Cracked or uncracked concrete? That is the question

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

Pull and Compression Zone Explained

Concrete is a material that cannot bend. If concrete is put under tension (it is pulled ➡️ tensile zone), cracks therefore occur. Imagine a concrete bridge that sags slightly due to traffic or its own weight. At the bottom of the sagging bridge, the concrete is stretched slightly, especially in the middle: it sits in the outer bend, as it were.

At the top (the inner bend) of the bridge, the concrete is compressed. This can cause cracks or fissures in the concrete at the bottom. This does not have to affect the safety of the bridge. The steel reinforcement will absorb the tensile force where the concrete cracks. However, this does have significant consequences for the load-bearing capacity and usability of post-installed anchorages.

All anchors suitable for concrete applications can be used in the compression zone . If there is a permanent, constructive application, an ETA-7 pressure zone approval on that anchor is required. However: the presence of a pressure zone must be proven constructively beforehand by means of a calculation. That calculation is complex, and goes much further than the simplified example of the underside of the said bridge.

Only ETA-1 (tensile and compression zone approval) anchors for constructive, permanent applications may be used in the tensile zone of concrete. The nice thing is: the presence of a tensile zone does NOT have to be proven in advance by means of a calculation. The presence of a draft zone can always be assumed, because this is the most unfavorable situation.

Anchorages in the tensile and compression zone

Cracks actually increase the diameter of drill holes. Because the same anchor is placed in a wider hole, it also becomes looser. This reduces the permissible load capacity. An ETA-1 approved anchor (which can therefore be used in both the tensile and compression zone) always has two load capacity values. High load capacity when applied in the compression zone of concrete, and lower for the tensile zone.

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

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

Types of anchors, suitable for the tensile zone

If there is a tensile zone, or is assumed because the concrete has not been calculated to prove the contrary, you are obliged to use an ETA-1-approved anchor for constructive, permanent applications. Such approval is always stated on the packaging. You can't tell by the 'type' anchor. There are push-through anchors ('clickers') 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 almost the same geometry, which are hardly distinguishable from each other. But with a completely different range of applications.

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

Only anchors that can expand or are completely enclosed in the drill hole by form locking can be approved for the tensile zone of concrete. Post-spreading anchors are, for example, through anchors such as the Sympafix TB1-PLUS. The excess of the conical section prevents the anchor from being pulled out of the hole, even if the drill hole is enlarged due to cracking. The form-locking principle can be found in the 'tension-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).

Would you like to know more about cracked and non-cracked concrete?

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