The Book of Concrete Screws
“Screwing can be great in the short term.”
EPISODE: Why do climbers use these?
The Bolting Bible
This is free resource by HowNOT2 and our way of contributing to the community. It's nice to understand what you are clipping and trusting with your life, even if you never plan on installing or removing bolts. We also believe that if you are going to spend time and money to bolt something, you should do it well.
This book is in a blog format. This page points to all 17 chapters, and the end of each chapter points to the next.
A downloadable PDF is available HERE.
Screwing Rock?
These work similarly to normal wood screws. Pre-drill a hole and the threads bite into the sides of the hole. The screws have a cutting thread of harder steel at the tip and the rest of the threads just follow along. A 3/8" screw requires a 3/8" hole so it's pretty straightforward.More Tools!?
Screwing Dirty Holes
The hole doesn’t have to be super clean like glue-in bolts require, but you should blow out the dust before installing them. Otherwise, you may not get it all the way in as the dust in the bottom stops it.Screwing WET Holes
You can get away with installing in a dry hole with an impact driver but it works better if you get the hole wet first. Just spit or squirt a stream of water in the hole before installing the screw. It is REQUIRED to get the hole wet first if you install by hand.Screwing Round Holes
You need a round hole for these to work right. You get the roundest hole with a 4 cutter drill bit instead of the bits that have 2 shoulders.
The Snowflake of Bolts
The rock can't be too hard and it can't be too soft. In fact, it is the only bolt that gets mad if the rock is too hard. It can strip the threads and not even go in all the way, or just be compromised. It's pretty obvious if you are forcing it.These are NOT safe in soft rock. We use these to hold down the piston of our hydraulic for testing and in sandstone, they kept coming out. If it pulls on one side in a cyclic manner, it will remove the teeth from the other side of the hole and lose its grip. We tested bolts in fractured limestone in a mercury mine and used concrete screws to hold down the piston. The piston anchor kept failing!!!
Install Tips
Where They Shine
Great removable bolts, not reusable bolts! These are the easiest bolt to remove and replace from a bolt stewards perspective. We use these a lot in BoltBusters to anchor down our hydraulic testers and to test all sorts of hangers. We needed a bomber bolt that was easy to remove after we were done. Concrete screws were great for this. They can help route developers put in temporary bolts until they install all the glue in's to make sure they like all the placements. They could be placed as permanent bolts as well.Can you REUSE them??? NO. Not really, Sometimes, Sort of. If you are in softer rock, you can reuse the bolt a few times but not the hole. If you are in hard rock, you can reuse the hole a few times but not the bolt. We pushed the limit of these when we reused the same holes about a dozen times to test hangers in concrete IN SHEAR (an important caveat). We could feel only the last 3 threads were grabbing at the bottom and the top would wiggle. The bolt was compromised and obviously, the hole was too. In tension, I don't think it would have done well.
After reusing the same hole enough in concrete it will come out like in this VIDEO (at very high forces). Pulling in tension snapped the head off at 43kn as you can see in our data below on test 142 but it is hard to do as the hangers typically fail first.
The Snowflake of Bolts
If these are so great, why doesn't everyone use them? Some are concerned that after many freeze/thaw cycles these bolts will become loose over time but there hasn’t been enough examples of this problem for us to be concerned about it. Several people have used and monitored them long term and have reported they don't loosen up over time.Keep in mind this is a newer fad and the ASCA or any old school bolter does not currently approve of these for the reason that extensive testing has not been done specifically for climbing. They have been extensively tested for construction so they aren't bad bolts, but which one is appropriate for climbing? There is no official consensus on that.
Not All Screws Are Created Equal
Simpson Titen HD
The shortest you'd want is 3" (75mm). a 4" (100mm) long screw is A LOT more to get in but also held a lot more in our tests when pulled in tension. If you've ever installed a 6" (150mm), please let me know. I've haven't heard of anyone doing that for rock.
I take that back, @mitchvorwerk hit me up and shared this use case for long and thicker screws: I've been using the 6" x 1/2" Titen HD Zinc Plated concrete screws as my temp anchors & temp bolts for about 1.5 years now. I love them for placements that I expect to replace with a 6" x 1/2" Bolt Products Twisted Leg glue-in bolt because I don't have to haul the drill up the wall again on glue day. Just twirl it out, brush, blow, brush, blow, and glue. I started out using the 12mm x 90mm Petzl Triplex SS removable bolts, but switched to the concrete screws so that I wouldn't have to futz with widening the hole and hauling the drill again. I use them with 1/2" Fixe PS hangers, but had to widen the hanger hole ever so slightly to get the bolt to fit. I do find that I can reuse the bolts a few times and they don't feel compromised. I don't use these as long-term bolts or put them out there for public use, just temporary bolts for exploration and development. The rock I'm bolting is moderately soft rhyolitic tuff in Paradise Valley, NV.
Simpson's Strong Tie TitenHD 304SS bolts have also been very impressive in Bolt Buster tests. If installed by hand, they are easier to get started but because the tip is not brazed, it's harder to keep going than Hilti's KH-EZ 316SS screw. That one is harder to get started but easier to get all the way in. Both are a super good enough bolt.
We have not tested Dewalt, Powers or Red Head's screws but we have heard they are not good. Red head is a 410 stainless that gets rust, Dewalt has weird pitch angle and Powers is meh. Multi-Monti is popular in Europe but we have not tested those yet. Not all brands are going to be super safe enough so please don't use the cheapest bolt you can find!
Strength
How strong are concrete screws? It really depends on the rock it is in, but here are the tests we have done with Simpson's Strong-tie Titen HD's. Most of our use cases are temporary so we have a lot more plated steel (PS) bolts tested than the stainless you would use for a permanent anchor. IF the bolt holds, we can get between 42kn and 52kn before the head shears off. 4" pulled in tension held A LOT more than the 3".
What's Next?
Chapter #7 - The Book of Mechanical Bolts