Bond strength between grout and smooth concrete

[cliff234]

What is an acceptable value to use for strength level bond stress between 4 ksi grout and smooth concrete?

I am designing floor framing comprised of hollowcore plank supported on steel. For reasons I will not get into, we cannot put longitudinal integrity steel in the plank joints at the beams (which is what we normally do). Instead, we are looking to detail 2’ long #4 bars in the plank cores at 24”. ACI 318 requires longitudinal ties be provided to develop 1.5k/ft (or 3k/2ft). The bars will extend 12” into each end of the opposing planks. The bond stress at the interface between the grout and the inside of the hollowcore cell will be about 25 psi (assuming 5” diameter cores and the fact that about 25% of the grout/plank interface will be missing because the top shell of the plank will be broken out to install the bars and grout).

I cannot find any information on the strength level bond stress between 4ksi grout and smooth concrete. What value can I use?

 

[KootK]

I do hollow core design and have not yet come across the information that you seek. What I can tell you is this:

1) I've stuck my hand into the cores of a lot of planks and found them to be a lot rougher than I'd expected.

2) Rightly or wrongly, it is ubiquitous within the precast industry to assume that the grout fill concrete is monolithic with the precast concrete.

What I've been doing is this:

3) Use a low shrinkage grout mix.

4) Assume that the smooth concrete values for shear friction investigations.

5) Be a little generous in the design.

If there's a real problem, I suspect that it's actually with the grouted cores creating a busting force that might pop neighboring, hollow cores.

 

[dik]

12" of bond for a #4 bar?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[CrabbyT]

I just pulled up some product data sheets (PDS) for various grouts by Five Star. All the instructions state that the concrete should be roughened prior to application. Could you acid etch the planks?

The PDS for Five Star High Performance Precision Non-Shrink Grout claims to have a bond strength of 2000 psi after 28 days.
[URL unfurl="true"]https://www.fivestarproducts.com/grout.html[/url]

The PDS for Five Star HP Epoxy Grout claims that the concrete should fail before the bond.
[URL unfurl="true"]https://www.fivestarproducts.com/products/epoxy-grouts/hp-epoxy-grout.html
[/url]

 

[dik]

As Koot notes, the inside of the extrusion 'cores' is slightly rough... I'd be more concerned about the acid etch. Have you talked to the precast supplier for any suggestions? I wouldn't think it would be a big problem if enough embedment... I think 12" is inadequate.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[cliff234]

KootK, CrabbyT and Dik - Thanks so much for the feedback.

Dik – Yes. 12” is not enough to develop the full strength of the #4 bar (10.8k), but it is enough to develop the 3k required by ACI 318. I'm a conservative engineer, and I always like to err on the conservative side; however the bar length is the length recommended by Girder-Slab. This is a hollow core floor supported on Girder-Slab D-Beams. The bars are threaded through openings in the web of the D-beam and serve to mobilize composite action between the plank and D-beam.

Also, I found my answer regarding bond between grout and plank. The second edition PCI Manual for the Design of Hollow Core Slabs (2nd edition) states on page 2-6 that an 80 psi strength level shear stress can be used between grout and the edges of the plank in longitudinal joints. While this is not a longitudinal joint, I expect that the surface roughness will be about the same.

Thanks again!

 

[dik]

Your bar size is too big, can you use 18GA wire... For ductility, I like to have the failure in the bar. Can you confirm the load with the HC supplier? It's his HC that will fail. Generally cores are a little rougher than the keyways... keyways can be 'glassy' smooth... it's part of the extrusion process. I'd consider using 18" min and not 12".

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[KootK]

While this is not a longitudinal joint, I expect that the surface roughness will be about the same.

Thanks for reporting back on that. I knew about the 80 psi but never thought to use it in this way. I guarantee you that the inside of the cores will be rougher than the keyways in most instances.