Drop in concrete anchors installation honts

Looks like many people will be using drop in concrete anchors for the first time. (Like these from Hilti). As someone who has installed untold boxes of these I thought I’d share some hints.

  • It’s fairly important for the anchor to be snug in the hole before setting. I often have to lightly tap them in with a hammer. Try to keep the drill perpendicular and avoid “reaming” the hole by repeatedly removing and inserting the drill.

  • The hole needs to be free of debris. After drilling clean the hole with a blower bulb (dollar store turkey baster works) or with a vacuum with a straw taped over the end of the hose that will fit in the hole. It’s surprisingly ineffective to just place the end of the vacuum over the hole. Be careful when using a blower bulb. The fine dust comes out rapidly. Wear eye protection, a dust mask and keep face away from hole as much as possible.

  • The anchors require a significant impact to set. I generally use a 2lb hammer and have to hit the setting tool 3-4 times. Make note of the sound when setting as it changes when properly set.

For our purpose where all the force is sideways you don’t have to spend big bucks on a high strength anchor. The high strength is mostly addressing pull out.

Anchors shouldn’t be installed within 1.5" of the edge of the concrete.


Cheers for the info and example anchor link.

Didn’t realize I can get away with just beating the anchors into a snug hole?

Anyone have thoughts on using construction/structural epoxy adhesive? Think I have some left over from other projects.

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The anchors still have to be set using the pin tool.


Makes sense.


I was looking at those anchors and have a question. After you use the setting tool, is the insert firmly located in the concrete, or does it need the screw tightened to keep it in place? In other words, does installing the screw in the last panel expand the insert further to tighten it in the hole, or is that all taken care of with the pin tool? Can you use a shorter screw or does the insert require a certain length of threads in the insert.

Once it’s been properly set it’s there independent of the bolt threaded into it.

If you plan to take the bolt in and out a lot it would be a good idea to apply a touch of thread lubricant. And just snug the bolt down, no need to go hog wild with the torque.

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Also very bad things can happen if you drill into tension concrete.

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There’s no way I wasn’t going to look that up… Guessing others will too, so…

Drilling into tensioned concrete, commonly known as post-tensioned concrete, can have serious consequences due to the high-strength steel tendons that are tensioned within the concrete to provide it with additional strength. These tendons are under extreme tension, and drilling into or through them can lead to several very bad outcomes:

  1. Structural Damage: Drilling can compromise the integrity of the concrete structure by weakening it. This can lead to cracks or even catastrophic failure over time, especially in load-bearing elements.

  2. Risk of Injury: If a drill bit contacts a tensioned steel tendon, the tendon could snap. The sudden release of tension can cause the tendon to whip out, potentially causing serious injury to anyone nearby.

  3. Expensive Repairs: Repairing the damage done by drilling into tensioned concrete is often complex and costly. It might involve specialized procedures to restore structural integrity and ensure safety.

  4. Legal and Financial Liabilities: Damaging a post-tensioned concrete structure can lead to legal consequences, including fines and the costs associated with repairs, downtime, and potentially compensation for injuries.

  5. Loss of Building Use: Significant structural damage may necessitate evacuations or limit access to buildings or bridges, affecting their usability until repairs are made.

To avoid these outcomes, it’s crucial to have detailed plans of the structure’s post-tensioning layout and to use non-invasive methods for detecting the position of tendons before drilling. Additionally, consulting with a structural engineer or a professional experienced with post-tensioned concrete is advised before attempting any modifications to such structures.

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I know for a fact that there are garages in Arizona that had tension concrete floor. Mine did. Usually it’s stamped near a door entrance.

Edit, @bar this maybe a good idea to add in the instructions as a warning.

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They are said to be rare in the northeast where I live. Perhaps that is because we have deep basements instead of slabs?

Hmm, for my understanding the tensioned steel reinforcements should be on the underside of the concrete slab or am I missing something?

It’s in the slab

Yes, sure inside the slab, but nearer to the bottom then to the top.