Paul Ziehl

Applications of acoustic emission evaluation for civil infrastructure

Due to the state of aging civil infrastructure systems structural health monitoring and nondestructive evaluation have received increased attention recently. Events related to bridge collapses in Pennsylvania (partial) and Minnesota (catastrophic) combined with the levee failures in Louisiana have justifiably drawn the attention of the policy makers and the public at large. Therefore it appears likely that both monitoring efforts of existing systems and the development of more resilient systems will be increased. In the case of civil structures (bridges, dams, levees, and buildings) the most common type of sensors used are strain gages and accelerometers. While these sensors can be useful if used correctly they are limited in the types of data that can be gathered and are not well-suited for many applications. In contrast acoustic emission sensors are very rarely used for civil applications but can in fact provide useful information either as a stand-alone data type or to supplement the data gathered from other sensors. This paper describes several case studies where acoustic emission has been successfully used in civil infrastructure applications and summarizes both the advantages and challenges that are inherent in the method for such applications.

Evaluation of Reinforced Concrete Beam Specimens with Acoustic Emission and Cyclic Load Test Methods

The cyclic load test (CLT) and acoustic emission (AE) are both relatively recent evaluation methods that lack historical data for validation of their evaluation criteria. A combined inspection, using the CLT loading procedure and evaluation methods with the AE evaluation methods, may increase confidence in the evaluations. This paper presents the results of the CLT and AE methods of evaluation applied to 14 reduced-scale reinforced concrete beam specimens that represent different cases in terms of failure mode (flexure and shear), loading pattern, concrete mixture design, and resulting material properties. Five AE evaluation methods are investigated to determine their ability to assess the damage level in the reinforced concrete specimens. Results show that the CLT method provides some advantages over the existing 24-hour load test, especially in that it allows for interrogation of the structural response to loading and therefore provides improved insight into structural behavior. For some of the flexural specimens, the CLT evaluation criteria did not fail prior to yield, but for the same specimens the calm ratio versus load ratio (AE) criterion did provide indications of increasing damage prior to yield. The deviation from linearity criterion was consistently failed prior to yield in all cases for the shear specimens. In general, deviation from linearity was a more stable evaluation criterion than either permanency or repeatability. More data from full-scale specimens and actual structures is needed in order to validate both the CLT and AE evaluation criteria.

Corrosion Damage Quantification of Prestressing Strands Using Acoustic Emission

AbstractSteel degradation due to corrosion in prestressed concrete bridges has been of major concern as it presents a threat to the integrity of structures adjacent to marine environments or where deicing salts are regularly used. To assess the potential for monitoring of the corrosion process, an accelerated corrosion testing program has been conducted. A series of specimens with dimensions 114×114×508  mm (4.5×4.5×20  in.) were subjected to constant potential application through the embedded steel strand while being continuously monitored with acoustic emission (AE). Depassivation of the strand was detected by monitoring the fluctuations in applied anodic current. Half-cell potential measurements assessed the probability of corrosion, and all results obtained were compared to acoustic emission data. The mass loss of the corroded strands was correlated to acoustic emission intensity analysis to quantify the degree of damage. Results show that acoustic emission is as sensitive as half-cell potential for d...

Acoustic-Emission-Based Characterization of Corrosion Damage in Cracked Concrete with Prestressing Strand

An accelerated corrosion study to assess the feasibility of acoustic emission (AE) for the detection of active corrosion in prestressing strand is described. Concrete prisms with an embedded steel strand were corroded by supplying a constant potential between the strand and a copper plate while the specimens were immersed in a 3% sodium chloride (NaCl) solution. Corrosion was detected using the half-cell potential (HCP), steel section loss, and visual inspection, and the results were compared to AE data. The location of active corrosion was determined experimentally based on the characteristic wave speed. An intensity analysis approach was used to plot the relative significance of the corrosion damage and a classification chart is presented. Results indicate that AE is a useful, nonintrusive technique for the detection and quantification of corrosion damage and may be developed as a structural prognostic tool for maintenance prioritization.

Evaluation of Strengthened Reinforced Concrete Beams: Cyclic Load Test and Acoustic Emission Methods

This paper compares two different nondestructive evaluation procedures for strengthened reinforced concrete beam specimens, the cyclic load test method (CLT) and the acoustic emission (AE) method, and reports on their ability to assess damage. The CLT method is proposed as an alternative to the existing 24-hour load test procedure. It makes use of a prescribed loading pattern and evaluation criteria of permanency, repeatability, and deviation from linearity. AE is a developing evaluation method that also makes use of prescribed loading procedures. An evaluation criterion for the AE method was developed and is described. A total of six beams were included in the investigation—two unstrengthened beams, two beams strengthened with precured strips, and two beams strengthened with unidirectional fabric. Findings showed that only one of three specimens evaluated by the CLT method failed any of the evaluation criteria. The values for permanency did not show any general trends with increasing load and were generally insensitive to damage. This was true even for the specimen that did not fail the CLT criteria. Despite initial expectations, the values of repeatability did not tend to decrease with increasing load. Damage was clearly indicated in the final loadset for both strengthened specimens that were evaluated by the AE method. The AE method has the ability to detect damage a considerable distance away from the sensor location and is very sensitive to damage progression. Although further experimental investigations are needed, the AE method appears to offer promise for nondestructive field evaluation.

Dual Mode Sensing with Low-Profile Piezoelectric Thin Wafer Sensors for Steel Bridge Crack Detection and Diagnosis

Monitoring of fatigue cracking in steel bridges is of high interest to many bridge owners and agencies. Due to the variety of deterioration sources and locations of bridge defects, there is currently no single method that can detect and address the potential sources globally. In this paper, we presented a dual mode sensing methodology integrating acoustic emission and ultrasonic wave inspection based on the use of low-profile piezoelectric wafer active sensors (PWAS). After introducing the research background and piezoelectric sensing principles, PWAS crack detection in passive acoustic emission mode is first presented. Their acoustic emission detection capability has been validated through both static and compact tension fatigue tests. With the use of coaxial cable wiring, PWAS AE signal quality has been improved. The active ultrasonic inspection is conducted by the damage index and wave imaging approach. The results in the paper show that such an integration of passive acoustic emission detection with active ultrasonic sensing is a technological leap forward from the current practice of periodic and subjective visual inspection and bridge management based primarily on history of past performance.

Design and Field Evaluation of Hybrid FRP/Reinforced Concrete Superstructure System

Fiber-reinforced polymers offer several advantages over conventional construction materials but are also faced with several challenges. These include increased first cost, relatively low stiffness, and a lack of field experience. To address these challenges and to advance the state of the art, a hybrid fiber reinforced polymer/reinforced concrete bridge was designed and constructed in Texas. The bridge design and field evaluation are unique in several respects. Design considerations, the bid process, and the results of intermittent live load evaluations that have been conducted over a period of approximately 2 years are presented. Recommendations for the design of future similar bridges are provided.

Probabilistic Prognosis of Fatigue Crack Growth Using Acoustic Emission Data

This paper presents a structural health monitoring methodology that uses acoustic emission (AE) features to predict crack growth in structural elements subjected to fatigue. This allows for the prediction of the failure of the structural element at the current load level. The methodology uses Bayesian inference to account for different sources of uncertainty such as uncertainty in the data (AE signal), unknown fracture mechanics parameters, and model inadequacy. The methodology is divided into two main components: a model updating component that uses available data to build a joint probability distribution of the different unknown fracture mechanics parameters, and a prognosis component in which this multivariable probability distribution is sampled to predict the stress intensity factor range at a future number of cycles. The application of the methodology does not require knowledge of the load amplitude nor the initial crack length. The methodology is validated using experimental data from a compact test specimen under cyclic loading.

On-Site Acoustic-Emission Monitoring for Assessment of a Prestressed Concrete Double-Tee-Beam Bridge without Plans

AbstractAcoustic emission was used in this study to assess the condition of a simple-span, prestressed concrete bridge located in southern New Mexico during a proof test. The 40-year-old bridge is county owned and no design plans were available. Acoustic emission (AE) data were collected under several loading conditions from two groups of sensors placed near the support and midspan of an interior double-tee beam. Strain measurements were also taken at midspan and used to direct the test and determine the maximum loading. AE data analysis methods were applied to evaluate the response of the structure under increasing loads and to develop crack maps. Analysis of the AE data showed signs of internal beam microcrack development near the supports (shear region) as well as in the midspan (moment region). The AE data indicated that damage in the form of crack growth was more prevalent in the region near the supports.

Triboluminescence for distributed damage assessment in cement-based materials

Triboluminescent materials are promising in the field of structural health monitoring for real-time crack detection and related damage assessment. This study presents a simple, but novel, image processing protocol to detect and quantify luminescence from crack formation in cement-based matrices. Mortar cubes of 2″× 2″ (5.1 cm × 5.1 cm) were loaded in compression with an external coating of manganese-doped zinc sulfide (ZnS:Mn) triboluminescent material. The concentration of triboluminescent material and rate of loading were varied to evaluate luminescence response. A digital single lens reflex camera was employed to capture luminescence from the resulting cracks, which formed and propagated during failure. The images were then analyzed with an image processor, and total luminescence/pixel along the cracks was quantified. Results show that overall luminescence increases with the increase in triboluminescent concentration as well as with the rate of loading. This article presents a novel method that can be applied to monitor crack formation in cement-based materials, providing reliable accuracy in luminescence quantification.