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].
Bridge deflection evaluation using strain and rotation measurements
Monitoring systems currently applied to concrete bridges include strain gauges, inclinometers, accelerometers and displacement transducers. In general, vertical displacements are one of the parameters that more often need to be assessed because their information reflects the overall response of the bridge span. However, the implementation of systems to continuously and directly observe vertical displacements is known to be difficult. On the other hand, strain gauges and inclinometers are easier to install, but their measurements provide no more than indirect information regarding the bridge deflection. In this context, taking advantage of the information collected through strain gauges and inclinometers, and the processing capabilities of current computers, a procedure to evaluate bridge girder deflections based on polynomial functions is presented. The procedure has been implemented in an existing software system - MENSUSMONITOR -, improving the flexibility in the data handling and enabling faster data processing by means of real time visualization capabilities. Benefiting from these features, a comprehensive analysis aiming at assessing the suitability of polynomial functions as an approximate solution for deflection curves, is presented. The effect of boundary conditions and the influence of the order of the polynomial functions on the accuracy of results are discussed. Some recommendations for further instrumentation plans are provided based on the results of the present analysis. This work is supported throughout by monitoring data collected from a laboratory beam model and two full-scale bridges.
Neutron Residual Stress Measurements on Rail Sections for Different Production Conditions
Rail sectioning with subsequent neutron diffraction experiments has been used to assess residual stresses in the rails. In this study we present the results of neutron stress measurements performed at the NIST Center for Neutron Research (NCNR) on rail sections from rails that were produced under various conditions. Specifically, these are air-cooled, air-cooled and roller-straightened, head-hardened and head-hardened and roller-straightened. More significantly, a head-hardened and roller-straightened rail was also studied after service to elucidate evolution of the service-induced residual stresses. In the latter case both a transverse-cut slice and the central region of a 0.53 m long piece were studied. Measurements on this piece are the first in which triaxial stresses have been determined for an intact rail. Neutron strain measurements with 3×3×3 mm3 spatial resolution were successfully employed for transversally cut slices to verify the difference in the stress state depending on the production process. Although examination of slices allows determination of only two-dimensional stresses in the plane of the slice, additional measurements on obliquely-cut slices, which were also carried out, and utilization of FEM gives the possibility of reconstructing the full triaxial stress distribution. Together, these approaches provide a better understanding of rail fabrication and the possibility of improving the durability and safety of rails in the future.Copyright © 2004 by ASME