Fastening in concrete vs. rock mass—Comparative determination of pull‐out loads for artificially created discontinuities in concrete

Possible discontinuities in the subsurface have a major impact on the load‐bearing behavior of post‐installed fastenings. For concrete, the behavior of post‐installed anchors and thus their design can be clearly addressed as concrete is assumed as a homogeneous fastening substrate. This is valid except, for example, in the area of building joints and at joints of precast concrete elements, where the formation of structural joints inevitably occurs and which basically correspond to a separation surface. In contrast, rock mass is characterized by the rock type, but is generally also significantly influenced by its discontinuities. These play a decisive role concerning rock mass stability and show a great impact on the load‐bearing capacity of rock, especially for fasteners with a shallow embedment depth. It is to be assumed that the same also applies to separating surfaces in concrete. Furthermore, the question arises as to the non‐destructive preliminary detectability of such weak zones. For carrying out a comparative study under controlled conditions, artificial interfaces of different geometries were generated in concrete in laboratory tests by inserting PTFE layers at different positions of the test member. Pull‐out tests of post‐installed fastening systems were carried out in their vicinity to determine the load transfer as well as the failure mode. It could be shown that discontinuities have a negative effect on pull‐out loads not only in the rock mass, but also in concrete. However, detection of these by means of a rebound hammer was only possible in the rock mass.