Rebar doweling epoxy

[Sparky4598]

I'm looking for a good strong epoxy for doweling #5 rebar into 3000psi concrete that is 8" thick. Some Google searches later I came across Hilti Hit-Re 500 which seems to be one of the strongest available and has very clear and easy to read data sheets to understand the strength but it is stupid expensive.

Then I came across RedHead T7+ which is about half the cost but I am having a lot of trouble understanding the code approval data sheet that contains the strength data. It doesn't list strength in a table, but seems to give equations to determine the developed strength, but I am just not following how to solve them.

Can anyone help me understand the RedHead datasheet better and/or suggest an alternative epoxy for doweling rebar that is comparable in price or cheaper than the RedHead T7+ but still has as good or better strength characteristics?

I'm not really expecting to be able to achieve full tensile strength of the #5 Grade60 rebar with only 6"-7" embedment in 3000psi concrete, but the more the better of course. 75% strength would be desirable as the rebar is a bit oversized for this application and we are just using it because we had extra.

 

[ANE91]

The equations for every conceivable limit state come from ACI 318 (used to be Appendix D; now in Chapter 17), and the flowcharts are on page 10 are pretty straightforward. You’ll be using Table 8 for bond strength, but you’ll also need to check concrete breakout strength (Table 6) and steel design strength (Table 5). Generally, the failure can happen in the concrete, in the epoxy (adhesive or cohesive), or in the steel — one of those will be the weakest link. At some point, it doesn’t matter how strong the epoxy is if the concrete is going to fail first, anyway.

It’s plug’n’chug. I can tell you for a fact that the differentiating factor for every epoxy-anchor manufacturer is the leeway provided in installation. There’s no benefit for Hilti to make an epoxy with 100ksi characteristic bond strength, for example, if 99.99% of it is lost due to an unclean hole. Their test data provides this information. A good engineered design accounts for all the relevant factors (i.e., substrate temperature, moisture conditions, hole cleanliness, overhead or not, sustained loads or not, etc.).

Which equation are you specifically struggling with? Bond strength is probably something like (Phi.ws*K.ws) * (Lambda.a*Tau.cr*Pi*d.a*h.ef) modified by geometry and edge effects. Almost all of these terms come from the tables in the ESR.

I ran these calcs every day for years. Unfortunately, the manufacturers have successfully lobbied to make this as complicated as possible, to their benefit. It’s not a simple table lookup. Google “Big Dig Tragedy.”

 

[ANE91]

I’ll also add that it’s probably half the cost for a reason. Testing is expensive. ESRs will never be 1–to-1; it’s impossible.

 

[Sparky4598]

The equations for every conceivable limit state come from ACI 318 (used to be Appendix D; now in Chapter 17), and the flowcharts are on page 10 are pretty straightforward. You’ll be using Table 8 for bond strength, but you’ll also need to check concrete breakout strength (Table 6) and steel design strength (Table 5). Generally, the failure can happen in the concrete, in the epoxy (adhesive or cohesive), or in the steel — one of those will be the weakest link. At some point, it doesn’t matter how strong the epoxy is if the concrete is going to fail first, anyway.

It’s plug’n’chug. I can tell you for a fact that the differentiating factor for every epoxy-anchor manufacturer is the leeway provided in installation. There’s no benefit for Hilti to make an epoxy with 100ksi characteristic bond strength, for example, if 99.99% of it is lost due to an unclean hole. Their test data provides this information. A good engineered design accounts for all the relevant factors (i.e., substrate temperature, moisture conditions, hole cleanliness, overhead or not, sustained loads or not, etc.).

Which equation are you specifically struggling with? Bond strength is probably something like (Phi.ws*K.ws) * (Lambda.a*Tau.cr*Pi*d.a*h.ef) modified by geometry and edge effects. Almost all of these terms come from the tables in the ESR.

I ran these calcs every day for years. Unfortunately, the manufacturers have successfully lobbied to make this as complicated as possible, to their benefit. It’s not a simple table lookup. Google “Big Dig Tragedy.”

Thank you very much for your reply!

"Which equation are you specifically struggling with? Bond strength is probably something like (Phi.ws*K.ws) * (Lambda.a*Tau.cr*Pi*d.a*h.ef) modified by geometry and edge effects. Almost all of these terms come from the tables in the ESR."

It's more so where the variables come from for the equation. I found the ESR for the Hilti epoxy and I was just using the equation (characteristic bond strength * Phi.d * Pi * d.a * h.ef) as a comparison between the two, but it would be nice to understand how to get the same values Hilti got in the tech spec sheet and use the whole equation correctly. I suppose that's where your k.ws and lambda.a and Tau.cr terms come in.

"I can tell you for a fact that the differentiating factor for every epoxy-anchor manufacturer is the leeway provided in installation."

So you are saying the Hilti may be a little stronger than the red head, but it's not enough to justify double the cost. That makes sense to me because the values for ideal dry conditions and uncracked concrete are quite a bit different, but the values for cracked water saturated concrete are very comparable with Hilti only being slightly stronger.

 

[ANE91]

I haven’t done the comparison in a long time, but you’ll find that published values based on their own internal testing are generally higher relative to those derived from Acceptance Criteria. That’s the whole idea behind AC 355.2/355.4 — it’s a “level playing field.” Believe it or not, not all jurisdictions/applications require ESRs. Consider certain remote industrial facilities, for example.