Concrete Adhesive Anchoring 3

[CTSeng]

For anchoring rebar or threaded rod to concrete most manufacturers offer both a 1:1 two part epoxy and ~10:1 acrylic epoxy. Both offering relatively similar capacities.

I'm interested in what others feel is the better product or for what situations one may be better over the other.

 

[Ron]

Both are good. Epoxy has longer history of performance. Both have similar failure modes and strength can be comparable for both, though epoxies are bit stronger for a given size. Mixing is critical for acrylic. Minor variation in mixing makes for large strength/setting time swings.

 

[Lutfi]

If you use adhesive type anchors, make sure that the contractor is required to follow manufacturer recommendations and perform hole inspection. The code only allows one half the allowable anchor value if hole is not inspected!

I like these anchors for columns anymore. I do not have to work with having to burn base plates because the "J" anchors were misplaced or shifted during concrete placement. They can use the base plate as template to drill and place the anchor bolts.

 

[JAE]

Ron - Somewhere I heard something about epoxy not doing real well with temperature swings up around 110 degrees - that the epoxy would almost begin to liquify.... you know anything about this? I've always wondered.

 

[SlideRuleEra]

Hilti, and their competitors, offer dual chamber glass capsules that contain a "vinyl urethane methacrylate" resin. The Hilti brand is an HVU capsule (part of their HVA adhesive anchor system). They come in various sizes for different size threaded rods / rebar.

The quantity of each component is premeasured and the glass capsule breaks (chemicals are mixed) at the precise moment that installation of the rebar / threaded rod begins. I have used this type product (successfully) for over 15 years for many heavy industrial applications, such as both building & machinery anchor bolts, rebar dowels, and even overhead anchors for supporting piping & electrical conduit.

The bulk versions (concentric & side-by-side tube applicators) work just as well but are more dependent on the skill of the installer to apply the proper quantity AND do so in a timely manner.

Hilti has extensive test data on glass capsule adhesive anchor performance. Since each capsule is essentially identical, the data should be reliable.

As stated the adhesives are affected by temperature. Hilti shows bond strength dropping linearly from 100% at 70 deg F. to 63% at 212 deg F.

 

[Ron]

JAE...both epoxies and acrylics are thermo-elastic materials, so temperatures in those ranges will cause deformation/creep to increase. Epoxies are perhaps a bit more sensitive after curing, but during curing it only causes the reaction to occur faster. Epoxy has a much greater exothermic range during curing.

 

[Corn]

Just an observation about expoxies. The glass temperature or the temperature that it will creep/flow/wilt/etc. is generally about 50F above the cure temperature. That is why generally, you see aerospace products oven cured and/or autoclaved. I would not reccommend any earth bound epoxy bonded joint for long term stress like a tensile structure anchor. The strength in hot weather would decrease if elevated temperature curing did not take place. This curing cycle takes many hours if not days and would be difficult, if not impossible for an object in the ground.

 

[Ron]

Corn...they're used every day. Each of the major anchorage manufacturers has an epoxy application and they perform well in most applications. I would not use them (and manufacturers also warn against this) for hanging sprinkler pipes or other applications for fire service, but for general structural anchorage, they work well.

 

[samdamon]

Gents-

I wouldn't use a polyester- or acrylic-based adhesive to carry any significant, permanent loads. These materials can cure quick and can have published load capacities comparable to epoxies, but I have heard that they are also somewhat more susceptible to poor performance resulting from jobsite errors, and also that the capacity of the adhesive can degrade significantly over time.

Epoxies also require care when placing, but they've been around longer and I believe there is a fair amount of data out there showing that epoxies keep their strength over time (Unless as noted by others they are heated).

I don't have a lot of experience yet with the new Hilti methacylate adhesives but did use it recently for an anchor bolt fix. Did not see it installed so can't comment further.

 

[sdz]

Different adhesives behave differently depending on site conditions. Some adhesives shouldn't be used in damp or diamond cored holes. Choose one that can be used in both these situations and that's less that can go wrong on site.

The injection systems are more flexible than capsual systems in that they can be used for different hole sizes and depths. I don't like glass capsuals because they are too easy to break before you get them in the hole!

 

[TFL]

Lufti,

What code and where only allows half the capacity for non-inspected holes?

thanks

 

[MBeckmann]

I am an epoxy chemist with over 20 years experience. Our company also makes polyester adhesives, so I know both technologies quite well. Epoxies do indeed have much better longevity than polyesters and acrylics, primarily due to the fact that their shrinkage rates are just a small fraction of those experienced with polyesters and acrylics. Due to their chemistry, polyesters and acrylics shrink a lot during cure (about ten times the amount of epoxy shrinkage); furthermore, this shrinkage continues virtually forever over the years. This does not happen with epoxy. Therefore, assume you have a joint with 4000psi strength today--if it's an epoxy, you can still expect a strength of 3000psi many years later; however, if it's a polyester or acrylic, you'll be lucky to have a joint strength of even 1000psi. Hope this helps clear things up a bit...

 

[pmkPE]

MBeckmann,

Do you know if the rated capacities of the epoxies are rated for immediate use such as the 4000 psi number you quoted or do they consider future devaluation and rate per the 3000 psi value considering the effects of time?

 

[MBeckmann]

pmkPE,
I'm not an engineer, but as I understand it when a joint is specified the engineer typically applies a large "safety factor" to the design--in other words, if a joint strength of, say, 1000psi is required, the spec may call for a minimum of 3000psi, yielding a "safety factor" of 3:1. The adhesive thus needs to be chosen that will maintain that 3000psi strength throughout the intended lifespan of the joint. In the example I cited above, and epoxy meets the requirements; the polyester or acrylic would not.

 

[JAE]

MBeckmann - the adhesive used in Hilti's products are vinyl urethane methacrylate....do you know if this has the same properties as "acrylic" per your comment above? I think I would forward your post above to Hilti and see what their engineers respond with.

 

[samdamon]

Jae-

Please don't quote me but my understanding is that the methacrylates marketed by Hilti (Such as HIT 150) are a second generation type of acrylic that is different from the vinyl esters and polyester adhesives available in the '80s and 90's. (SIKA also markets a modern methacrylate adhesive.)

The new methacrylate compounds are supposedly more more resilient. I'm not aware of any data on how well they keep their capacity over a long time though.

 

[JAE]

SamDamon - that's essentially my question to MBeckmann in that he implies that acrylics shrink over time and I was wondering if the methacrylates are somehow different.

 

[MBeckmann]

My comments apply to ALL "addition"-type polymers, a group which includes acrylates, vinyls, polyesters, methacrylates, and vinyl esters. Modification of these polymers (with materials such as urethanes, to produce 2nd generation products as mentioned above) can indeed dramatically influence properties, such as shrinkage. I do not have sufficient experience with these 2nd generation materials to predict their ultimate performance over time, but I would suspect they still exhibit much higher shrinkage and consequent loss of strength over time than would a high-quality epoxy. I think if you pose these questions to Hilti and Sika that they would agree.

 

[samdamon]

Beckmann- Thanks for your comments. This is news to me.

 

[Lutfi]

tfl,

That was the Standard Building Code (1999 edition, at least the one I had). We used SBC in Florida before we came up with Florida Building Code.

The rational behind that is that if the hole is not inspected (i.e. done prepared properly in accordance with the manufacture’s recommendations) the published value may not be achieved.

Regards,