Acoustic emission testing of fractures in pre-stressed concrete wire

Daniel Weger, summer semester 2013

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Acoustic emission testing (AET) uses methods from seismology and communication engineering to collect and localise events. Using appropriate sensors, the noises created by fractures in pre-stressed concrete wire can be recorded and evaluated. ^{[1] [2]}

Fundamentals of acoustic emission testing

Noises created by stress on and damage to a component can be recorded and evaluated using appropriate sensors. As installing sensors and analysing the signals usually takes place without damaging the component, acoustic emission testing is traditionally considered to be a method of non-destructive testing. In contrast to most other non-destructive testing processes, acoustic emission testing represents a "passive" method as the site of damage produces a signal which is recorded by the sensors and makes it possible to localise defects. The localisation of the signal source is by no means trivial and uses seismological methods. ^[1]

In acoustic emission testing, sensor networks and arrays are used for making a recording. This use of an array is simpler and makes it easier to localise the acoustic source. ^[2]

Many AET evaluation methods are related to methods used in geophysics. Only after successfully localising the acoustic source is a reliable interpretation of the acoustic signal possible. Correctly determining the time period of the signal's initial use is crucial. ^[1]

Acoustic emission testing is based on irreversible and thus inelastic deformations in a component that is under load. The field of application for AET is thus mainly in the area of detecting crack initiation, analysing crack growth or crack surface friction. As these processes are usually linked to the build-up of previous internal stresses due to mechanical or thermal causes, acoustic emission tests are very suitable for monitoring components or other structures under load during routine operation. ^[1]

Parameter and signal-based acoustic emission testing

Acoustic emission testing can be useful as part of material testing for detecting and evaluating defects in materials or for analysing damage progression. It is possible to observe damage progress over time and space. Under favourable conditions, evidence about causes of sound events in terms of fracture mechanics can be determined from the wave trains of acoustic emissions.

What is termed parameter-based AET is the conventional, well-known form. However, its meaningfulness is very limited. More or less meaningful parameters are extracted from the acoustic emission signals. The actual signals are thus lost and can no longer be used for later analysis.

In the case of signal-based AET, complete signal forms are recorded. The drawback of this method is that, owing to the measuring methods used, fewer signals are often recorded in comparison and their evaluation requires considerably more effort. ^[1]

Detection of cable breaks on suspension bridges

The automatic detection of cable breaks on suspension bridges through the use of AET represents an interesting area of application for AET. Just like pre-stressed concrete wires in pre-stressed components, cables of suspension bridges are always subject to tension and usually consist of bundled tension wires or cable strands made of high-tensile steel that has been combined to form cables in suspension bridges. The conventional electrical or visual testing methods are limited to use on the layers of cables close to the surface. These testing methods can only provide results at freely accessible or selectively, destructively exposed sites. It only shows the condition at the time of testing and does not facilitate observations over a longer period of time. In addition, these methods are very time-consuming and expensive.

The installation of an acoustic monitoring system like AET has many advantages. Sensors are fixed on the suspension bridge cables and any noises created are permanently recorded. The difficulty with this is then filtering out damage events from the multitude of detailed signals. On the one hand, there is what is termed background noise. Above all, this consists of the typical noises of the bridge owing to wind movement and other environmental factors. In addition, there are acoustic signals caused by traffic or work on the bridge. After calibrating the acoustic velocity in the material and further preliminary investigations, a relatively certain localisation of the wire fractures along the cable can be achieved provided that there is at least one sensor on each side of the acoustic source (fracture point).

It should be taken into consideration that many influences change the wave velocity within a material. Not only steel quality, but also age, preloading, environmental factors and even other manufacturers can change the wave velocity. In future, automatic AET methods are meant to be capable of taking over the monitoring of cables on suspension bridges. ^[4]

AET for detecting fractures in pre-stressed concrete wires in reinforced concrete bridges

Acoustic emission in concrete and re-enforced concrete

Acoustic emissions are released during crack formation. The elastically stored energy is emitted in micro and macro cracks. The problem with concrete, in contrast to metallic materials, is its relatively large attenuation and scattering. Only wave lengths that are larger than the inhomogeneities (gravel pockets etc.) or very high-energy acoustic emissions can stand out from the background noise and range more than a few decimetres. Cracks can also be overcome by soundwaves only with a considerable loss of energy and thus loss of information.

Additional acoustic emissions in the composite zone are emitted in re-enforced concrete when the composite disintegrates between the concrete and the re-enforcement. Similarly, sound waves can occur because of a fracture in the re-enforcement owing to overloading or fatigue.

In re-enforced concrete, fractures in pre-stressed steel also create acoustic emissions. Fractures in pre-stressed concrete in both grouted and non-grouted tendons can be recorded. Here, wave propagation can be very complex due to the heterogeneous composition of aggregate particles, rebar steel, the cement matrix, tendons and casings from sheet metal or plastic.

The fields of application for acoustic emission testing in re-enforced concrete superstructures range from tracking crack initiation in concrete to crack surface friction to cracking of rebar or pre-stressed steel. Many tests have shown that AET is suited for recording damage processes in principle. ^[2]

Comparison of laboratory and practical applications on a bridge

The internal structure of the test piece and its exact dimensions are known in tests carried out on laboratory test pieces. A bridge is much more complex. Furthermore, the actual state of the re-enforcement and tendons is not certain. It is possible that there are no plans for the object being tested or the tendons and other built-in components have been used differently as recorded in the plan. In the laboratory, the source of noise emissions is determined using the arrival time of the p-wave (compression wave) which propagates directly between source and sensor. In bridges, the wave can only move a limited amount between source and sensor directly. There is wave interference with regards to their velocity and their route through hollow blocks, gravel pockets, re-enforcement and tendons.

The high frequencies through concrete are damped down due to the longer route of the signal on a bridge. Thus, sensitivity and frequency characteristics need to be tuned to the expected signals which will deviate from laboratory results. When selecting sensor types, their quantity and positioning, it is important to determine whether the whole bridge or just parts of it are to be monitored. In addition, not all surfaces are freely accessible in a real structure.

In laboratory tests, external loads generate acoustic emissions. In pre-stressed re-enforced concrete bridges, the acoustic emissions are created by slow corrosive processes and by fractures resulting from mechanical overstressing of the remaining cross section. In the laboratory, the level of background noise and the number of interference sources are small, thus making the quality of the recorded data very high. In contrast, there are more background noises on a bridge. The strength of the background noises depends heavily on the weight and the speed of the vehicles. In laboratory tests, scientists use laborious processes in an attempt to achieve the highest possible degree of accuracy. Applications in the field intend to primarily determine potential serious damages, where an adequate degree of accuracy suffices. ^[2]

Suitability of acoustic emission testing for monitoring fractures in pre-stressed concrete wire in reinforced concrete bridges

Despite a variety of environmental noises, it is possible to record, classify and localise both spontaneous and artificially created fractures in pre-stressed concrete wires. An accumulation of fractures in wires and the resulting damage to the bridge can be recorded. Fractures in wires can not only be detected in badly grouted tendons, but also fully grouted ones. Among other things, problems in detection can occur owing to gravel pockets and hollow blocks which impede the route of the acoustic emissions. The results can be verified well by using potential field measurements.

Factors influencing localisation ^[2]

• Structure/bridge geometry
• Sensor position
• Sensor placement
• Background noise
• Condition of the bridge

(grouting defects, gravel pockets, cracks, hollow blocks, foreign bodies...)

Additional possible fields of application ^[2]

• sensitive and exposed structures

(suspension bridges, cable-stayed bridges)

• individual cable anchoring or tendon anchoring and rock anchors at risk

Abbreviations

Literature

1. Grosse, C.: Grundlagen der zerstörungsfreien Prüfung. P. 138. Lehrstuhl für zerstörungsfreie Prüfung der Technischen Universität München. München, 2012.
2. Fricker, S.: Schallemissionsanalyse zur Erfassung von Spanndrahtbrüchen bei Stahlbetonbrücken. Dissertation, p.168. ETH Zürich. Zürich, 2009.
3. Grosse, C.: Quantitative zerstörungsfreie Prüfung von Baustoffen mittels Schallemissionsanalyse und Ultraschall. Dissertation (Ph.D. thesis), 168 p. University of Stuttgart. Stuttgart, 1996.
4. Krüger, M.; Lehmann F.; Grosse C.: Acoustic emission monitoring of wire breaks at the humber suspension bridge. Proc, 12p. 13th International Conference on Structural Faults & Repair (Ed. M. Forde). Edinburgh, 2010. ISBN 0-947644-67-9