Development of an Embedded Sensor Holder for Concrete Structures Monitoring

The concrete material presents a different behaviour in compression and in tension. Its tension resistance is, approximately, ten times less than the compression one. So it is very common to see cracks in reinforced concrete structures in service conditions. The deformation field presents a discontinuity in each crack zone. This fact leads to the motivation of conceiving and developing a new sensor holder which main objective is to guarantee that the measurements of concrete deformation may be performed either in compression or in tension with or without concrete cracking. Sensor holders are an efficient and robust way to apply sensors for concrete structures monitoring. Such holders may be considered as an interface system, between the sensor and the involving material. The sensors can be introduced in concrete structures during the execution process, in a way that an internal measurement of the parameter is realized [1]. The main objectives of the sensor holders are: (i) offer an adequate robustness against possible damages that can occur during the structure execution process; (ii) protect against chemical attacks during the service life; (iii) guarantee a representative deformation measurement of the involving concrete; (iv) measure any deformation induced to the holder and also, v) prevent that the constituent material rigidity do not modify the local deformation of concrete [1]. To fulfil the referred main objectives two types of sensor holders are preferred for embedding into concrete. One of them is designated as of I type, characterized by a high diameter of their extremities in respect to the cylindrical body. The other one is designated by W type, which is characterized by roughness body with a constant diameter. Lesoille et al [2] performed a numerical study where this two type of sensor holders were compared. According to that, sensor holders of type I and constituted by one single material, are very sensible to the adhesion conditions of the interface sensor holder – involving material. This phenomenon is amplified when the holder is made of steel. The sensor holder of type W presents symmetric behaviour in compression and tension. However, such holders only work with sensors that are able to integrate the deformation throughout the whole length. For all studied holders the sensors are fixed inside for all length. Another kind of sensor holders, are the ones in which the sensors are fixed in it extremities being free in the remaining zone. Those holders are usually of I type, made of steel and adherent to the involving material. For such holders it is verified that when long term monitoring is performed the sensor, under tension, tends to relax diminishing its sensitivity. This type of holders is generally used with vibrating wire sensors [3].