Concrete finishing 4

[Structuralconcrete]

Hello all,
Not sure how many of you are familiar with finishing concrete slabs but I have a dilemma that has yet to be solved.
We have poured several slabs in this project and unfortunately they have not gone as planned. The first interior slab we poured was a metal deck and we had small silver dollar sized areas of delamination.
Keep in mind we have poured millions of square feet of concrete slabs and only recently started running into this issue. The first thought was maybe the mix design has some issues. The design was tweaked for the second pour and that one went perfect but apart from the second pour all other slabs have had these small spots of delamination (2-3 spots).
Its been a a couple months of pouring and after changes to the mix these random spots continue to appear. What could be causing this to come about so sporadically?

 

[hokie66]

Popouts as you described are caused by expansion of aggregate. Some of your aggregate probably contains chert or some other expandible material. Alkali silica reaction is also suspect. You would need petrographic analysis to be sure of the cause.

https://cti-ia.net/what-are-aggregate-popouts-how-can-i-avoid-them/

 

[dik]

In this locale, chert is a common cause.

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

-Dik

 

[Structuralconcrete]

That is interesting. So the pop outs I'm facing typically are visible within the next couple days after placement and only 1/8" thick of peeling is occurring

 

[Enable]

Thanks hokie / dik. Not something I have come across previously. Very neat.

Not to hijjack the thread, but dik you say it's common in the western provinces? Does CSA A23 not limit the amount of said aggregate in the mix or is it left up to the supplier as long as they meet the other prescriptive requirements? If allowed by A23 how should one specify the mix such that issues dont arise with architectural concrete?

 

[Structuralconcrete]

@hokie66
Do you have any previous experience with manufactured sands in a mix design?

 

[dik]

Common to the provinces may be a stretch... common to locales, maybe. There may be a source of aggregate 100 miles away with no alkali reactive aggregate.

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

-Dik

 

[Structuralconcrete]

@dik
I did a small amount of reading on chert. If it were chert there would be a larger piece of pop out? Am I understanding correctly? When chert occurs the surface along with the aggregate end up popping out, at least that is what I gathered.

 

[Ron]

Agree with hokie66's assessment. Alkali-silica reactivity or alkali-carbonate reactivity usually cause this. Lift up one of the delaminations and it will be a slightly conical shape with a soft white or yellowish/greenish powdery or gooey substance at the apex. Powdery if now very dry, gooey if the concrete is exposed to moisture.

The usual culprit is small pieces of chalcedony chert in the coarse aggregate. As hoki66 notes, a petrographic examination is necessary.

In its most subtle form, it is a surface aesthetic nuisance. In its most aggressive form it can be destructive to the entire cross-section of the concrete.

 

[Ron]

I replied to the thread without reloading the current page. One question....is there a piece of aggregate at the center of the anomaly? You mentioned "peeling". If there is no coarse aggregate at the center of the popout, then you might have blistering, a finish issue. Blistering is usually caused by trapped bleed water and overtroweling, but it is also usually not consistently small and round. The shapes can be random and they can be fairly large.

Can you post photos?

 

[Structuralconcrete]

 

[hokie66]

No reference for size, but those look larger than a silver dollar. As per Ron's last post, I now think your problem is finishing gone awry. Back to school for those finishers.

 

[dik]

Definitely not alkali aggregate reaction... much more like blistering...

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

-Dik

 

[Ron]

Yep...blistering and scaling. This is caused by poor finishing techniques. Bleed water got trapped under the surface. Hard troweled too quickly.

 

[dik]

I don't think I've seen better photographic examples...

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

-Dik

 

[Enable]

150% that is a finish problem. Either on too early as Ron suggested or too much water at the time of finish or if it is air entrained they densified the surface too much with the trowel. What is the mix? Are you using a super-p admixture? If not, the finishers have a tendency to throw water on the surface like they are blessing it.

I'd chain drag the entire surface. While those may be the apparent scaling areas at present, my money is on the problem being much more systemic. The consequences of which might only be seen in service.

 

[Ron]

@dik....I agree. Those are excellent examples of the issue.
@structuralconcrete.....would you be willing to share the originals of those photos? I am starting editing of the next edition of my textbook and there might be a place for those in the book. You would be credited in the caption.

 

[Structuralconcrete]

I've done the chain drag test and it is isolated to just those areas. What confuses me is that its so isolated. These pictures are as bad as it gets the majority of these areas are reduced down to silver dollar size though.
If it is indeed a finish issue, why is it just isolated to like 2-3 locations?

 

[Ron]

@structuralconcrete....that's not unusual. Water distribution varies within a concrete mix as with all the other constituents. If the concrete is not adequately mixed, you get variations in air voids, water gain voids, aggregate distribution and random locations of anomalies develop as a result. This can result from insufficient drum rotations or dirty/impacted or worn mixer blades.

It can also result from the finishers throwing water on the surface during finishing to re-wet the surface or throwing dry cement onto the surface to dry up spots of heavy bleed water.

For these reasons, it is good to do an audit of a ready-mix supplier if you are placing a large area of concrete. The National Ready Mix Concrete Association has a good checklist for doing a plant audit. I've done many over the years. One particular outcome of a plant audit is that they will know you are serious about concrete quality so they tend to pay more attention to your deliveries. Further, the concrete mix must be properly paired with the application and the placement methods to be used. For instance, for slabs on grade, use the largest coarse aggregate that can reasonably be placed for the application. This will keep the cement content lower, the water content lower and will reduce shrinkage.

Concrete is not as simple as the concrete subs would like to think. Remember....good concrete is made with quality aggregates, good portland cement and clean water. Bad concrete is made with the same ingredients.

 

[Structuralconcrete]

@Ron
Your feedback has been very insightful. One comment I do have is that during the finish process the finishers have not done any of the above. They did not use any water to wet up dry spots or use powder to dry up wet spots. I find it hard to believe that it was indeed a finish issue although I do not rule it out.
So I've mentioned several times that I am fairly new to concrete, I have learned a lot over the last 6 months. One thing I notice during the placement of the slabs where this "peeling" has occurred is that the mix loses 1.5-2" of slump from the truck to the end of the hose. Could this be the cause of dirty materials used for the mix?

Also, what are the subjects you are covering in your book?