Shear-Friction and Intentionally Roughened Surface 2

[LPPE]

Citing ACI 318-02, Section 11.7.4, there is a coefficient of friction factor, with different values for hardened concrete, roughened or not.

Is it possible to intentionally roughen an existing hardened concrete surface? Suppose I have the smooth vertical face of an existing concrete wall or footing, to which I am placing fresh concrete against it. How does one roughen the existing smooth surface after the fact? Is it even possible without damaging the existing concrete?

 

[JAE]

I think you could take a small (15 lb) chipping hammer and create an intentionally roughened surface with 1/4" amplitude per ACI. The smaller the better. A larger hammer would possibly create internal fractures in the underlying concrete.

I think this would possbly be less effective than a roughened surface created with wet concrete, so I might be tempted to use a smaller coefficient - perhaps 0.8.

 

[InDepth]

Yes, completely possible. You can do it with chipping hammers. Typically you need to specify "1/4 inch amplitude roughness (down to competent concrete)." Contractors can achieve this, but it requires a lot of effort/time. Usually it is better to provide drill and expoxy dowels with appropriate embedment and lap splicing (would recommend this if in a seismic zone). I would be conservative and use the non-roughened, it will save cost and mistakes.

Other options include keys, coating with a latex bonding agent, etc...

If you provide the application, that would help whitle down a better answer.

References: You can take a look at the ACI Manual of Concrete Practice as well as the Concrete Repair Manual for more info.

 

[kslee1000]

Try contact contractors with experience on water/sand blasting. It might works.

 

[InDepth]

Sand blasting is OK, but in my experience never achieves the 1/4 amplitude with exposed aggregate. Water "erosion" is possible and can limit micro-cracking if its a concern....but if you are inside a building....where would the water go?

 

[kslee1000]

Dug a hole, buy a sump pump from HD.

 

[DTGT2002]

I had never really considered that section of code with respect to adding on to existing construction, but cannot see why it would not apply.

While it seems trivial, defining and acheiving the 1/4" amplitude is difficult at best. When you type 1/4" amplitude, what do you imagine? Neat rough peaks undulating at 1/2" on center in both directions or smooth even peaks and valleys aligned as rows across the surface?

For our purposes, we have make reference to the various ICRI rubber mould profiles available as a reference point and had mockup areas produced to define an acceptable amplitude.

To acheive a clean roughened surface without obvious fractures is hard to do with a chipping hammer. Scoring the concrete or cutting grooves is attainable with the hammer, but for a finer surface without obvious cracking, a hand held needle scaler is optimal, with substantial man hours and skill needed to avoid over working the surace but still provide a consistent rough finish - if consistent and rough can be used together.

That said, with green concrete, a reasonable surface can be produced by high pressure water jetting from a 5000 psi pressure washer - without the expense of the specialized 10,000 psi jets and cutters. But that is for green concrete.

Depending on your constraints, I would hope you can acheive your shear strength through rebar. Failing that, find a contractor aware or the requirement and able to acheive the desired finish and able to provide past examples of projects which the planned technique was used. I would expect the use of chipping hammers and needle scaling to arrive at the desired finish.

Someone who does not provide a detailed means of how they will do the work likely will not know how to acheive the needed finsh and will instead provide a plane with substantial microcracking and delaminating layers, though you may never have the opportunity to see it.


Assuming rebar will be drilled and expoxied into the existing concrete, is the capacity of the current rebar fully utilized at the interface and the 1.0 factor is required to satisfy the design or could a nominal amount of rebar be added to accept the 0.6 factor?


ICRI Technical Guideline 3732 can be a valuable resource for understanding the anticipated means of acheiving various surface finishes. It specifically speaks to Selecting and Specifying Concrete Surface Preparation for Sealers, Coating, and Polymer Overlays. However, the concepts of amplitude and anticipated surface profiles are consisent with what is required of 318.

I picked it up as part of ACI's Concrete Repair Manual, but it can be aquired from ICRI directly.

Hope this wasn't too rambling.

Daniel

 

[JAE]

InDepth,
I guess I'm not seeing your point that drilled and epoxy dowels "are better". Better than what?

The whole point of the original question was for shear friction which MUST have drilled and epoxied rebar to even work.

So the question isn't roughened surface verses dowels. Shear friction would require them both. The question is - can you achieve the roughened surface and still rely on the [μ] factor = 1.0 for 1/4" roughened surface or something less?

 

[WpgKarl]

Here is an article about composite action of topping and precast members - without intentional roughening....

Interesting tests

 

[hokie66]

That is a good reference, WpgKarl, and I think it reflects current practice for horizontal shear for a topping slab composite action. But the OP has asked a different question, one where the answer is more difficult. In new construction, the joints can be located where the shear is minimal, but when adding on to an existing structure, vertical shear is critical. If the loads are significant, I always insist on separate support rather than trying to "weld" to the existing concrete.

 

[LPPE]

Good points by all, and a great discussion here, IMO, that hasn't been discussed on this board before.

Of course, the actual situation I'm up against is never as simple as XYZ. But basically, I have about 60 linear feet (not all in a straight line) of existing concrete footings that are about 3 feet thick. High building loads, poor soil conditions. My new concrete foundations need to attach to the original footings. Due to the nature of the new structure, I have column axial loads, potential uplift, and moments in both axes. The new footing to existing footing "connection" is an important one.

I have designed the dowels using the 0.6 factor instead of the 1.0 factor, but hoping to still (easily) get the roughened surface, and have that as an additional "factor of safety" (or my own idiot factor, however you wish to call it!).

I think the bottom line is that, to answer the original question, is NO, it is not easy to "intentionally roughen" an existing vertical concrete surface to the requirements of Section 11.7.4. Maybe if you have a small area, like maybe attaching a grade beam to another perpendicular existing grade beam, you could have a laborer chip or roughen the surface by hand. But in my situation, there's no way there's time to chip away at 60 feet of 3 feet thick footings by hand, and it doesn't sound like there's any mechanical ways available without potentially damaging the existing footings.

 

[dgkhan]

Long back when I used to work on site, I saw contractor using steel wire brush mounted on an electric drill and to be honest with you, it worked very well. Just use very hard wire brush.

 

[kslee1000]

It is my opinion, hammer chipping (except small area done by hand tools as mentioned) is ineffective (labor intensive) and with the risk of damaging the existing concrete (causing visible, or internal structural cracks that are difficult to repair).

For your case, sand/water blasting after spotly light chipping shall achieve the extra safety you are looking for.
But for extreme important cases, you may add more safety by providing deepened shear pockets, done by hand chipping (no machines), along the length. Take time to cure the new concrete, shrinkage could be a killer.

 

[beton1]

There are attachments that can be used with light chipping hammers. Hilti for example has a 'bushhammer' or 'scabbler' attachment. We have used it for roughening surfaces prior to repair work. It may not be exactly a 1/4", but will provide a roughened surface and is easier than chipping, and won't cause microcracks. You can 'angle' the chipping hammer to gain some "non-uniformity" over the 1/4" required.