Roughened finish (shear friction) 1

[JohnRwals]

Hello!

To achieve monolithic concrete or composite action, generally “surface intentionally
roughened to an amplitude of 0.25 inches” is applied.
How can I make this surface preparation over green concrete in the precast concrete plant?
As a rake/rough broom has been used, it makes loose concrete paste a lot.
Any reasonable solution to recommend?

Thanks for your time!

-JRW

 

[jayrod12]

We have worked with our pre-caster's locally to determine that their shop broom finish generally appears to meet the requirements for bonding of the concrete topping. This is generally confirmed via pull testing of the in place topping during construction.

 

[KootK]

When I dug deep on this topic a while back, I found evidence to suggest that no deliberate roughening of the plank is required in many cases. Generally, what is known as "machine cast" finish will suffice. So the booming stuff is kind of a waste of effort in many respects.

 

[jayrod12]

Interesting take Koot. I'll admit I wasn't part of the discussions or agreements between the precasters and our office. I tend to just care about the pull test results.

 

[KootK]

Some recent articles on the topic.

Link

Link

Very hard working plank will, theoretically, tend to fail a nominal shear bond near the plank end(s). Even that's a bit suspect since it's intimately related to assumptions about what the effective width of the plank is for concentrated loads etc as one nears the support.

@jayrod12: how do you call out the pull off testing requirement? That's not commonly done in my experience but seems like a fine idea.

 

[jayrod12]

We have this included in our general notes. And often it finds it's way into the book specification as well.

 

[dold]

The investigators of the Florida (FIU) Pedestrian Bridge failure did quite a bit of testing and experimenting on roughened joints and determined that there was a significant difference in capacity between roughened vs non-roughened (or insufficiently roughened). I believe they determined that this was one of the primary causes of the failure (aside from design flaws).

Some links:

Fascinating laboratory testing of the roughened vs non-roughened interfaces. Section 4.2.3 discusses the difference in strength <<< my favorite

Eng-tips disaster thread - also see the previous 14 threads with loads of discussion

Another Eng-tips thread - scroll down about half way for some diagrams

NTSB report - discussion of shear friction throughout the doc

NTSB digest

NTSB investigation docket - most/all documents involved with the investigation

 

[dik]

...and don't wet the concrete surface prior to applying the topping.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[JohnRwals]

Thank you very much for many responses!

I would like to add one more condition...Self-consolidating concrete
Are these opinions applicable to self-consolidating concrete?
I think this is huge gray area, which will bring a lot of litigations
as SCC is getting more popular.
I remember reading a dissertation saying surface scratch does not influence composite action much
in the conventional concrete, also.

-JRW

 

[jayrod12]

I'm of the opinion that regardless of SCC or not, there is a way to finish or modify concrete such that it will accept a bonded topping. Surface scratch is insufficient in my opinion as well. There should be an intentional roughening and ensuring that the surface is clean and able to accept/absorb some of the paste of the topping.

 

[bugbus]

Common practice in Australia is to apply a surface retardant on the precast element once cast, and come back later to spray away the cement and fines with a hose to achieve the 3 mm roughness. Sometimes a transverse broom finish on the tops of girders is enough.

 

[wrantler]

Precasters here do what @bugbus states. They can apply it to the forms also if needed. it is then blasted with highpressure water after. They say it is very easy to do...I am of the opinion that the roughened surface is key.

I have also seen sandblasting.

Contractors use the retarder method to do your fancy exposed aggregate driveways. When I worked in the concrete industry we had a special mix for exposed aggregate driveways/sidewalks.

 

[Brad805]

Some of this depends on the product. If it is an extruded product they have fewer options to do much in the shop because of the mix design. We do a lot of manually finished products and if there is a topping they use brooms they made to create a very rough finish. I think the bigger problem is what is done on site before casting the topping. We were at one project where another trade had cast the topping and they did nothing before casting. It had rained before I was there and I could get the entire topping slab to move up and down due to the lack of bond and water entering the space between the two slabs. Clearly the bond was zero in many areas in that case.

 

[JohnRwals]

This detail came from OK DOT.
They want to make a special trowel to make these shapes.
Many details including ACI refer to the geometry like its amplitude, size and interval.
But, I think more critical point is to make aggregate exposed.
Let's assume that concrete top surface is just cement paste as aggregates settle to the bottom.
And then, roughened surface will mainly depend on chemical bonding with no aggregate interlocking,
which will expect little bonding strength.
Also, I wonder why DOT expects horizontal shear strength with roughened surface rather than dowel bars.
I think they can use U bars protruding above interface, which will be more positive, better and stronger
for interface shear strength.

-JRW

 

[JLNJ]

Like KootK, I was convinced a long time ago that the "as-cast" condition is sufficient to make the topping composite with the plank. No bonding agent necessary.

The other thing I learned way back then was to specify exactly what you want in the bid docs so that everyone's expectations are aligned.