Non-Destructive Testing Methods to Identify Voids in External Post-Tensioned Tendons

임석빈,Stefan Hurlebaus 대한토목학회 2012 KSCE JOURNAL OF CIVIL ENGINEERING Vol.16 No.3

A considerable number of Post-Tensioned (PT) bridges have been constructed because PT systems enable them to carry significant traffic loads and have an aesthetical structure. However, strand corrosion has been a long-standing issue because it may lead to the failure of tendons and the deterioration of structural performance. The corrosion typically occurs in voided locations with exposed strands; thus, the inspection of voids in external PT tendons is important and necessary in order to protect strands before corrosion occurs. Based on literature review, several Non-Destructive Testing (NDT) methods are compared for effectiveness of identifying voids in external PT tendons, and the Impact Echo (IE), ultrasonic, and sounding inspection methods are then selected and assessed using small-scale and mock-up specimens. From the experimental results, the wave-based inspection methods, IE and ultrasonic methods, are difficult to apply in the field because the imperfect bonds between ducts and grouts obstruct the transmission of waves. However, the sounding inspection method is not affected by the discontinuities and successfully identifies voids in test specimens. Thus, the sounding inspection can be an effective tool for identifying voids because of its easy application in the field.

Enhanced damage index method using torsion modes of structures

임석빈,Stefan Hurlebaus,Harding C. Cloudt,Jeffrey A. Fogle 국제구조공학회 2013 Smart Structures and Systems, An International Jou Vol.12 No.4

A growing need has developed in the United States to obtain more specific knowledge on the structural integrity of infrastructure due to aging service lives, heavier and more frequent loading conditions, and durability issues. This need has spurred extensive research in the area of structural health monitoring over the past few decades. Several structural health monitoring techniques have been developed that are capable of locating damage in structures using modal strain energy of mode shapes. Typically in the past, bending strain energy has been used in these methods since it is a dominant vibrational mode in many structures and is easily measured. Additionally, there may be cases, such as pipes, shafts, or certain bridges, where structures exhibit significant torsional behavior as well. In this research, torsional strain energy is used to locate damage. The damage index method is used on two numerical models; a cantilevered steel pipe and a simply-supported steel plate girder bridge. Torsion damage indices are compared to bending damage indices to assess their effectiveness at locating damage. The torsion strain energy method is capable of accurately locating damage and providing additional valuable information to both of the structures’ behaviors.

Simulation of Repair Grout Flow in External Tendon System

임석빈,Stefan Hurlebaus 대한토목학회 2012 KSCE JOURNAL OF CIVIL ENGINEERING Vol.16 No.7

The corrosion of voided tendons has been a long-standing issue in Post-Tensioned (PT) systems because it can lead to the failure of tendons and deteriorate bridge performance. Repair grouting in voided tendons can be one of many rehabilitation options if there are no galvanic couples between the new grouts and original grouts. To identify an appropriate repair grouting method for voided tendons, numerical analysis using a commercial Computational Fluid Dynamics (CFD) program, FLUENT, is utilized in this research. The geometry of voided tendons is a complicating factor when creating reliable models for numerical analysis; thus, a research for the simulation of repair grout flow proposed a simplified model and verified the model with experimental results. Thus,this research performs numerical analyses using the simplified model and assesses various parameters of repair methods and materials. The numerical results indicate that the Pressure-Vacuum Grouting (PVG) method with repair grouts having low viscosity exhibits the best performance for filling voids. If it is determined that filling voids with repair grout is appropriate and prevents future corrosion, it is recommended that voids in the field be filled using the PVG method with grouts exhibiting low viscosities.

Noncontact techniques for monitoring of tunnel linings

White, Joshua,Hurlebaus, Stefan,Shokouhi, Parisa,Wittwer, Andreas,Wimsatt, Andrew Techno-Press 2014 Structural monitoring and maintenance Vol.1 No.2

An investigation of tunnel linings is performed at two tunnels in the US using complimentary noncontact techniques: air-coupled ground penetrating radar (GPR), and a vehicle-mounted scanning system (SPACETEC) that combines laser, visual, and infrared thermography scanning methods. This paper shows that a combination of such techniques can maximize inspection coverage in a comprehensive and efficient manner. Since ground-truth is typically not available in public tunnel field evaluations, the noncontact techniques used are compared with two reliable in-depth contact nondestructive testing methods: ground-coupled GPR and ultrasonic tomography. The noncontact techniques are used to identify and locate the reinforcement mesh, structural steel ribs, internal layer interfaces, shallow delamination, and tile debonding. It is shown that this combination of methods can be used synergistically to provide tunnel owners with a comprehensive and efficient approach for monitoring tunnel lining conditions.

Effectiveness of strake installation for traffic signal structure fatigue mitigation

Wieghaus, Kyle T.,Hurlebaus, Stefan,Mander, John B. Techno-Press 2014 Structural monitoring and maintenance Vol.1 No.4

Across-wind response is often the cause of significant structural vibrations that in turn cause fatigue damage to welded and other connections. The efficacy of low-cost helical strakes to mitigate such adverse response is presented for a traffic signal structure. Field observations are made on a prototype structure in a natural wind environment without and with helical strakes installed on the cantilevered arm. Through continuous monitoring, the strakes were found to be effective in reducing across-wind response at wind speeds less than 10 m/s. Estimates of fatigue life are made for four different geographical locations and wind environments. Results for the class of traffic signal structure show that helical arm strakes are most effective in locations with benign wind environments where the average annual wind speed is not more than the vortex shedding wind speed, which for this investigation is 5 m/s. It is concluded that while strakes may be effective, it is not the panacea to mitigating connection fatigue at all locations.

Assessment of modal parameters considering measurement and modeling errors

Qindan Huang,Paolo Gardoni,Stefan Hurlebaus 국제구조공학회 2015 Smart Structures and Systems, An International Jou Vol.15 No.3

Modal parameters of a structure are commonly used quantities for system identification anddamage detection. With a limited number of studies on the statistics assessment of modal parameters, thispaper presents procedures to properly account for the uncertainties present in the process of extracting modalparameters. Particularly, this paper focuses on how to deal with the measurement error in an ambientvibration test and the modeling error resulting from a modal parameter extraction process. A bootstrapapproach is adopted, when an ensemble of a limited number of noised time-history response recordings isavailable. To estimate the modeling error associated with the extraction process, a model predictionexpansion approach is adopted where the modeling error is considered as an “adjustment” to the predictionobtained from the extraction process. The proposed procedures can be further incorporated into theprobabilistic analysis of applications where the modal parameters are used. This study considers the effectsof the measurement and modeling errors and can provide guidance in allocating resources to improve theestimation accuracy of the modal data. As an illustration, the proposed procedures are applied to extract themodal data of a damaged beam, and the extracted modal data are used to detect potential damage locationsusing a damage detection method. It is shown that the variability in the modal parameters can be consideredto be quite low due to the measurement and modeling errors; however, this low variability has a significantimpact on the damage detection results for the studied beam.

Determination of the repair grout volume to fill voids in external post-tensioned tendons

Seok Been Im,Stefan Hurlebaus 국제구조공학회 2012 Structural Engineering and Mechanics, An Int'l Jou Vol.42 No.1

Recently, investigated failures of external post-tensioned (PT) tendons have called attention to the corrosion of strands in PT bridges, and the prevention of ongoing corrosion is required to secure their structural integrity. Since voids inside ducts can be a source for the ingress of water or deleterious chemicals, the vacuum grouting (VG) method and a volumeter for estimating amount of repair grouts were employed to fill voided ducts. However, the VG method is expensive and time-consuming for infield application because it requires an air-tight condition of entire ducts. Thus, latest research assessed three different repair grouting methods, and the pressure vacuum grouting (PVG) method was recommended in the field because it showed good filling capability in voided ducts and did not require an air-tight condition. Thus, a new method is required to estimate the volume of repair grouts because the PVG method is not applied in air-tight ducts. This research assesses the relationship between voided areas on ducts identified with soundings and required grout volume for repair using experimental results. The results show that the proposed equations and assumptions for estimating repair grout volume provide a sufficient amount of repair grouts for filling voided ducts.

Assessment of modal parameters considering measurement and modeling errors

Huang, Qindan,Gardoni, Paolo,Hurlebaus, Stefan Techno-Press 2015 Smart Structures and Systems, An International Jou Vol.15 No.3

Modal parameters of a structure are commonly used quantities for system identification and damage detection. With a limited number of studies on the statistics assessment of modal parameters, this paper presents procedures to properly account for the uncertainties present in the process of extracting modal parameters. Particularly, this paper focuses on how to deal with the measurement error in an ambient vibration test and the modeling error resulting from a modal parameter extraction process. A bootstrap approach is adopted, when an ensemble of a limited number of noised time-history response recordings is available. To estimate the modeling error associated with the extraction process, a model prediction expansion approach is adopted where the modeling error is considered as an "adjustment" to the prediction obtained from the extraction process. The proposed procedures can be further incorporated into the probabilistic analysis of applications where the modal parameters are used. This study considers the effects of the measurement and modeling errors and can provide guidance in allocating resources to improve the estimation accuracy of the modal data. As an illustration, the proposed procedures are applied to extract the modal data of a damaged beam, and the extracted modal data are used to detect potential damage locations using a damage detection method. It is shown that the variability in the modal parameters can be considered to be quite low due to the measurement and modeling errors; however, this low variability has a significant impact on the damage detection results for the studied beam.

Summary Review of GPS Technology for Structural Health Monitoring

Im, Seok Been,Hurlebaus, Stefan,Kang, Young Jong American Society of Civil Engineers 2013 Journal of structural engineering Vol.139 No.10

Determination of the repair grout volume to fill voids in external post-tensioned tendons

Im, Seok Been,Hurlebaus, Stefan Techno-Press 2012 Structural Engineering and Mechanics, An Int'l Jou Vol.42 No.1

Recently, investigated failures of external post-tensioned (PT) tendons have called attention to the corrosion of strands in PT bridges, and the prevention of ongoing corrosion is required to secure their structural integrity. Since voids inside ducts can be a source for the ingress of water or deleterious chemicals, the vacuum grouting (VG) method and a volumeter for estimating amount of repair grouts were employed to fill voided ducts. However, the VG method is expensive and time-consuming for infield application because it requires an air-tight condition of entire ducts. Thus, latest research assessed three different repair grouting methods, and the pressure vacuum grouting (PVG) method was recommended in the field because it showed good filling capability in voided ducts and did not require an air-tight condition. Thus, a new method is required to estimate the volume of repair grouts because the PVG method is not applied in air-tight ducts. This research assesses the relationship between voided areas on ducts identified with soundings and required grout volume for repair using experimental results. The results show that the proposed equations and assumptions for estimating repair grout volume provide a sufficient amount of repair grouts for filling voided ducts.