Core drilling questions

Hi all,

I have a question about core drill bits: How long do they last? I've got literally thousands of holes, 7/8" diameter, 3 1/2" deep to drill into concrete and epoxy all-thread rods for a job, and none of the factory reps seem to want to give me a straight answer on how many holes I can get from one bit. The Hilti guy wouldn't even get pegged down between 50 and 500. I can assume I'll hit the occasional rebar, but the plans outline the bolts not being in line with the rebar. And we all know that if it's in the plans that way, that's exactly how it was built, right? I know I'll need a bunch of bits (right now I'm figuring 200 holes per bit), but at a hundred bucks apiece, I want to estimate enough of them into the job that I won't take a huge hit buying bits.

I don't want to use a roto-hammer as most of these bolts are 4" away from the edge of the concrete pedestal, and I'm worried both about precision (not having the drill straight or having it wander) and breaking off the edges of the concrete pedestals that I'll be drilling on. Also, if I hit rebar with the roto-hammer, I'm in a bad way, because these bolts can't be moved.

Third, how long does it take? How many minutes to core drill a hole, remove the core, clean out the hole, insert some epoxy and set an anchor bolt to grade? If my per-hole estimate is wrong by one minute, that equals about 9 days of production. I realize it's just a guess, but I want it to be an educated one, and frankly, I'm not very educated on the production aspects of 7/8" core drilling. Currently, the average of people I've surveyed is about 10 minutes.

If any of you have any thoughts or guidance on this issue, I'd appreciate your wisdom and opinions.

Thanks,

Phil

Phil-

You're on the right track ...asking all these questions in advance. That's the good news.

The bad news as you mentioned is being off by a a minute per hole makes a huge difference.

Hilti rotary hammer are awesome tools you can drill fast & straight. But not through rebar.

Hilti core drilling setups are awesome as well.

Having drilling 100's of holes personally, I would suggest a two prong approach

Use the rotary hammer, its faster, cleaner...no water mess. If you hit rebar stop & finish with the coring system.

Is the coring system a Hilti vacuum base? If so make sure you have the room & appropriate surafce finish for it to suck down.

Setup, placement & suck down is much slower for core drill than point & shoot with the hammer drill.

Rotary hammer will give at least 100 holes 3.5" deep IF you don't abuse the bits. Same with core drills, hitting rebar will reduce life by 50% absolute worst case.

Don't lean on the hammer drill, a Hilti hammer drill works fine on its own weight.

I drilled 52 holes 1.125" diameter by 12" deep...used 2 Hiilti hammer bits, hit partial rebar in 5 holes & cored them no sweat.

If your Hilti hammer is variable speed you can easily accurately place holes. Since your holes are only 3.5" deep I doubt you'll hit much rebar unless they placed really close to the surface. I'm figuring its at least 2" deep.

I suggest you do some process development prior to bid submittal. It will be worth the effort

How many men in the drill, prep & place team?

What chemical anchor are you using? Who supplies the material? If you supply the material can the hole be

mechanical dispenser or pneumatic? Anchors precut? Anchor setting tolerance? hole pattern x-y tolerance? protrusion tolerance?

I suggest you "productionize" the process; efficiency is the name of the game, your crew skill level & motivation are key factors. Simple aids to production will greatly improve efficiency, work flow organization is key as well.

As you can probably tell, I've done this sort of thing before but every situation / job site is a little different and for 1000's of holes, minutes make a huge difference, even 10 seconds per hole matter. I design & specify these sorts of processes.

Your average time estimate is 10 minutes, what was the range in times?

Your 10 minute number, at this point, without prior experience or some process development, is pretty much a WAG (wild ass guess)

be careful, I'd hate to be very far off on the estimate per hole on

cheers Bob

Phil-

All my rambling is all well & good for developing a really good number but if you're not needing to have a really sharp pencil to win the job job it wasn't all that useful.

Are we talking about a fixed price & you pocket any excess? :)

IMO based on experience, for multiple groups of multiple holes, your

If you have a decent crew & give them some sort of incentive (& work flow guidance) to work faster I'm sure you can beat that number by a

BTW a 4" edge distance in decent concrete is plenty for a 7/8" hole. I hammer drilled 1" holes in unreinforced concrete with only 2" edge distance.

I whimped out on a 2" hole (x 10" deep) with 6" edge distance and used the core drill.

cheers Bob

Bob,

My comments are interspersed. Thanks for the insight. Phil

Thanks.

Snip

It is and I do.

Snip

Easier said than done with a crew of carpenters.

According to the plans, the rebar shouldn't be in the path of the bolt holes anyway. And we all know that if it's shown that way on the plans, that's how it was built, right?

I was thinking two--one to do the actual drilling, one to tend the hoses, cords, bits, etc. When the drill guy gets tired, they can switch.

It specs a 7/8" hole. I'm sure the owner would give us a break on it, though, if we needed 13/16". We're supplying the epoxy, also called out in the plans.

Electric dispenser, precut anchors, x-y pretty tight (within 1/8" and protrusion probably 1/4".

We've already discussed templates, layout procedures and order of operations. Most of the anchors are in groups of 10 or 12, so we'll make a 12-hole template or three, double nut the anchors onto the template, drill the holes and set 12 bolts at a time. While the epoxy is drying, we'll drill 36 more holes.

While none of the folks I've talked to have drilled holes at a level like this, the consensus is 10 minutes to drill the hole, clean it out, squirt some epoxy into the hole, and set the bolt.

Obviously, while we'd like to be dead-on with our estimate, I'm mostly just trying to get into the ballpark with the time estimate. We have a schedule we have to meet, and I was wondering how many drilling rigs I needed, etc. With our current manpower shortage, I want to have as few guys on this as possible, and just have them work for the duration of the job on this one item. A few man-days one way or the other won't break the bank on this job (it's a big one), but closer to the "true" value of the work is always better. That way, there's less variability in our estimate, and the smaller contingency we need to build in. With less risk, we can bid at a lower margin, and hopefully get the job. But I'm sure you knew all that already, Bob. :)

Yes. This is a lump sum job.

That's good to know. Also, I don't think I mentioned it, but this concrete is no longer green. It's currently about a month old, so it's pretty well set up. If we get the job, we won't get in there for at least another month. So that's good.

Sissy.

Thanks again, Bob, for the insight. I hadn't considered the 2-pronged approach. If we can save a few bucks that way, that will be great.

I'll be sure and post an update if we get the job.

Take it easy,

Phil

Phil-

Good luck on getting the job.

Having an electric dispenser will be a big help. On those 52 each 1

I would suggest drilling & "rough" cleaning a large number of holes. Have a good shop vac with lots of extra filters (washable ones). Sweep up most of the debris with dust pan & brush.

A modified 2" (or 3") PVC elbow can be used as a vacuum hose nozzle. Drill a hole thru the back side of the elbow to accept a 1/4" tube on a modified blow gun.

Using this setup you can blow the holes & capture the debris (so the crap doens't shoot up into everyone's face)

Sticking with a single process with save a LOT of time, switching between tasks really eats up time.

SO drilling & rough cleaning is one process (one guy). Drilling is really quick, clean & easy with a hammer drill. The most tiring thing will be leaning over the drill. I tend to use the REALLY long bits, so I can stand at a more comfortable height & controlling drilling squareness is easier.

If you go with shorter drill bits, encourage the guy to get comfortable, sit on a bucket or use knee pads

A guy with the vac can follow when the driller gets far enough ahead. The cleaning process should be:

the correct sequence is: blow brush blow brush

the last thing to go in the hole (before the epoxy) must be the brush.

if you blow last, it coats the hole with dust & weakens the epoxy bond

when the "hole cleaner" catches up with the driller, he can switch to setting anchors.

Depending on whose product you're using & how big the hole diameter is...the anchors can stand up by themselves.

Be careful on hole depth...too deep & you're screwed.....it makes controlling anchor protrusion a bitch.

It is easier to shorten a anchor that sticks out too far than deal with a "shorty". Discuss QC with whoever will check or sign off on mistakes. I'd m

Whose cutting or supplying the anchors? Are they off-the-shelf Hilti threaded rod or just cut threaded rod? Variable anchor lengths will be a problem to deal with.

cheers Bob

Interesting, Bob. Simpson's instructions for their SET epoxy say to "blow brush blow". Do you think that they have it wrong?

Cheers, Wayne

My experience plus my interaction with home office Hilti engineers ....they said brush, blow, brush

They explained that the "final brush" drops the drops the dust off the hole wall.

I added the extra blow / vacuum on the front end but kept the Hilti "finish with brush"

Again my experience based on drilling, cleaning, bonding & testing anchors in concrete in the lab.

flour (dust) is used as a release agent when working with dough

paint doesn't stick to dust; I use a brush & a tack cloth when prepping for paint

YMMV.....draw your own conclusions

counter argument / theory? :)

cheers Bob

Oh, I don't really have one, although one can imagine a counter argument. Mostly I was surprised that Hilti and Simpson give opposite instructions in this regard. I guess the obvious question is what is the bonding mechanism of the epoxy to the concrete? But this is academic.

Cheers, Wayne

Wayne-

Hilti says brush,blow for their epoxy systems. End it by brushing and the hole won't be clean enough. You don't need to brush at all, only blow if you use the HVA capsules.

Tim

Absolutely, dust inhibits adhesion. But then the question is which procedure, brushing or blowing, yields a dust pattern that has the lower impact on adhesion. You mentioned brushing last to dislodge any loose dust on the sides, where it will fall to the bottom.

Arguments in favor of blowing last could be (1) blowing will remove any loose dust at the bottom and won't significantly redeposit it on the sides or (2) brushing may yield fine dust that is suspended in the air, and blowing is the only way to remove this fine dust.

Anyway, I don't really know anything about this. But Simpson recommends blowing last, and I see someone else said that Hilti's written instructions say the same thing. I assume the only way to resolve the question would be to do tests. Or perhaps it is most important to just use whatever procedure was used when the published strength data was developed.

Cheers, Wayne

For the HIT RE 500 epoxy (to pick a random one, I'm not familiar with their products), see page 15 of this file:

I'm not trying to dispute your statements, I was just surprised to see at the discrepancy between your theory (which makes sense) and the manufacturer's instructions. It leaves me a little confused.

Cheers, Wayne

I've sure set a lot of chemical anchors, but not in the thousands on any one job. Most of my applications have been indoors so we developed a brush and suck method. A plumber's wire brush, cut off the handle, chuck it into a battery drill and rig whatever tube on a good shop vac to go to the bottom of the hole has worked well for us. As has been said, controlling the depth is critical if you can't run wild. What on earth are you setting that only needs a 1/4" exposure and 1/8" tolerance? I think the biggest issue will be maintaining that type of tolerance. I don't think I would even consider a core drill, it sounds like a heavy hammer drill or rotary hammer job to me unless they are so close together that they generate a fracture line. You might consider two bits in two drills, a lead hole with a 1/4" bit in a light weight SDS drill for accuracy followed by the 7/8 in a heavier drill. Another concern is concrete depth if you are drilling 3 1/2" are you approaching blowing out the bottom? Relton and others make special rebar cutting bits that work in the same size hole you are drilling:

That's not what the Hilti factory new adhesive development engineer told me in 1997.

brush, blow, brush

has got to leave a more dust free surface than

brush, blow

maybe they did more testing but when I was doing testing for them it was

brush, blow, brush

and that's what I've done (& still do) and the anchors test out just fine.

cheers Bob

Wayne-

its not my theory, its what the Hilti development engineer taught me in 1997

looks like they have chosen to omit the final brush set and now suggest brush, blow

oh well,

I will still brush, blow, brush until someone shows me data that the final brushing is detrimental to anchor performance.

cheers Bob

I hear you Bob. There's usually more than one way to do something and still end up with a quality product. But, when an architect or engineer specs a certain product, we're obligated to follow the manufacturers instructions. That way, provided things are done correctly, it's alot harder for problems to come back and bite the installer. Sometimes, as in your case, things test out just fine. However, I've also seen the other side. Mostly with metal roofing, flashings, etc. where guys either took shortcuts or thought they knew more than the manufacturers. It's no fun going back to repair or replace someone elses shoddy work in order to get the warranty reinstated or stop leaks. I just figure that after spending thousands or millions of dollars on research and development of a product or a system, the least I can do is install per their instructions. It can save me alot of pain down the road.

Have a good one Tim

Tim-

I am an engineer AND I have done 100's of installed & tested anchors (pulled to failure) for various mfr's.

The procedure was brush, blow, brush

now it looks like they say in their instructions brush, blow (omitting the final brush)

looks to me like a short cut...........I'm giving my Hilti contact a call on Monday to get the straight story.

cheers Bob