Where are the cracks in the definition of “cracked concrete”?

Classifying concrete as cracked or uncracked is one of the most expensive decisions in an anchor design — yet the industry treats almost all concrete as cracked by default. This article asks whether that default is actually justified.

Imagine two insurance companies that control 65% of the global market in their field, where every legislator and expert either works for them or eventually will. Now imagine there is a relatively rare disease that could justify doubling the price of the insurance and tripling the profits. The experts funded by those companies rule that every patient “at risk of contracting the disease” must buy the insurance — without ever defining clearly who is actually at risk.

Does the disease exist? Yes. If you want to be certain you are covered, is it better to buy the insurance? Yes. But is there any statistical basis for compelling all of humanity to buy it, when actual cases of the disease are isolated? This article deals with exactly that question — in the context of cracked concrete.

Deciding at the design stage whether the concrete is cracked or uncracked is a design decision that has a major effect on the design, and an even greater effect on the cost of the anchoring.

There is no doubt about the underlying phenomenon. It has been studied at length worldwide; a wide crack does reduce the load capacity of an anchor, and the international standards define this clearly. The question is not what the load capacity of an anchor installed in a crack is — it is what the probability is that an anchor in a normal application will end up installed in a crack of the kind the standards describe.

Most, if not all, of the research on the subject is funded by anchor suppliers — the same suppliers who earn up to four times more when the concrete is classified as cracked. It is hardly surprising that they are keen to convince designers that concrete is cracked at all times. Let us examine what the standards actually say, review the various studies, and look at the factors that create or widen a crack at the point where the anchor is installed.

What the European standards say

The European concrete standard and the European anchor standard are consistent with one another: both define cracked concrete by the possible appearance of a crack up to 0.3 mm wide. Notably, neither standard makes any reference to how deep that crack runs — a point we return to below.

The British standard’s guidance (BBA Guidance no. 39)

The British guidance is more nuanced, and frames the whole question in terms of probability:

“…in a building in use, i.e. where decanting does not take place, anchors can be installed in any location provided it is unlikely that a significant increase in loading will occur after the installation of anchors.”

This explicitly includes anchors approved only for uncracked concrete.

“Imposed loads applied to floors vary with time and space, and thus any realistic consideration of loading will be in terms of probability.”

Cracks in concrete occur at a fairly regular spacing, so whether an anchor lands exactly on a crack is itself a matter of probability. The more anchors there are in a connection, the lower the probability that any given one sits in a crack.

“The load carried by anchors is generally independent of the imposed loading applied to the structure, and thus there is joint probability involved.”

The larger the load on the anchor, the more it tends to affect the concrete. On the other hand, larger loads usually mean deeper anchors — and the deeper into the concrete you go, the lower the likelihood that an existing crack containing the anchor will widen over the years.

The classic example: a beam on two supports

The best-known example in the anchoring field is a concrete beam on two supports: the upper part in compression, the lower part in tension. This gives rise to the simplistic view that concrete in the ceiling is in tension and must be classified as cracked, while concrete in the floor is in compression and is therefore uncracked.

Is that always true?

• It is certainly not true in prestressed slabs.
• Tension and compression in the region of the supports are far lower, and above a support the sign can even reverse — in which case it is the concrete in the “floor” that is locally in tension. And if the supports are not at the ends of the beam, the distribution changes again.
• Tension and compression are also much greater in the outer layers of the concrete, and at a greater distance from the supports.
• The European standard considers an anchor installed in concrete in which a 0.3 mm crack appears through the full thickness of the section after installation. In reality, cracks do not run the full depth — they are limited to a very shallow depth.

Even if we imagine a beam 400 mm thick, assume the top 200 mm is in tension and the bottom 200 mm in compression, and install an anchor 300 mm long — then installing from the floor side, I will actually reach the concrete region that is in tension, and vice versa. All of the research and all of the standards are built around the simplistic picture, when the reality on site is far more varied.

When does the crack actually matter?

For concrete cone failure, the presence of a crack certainly can produce the reduction that the cracked-concrete definitions describe, because the crack can split the concrete cone, just as it would in concrete that is entirely in tension.

A number of recent studies — for example by Akanshu Sharma — show that the ETA anchoring standard substantially understates the contribution of the reinforcement in the concrete cone calculation. This is fairly logical: the European ETA anchoring standard does not account for reinforcement in concrete cone failure, even though the reinforcement’s job is precisely to absorb tension in the concrete. In another study, Rasoul Nilforoush explains that the reinforcement’s contribution is greater the thinner the member is.

This means that if the concrete beam is thick, there is a strong likelihood of heavy or closely spaced reinforcement that will significantly improve the concrete cone failure capacity. And if the beam is thin — even though the reinforcement contributes proportionally more — a crack is correspondingly less likely to run deep enough to matter.

How different anchor types affect crack formation

Wedge anchors and heavy-duty (dynamic) anchors

The zone of influence of a wedge anchor within the concrete is very wide. Tension is applied during installation, in order to open the clips, and it persists even without any external load (decaying over time to about 60% of the initial value). This is why installing a wedge anchor close to a concrete edge can sometimes break the concrete before the full load is even applied to the anchor.

Image source: “Turning the screw on stud anchors”, Engineers Journal, 05-05-15.

Directly threaded concrete screws and chemical anchors distribute the stresses along the bond or the thread, and apply no tension unless external tension is applied to the anchor. Their influence on the concrete is much less widespread.

Although I have not found research that confirms this directly, I believe that the permanent tension induced by a wedge anchor or a heavy-duty anchor, combined with their wide zone of influence, are contributing factors in crack formation at the anchor location. Even when concrete screws or chemical anchors are under tension, that tension peaks at about one third of the anchor’s load capacity. That is not the case with wedge anchors, where the tension generated in the concrete during installation can approach the anchor’s full load capacity.

Conclusion

On the face of it, the European standards that define cracked concrete are consistent between the concrete standard and the anchor standard, both defining it by the possible appearance of a crack 0.3 mm wide. But while a crack of very shallow depth is enough to have the concrete classified as cracked — because no reference is made to crack depth at all — its effect on an anchor installed in that same crack will be minor if the depth is shallow. The standards do not distinguish between surface cracks and deep cracks. As a result, the anchor standards treat the crack as running the full depth of the section, when in practice, in the great majority of cases, it does not.

Naturally, if there is real doubt, it is always legitimate to design for cracked concrete to be safe. But in my opinion, in most cases the concrete is uncracked — or the design can be arranged so that it qualifies as uncracked.

Practical design guidance

For anchors installed in concrete regions that are under tension, the classification depends on the details of the connection:

For anchors installed in concrete regions that are under compression, the concrete is uncracked.

Sources

• BBA Guidance no. 39
• “Comprehensive Experimental Investigations on Anchorages with Supplementary Reinforcement” — Akanshu Sharma, Rolf Eligehausen, Jörg Asmus, Institute of Construction Materials, Stuttgart, 2017.
• “Influence of Supplementary Reinforcement on the Tensile Capacity of Headed Anchors in Uncracked Concrete” — Rasoul Nilforoush, Martin Nilsson, Lennart Elfgren, Journal of Structural Engineering 2018, 144(4):04018012.