural direction of moisture transport the error in RH readings will be more than ±10 % from the theoretical value for an undisturbed moisture profile. These results are based on a characteristic sensor size of around 50 mm. Results Results from the SensoByg project are in the making and therefore, not ready for presentation here. A series of important issues have been identified for the successful applications of wireless sensors in the construction industry: • The development of proper methods for installation of sensors in newly cast concrete is important. The environment on a building site is harsh and aggressive for delicate electronic devices. This should be addressed when installation methods are chosen. • Fixing sensor units so that the correct location of the measurements is ensured. • The sensor casing should be robust enough to resist the impact of falling concrete as well as impacts from the casting equipment and vibrators. At the same time it should allow water vapor to enter the sensor unit but keep fluid water and cement slurry from entering the casing. • Both concrete, reinforcement and water influences the wireless signal strength and imposes strict demands on the sensor design. The balance between sufficient battery life and signal performance is crucial. • Methods for post-installation are also needed for various building elements such as concrete bridges and tunnels, lightweight stud walls, bathroom walls and wooden floors. It is recognised that post-installation in existing structures is probably where the largest potential lies. • Precision, stability and accuracy of sensor units should be verified. In many cases the sensor is hidden inside structural elements and no possibility of recalibration and maintenance exists. 131
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Industrialised, Integrated, Intelligent sustainable Construction - I3con

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