Steel structures often need to be fastened into concrete structures afterwards – everything from suspending installations to anchoring beam supports. The types of post-installation fastenings are almost endless: expansion anchors, concrete screws, chemical anchors, and so on. In this jungle, it’s easy to overlook one important aspect: whether the concrete should be considered cracked or non-cracked.

Concrete is brittle and has relatively low tensile strength. This is managed by reinforcement inside the concrete, which takes up the tensile forces and holds the cracks together when they appear. Cracks in the concrete don’t just change how the structure itself behaves – they also affect the performance of the fastenings. Cracks influence how stresses are distributed locally around the fastening where it transfers the load, which reduces the concrete’s capacity. In some cases, the capacity of the same fastening may be only half in cracked concrete compared to non-cracked. When concrete is cracked, there’s a high chance that a crack runs along the drilled hole of the fastening. This widens the hole slightly and makes it flex under load, which increases the risk of the fastening slipping – and if it hasn’t been tested and approved for cracked concrete, it may fail completely without warning.

As mentioned, testing and approvals determine the field of use. It’s not something you can see with the naked eye. Two products may look almost identical but still be approved for different applications (see Figure 1). Sometimes fastenings are referred to as “option 1” (cracked) and “option 7” (non-cracked). The names come from common testing program options in the assessment documents for ETA, which define what performance and applications can be declared. If the fastening hasn’t been tested in cracked concrete, its performance there is unknown, and it should not be used in cracked zones. If it has been tested in cracked concrete, tests are also performed in non-cracked concrete. Testing in cracked concrete is therefore more extensive and complicated, but it covers more and works for both cases.

So when should you assume the concrete is cracked? Cracks appear in areas where tensile stress exceeds the tensile strength of the concrete. Common places are above supports in continuous beams and slabs, or at the bottom surface in the middle of the span between supports (see Figure 2). When cracks appear due to restrained movement – for example, from drying shrinkage or temperature changes – it’s harder to predict exactly where they will occur.