MIRA -Ultrasonic Pulse Echo Imaging

MIRA- Ultrasonic Pulse Echo Imaging

Introduction

This device is a revolutionary approach to concrete ultrasonics. The best way of describing its capabilities is by analogy. It is the construction equivalent of ultrasonic scanning in the medical world. There are various parts that go to why this device is so advanced:

The ultrasonic transducers are only 2mm diameter
This is made possible through the use of side scanning shear wave emission technology developed by ACSYS’s. Conventional ultrasonic equipment would typically use 50mm diameter transducers to produce compression waves.

The ultrasonic probes require no coupling gel
Dry point contact transducers mean that instead of having to use coupling gel to ensure the transmission of waves into the concrete as in standard ultrasonics. The probe can be moved from position to position with next to no surface preparation.

The use of shear waves for the assessment rather than compression waves
This provides sufficient energy that a pulse echo mode can be used. Ultrasonics in concrete typically uses direct transmissions which requires access to both faces and is limited to one time consuming reading at a time.

The use of a multiple array of probes in one head
Only possible because of the small ACSYS probes but made to work by the probes specially designed spring mounting to ensure uniform contact. This enables around 100 pulse echo ultrasonic results to be obtained in under 2 seconds with each scan. Hence readings are taken at close (30mm) spacing. Typically ultrasonic in concrete are taken one at a time at 500mm centres in order to make processing manageable.

Data is collected and undergoes initial processing using an on board computer
The processed signals from each scan are can be downloaded to a desktop computer for further processing using the new and improved 3D imaging software available. This makes the measuring head light and easy to use, and testing can be undertaken by one person. Conventional ultrasonic testing require two people.

The software automatically combines multiple scans
Thousands of results can be combined in one file of the element being tested. Taking scans at 50mm centres provides overlap and at a few seconds for each reading an area 400mm wide (probe width) by 1000mm long can be completed in minutes.

The software builds 3D sectional images of the concretes internal geometry
These images can be played as a video as if the viewer were travelling through the concrete, displayed as any section through the concrete or 3D scans can be used to show progressive sections through the concrete.

The Mira is not one new technology. It is many new technologies seamlessly combined to create a truly revolutionary concrete investigation tool.

How it Works in Practice

The technology is new. Engineers are not used to being able to see the inside of concrete in 3D and interpretation of the images is not always easy. The hundreds of ultrasonic measurements taken are analysed by the software to determine how many signals are reflected by different points in the concrete. The computer builds up a coloured image of the concrete with different colours representing the number of waves that are reflected from each location in the concrete. If there are no reflections from a point the image is blue. If there are many reflections from a point the image is red. Any air/interface may reflect a signal but certain interfaces reflect more strongly. Air concrete interfaces will reflect virtually all of the signal.

Applications

The Mira system can be used to locate voids, honeycombing, hollow conduits, poorly grouted prestressing ducts, poor quality construction joints, poorly bonded repairs, cracking concrete thickness, cables.

Reinforcement- Well bonded reinforcement does not seem to reflect signals but reinforcement with a large transition zone or with underlying bleed channels will. The degree of the poor interface is difficult to judge and whether this poor interface is important is an engineering decision.
Honeycombing- Even light honey combing appears to reflect a wave and it does not’t seem to require great severity to give a “red” area. This is probably useful as from an engineering perspective even minor voidage causes significant reduction in concrete strength and durability. However, isolated spots of voidage behind the reinforcement (beyond the cover zone) may not be significant. The advantage of the MIRA is the concrete can be viewed in a continuum in all three dimensions. It is the extent and continuity of voidage that is most important to the engineer. If voidage exists through the wall it may cause leakage issues. A large area of minor voids close to rebar depth could be a durability issue and voidage in a critical area, eg corbel intersection with main element, may be important.
Poorly Grouted Prestressing Ducts- The grouting systems used in Australia until around 2007 were poor. The system used were the same systems that led to the banning of post-tensioning in bridges in the UK. In Australia less concern was suggested due as, unlike Europe, Japan and North America, Australia did not use de-icing salts. The de-icing salts were considered to be the main source of prestress corrosion in poorly grouted ducts. The grouting systems used in Australia now follow the improved systems introduced in North America and Europe and poorly grouted ducts should not be an issue for new structure. However there is a potential issue of the long term performance older post-tensioned bridges, particularly in areas where marine spray could lead to ingress of water with a high chloride content. Where failure risks are high the Mira, possibly in conjunction with Impact Echo and Boroscopes, would be a useful system for investigating the condition of post-tensioning cables.
Construction Joints- The perfect construction joint will transmit a signal in a similar fashion to concrete while poor joints will reflect a large portion of the wave energy. Although the Mira can be used to get an indication of the joints performance caution is needed, particularly where the materials on either side of the joint are different as wave energy can be reflected due to this difference. The Mira results can be treated as indicating areas for more detailed testing.
Conduits- Empty conduits provide an interface that will strongly reflect waves. The sensitivity of the Mira at showing voids can be seen from tests where 25mm conduits are clearly visible
Thickness- The air concrete interface of walls and suspended slabs give a very strong wave reflection as does the interface of the inter wall of a pipe, even when running full. The picture to the right shows an image of a pipe running full and the pipe thickness around a half circumference can be seen in the D-scan. Although the shear wave may have sufficient energy to reflect of the opposite face up to 2m away 0.5m is more typically the limit, and even then if there are defects in the concrete the rear surface may not be seen. The thickness of a slab on grade will generally be detected as the concrete will be a very different material to the material it is sitting on. However, blinding concrete may give an overestimation of thickness where the interface between it and the structural concrete cannot be seen.

Clearly very careful thought to what the results mean in engineering terms is required bearing in mind that defects identified would not have been recorded using previous technology.

Visual Presentation of Ultrasonic Results

The images are presented as different colour representing the quantity of reflected waves from each point. The standard display has blues no reflections, red as many reflections with green and yellow showing increasing intensities in between. Different hues of each colour also show variations in intensity within each colour band. This concept is relatively easy to grasp.

The images are shown as sections through the concrete in all three dimensions. As we see in three dimensions this sounds like a simple concept yet it takes some practice to start to quickly interpret these three sections. The ability of the instrument is also enhanced by being able to view the sections as a thin sections or section giving an average for a thick section.

The standard visual presentation format is the B-scan, C-scan and D-scan for the pipe thickness test shown above. As shown in the picture to the left the B-scan is a section through the element with a face parallel to the probes long axis, the C Scan is a section through the element with a face perpendicular to probes long axis and the D-scan is a section through the element parallel to the elements surface. The B,C and D scans can be looked at as in the standard view or a 3D view of a particular scan can be added as shown in the pipe example. To make it easier for novices these 3D scans can be overlain using any graphics package so that the continuity or discontinuity of defects can be easily seen. An example of a tunnel wall inspection is shown to the right.

The location of each scan is selected by the viewer by clicking on the ruler on the side of the scans. By clicking and dragging on the ruler the sectional image changes to give an average representation over the thickness selected. This can help show any patterns. The picture to the left uses this method to highlight a band of poorer compaction above a joint.

Where there are localised issues the Mira can be used to scan the whole area but when there are more general concerns some form of rapid scan testing testing, e.g. impulse response, can be used to identify areas of possible concern. The Mira can then be used for more detailed analysis in areas of concern and compared with areas that are not of concern. The results for such a case are shown below.

Form supplied

The full system contains:

• А1040 MIRA-tomograph ultrasonic unit
• Detachable battery
• Charging unit
• USB connection cable
• Check sample
• Notebook
• Sun shield
• Safety rope
• Additional stick-holder
• Transportation Case

Additional Information

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