Cracked or uncracked concrete? That is the question

Structural anchors can have a so-called ETA approval for cracked concrete (also called the tension zone) and uncracked concrete (also called the compression zone or compression zone). But what does that distinction mean? And why is it relevant at all?

Tension and compression zones explained

Concrete is a material that cannot bend. Therefore, when concrete is subjected to tension (pulled upon ➡️ tension zone), cracking occurs. Imagine a concrete bridge that sags slightly due to traffic loads or its own weight. At the underside of the sagging bridge, the concrete is stretched slightly, especially in the middle: it is, as it were, located in the outer curve. At the top side (the inner curve) of the bridge, the concrete is actually compressed. Consequently, cracks or fissures may form in the concrete at the underside. This need not have any consequences for the safety of the bridge. The steel reinforcement will absorb the tensile force at the point where the concrete cracks. However, this does have significant consequences for the load-bearing capacity and usability of retrofitted anchors. All anchors suitable for applications in concrete can be used in the compression zone. If there is a permanent, structural application, an ETA-7 compression zone approval is required for that anchor. However: the presence of a compression zone must be structurally proven in advance by means of a calculation. That calculation is complex and goes much further than the simplified example of the underside of the bridge mentioned. In the tension zone of concrete, only ETA-1 (tension and compression zone approval) anchors for structural, permanent applications may be used. The good news is: the presence of a tension zone does NOT need to be proven in advance by means of a calculation. The presence of a tension zone may always be assumed, because this is the most unfavorable situation.

Anchorages in the tension and compression zone

Cracks actually increase the diameter of boreholes. Because the same anchor ends up in a wider hole, it also becomes looser. As a result, the permissible load capacity decreases. An ETA-1 approved anchor (which may therefore be used in both the tension and compression zones) therefore always has two load capacity values. A high load-bearing capacity when applied in the compression zone of concrete, and a lower one in the tension zone.

In the graph above, the blue lines show the results of a tensile test of 2 anchors in the concrete compression zone, and the red lines show the same anchor in the tension zone. The y-axis represents the force exerted on the anchor, and the x-axis represents the displacement of the anchor in mm (the anchor releases at the peak). The difference in load capacity in the two different situations is therefore very substantial.

By way of illustration: the ETA-1 approved Sympafix SB2-PLUS 8x100 concrete screw may be loaded up to 6.7kN in the concrete compression zone, whereas in the concrete tension zone this is 4.2kN. If no structural calculation proof has been provided that the anchoring takes place in the compression zone, the load must therefore remain below 4.2kN per anchor. After all: without calculation, a tension zone may always be assumed.

Types of anchors suitable for the tension zone

If a tension zone is present, or if it is assumed because the concrete has not been calculated to prove otherwise, then you are required to use an ETA-1 approved anchor for structural, permanent applications. Such approval is always stated on the packaging. You cannot tell this from the ‘type’ of anchor. There are through-anchors (‘snap anchors’) with such an ETA-1 approval (the Fischer FAZ and the Hilti HST, for example), but also with only an ETA-7 approval (the Fischer FBN and the Hilti HSA). Anchors from the same manufacturer and with virtually the same geometry, which are hardly distinguishable from each other. But with a completely different range of applications.

Symptompafix's ETA-approved steel anchors always have an ETA-1 certificate. Anchors with only an ETA-7 approval are simply not included in the range. This ensures the user does not accidentally install an anchor that is insufficiently certified.

Only anchors that can expand or are fully enclosed in the borehole by form locking can be approved for the tension zone of concrete. Examples of expanding anchors are through anchors such as the Sympafix TB1-PLUS. The oversize of the conical section prevents the anchor from being pulled out of the hole, even if the borehole widens due to cracking. The form-locking principle is found in the ‘stress-free’ anchors: screw anchors (such as the Sympafix SB2-PLUS) and chemical anchors (such as the Sympafix C100-PLUS, UC200-PLUS Ultrabond and X150-PLUS).  

Want to know more about cracked and uncracked concrete?  

We have a structural solution for every construction project. Feel free to contact us via sales@sympafix.com or 072 - 303 0500.