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

Imagine two insurance companies that control 65% of the global market in their field, and where every legislator and expert in that field 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 affect the load capacity of an anchor, and 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 suppliers 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 do the European standards say?

The first definition is based on a mathematical formula:

σ_L + σ_R ≤ 0

σ_R — stress in the concrete resulting from deformations (0 N/mm² in a compression zone; 3 N/mm² if unknown)
σ_L — stress in the concrete resulting from an applied force (for example, from the installation of an anchor)

Because the stresses are generally not known, the British standard added guidance to help designers:

The British standard’s guidance

“The capacity of the anchors is not influenced by the presence of cracks at the time of the installation, but rather the subsequent movement of these cracks. Only loads applied after the installation of the anchors are of interest.” In other words: for the concrete to be classified as cracked for the purposes of anchor design, the loads applied to the anchor must cause a crack to appear or widen — or the stresses in that region of the concrete must do so. This means that, contrary to the common assumption, in an older building the likelihood of a crack forming that was not caused by the anchor itself is lower than in a new building.
“In an existing 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 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. Whether an anchor lands exactly on a crack is therefore 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 will tend 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 chance of encountering cracks.

The guidance concludes: “Uncracked concrete is concrete in which the anchor has not been installed in a crack, or in which there is no high likelihood that an existing crack containing the anchor will widen over the years.”

So the classification of concrete as cracked or uncracked is probabilistic, based on factors influencing the concrete. Let us work through those factors, starting with the ones unrelated to the anchor itself and relating instead to the region of concrete in which the anchor is installed.

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 crack 0.3 mm wide appears through the full thickness of the concrete after the anchor is installed. 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, and assume the top 200 mm is in tension and the bottom 200 mm in compression, and I 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 the research and all the standards are based on concrete in homogeneous tension. That may reflect a small anchor installed at shallow depth, but larger anchors are usually installed deeper, and the installation zone sometimes contains a layer of concrete in tension and a layer in compression.

Tension and compression zones in concrete

When does the crack actually matter?

The reduction in anchor load capacity in cracked concrete under a pull-out failure comes from expansion processes that cause movement and release of mechanical anchors, or debonding of the adhesive from the concrete in chemical anchors. If the region where the connection is made is one that cannot open up (0.3 mm being the width at which concrete is defined as cracked) — because the concrete there is in compression — that simply cannot happen.

As for concrete cone failure, the presence of a crack certainly can produce the reduction that the cracked-concrete definitions describe, because the crack can cause the concrete cone to split, 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 is less dense — its effect will still be significant. Either way, the case for reducing the concrete cone capacity in uncracked concrete is weak. Note that for chemical anchors the situation is more complex, because part of the bonded length sits in a concrete region under tension and part does not. It may therefore be appropriate to apply at least a partial cracked-concrete reduction, in proportion to the percentage of the bond length located in the tension zone.

How different anchor types affect crack formation

It is well established that the tension zone an anchor creates causes a crack to appear at the anchor location. But is there a difference between anchor types in this respect?

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

Wedge anchors and heavy-duty (dynamic) anchors

In wedge anchors and heavy-duty anchors, all the stresses during tensioning are concentrated in a small area around the expansion clips — but the effect within the concrete is very wide. Tension is applied during installation, in order to open the clips, and 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

Concrete screws and chemical anchors - effect on the concrete

Directly threaded 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 and 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.

More images from the study

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 anchors 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, the crack depth is very limited.

This explains why in ordinary site testing you will never actually feel the effect of “cracked concrete”: if it exists at all, it is a phenomenon that develops over time, with a probability tied to the type of design and the anchor locations. But because the European standards make no reference to crack depth or to the probability of a crack appearing, designers are pushed into classifying the concrete as cracked in most cases, purely to be safe.

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

Seismic design: the concrete is cracked, in every case.
Anchors installed in concrete regions under tension:

Treat the concrete as uncracked when:

The load is distributed across a large number of anchors, preferably concrete screws or chemical anchors.
A mechanical anchor is used that engages in a compression zone (i.e. deeper).
In the anchor zone there is no likelihood of a crack forming to a depth close to the embedment depth.
No significant change in loading on the concrete or the anchor is expected after installation.

Treat the concrete as cracked when:

The connection uses few, short anchors in a tension zone.
Significant load changes on the concrete or the anchor are expected after installation (which would produce tension and cracking).
The concrete was designed to function as cracked concrete in the anchor zone, and there is reasonable cause to suspect a deep, wide crack will appear there.

Anchors installed in concrete regions under compression: the concrete is uncracked.

Sources

BBA Guidance no. 39

Study by Akanshu Sharma

“Comprehensive Experimental Investigations on Anchorages with Supplementary Reinforcement” — Akanshu Sharma, Rolf Eligehausen, Jörg Asmus, Institute of Construction Materials, Stuttgart, 2017

Study by Rasoul Nilforoush

“Reinforcement on the Tensile Capacity of Headed Anchors in Uncracked Concrete” — Rasoul Nilforoush, Martin Nilsson, S.M. ASCE, Lennart Elfgren, Journal of Structural Engineering 2018, 144(4):04018012

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