Nondestructive Load Testing Research Articles

Diagnostic Load Testing for Improved Accuracy of Bridge Load Rating

Bridge load rating provides a standardized procedure to determine the safe load-carrying capacity of a bridge, thereby allowing engineers to establish posting and permitting requirements. The structural condition of components, material properties, loads, and traffic conditions all contribute to the load rating, which describes the capacity of the controlling component of the structure. The primary purpose of bridge load rating is to ensure public safety. The bridge industry has permitted nondestructive load testing, at the discretion of the owner, to establish the safe load-carrying capacity of a given bridge when coupled with analysis and sound engineering judgment. The direct measure of structural response through diagnostic live load testing is viewed as a more accurate method of determining capacity and requires minimal assumptions regarding load distribution. Diagnostic live load testing often results in higher loading postings when compared to traditional analysis because of the elimination of conservative assumptions. A number of data acquisition (DAQ) systems are available for implementation in the load rating process. When correctly implemented, the systems provide an improved understanding of the live load response. Furthermore, deferring repair or replacement through diagnostic load testing can deliver immediate and long-term cost savings. As such, implementation of a diagnostic load testing program may have significant short- and long-term paybacks for both bridge owners and users. The current study employed three DAQ systems to demonstrate how diagnostic load testing can be used to augment traditional load rating procedures. The study revealed that diagnostic load testing of a rural, simple-span steel girder bridge resulted in an improved load rating of 273%, refining the test truck inventory rating from 1.5 to 4.1. Similarly, diagnostic load testing of a rural, simple-span steel pony truss bridge resulted in an improved load rating of 170%, going from a test truck inventory rating of 4.5 to 7.6. Analysis of the field-collected data demonstrated the improved ratings were attributable to a number of factors not considered during traditional load rating, such as site-specific factors and contribution of nonstructural components as well as a true measure of the dynamic amplification, load distribution, and composite action. The positive results showcase the possible improvement to load rating by using diagnostic load testing as compared to traditional load rating procedures, leading to a safer bridge inventory.

A Global Integrity Parameter with Acoustic Emission for Load Testing of Prestressed Concrete Girders

Structural evaluation of existing infrastructure has become a critical subject in civil engineering. In recent years, significant efforts have been placed on developing nondestructive techniques such as acoustic emission to monitor and effectively assess the integrity of a structure without causing significant damage. However, acoustic emission methods face challenges regarding the subjectivity of associated performance evaluation criteria and a lack of measurable parameters directly related to the mechanical response of the system. It has been previously suggested that an integrated approach of the cyclic load testing method with acoustic emission techniques may overcome these difficulties and constitute a more effective, nondestructive load testing methodology. The current investigation analyzes experimental data gathered from flexural testing of six full-scale prestressed girder specimens (lightweight and normalweight) and presents a potential approach for damage detection and assessment within the minor to intermediate damage zones based on acoustic emission data.

Field Study on the Behavior of Destructive and Non-Destructive Piles Under Compression

This paper presents a series of full-scale load tests on long bored piles instrumented with strain gauges along the shafts, including eight field tests of piles loaded to failure and one non-destructive pile load test. The load-displacement response, skin friction, end resistance, and the threshold of the pile-soil relative displacement for fully mobilizing skin resistance were discussed. A simple softening model was proposed to describe the degradation behavior of the skin friction along the pile-soil interface and the load-displacement relationship developed at the pile base. It is found that the shaft resistance degradation investigated in the non-destructive load test only occurs at a shallow depth, and the skin friction of deeper soil is not fully developed. However, unlike the results of the non-destructive load tests, the softening is accompanied by a reduction in skin friction and observed to be along the whole pile depth. The thresholds of pile-soil relative displacement for fully mobilizing skin resistances in different soils have been found to be in the range 0.6% to 2.4% of the pile diameter. Moreover, in practical applications, a bilinear model is assumed to be feasible in analyzing the load-settlement relationship developed at the end of non-destructive pile, whereas the load transmission curve of the soils below the pile base corresponds to a softening model in the field tests of piles loaded to failure.

Track Substructure Assessment using Non-Destructive Load Tests. A Portuguese Case Study

Nowadays, there is an increased interest in developing performance based specifications for quality control during the construction, that can provide more realistic information about the life cycle behaviour of the track.In this research, Falling Weight Deflectometer (FWD) tests were undertaken during the construction of a 29km new railway line, at the top of the substructure and in different months. Based on FWD deflections, structural models of the track were established through back-calculation, and the variation of the moduli for different testing campaigns was evaluated. The main results obtained are presented and analysed in this paper.

Evaluation of a Noncomposite Steel Girder Bridge through Live-Load Field Testing

This paper presents the field evaluation of a damaged noncomposite steel girder bridge that is part of the interstate highway system in Vermont. This bridge is typical of late 1960s construction and spans over a two-lane state highway near the town of Weathersfield, Vermont. The superstructure contains three-span continuous girders supported on abutments at the ends and on RC multicolumn interior bents. Strains were measured during live-load testing that was conducted to better understand the bridge behavior. The field results were compared with results from finite-element models created using common engineering assumptions. In addition, the load-distribution characteristics of girders that were damaged by an overheight truck traveling on the state highway under the bridge were evaluated. The results indicate that alternate load paths were developed within the bridge superstructure as a result of damage from the truck impact. For the loading magnitude applied during the load tests, evidence of composite action was observed and participation of curbs on the response of the bridge was noticed. Bridge skew and partial restraint generated at the supports also contributed to differences in the observed and calculated responses. These nondestructive load testing results were used to provide confidence on the load-carrying capacity of the bridge and to avoid costly bridge closures and detours.

Bridge Deck Load Testing Using Sensors and Optical Survey Equipment

Bridges are under various loads and environmental impacts that cause them to lose their structural integrity. A significant number of bridges in US are either structurally deficient or functionally obsolete, requiring immediate attention. Nondestructive load testing is an effective approach to measure the structural response of a bridge under various loading conditions and to determine its structural integrity. This paper presents a load-test study that evaluated the response of a prefabricated bridge with full-depth precast deck panels in Michigan. This load-test program integrates optical surveying systems, a sensor network embedded in bridge decks, and surface deflection analysis. Its major contribution lies in the exploration of an embedded sensor network that was installed initially for long-term bridge monitoring in bridge load testing. Among a number of lessons learned, it is concluded that embedded sensor network has a great potential of providing an efficient and accurate approach for obtaining real-time equivalent static stresses under varying loading scenarios.

Load testing and GPR assessment for concrete bridges on military installations

The US Army owns and maintains approximately 2000 bridges on its installations spread out in the United States and around the world. From this inventory, 67% are concrete bridges, most of which do not have design or as-built plans to effectively evaluate their condition/capacity. In addition, military vehicles have unique characteristics that are difficult to take into account when analytical analyses are performed. For these purposes, the US Army Engineer Research and Development Center has been adapting non-destructive techniques. The most promising concept explored thus far involves bridge rating through non-destructive load testing in combination with Ground Penetrating Radar (GPR) techniques. Field load testing techniques are valuable for evaluating the capacity of existing bridges and accurately characterize the structure’s live-load response while GPR techniques provide valuable information regarding the internal reinforcement as well as the condition of the concrete. This article will provide a brief description as well as the results from recent evaluations performed on reinforced concrete bridges at military installations.

Study on Safety Evaluation of an Aged Steel Truss Bridge with Cracking Members

The safety of aged steel structures, especially those with cracking members, has aroused a great deal of attention. In this paper, the application of fracture mechanics for evaluating the ultimate load-carrying of steel material is proposed. Three-point bending tests of three specimens with the mode-I fracture are done to get the ultimate load-carrying capacity, when specimens are destroyed. Fracture ductility KIC, and the steel allowable stress [σ] with certain crack length are formulated. Magnetic method for measuring cracks of steel members is adopted for an aged steel truss bridge, and material composition of specimens is tested, and field nondestructive load test involved static and dynamic test are carried into execution. By comparing the results of the calculation and those of the field test, some conclusions are obtained, which are likely to contribute to safety evaluation of aged steel structures, or contribute to designing similar bridges.

Forensic Investigation of Ultrathin Whitetopping Pavements in Florida

Developed in the early 1990s, ultrathin whitetopping (UTW) is a relatively new technique for asphalt pavement rehabilitation. To evaluate the applicability of UTW pavement in Florida, in 1997, the Florida Department of Transportation (FDOT) constructed an experimental UTW pavement in a weigh station along I-10, located in north Florida. The performance of these test sections, however, was less than ideal, with the observation of some early cracking on the concrete surface, which developed into severe cracking with time. Therefore, a forensic investigation was conducted to determine the causes of the problems in these UTW sections, so that lessons could be learned from this experimental project, the use of UTW under Florida's conditions could be adequately assessed, and UTW technology could be properly applied in the future. The scope of work consisted of field evaluation, laboratory testing, and pavement design evaluation. Field evaluation included a pavement condition survey, pavement temperature measurement, nondestructive load testing using a falling weight deflectometer, and slab thickness determination. Laboratory tests were performed to determine concrete and asphalt material properties. Other design and traffic data were also acquired from FDOT. Data collected from the field evaluation and laboratory testing were used in conjunction with a mechanistic UTW pavement design/evaluation procedure to determine the possible causes for premature failure. From this comprehensive evaluation, the primary cause for the failure was found to be inadequate UTW pavement design. The inadequacy of the combination of thickness and slab dimensions contributed to the early cracking of the UTW pavement.

Methods of Monitoring and Evaluating Structural Performance

The American Society of Civil Engineers/Structural Engineering Institute Committee on Methods of Monitoring and Evaluating Structural Performance developed a one-day track on bridge testing and health monitoring for Structures Congress 2005. This track consisted of four technical sessions, each on a different monitoring and evaluation approach. These four sessions covered: nondestructive load testing and strength evaluation of bridges; in-service evaluation of bridges; field monitoring for fatigue evaluation of existing bridges; and corrosion in bridges. A subset of the above papers was selected for this special issue of Journal of Bridge Engineering. These papers contribute to the state-of-the-art for nondestructive bridge testing, evaluation and health monitoring.

Analytical and Field Investigation of Roma Suspension Bridge

The Roma-Ciudad Miquel Aleman International Suspension Bridge is a historic unstiffened suspension bridge between Texas and Mexico with a 192m (630ft) main suspended span, originally constructed in 1928. In 1997, the bridge was inspected and a full-scale nondestructive load test was conducted. The resulting experimental data are evaluated and compared to the results of analyses by finite-element method modeling. The history of the bridge is reviewed, with emphasis on modifications and retrofits to the structure. The unique behavior and attributes of unstiffened suspension bridges are discussed in the specific context of this particular bridge.

Performance of Strand Splice Repairs in Prestressed Concrete Bridges

An experimental investigation was carried out to compare the performance of repaired prestressed bridge girders containing spliced prestressing strands. A field-impact-damaged girder was removed from a railroad bridge structure and utilized for investigation of strand splice repairs. All of the splices investigated provide a means of tensioning the repaired strand or strands. In a series of nondestructive load tests, four different splices were employed to repair intentionally damaged strands. Comparison was made between four strand splices used to repair damaged strands. Overall structural performance as well as ease and reliability of splice installation were compared. Results of the study indicate that certain types of splices can be installed more easily and reliably than others and that splice repairs reduce the level of stress experienced by undamaged strands.

Evaluation of Existing RC Slab Bridges

The objective of this study is to evaluate the American Association of State Highway and Transportation Officials (AASHTO) rating procedure for reinforced concrete (RC) slab bridges by using nondestructive load testing. Three RC slab bridges, each with three continuous spans, were selected for this study. These bridges had minor to severe deterioration. For each bridge, the rating factors were calculated using AASHTO specifications. Nondestructive load tests were carried out on all three bridges. Four dump trucks, each filled with gravel, were used for loading. The trucks were placed in three different load positions to cause maximum positive moment in the middle span, maximum positive moment in the end span, and maximum negative moment at the intermediate supports. The analytical models were updated using the results from load testing and then were used to calculate new rating factors for each bridge. The current AASHTO rating procedure was found to be too conservative.