EXPERTISE

Crack identification
Identifying cracks and fissures in scanner recordings

SPACETEC scanners are capable of identifying cracks as small as 0.3 mm in width. This may be surprising at first glance, because the size of a single dot on the tunnel surface at the highest resolution of 10,000 pixels per scanner line is only about 3 mm. How can such delicate structures be identified?

It works because a crack as a whole is much bigger than the pixel of the scanner. Normally, cracks and fissures in tunnels have lengths measuring in metres. With a dot size of 3 mm, a crack one metre in length is captured by about 330 dots.


The crack takes only a small fraction of the area of each pixel. As a result, the corresponding dot appears somewhat darker than its surrounding.



The result is a track of slightly darker image dots. This track is identified as a crack by its characteristic curve.

The picture clearly shows the cracks running at right angles to each other. In most cases they can also be distinguished well from other line structures.


The enlargement clearly shows the individual dots forming the crack trace.

Cracks can therefore be identified easily with the tunnel scanner. What cannot be measured, however, is the width of the crack.

Incidentally, the above interrelations not only apply to SPACETEC scanners but in general to all scanners with similar resolution.

Therefore:
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The higher the resolution, the better the cracks are identified.
The higher the resolution, the bigger the data volume
The higher the resolution, the slower the measuring speed.


Advantages of crack identification with the help of scanner recordings

In conventional methods, cracks are drawn into the findings charts by hand and marked with the width of the crack.

In crack identification with scanner recordings, the cracks are drawn in above a true-to-scale image of the tunnel surface, with their progress, size and exact position being recorded and documented.

The advantages over conventional methods:

- The result is objective and independent from the apppraiser's individual estimate.
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The results can be compared more easily with later inspections.

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The accurate capture of progress and position allows the crack pattern to be shown throughout whole tunnels. This approach delivers additional information.

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The true-to-scale capture in the PC allows quantitative analyses. Data from different tunnels can be compared easily.


In combination, these advantages ease the decision-making process involving actions to be taken.




True-to-scale crack charting above a scanner recording

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