Hiba Abdel-Jaber

Analysis of the status of pre-release cracks in prestressed concrete structures using long-gauge sensors

Prestressed structures experience limited tensile stresses in concrete, which limits or completely eliminates the occurrence of cracks. However, in some cases, large tensile stresses can develop during the early age of the concrete due to thermal gradients and shrinkage effects. Such stresses can cause early-age cracks, termed ?pre-release cracks?, which occur prior to the transfer of the prestressing force. When the prestressing force is applied to the cross-section, it is assumed that partial or full closure of the cracks occurs by virtue of the force transfer through the cracked cross-section. Verification of the closure of the cracks after the application of the prestressing force is important as it can either confirm continued structural integrity or indicate and approximate reduced structural capacity. Structural health monitoring (SHM) can be used for this purpose. This paper researches an SHM method that can be applied to prestressed beam structures to assess the condition of pre-release cracks. The sensor network used in this method consists of parallel long-gauge fiber optic strain sensors embedded in the concrete cross-sections at various locations. The same network is used for damage detection, i.e. detection and characterization of the pre-release cracks, and for monitoring the prestress force transfer. The method is validated on a real structure, a curved continuous girder. Results from the analysis confirm the safety and integrity of the structure. The method and its application are presented in this paper.

Systematic method for the validation of long-term temperature measurements

Structural health monitoring (SHM) is the process of collecting and analyzing measurements of various structural and environmental parameters on a structure for the purpose of formulating conclusions on the performance and condition of the structure. Accurate long-term temperature data is critical for SHM applications as it is often used to compensate other measurements (e.g., strain), or to understand the thermal behavior of the structure. Despite the need for accurate long-term temperature data, there are currently no validation methods to ensure the accuracy of collected data. This paper researches and presents a novel method for the validation of long-term temperature measurements from any type of sensors. The method relies on modeling the dependence of temperature measurements inside a structure on the ambient temperature measurements collected from a reliable nearby weather tower. The model is then used to predict future measurements and assess whether or not future measurements conform to predictions. The paper presents both the model selection process, as well as the sensor malfunction detection process. To illustrate and validate the method, it is applied to data from a monitoring system installed on a real structure, Streicker Bridge on the Princeton University campus. Application of the method to data collected from about forty sensors over five years showed the potential of the method to categorize normal sensor function, as well as characterize sensor defect and minor drift.

Evaluating the coefficient of thermal expansion using time periods of minimal thermal gradient for a temperature driven structural health monitoring

Structural Health Monitoring aims to characterize the performance of a structure from a combination of recorded sensor data and analytic techniques. Many methods are concerned with quantifying the elastic response of the structure, treating temperature changes as noise in the analysis. While these elastic profiles do demonstrate a portion of structural behavior, thermal loads on a structure can induce comparable strains to elastic loads. Understanding this relationship between the temperature of the structure and the resultant strain and displacement can provide in depth knowledge of the structural condition. A necessary parameter for this form of analysis is the Coefficient of Thermal Expansion (CTE). The CTE of a material relates the amount of expansion or contraction a material undergoes per degree change in temperature, and can be determined from temperature-strain relationship given that the thermal strain can be isolated. Many times with concrete, the actual amount of expansion with temperature in situ varies from the given values for the CTE due to thermally generated elastic strain, which complicates evaluation of the CTE. To accurately characterize the relationship between temperature and strain on a structure, the actual thermal behavior of the structure needs to be analyzed. This rate can vary for different parts of a structure, depending on boundary conditions. In a case of unrestrained structures, the strain in the structure should be linearly related to the temperature change. Thermal gradients in a structure can affect this relationship, as they induce curvature and deplanations in the cross section. This paper proposes a method that addresses these challenges in evaluating the CTE.

Identification of steady-state uniform temperature distributions to facilitate a temperature driven method of Structural Health Monitoring

Structural Health Monitoring seeks to characterize the performance of a structure from combinations of recorded sensor data and analytic techniques. Temperature is normally considered noise in this analysis, obstructing the goal measuring the elastic response of the structure. While these elastic loads do help characterize a portion of structural behavior, the thermal loads on a structure can induce comparable strains to these elastic loads. Characterizing a relationship between the temperature of the structure and the resultant strain and displacement can provide a deep understanding of the structural condition. In order to begin characterizing this 3-dimensional relationship, time periods with relatively steadystate, uniform temperature distributions need to be identified from the measured data. These periods of uniform temperature distribution in the structure show a thermal response as free as possible from thermal gradients across the structure. These steady-state periods help create a signature of the structure when analyzed with the relevant strain and displacement measurements of the structure. An algorithm for finding these uniform distributions was created to identify these desirable time periods with data of interest. Finding time periods with a completely uniform temperature distribution can be unreasonable, so a suitable temperature interval was chosen to produce a set of data with a reasonable approximation to a uniform distribution, while still providing a large enough set of data to produce meaningful results. These time intervals provide the necessary temperature, strain, and displacement measurements to characterize a signature for the structure, providing a more in-depth analysis in SHM.

Monitoring of long-term prestress losses in prestressed concrete structures using fiber optic sensors:

This study presents a method for on-site assessment of prestress losses in prestressed concrete structures. The study is motivated by the increased use of prestressed concrete, the importance of prestressing force levels as a parameter, and the lack of formalized methods for its on-site assessment. The proposed method uses strain measurements from long-gauge fiber optic sensors to study strain changes at the centroid of stiffness (i.e. centroid of composite section) of the cross-sections. Its advantages include (1) robustness to operational load on the structure caused by seasonal and daily temperature variations, in addition to loading; (2) rigorous quantification of uncertainties associated with measurements and parameters; and (3) applicability to a wide range of beam-like structures. The application of the method is illustrated through application to measurements collected over a 7-year period from strain sensors embedded in Streicker Bridge, a post-tensioned concrete pedestrian bridge on the Princeton University campus. Application of the method indicates that prestress losses measured by sensors are of comparable magnitude to design estimates, which implies that estimates are not necessarily overly conservative.

Structural health monitoring methods for the evaluation of prestressing forces and prerelease cracks

Prestressed concrete bridges currently account for 45% of bridges built in the last five years in the United States. This has resulted in an increase in the number of deficient bridges composed of prestressed concrete, which requires a better understanding of the on-site performance of this building material. The use of new materials, such as high performance concrete, in conjunction with prestressing provides additional motivation for the creation of structural health monitoring (SHM) methods for prestressed concrete. This paper identifies two parameters relevant to prestressed concrete, along with methods for their evaluation. The parameters evaluated are the prestressing force value at transfer and the width of pre-release cracks, both of which are indicators of structural performance. Improper transfer of the prestressing force can result in tensile stresses in the concrete that exceed capacity and result in cracks and/or excessive deflections. Pre-release cracks occur in the concrete prior to transfer of the prestressing force and are mainly caused by autogenous shrinkage and thermal gradients. Closure of the cracks is expected by virtue of prestressing force transfer. However, the extent of crack closure is important in order to guarantee durability and structural integrity. This paper presents an integral overview of two novel methods for the statistical evaluation of the two monitored parameters: prestressing forces and the width of pre-release cracks. Validation of the methods is performed through application to two structures, both of which are components of Streicker Bridge on the Princeton University campus. Uncertainties are evaluated and thresholds for unusual behavior are set through the application.

Monitoring of prestress losses using long-gauge fiber optic sensors

Prestressed concrete has been increasingly used in the construction of bridges due to its superiority as a building material. This has necessitated better assessment of its on-site performance. One of the most important indicators of structural integrity and performance of prestressed concrete structures is the spatial distribution of prestress forces over time, i.e. prestress losses along the structure. Time-dependent prestress losses occur due to dimensional changes in the concrete caused by creep and shrinkage, in addition to strand relaxation. Maintaining certain force levels in the strands, and thus the concrete cross-sections, is essential to ensuring stresses in the concrete do not exceed design stresses, which could cause malfunction or failure of the structure. This paper presents a novel method for monitoring prestress losses based on long-gauge fiber optic sensors embedded in the concrete during construction. The method includes the treatment of varying environmental factors such as temperature to ensure accuracy of results in on-site applications. The method is presented as applied to a segment of a post-tensioned pedestrian bridge on the Princeton University campus, Streicker Bridge. The segment is a three-span continuous girder supported on steel columns, with sensors embedded at key locations along the structure during construction in October 2009. Temperature and strain measurements have been recorded intermittently since construction. The prestress loss results are compared to estimates from design documents.

Validation of long-term measurements from FBG sensors

Temperature monitoring has been of increased importance in recent years due to the need for temperature measurements in order to compensate other measurement parameters, such as strain, and the increased attention to understanding thermal behaviors of structures in order to assess their performance and condition. To ensure the accuracy of thermal compensation and study of thermal behavior, reliable long-term temperature measurements are required. In this paper, two methods that are aimed at validating long-term temperature measurements are created and their application is presented. The methods differ in the type of data they use for the purpose of validation. The first method relies on the existence of two independent temperature sensors at the same location. Validation is performed by comparing the measurements from the two sensors to one another, and discrepancies between the two data sets indicate malfunction or drift in at least one of the sensors. The second method is applicable to the more general case where only one temperature sensor is available at a given location. The method thus utilizes ambient temperature data from a nearby weather tower to validate measurements from the sensor. The two methods are applied to temperature measurements from FBG sensors installed on Streicker Bridge on the Princeton University campus. The methods successfully identified and characterized malfunction and drift in some of the sensors and confirmed stable measurements in other sensors.

Validation of Long-Term Data from FBG Temperature Sensors

Monitoring temperature in structures is essential for understanding their thermal behavior and performing thermal compensation for any other type of sensors used for structural health monitoring (e.g. strain). This is particularly important for long-term monitoring applications. Nevertheless, a universal procedure for the validation of temperature data from a monitoring system has not been created and implemented. In this paper, preliminary research on two methods that aim to validate temperature readings from temperature sensors is presented. The methods differ in the type of data available for validation: (1) data from two independent monitoring systems are available, and (2) data from a monitoring system and a nearby weather tower are available. The methods use linear regression and confidence intervals on the linear regression model parameters to quantify annual changes in readings, which are then used to characterize long-term performance of the sensor such as changes in sensitivity or stability, or other unusual behaviors. The methods were applied to data collected over five years from fiber optic sensors based on Fiber Bragg Gratings (FBG) installed on Streicker Bridge on the Princeton University campus, and similar qualitative conclusions were derived from the two methods at instrumented locations where two independent monitoring systems are installed. This validation of the two methods was used as a confirmation that the second method, for which data is generally available, can be used in lieu of the first in cases where two independent monitoring systems do not exist. Based on this validation, the second method was independently used to test data at other locations on the bridge, where only one type of sensors is installed. The validation procedure showed and quantified minor changes in sensitivity and stability in measurements from some instrumented locations after the first two years. doi: 10.12783/SHM2015/62

Monitoring of pre-release cracks in prestressed concrete using fiber optic sensors

Prestressed concrete experiences low to no tensile stresses, which results in limiting the occurrence of cracks in prestressed concrete structures. However, the nature of construction of these structures requires the concrete not to be subjected to the compressive force from the prestressing tendons until after it has gained sufficient compressive strength. Although the structure is not subjected to any dead or live load during this period, it is influenced by shrinkage and thermal variations. Thus, the concrete can experience tensile stresses before the required compressive strength has been attained, which can result in the occurrence of “pre-release” cracks. Such cracks are visually closed after the transfer of the prestressing force. However, structural capacity and behavior can be impacted if cracks are not sufficiently closed. This paper researches a method for the verification of the status of pre-release cracks after transfer of the prestressing force, and it is oriented towards achievement of Level IV Structural Health Monitoring (SHM). The method relies on measurements from parallel long-gauge fiber optic sensors embedded in the concrete prior to pouring. The same sensor network is used for the detection and characterization of cracks, as well as the monitoring of the prestressing force transfer and the determination of the extent of closure of pre-release cracks. This paper outlines the researched method and presents its application to a real-life structure, the southeast leg of Streicker Bridge on the Princeton University campus. The application structure is a curved continuous girder that was constructed in 2009. Its deck experienced four pre-release cracks that were closed beyond the critical limits based on the results of this study.