Relative humidity measurements in concrete
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A commonly used way to monitor the concrete s moisture condition is to measure Relative humidity (RH). Measuring relative RH in concrete, and understanding the underlying mechanisms are complex tasks. Field measurements are especially challenging because of limited control of exposure factors compared to laboratory measurements. The aim of the MSc project is to come up with recommendations for how and when to perform RH measurements in concrete in field. The MSc project should also contribute to an improved understanding of how field measurements of RH in concrete vary with the exposure conditions. In addition, a critical discussion on the reliability of the RH sensors will be presented. Two RH measurements systems used: HumiGuard and Vaisala. From the start of the MSc project the main focus was field measurements, but early on it became clear that we needed to understand the RH measuring systems better in order to be able to interpret the field results. A series of tests in laboratory were conducted in this project to get a better understanding of the workings of the RH measuring systems used e.g. initial calibration of sensors, drift, and temperature response in laboratory. The field measurements that were conducted gave interesting information about the behaviour of RH in boreholes in concrete samples. Some of the samples are exposed to the weather and sun, and some are protected from rain and sun exposure. For the field measurements only HumiGuard sensors were used. During the laboratory tests, large differences in the behaviour of two RH measurements systems (HumiGuard and Vaisala) were observed. The Vaisala responds fast, but drifts while installed in the concrete. The HumiGuard on the other hand reacts much slower, and stabilizes over time. We found that there are many uncertainties with using the HumiGuard system and that there are many sources of error: The initial scatter of calibration values corresponding to 84.5% RH, difference in drift for the individual sensors, the inertia of concrete and sensors and the exposure conditions for the concrete measured in. Using the HG sensors like they are used commercially today. e.g. drying of floors in buildings, with stable conditions and enough time for the sensors to stabilise and control of the initial µS value for each sensor; measurements within the uncertainty specified (2.5%) might be obtained. However, we would conclude that using for field measurements with the variations in temperature we get, it seems that the sensors are not responding fast enough and errors must be expected on the results. After writing this report, we found that RH measurements are uncertain and difficult to control. We hope that this report can be a piece to the puzzle towards more reliable RH measurements.