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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences
I. INTRODUCTION
A crack in the composite material concrete activates a complex
redistribution between concrete matrix and reinforcement. Only
by cracks the ultimate load can be reached. Knowing cracks by
number, destination, width and edge displacement, conclusions
about the load reaction of the concrete structure can be drawn
and finally dimensioned.
Photogrammetry is used to observe targets at the surface of a
structure during a deformation test.
A grid of targets is the base for the calculation of crack
position, crack width and crack edge displacement. In
comparison with conventional strain gages and position
encoders a two-dimensional surface can be measured. In
addition a grid of deformations allows transmitting the
displacements of a real test to a numeric simulation.
The aim of the research is therefore the measurement of
deformations at the surface of concrete structures with
photogrammetry to observe crack pattern, crack width and
crack edge displacement. The research is part of the
*Sonderforschungsbereich 532- Textilbeton" specialized for the
new textile reinforcement.
2. APPLICATION AREAS
Structures of concrete are usually broken. The whole crack
width is distributed to many very narrow cracks so that the
appearance and the protection against corrosion can be fulfilled.
Concrete breaks at a low tensile stress. The load is taken in the
crack totally by the reinforcement and is transmitted in the
neighbourhood from reinforcement to concrete by bond (figure
1). The tensile stress in concrete rises with the distance to the
crack, till the tensile strength is reached again and a new crack
opens. A strong bond generates many tiny cracks.
Figure 1: Stress of concrete and reinforcement in the region of
crack
Otherwise the bond behavior is recognizable by the way and the
width of cracks. By different types of tests the properties of
material and bond is analyzed (figure 2) and are used for the
assessment of several concrete structures.
:. Vol XXXV, Part BS. Istanbul 2004
(e)
A 5 CTUÀ
Diverse types of tests (in cooperation with the
Institutes of structural concrete Aachen and
Kaiserslautern)
Figure 2:
(a) Tension, (b) shear, bending, (c) plate tests and other special
tests like (d) for the research of crack edge displacement or (e)
of a recess are performed. All test specimens are continuous
loaded and are observed by a photogrammetric multi-camera
system.
The crack width depends on the used reinforcement. At the
researched textile reinforced (reinforced with fibres of glass or
carbon) specimens starts the crack width with about 20 um, the
human eye can detect cracks starting with about 40 pm. It is
also not possible to observe all cracks simultaneously and
measure detail information like crack width or crack edge
displacement. A wide application area arises for the technique
of photogrammetry.
3. PHOTOGRAMMETRY
Photogrammetry is known as a contactless measurement
technique. Before starting the test the specimens are prepared in
the area of interest with a regular grid of targets (target diameter
holds 3 mm and distance between targets 5 mm, see figure 3).
During the test the field of targets is simultaneously
photographed by up to three digital cameras (Kodak DCS Pro
14n with 35 mm or 28 mm lenses) with a time interval of 10
seconds. Due to the increasing load cracks are arising and, to
this, the distances between related targets are changed. Figure 4
shows the arrangement of the cameras during a plate test.
= es A
Figure 4: Test assembly with cameras and plate test specimen
after failure
