Cracked concrete and anchoring: frequently asked questions

Anchoring and cracked concrete — frequently asked questions

Is concrete in tension automatically cracked concrete?

Not necessarily. Tension raises the likelihood that a crack will appear, but classifying concrete as cracked depends on the probability of a significant crack appearing at the anchor position after the anchor is installed.

Is old or damaged concrete cracked concrete?

No. Old or damaged concrete is not covered by the European standards at all. It can certainly cause a significant reduction in the load capacity of an anchor fixed into it, but that has nothing to do with concrete being classified as cracked.

Is cracked concrete simply concrete that has cracks in it?

No. Concrete with cracks you can see with the naked eye is unsound concrete, and how an anchor performs in it is not covered by the European standards — we recommend carrying out pull-out tests to assess the anchor's capacity in such concrete. That said, concrete with very fine cracks can be a factor that raises the likelihood of the concrete counting as cracked — because cracked concrete means concrete in which a new crack appears, or an existing crack widens, at the anchor position.

Is concrete in a ceiling cracked?

Not necessarily. Concrete in a ceiling, far from any support, is in tension — which does raise the risk of the concrete being cracked. But in the region of a ceiling close to a support, the concrete is usually in compression. There is also an effect from the length of the anchor relative to the thickness of the concrete: concrete that is in tension on one face is usually in compression on the other, so the zone in which the anchor actually engages, and the type of anchor, both matter.

Is concrete in a floor slab uncracked?

Not necessarily. See the answer about ceilings — a floor can be in tension too.

Is young concrete (less than 28 days old) cracked concrete?

No — there is no connection. Concrete less than 28 days old is not an approved base material for installing an anchor under the European standard at all.

Is Spancrete (spun concrete) cracked concrete?

No. Spancrete is uncracked concrete — but it is better to use anchors designed specifically for it.

If an anchor is approved for cracked concrete, does that make it good in cracked concrete?

Not necessarily. An approved anchor is one that has been tested in cracked concrete. That tells you nothing about how good the anchor is in cracked concrete — only that its capacity in cracked concrete has been measured.

Is concrete in compression always uncracked?

Yes, barring exceptional cases — because concrete in compression generally will not permit a deep crack of the kind that could affect how an anchor performs.

Can the classification of the concrete change according to the type of anchor?

In theory, yes. Once a wedge anchor is installed, its expansion clips create a tension zone at the end of the anchor. That means the region it sits in is in tension even when no load at all is applied to the anchor. A concrete screw (such as the Adit TFE) or a chemical anchor (such as Adit Chemfix100), by contrast, only induce tension in the concrete when tension is actually applied to them. Although the logic is clear — you are better off avoiding a wedge anchor if you want to minimise the chance of a crack forming — we do not in practice distinguish between anchor types. The reason is simply that we have no tools with which to measure the effect of the anchor type on the probability that a crack will appear.

Can an anchor approved only for uncracked concrete be used in cracked concrete?

Under the European standards, no — there is no permission to install an anchor rated for uncracked concrete in cracked concrete. But because the definition of cracked concrete is probabilistic, there are two cases in which we think it is reasonable to disregard the cracked classification: (1) where the load applied to the anchor is in shear only; and (2) where the anchor's pull-out capacity in uncracked concrete is more than three times what is required — because there is no anchor whose capacity in cracked concrete is less than a third of its capacity in uncracked concrete.

Can an anchor's capacity in cracked concrete be tested on site?

It cannot. The case the European standard describes — an anchor installed in a 0.3 mm crack running its full length — is a situation that is almost impossible to reproduce in a standard building. That is also why, in my opinion, genuine cases of cracked concrete are very rare in reality.

Who gains and who loses when concrete is classified as cracked?

Where the concrete is not truly cracked: the supplier gains, because the cost of the detail is multiplied by three; the contractor loses, for the same reason; and the designer loses, because it can force unnecessary complexity into the detail. In the rare cases where the concrete really is cracked, of course, everybody gains.

How do I know whether to design for cracked or uncracked concrete?

“Cracked concrete” does not mean the concrete is visibly broken. It means the anchor sits in a tension zone, where micro-cracks can form around it. As a general rule, slabs and ceilings in tension, and zones subject to dynamic or seismic loading, are treated as cracked concrete; regions in compression are generally treated as uncracked. For the reasoning behind this, see our detailed article on the definition of cracked concrete.

When should I use a wedge anchor rather than a chemical anchor?

The great advantage of the wedge anchor is speed: once the hole is drilled and the nut tightened to the specified torque, the anchor is immediately ready to carry its full load. A chemical anchor, by contrast, requires a wait of hours for the adhesive to cure. So where the installation is dry and clean, in sound concrete, and the programme is tight, the wedge anchor is usually the more economical solution.

Can a wedge anchor be installed in a hollow block wall (pumice or Ytong)?

Absolutely not. A wedge anchor generates very large splitting pressure against the walls of the borehole. Installed in hollow concrete block, or in lightweight aerated block (Ytong), it will burst the block apart from the inside, and the anchor will simply pull out.

Does the borehole need cleaning before a wedge anchor is inserted?

Yes. A mechanical anchor is somewhat more forgiving of dust than a chemical anchor, but drilling dust accumulating at the bottom of the hole will stop the wedge anchor reaching its required embedment depth. Dust can also prevent the expansion clips from opening properly against the concrete. Always pump or blow the dust out before inserting the anchor.

Is the MTP-X wedge anchor approved for seismic use?

Yes. The MTP-X carries the most stringent European approval categories (ETA), including seismic approval in categories C1 and C2. That matters for public infrastructure and engineering projects. Bear in mind that for structural elements under seismic design, a C2 approval is the one required — C1 covers non-structural elements only. See our article on how to interpret seismic approval.

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