Post Installed Anchors in Green Concrete
Post Installed Anchors in Green Concrete
(OP)
I could probably find this information in a Hilti manual or by calling a rep but I'm going to start here. I guess I just have a fundamental mistrust of sales literature / people.
Let's say that I've designed a post installed anchor and selected it from a manufacturer's catalogue based on a 30 MPa concrete strength. If the anchors get installed into 20 MPa concrete that hasn't yet reached it's 28 day strength, what does that mean for the anchor capacity in the completed structure? Am I limited to capacities based on 20 MPa? Or will the capacity of the anchors eventually reach that consistent with the 30 MPa assumption? This assumes that the concrete eventually gets there of course.
I'd like comments pertaining to both expansion anchors and drill and epoxy anchors. My intuition is that a drill & epoxy anchor will eventually make it's way to the 30 MPa capacity while an expansion anchor may not.
Let's say that I've designed a post installed anchor and selected it from a manufacturer's catalogue based on a 30 MPa concrete strength. If the anchors get installed into 20 MPa concrete that hasn't yet reached it's 28 day strength, what does that mean for the anchor capacity in the completed structure? Am I limited to capacities based on 20 MPa? Or will the capacity of the anchors eventually reach that consistent with the 30 MPa assumption? This assumes that the concrete eventually gets there of course.
I'd like comments pertaining to both expansion anchors and drill and epoxy anchors. My intuition is that a drill & epoxy anchor will eventually make it's way to the 30 MPa capacity while an expansion anchor may not.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.






RE: Post Installed Anchors in Green Concrete
How does it work for 20, if calcs need 30?
to me its no different than a cracking calc. At 20 mpa it might crack, where at 30 it would be fine.
If you need 30, wait for 30.
RE: Post Installed Anchors in Green Concrete
RE: Post Installed Anchors in Green Concrete
I would expect an epoxy anchor to fall short of the expected strength. I've heard that ACI 318-11 required concrete to have cured a minimum of 21 days prior to installing an epoxy anchor. As I understand it, it's not related to the strength of the concrete, but rather the higher levels of moisture and how this affects the bond strength.
Whether you believe what they have to say or not, I`m curious what Hilti would say. I would expect to hear, "We haven't tested that" but you never know. You can also ask to speak with one of their engineers rather than the sales staff that typically answer the phone.
RE: Post Installed Anchors in Green Concrete
My thinking with the expansion anchor is that as the expansion occurs, the surrounding concrete might crack locally and otherwise give way a bit to the expansion process. If more of that giving way would occur at 20 MPa than it would at 30 MPa, I would think that it would a) affect the final strength and b) not be recovered as the concrete matures.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Post Installed Anchors in Green Concrete
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Post Installed Anchors in Green Concrete
"The design performance of adhesive anchors cannot be ensured by establishing a minimum concrete compressive strength and the time of installation in early age concrete. Therefore, a minimum concrete age of 21 days at the time of adhesive anchor installation was adopted."
I understand your logic re: local cracking, but I`m not sure that I share your concerns.
If you're installing the anchor in properly cured, old concrete, I can imagine small/micro cracks in the concrete.
If you're installing the anchor in brand new concrete, I can imagine these cracks being slightly larger - but enough to impact the capacity?
In the 30MPa concrete, what's the failure mechanism of the expansion anchor?
RE: Post Installed Anchors in Green Concrete
The application is direct tension with lots of edge distance. I assume that means some manner of concrete breakout failure.
As a mental experiment, consider casting an already expanded expansion anchor into wet concrete. When the concrete dries around the anchor, will the anchor possess it's rated capacity? Certainly, the contact stress between the anchor and the surrounding concrete would be less. I feel that the 20 MPa case must lie someplace along the non-linear continuum between fresh concrete and 30 MPa.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Post Installed Anchors in Green Concrete
I'll be very keen to learn differently if anyone has sources!
RE: Post Installed Anchors in Green Concrete
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Post Installed Anchors in Green Concrete
RE: Post Installed Anchors in Green Concrete
1) Steel failure
2) Pullout
3) Concrete failure.
I have a hard time believing that steel failure (of course) or pullout would be affected by the strength of the concrete at the time the anchor is set.
If one of these controls your design - maybe you have nothing to worry about.
If your failure mode is the concrete cone/breakout - we're back to square one with the above discussion.
RE: Post Installed Anchors in Green Concrete
RE: Post Installed Anchors in Green Concrete
I`m picturing this as being an occasional tension load, created by wind or seismic such that there wouldn't be long term tensile loading subject to creep. If this is the case, I`m still leaning towards being able to use the 30 MPa.
If the tensile load is constant from the date of installation, maybe the creep plays a more significant role.
Has there been a call to the manufacturer?
RE: Post Installed Anchors in Green Concrete
@Once: I expect that when the load is applied would make a difference. I'm hanging some equipment with these anchors. The equipment won't show up until well after the 28 day strength is reached.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Post Installed Anchors in Green Concrete
If the maximum stress is immediate, you're going to have more trouble as the majority of concerns should revolve stressing prior to design strength. Where the higher end and peak stresses occur after load is imparted onto the concrete fastener, you're far more likely to be achieving full strengths despite the early install.
I would love to have a simple chart of anchor type and stress effects... Maybe I've just thought of a little side project to create a FAQ.
RE: Post Installed Anchors in Green Concrete
For mechanical expansion anchors, installation should not take place until the concrete has aged at least seven days. If you let the concrete mature to its design strength before loading it, you get the full capacity (30 MPa). If you load the anchor before the concrete reaches design strength, you're stuck with whatever capacity would be associated with the strength of the concrete when the anchors were installed (20 MPa). Hilti feels that this last statement is probably conservative which is fine by me.
Apparently the rules for adhesive anchors are identical to those for mechanical anchors. The Hilti representative that I spoke to wasn't familiar with the 21 day requirement mentioned above (ACI 318-11 RD2.2). Hilti feels that adhesive anchors can be installed at seven days without reduction. That being said, they have wet install factors that do need to be accounted for.
Power-actuated fasteners, which we have not yet considered here, are different. You can install them after seven days but you are permanently stuck with the capacity associated with the concrete strength achieved at the time of install (20 MPa). It seems that power-actuated fasteners achieve a sort of fusion weld to the concrete just like they do with steel. The quality of that fusion weld is a function of the concrete strength at the time of install of course.
I invited my new Hilti friend to join our discussion here, at Eng-Tips. If he turns up, we can pepper him with more arcane questions then. It sounds as though Hilti has tested most of this stuff already which is reassuring.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Post Installed Anchors in Green Concrete
Is to say that you get whatever strength the concrete has achieved without a further reduction, correct? So in this case (for our example values above) we would have strength based on 20MPa concrete?
RE: Post Installed Anchors in Green Concrete
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.