Smooth finish on concrete surfaces 1

[WARose]

Something occurred to me the other day (not for the first time): how do cast in place reinforced concrete members like beams & columns (whose surfaces are covered by forms) get that surface that doesn't show any aggregate? With a surface not covered by forms (like mats) you can get that look with trowel work.....but I've seen some really good finishes on non-exposed surfaces.

 

[Enable]

Theoretically proper vibratory efforts combined with fresh formply will do the job. However, good luck with that. The only way to get a glass finish is A) new formwork, and B) use SCC.

If you want a finish with no bug holes (but not necessarily smooth) then you can remove vertical forms after the concrete has initially set and brush the side as they do with curbs.

 

[dik]

WARose summed it up... good forming and placing techniques... normally aggregate is not exposed on the surface; you can use retarders and wash if you want it exposed... you actually have to work for it.

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

-Dik

 

[KootK]

The mechanics of how aggregate mostly winds up not at the surface is an interesting question. I don't know the answer but I'll hazard a guess (or two).

Vibration should create a lateral shifting of the concrete and aggregate within the forms back and forth and up and down. When that happens, perhaps bits of aggregate resting against the forms temporarily move away from the forms. Maybe that creates a slight vacuum pressure between the aggregate and the forms that draws in cement paste.

It doesn't take a whole lot to turn a normal concrete suface into an exposed aggregate surface so the aggregate can't be too far from the formwork at set.

Another, maybe simpler theory: is the aggregate just really well coated with cement paste from rolling around in it and that's what we see at the surface?

 

[epoxybot]

Well, the inverse would be bug-holes & honeycombing, so maybe it all comes down to fineness modulus.

 

[hokie66]

When you see an off formed surface that looks completely smooth, it is generally because it has been rubbed. A bit of cement rubbed in.

 

[KootK]

Ahh... is that the sack rubbing that one sees in the specifications?

 

[Enable]

A burlap rub does not provide a smooth finish at all. It takes out minor imperfections (such as bug holes) and replaces it with a consistent(ish) swirl with raised cement.

 

[Ingenuity]

As stated by others, vibration, paste content, and form-face type all play a role. With respect to form ply, high density overlays to form ply give a smooth mirror-like finish, medium density overlays give a smooth but matte finish. I personally prefer the matte finish of MDO ply - and less expensive than HDO.

 

[WARose]

Thanks.

I'm seeing a lot of recommendations (on the web) for a product called Tamoseal.

I also saw this:

https://www.youtube.com/watch?v=FzHlwKCx1Qg

I have to admit.....I rarely cover surface finish in my general notes or specs.

 

[hokie66]

Yes, that video shows a rubbed finish. Smooth may be the wrong word, maybe uniform is better.

 

[Enable]

Lets be clear: there is a difference between a glass finish and a consistent finish

Glass Finish
Can only be achieved by new formwork (typically formply) and either a combination of good vibration (theoretically possible but unlikely to pan out in practice) or an SCC mix.

Consistent Finish
If you want something that is not glass, but is consistent though not an aggregate exposed look (i.e. really only bug holes are removed), then you apply a rub after the forms are stripped. Rubs are applied in any number of ways but are predominantly done by mixing a cement based slurry and rubbing onto the wall by hand with burlap or XPS insulation in a swirl pattern.

There are guys on form sites whose entire job it is to rub after the forms are stripped. It also hides issues one doesn't want the consultant to see (pro tip: if you are worried about an element in particular make sure you are there the day forms are stripped as otherwise it'll likely be hidden)

Products for a rub
Never used the Tamoseal and that does not look like the correct product. That is a repellent coating. You want a cement based polymer modified product.

Tamms thin patch from Euclid is typical.

I generally am not partial to rubs because they tend to be maintenance issues. The only rub that I have ever found to be durable over time in the Canadian climate is Uniplast by Durabond. It is a polymer based product mixed with latex as opposed to water. That shit wont come off...ever. Problem of course is getting it on things around the element you are rubbing (seriously wont come off with a 5000 PSI powerwasher after 1 hr...it's tough stuff).

 

[WARose]

Never used the Tamoseal and that does not look like the correct product. That is a repellent coating. You want a cement based polymer modified product.

Looks pretty good to me:

https://www.youtube.com/watch?v=WhuMm66CZZY

If it seals the surface and leaves a good finish....not sure what the downside is.

 

[canwesteng]

I've used sack rubbed finish for "architectural" finish on industrial sites. Always looked pretty good to me. Maybe an architect would want a smooth glass finish on a high-rise or something, but I don't see how you could tell the difference from any distance away.

 

[Ingenuity]

I never rub my form finishes, but I do occasionally use a carborundum stone at form lines, if warranted.

Here is a pair of pedestals I formed and placed last month - pedestal forms were 3/4" MDO (about 12 months old plyform that was left in the field), with many many reuses. Form oil was applied about 3 hours before the forms were set. Concrete was 4,000 psi, 3" slump. A 2-5/8" dia vibrator head was used. Due to overhead access restrictions, we had to wheelbarrow-and-shovel the concrete from the concrete transit mixer to forms.

The base plate grout bed above the pedestal was formed with (old and somewhat corrosion-pitted) mild steel angles and magnetic chamfer strip using field-batched SCC - 10 sack mix. No vibration.

Certainly no Class 1 finish (it is a quarry, and not architectural concrete), but with some experience, the right materials and equipment, it is pretty simple to achieve a consistent finish.

 

[Enable]

WARrose: I'm not saying it wont work. It's just not typical (in Ontario) and if you look at the video on Euclid's website as well as your video bug holes are still very present. Though I have no experience with the product so it might be possible to thicken it up so that's not an issue. Donnu.

Ingenuity: Stones work great. Those pedestals also look good. I imagine you cleaned your forms immediately after use or else reskinned? You can most assuredly reuse formwork (as you show here) and get a good, solid finish. But if the question is Glass I would say no. When people ask for smooth this is what I take it to mean. But as evidenced by this thread some people think smooth to mean rub so all depends on exactly what someone wants.

EDIT - There is also a question what element you are placing. The pedestals Ingenuity presents are small and you have ready access to the top / can hammer tap all the way around. Plus making sure a vibrator gets everywhere is not a problem. Try to pour a tall, long wall and get a glass finish with regular means. Possible I suppose but I've never seen it done.

 

[dik]

and careful with patching... it can make things look a lot worse. Generally to get a high quality finish, you are looking at precast...

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

-Dik

 

[hokie66]

If is is an architecturally exposed finish, the architect should be specifying and inspecting the work. What is acceptable to me is often not acceptable to an architect.

 

[JoelTXCive]

Don't the finishers spray the forms down with an oil or release agent of some sort right before they pour? Maybe that marginally helps pull paste to the surface?

 

[hokie66]

Form oil is used, but not "right before they pour". You don't want form oil on the reinforcement.