It’s quite common that steel structures need to be anchored to concrete structures afterwards - everything from hanging installations to fastening beam supports. The types of post-installed fasteners 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 a brittle material with relatively low tensile strength. This is compensated for by reinforcement inside the concrete, which absorbs tensile forces and holds cracks together when they form. Cracks in the concrete not only affect how the structure itself behaves, but also how the fasteners perform. Cracks influence the way stresses are distributed locally around the anchor point where loads are transferred, which reduces the concrete’s load-bearing capacity. In some cases, the capacity of the same anchor can be half in cracked concrete compared to non-cracked. When the concrete is cracked, there’s a high chance that a crack runs along the drill hole for the fastener. This slightly widens the hole and makes it flex under load, increasing the risk of slippage - and if the fastener isn’t tested and approved for cracked concrete, it may loosen completely without warning.

As mentioned, testing and approvals determine the correct area of use. It’s not something you can see with the naked eye. Two products may look almost identical but still be approved for completely different purposes (see Figure 1). Sometimes you’ll see the terms “Option 1” (cracked concrete) and “Option 7” (non-cracked concrete). These names come from common test programs in the ETA assessment documents, which define what performance and applications can be declared. If the anchor hasn’t been tested in cracked concrete, its performance there is unknown and it shouldn’t be used in those areas. If it has been tested in cracked concrete, it’s also tested in non-cracked concrete. Tests in cracked concrete are therefore more extensive and complex but cover both conditions.

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