A little more informaton from the Engineering News Record.
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Big Dig Boston ceiling collapse 21
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The issue of concrete/epoxy bond seems to be the focus point of the failure. It also appears that the fact that the anchors were core drilled instead of hammer drilled weakened more the problematic adhesion bond. The water used in the core drilling process may also contribute to the weakened bond.
Another issue, likely underestimated initially, is the huge incompatibility in between thermal expansion coefficients in between epoxy and concrete/steel.
Same years ago my very educated friend demonstrated to me the simple test of the bond in between epoxy coating and steel rod. We moved the epoxy coated rebar several times from liquid nitrogen into warm water. The epoxy coating cleanly delaminated from the rebar.
The infamous anchors likely behave in the same manner. Heavy steel anchors act as a heat conducting bridges, introducing rapid temperature changes to the anchors and surrounding material. In the process, the adhesion bond in between epoxy and concrete releases, and anchor could cleanly pull out. Should the holes be percussion drilled, the jagged surface is less prone to this type of failure.
As conclusion, the new (or additional) anchors shall be or percussion drilled, or core drilled, but the hole should be roughened and cleaned before installation.
Another issue, likely underestimated initially, is the huge incompatibility in between thermal expansion coefficients in between epoxy and concrete/steel.
Same years ago my very educated friend demonstrated to me the simple test of the bond in between epoxy coating and steel rod. We moved the epoxy coated rebar several times from liquid nitrogen into warm water. The epoxy coating cleanly delaminated from the rebar.
The infamous anchors likely behave in the same manner. Heavy steel anchors act as a heat conducting bridges, introducing rapid temperature changes to the anchors and surrounding material. In the process, the adhesion bond in between epoxy and concrete releases, and anchor could cleanly pull out. Should the holes be percussion drilled, the jagged surface is less prone to this type of failure.
As conclusion, the new (or additional) anchors shall be or percussion drilled, or core drilled, but the hole should be roughened and cleaned before installation.
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- #64
HagarTheSmellable
Structural
- Jun 2, 2006
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As usual, the variety of opinion on this has addressed almost every question that popped into my pinhead.
One of them I'm still curious about. Could damaging sympathetic vibrations have been induced into the panels because of the (probably) constantly changing exhaust volume requirements? If indeed those fans are load-managed instead of operating at a constant level 24/7.
I'd prefer leaving the equipment bare and beautiful -- that's MY type of sculpture. Leave the cosmetics to lady shoppers. But, politicians like certain types of cosmetics too.
One of them I'm still curious about. Could damaging sympathetic vibrations have been induced into the panels because of the (probably) constantly changing exhaust volume requirements? If indeed those fans are load-managed instead of operating at a constant level 24/7.
I'd prefer leaving the equipment bare and beautiful -- that's MY type of sculpture. Leave the cosmetics to lady shoppers. But, politicians like certain types of cosmetics too.
Count me as one who has never used an adhesive anchor in an overhead application, and never in critical tension. Whatever the manufacturers say (and they don't say much about overhead installation), there are more things that can go wrong with adhesive anchors than mechanical anchors.
I believe the new anchors will be undercut anchors. These will probably have to be longer than the original bolts to get past the reinforcing, as I don't think undercutting reinforcing is a done thing.
Wonder who is leading the forensic investigation? Hope as well as looking at the bolts, they are considering the whole system, e.g. fluctuating loading, stability.
I believe the new anchors will be undercut anchors. These will probably have to be longer than the original bolts to get past the reinforcing, as I don't think undercutting reinforcing is a done thing.
Wonder who is leading the forensic investigation? Hope as well as looking at the bolts, they are considering the whole system, e.g. fluctuating loading, stability.
One article said that the replacement bolts would be Hilti. That's a good indication that the failing bolts were not Hilti. Has anyone heard the brand?
I use epoxy bolts in horizontal and vertical-down applications. Horizontal applications are principally shear, but reasonable tension as well. Vertical down applications are uplift tension and some shear. So far so good under fairly high wind loads.
I've also tested quite a few epoxy bolts in pullout resistance. Failure mode is almost always concrete-epoxy interface.
I use epoxy bolts in horizontal and vertical-down applications. Horizontal applications are principally shear, but reasonable tension as well. Vertical down applications are uplift tension and some shear. So far so good under fairly high wind loads.
I've also tested quite a few epoxy bolts in pullout resistance. Failure mode is almost always concrete-epoxy interface.
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- #70
regarding the use of the epoxy or adhesive anchors in tension: do any of you allow the anchors to be used for baseplates where uplift occurs and the contractor misplaces or damages the cast-in-place anchor bolts? I do not, but wondering what others allow.
as far as Hilti's replacement anchor: what will that be? will they use the epoxy anchor again, but with greater QC? or the HUS-H threaded bolt which is similar to the Simpson Titan HD or Powers Wedge bolt?
as far as Hilti's replacement anchor: what will that be? will they use the epoxy anchor again, but with greater QC? or the HUS-H threaded bolt which is similar to the Simpson Titan HD or Powers Wedge bolt?
- Thread starter
- #72
archeng59 - not epoxy - they'll use an undercut anchor.
I believe its this one:
Undercut Anchor Link
I believe its this one:
Undercut Anchor Link
MikeHalloran
Mechanical
I understand why they need to do it _now_, but I'm puzzled about why there was a need to drill blind fasteners into the tunnel ceiling in the first place, given that it's a cut and cover tunnel.
Mike Halloran
Pembroke Pines, FL, USA
Mike Halloran
Pembroke Pines, FL, USA
- Thread starter
- #76
archeng59...yes, I allow the use of epoxy anchors for uplift, but it's limited based on the size and configuration of the connection.
As for fixing contractor screw-ups in anchor bolt placement, I typically take a harder line on repair, though if a practicable epoxy solution is proposed, I'll consider it. However, if the contractor screwed up the location, he won't suddenly get smart and do all the right things that make epoxy anchors work. Epoxy anchors to replace misplaced bolts are difficult for clearance, bonding, cleaning, size, and embedment issues.
As for fixing contractor screw-ups in anchor bolt placement, I typically take a harder line on repair, though if a practicable epoxy solution is proposed, I'll consider it. However, if the contractor screwed up the location, he won't suddenly get smart and do all the right things that make epoxy anchors work. Epoxy anchors to replace misplaced bolts are difficult for clearance, bonding, cleaning, size, and embedment issues.
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- #78
boffintech
Civil/Environmental
I have nearly 20 years of experience performing both placement and post-placement inspections of rebar, post-tensioning systems, concrete, masonry, etc. I’ve said it once and I’ll say it again: If structural engineers who specify epoxy for dowels and the like believe that the work is being done correctly then they live in a world unfamiliar to me. The key instructions to correctly epoxy in a dowel are either unknown to or widely ignored by contractors. You would be astonished at the number of so-caled epoxied dowels that I can pull out with one bare hand.
The following is of particular importance to a correctly epoxied dowel: The hole is cleaned with oil-free compressed air, brushed with a nylon brush, and cleaned again with oil-free compressed air. Generally speaking on a job site oil-free compressed air can only be obtained from a wet-dry vac or an air-compressor with the correct filters installed. Explaining to a contractor that the on-site air compressor can not be used for hole cleaning because the air contains oil that will act as a bond breaker between epoxy and concrete is like banging your head against a concrete wall and we haven’t even gotten to the brush part. A brush for cleaning the drilled holes is as elusive as a non-working rebar foreman; they rarely, if ever, exist on a job site.
I arrived at one particular job site last week to find the contractor in the process of placing #4 rebar dowels in 1.25” diameter drilled. The holes were drilled for 1” diameter SMOOTH dowels (the interface of an interior slab and an exterior slab where large trucks enter a service area). The contractor had no oil-free compressed air and no brush to clean the holes. On another job a pile cap had 18 #11 column dowels misplaced so all were cut off with a plan to drill and epoxy new dowels in the correct location. The contractor had no oil-free compressed air and no brush to clean the holes.
Over the years I’ve caught numerous contractors using the following procedure for placing epoxied dowels. 1) drill hole 2) hammer rebar into hole 3) squeeze epoxy onto the little mound of dust around the top of the hole. Seriously! I’ve also seen numerous dowels in place that have two different colors of epoxy on either side of the dowel/hole which means that the epoxy was squeezed into the hole without the mixing tube attached to the dispenser! I’ve seen numerous contractors attempt to mix the two parts of the epoxy mix on a board and then try to cram the “mix” into the hole with a little stick..
A couple of months ago I looked at a fix on a 300’ long retaining wall with #7 dowels at 6” and #5 dowels at 6”. When the foundation was placed (in three separate 100’ concrete placements) the dowels were placed in opposite faces of the wall and the EOR wanted new dowels #7 dowels epoxied in the correct face. I was able to dislodge 3 of the first 10 “epoxied” dowels I tested with two bare hands. I knew to test them because upon arrival to the site I bypassed the superintendent and went straight to the labor and asked how they installed all those dowels. This is a favorite tactic of mine, “How’d you do it?” I discovered that there was no on-site oil-free compressed air, no hole cleaning brush, and no correct size drill bit. OK, so I didn’t make it on that contractor’s Christmas card list.
The following is of particular importance to a correctly epoxied dowel: The hole is cleaned with oil-free compressed air, brushed with a nylon brush, and cleaned again with oil-free compressed air. Generally speaking on a job site oil-free compressed air can only be obtained from a wet-dry vac or an air-compressor with the correct filters installed. Explaining to a contractor that the on-site air compressor can not be used for hole cleaning because the air contains oil that will act as a bond breaker between epoxy and concrete is like banging your head against a concrete wall and we haven’t even gotten to the brush part. A brush for cleaning the drilled holes is as elusive as a non-working rebar foreman; they rarely, if ever, exist on a job site.
I arrived at one particular job site last week to find the contractor in the process of placing #4 rebar dowels in 1.25” diameter drilled. The holes were drilled for 1” diameter SMOOTH dowels (the interface of an interior slab and an exterior slab where large trucks enter a service area). The contractor had no oil-free compressed air and no brush to clean the holes. On another job a pile cap had 18 #11 column dowels misplaced so all were cut off with a plan to drill and epoxy new dowels in the correct location. The contractor had no oil-free compressed air and no brush to clean the holes.
Over the years I’ve caught numerous contractors using the following procedure for placing epoxied dowels. 1) drill hole 2) hammer rebar into hole 3) squeeze epoxy onto the little mound of dust around the top of the hole. Seriously! I’ve also seen numerous dowels in place that have two different colors of epoxy on either side of the dowel/hole which means that the epoxy was squeezed into the hole without the mixing tube attached to the dispenser! I’ve seen numerous contractors attempt to mix the two parts of the epoxy mix on a board and then try to cram the “mix” into the hole with a little stick..
A couple of months ago I looked at a fix on a 300’ long retaining wall with #7 dowels at 6” and #5 dowels at 6”. When the foundation was placed (in three separate 100’ concrete placements) the dowels were placed in opposite faces of the wall and the EOR wanted new dowels #7 dowels epoxied in the correct face. I was able to dislodge 3 of the first 10 “epoxied” dowels I tested with two bare hands. I knew to test them because upon arrival to the site I bypassed the superintendent and went straight to the labor and asked how they installed all those dowels. This is a favorite tactic of mine, “How’d you do it?” I discovered that there was no on-site oil-free compressed air, no hole cleaning brush, and no correct size drill bit. OK, so I didn’t make it on that contractor’s Christmas card list.
I have a question about accountability in these such matters...
(although I think I know the answer) What degrees of responsibility should fall on the engineers/designers/planners versus on the construction companies/crews when the construction is not done according to design/specification?
Or, how much should the engineering side of it be held responsible for things, under the presumption that they should 'know' that the labor won't do what is specified (whether it's due to ignorance or complacency, etc.)?
In other words, if and when the engineering should have 'known better'?
(although I think I know the answer) What degrees of responsibility should fall on the engineers/designers/planners versus on the construction companies/crews when the construction is not done according to design/specification?
Or, how much should the engineering side of it be held responsible for things, under the presumption that they should 'know' that the labor won't do what is specified (whether it's due to ignorance or complacency, etc.)?
In other words, if and when the engineering should have 'known better'?
Boffintech...Let me know if you're ever looking for a job in NE or Central Florida!
You're right. Most of the time they don't install epoxy dowels or anchor bolts correctly. For repair work, I specify independent inspection.
Luftweg...we can't just rotely assume incompetence. There are those conscientious contractors and labor who strive to do the right thing. Further, it isn't always a commission error...often it is just ignorance.
Our obligation is to specify clearly and set the parameters. Their obligation is to do it right within those parameters. To quote one of my favorite Presidents (Ronald Reagan)....Trust, but verify!
You're right. Most of the time they don't install epoxy dowels or anchor bolts correctly. For repair work, I specify independent inspection.
Luftweg...we can't just rotely assume incompetence. There are those conscientious contractors and labor who strive to do the right thing. Further, it isn't always a commission error...often it is just ignorance.
Our obligation is to specify clearly and set the parameters. Their obligation is to do it right within those parameters. To quote one of my favorite Presidents (Ronald Reagan)....Trust, but verify!
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