In the construction industry and in the respective specialist departments, research with respect to repair and retrofitting becomes more and more important. For example, the bearing capacity of existing concrete structures may be increased considerably by applying concrete supplements to existing construction parts, thus ensuring their safety. This helps to significantly extend the service lifetime of such structures.

The success of the repair methods mainly depends on the quality of the joint between the old and new concrete. Important parameters in this process are the preparation of the existing old surface, the procedure when applying the new concrete and the ingredients of the concrete supplement (e.g. addition of superplastisizer, microsilica, cement type, etc.).

In order to better understand the possibilities and limits of the repair technology, the Department of Concrete Structures at the Technische Universität München, Germany, presently carries out tests using different concrete qualities and various methods of specimen preparation (different degrees of roughening the surface of the old concrete by sand or water blasting, varying the water content of the old concrete structures before applying the new concrete).

The splitting tensile test described below provides for checking the quality and stability of such a joint.

For this test, cubes with an edge length of 15 cm are cut out from plate-like components such that the joint between the old an new concrete is exactly in the center of the cube.

The ARAMIS measuring system is used for the deformation analysis of the specimen surface and for determining the fracture behavior including the crack development. The specimens need to be prepared for the measurement. For this purpose, a color spray is applied by softly pressing the spray button so that an irregular contrast pattern results (Fig. 1).

The specimen is then mounted into the compression test machine such that the joint is in the center, running perpendicularly from top to bottom. Between the pressure plates and the surface of the specimen a 10 mm wide and 4 mm thick steel strip (alternatively hard felt strip) is placed concentrically on the joint on both sides, such that the entire pressure is conducted into this joint zone.

Now, when applying the pressure, this setup causes the force to spread out in a wedge-shaped manner within the specimen. Thus, in the area of the joint, tensile stress occurs (therefore the name splitting tensile test).

Depending on the quality of the concrete and on the treatment of the surfaces, typical failure types result when increasing the pressure. If the specimen breaks exactly in the joint, this is called an adhesive failure. If the cracks exclusively run through the "old" or the "new" concrete, this is called a cohesion failure. Frequently a combination of both failure types occur (combined fracture).

Prior to recording the images, the ARAMIS test setup is calibrated using a certified calibration panel. During the pressure is applied, image pairs and the current test pressure are synchronously acquired in regular intervals.

As soon as the image acquisition is finished, the images are evaluated using the ARAMIS software. For this purpose, small "test areas" of the left camera image with their individual pattern are assigned to the synchronously recorded stereo image of the right camera. For each of these assigned positions, the software calculates a 3D position on the specimen such that after the stereo image pair is evaluated, the current shape of the visible specimen area is acquired and measured.

In addition, the "test areas" of the first image pair are assigned to the images showing the increasing load and are defined as correlation value. This is how the ARAMIS software determines the deformation of the test pattern and thus the local deformation of the specimen front side.

By eliminating the rigid body movement of the specimen, the local deformation of the specimen surface can be made visible.

Interesting for the scientist is the local deformation (strain), increasing until a crack results, and thus quantitatively capturing the development of defects and graphically visualizing them (see Fig. 9). Figure 9 shows strain values in horizontal direction as overlay to the image of the specimen help to understand the fracture process. Left and right at the bottom you can see the shadow of the small fixture for the steel strip, causing two areas that cannot be evaluated.

To better understand the fracture process, the determined strain values are displayed in Fig. 9 as an overlay to the currently visible specimen surface. Depending on the size of the selected "test area", smaller cracks will be captured and visualized as local strain concentrations during the evaluation as well. Fig. 9 clearly shows that next to the fracture surface several local cracks develop.

Subsequent to the splitting tensile test the shape of the failure plane was scanned with the ATOS system, which generates a dense data mesh from just a few images of the fracture surface. Based on this shape data, the 3D shape of the cracking can be measured and assessed, This specimen broke at 182.5 kN.

Using this or a similar measuring and test setup you may also easily measure and assess other materials as well as reinforced specimens (e.g. applying steel or polymeric fibers).

By courtesy of the Department of Concrete Structures at the Technische Universität München, Germany.