Surface irregularities in concrete wall 5

[concreteworld]

Hello All,

We are troubled with change in color of concrete in METRO tunnel wall. The wall is 30 m long and casted in multiple lifts 500-1000mm thick at one time. In this 6.5m meter wall, we see color layers have different colors, from light to dark grey. The concrete supplier says that it is due to varying color of flyash in the mix and the surface and color of formwork used.

what could be other possible reasons for such changes in color of concrete other than inconsistent W/C in different batches?

 

[TehMightyEngineer]

Different batches will always have small variations in darkness of the finished concrete. Though, these should be really small shade differences. w/c ratio could produce color variations but you could also have color variations with no change in w/c.

I'd agree with the contractor; my first thought would be different colors produced by the fly ash used in each batch. Are you seeing the color change between each lift in a pour, between cold joints, and/or between form joints?

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[concreteworld]

Please see the attached photo of the tunnel wall. Flyash color cannot be controlled as per the flyash supplier. What solutions we can implement to have uniform color of concrete walls?

 

[hokie66]

Looks fine to me. It is a concrete wall, not a piece of art. The colour will somewhat even out over time, but if you want it all the same colour, plan to paint it.

 

[TehMightyEngineer]

That looks very typical for the finish left by formwork having differential drying and surface conditions. As hokie said that should almost definitely even out over time. For future pours if they use oiled, clean steel formwork or otherwise cleaned and oiled up their current form system they might get more uniform results but I don't believe what they're currently using will be a problem.

Also, I will respectfully disagree with hokie on one point; don't paint concrete! (Pet peeve of mine) It's a durable surface finish and that requires little to no annual maintenance for appearance; why replace it with a surface finish that requires regular attention to keep looking nice? Stamped or form lined concrete is a much better way if you want to break things up visually.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[TehMightyEngineer]

If you absolutely must "repair" what you have I'd actually contact an architectural sand blasting company. The can apply a stencil over your concrete wall giving you pretty much whatever design you want. Coat the wall with a silane sealer and then you'll have the best looking wall in town.

100% overkill but it's your wall, not mine.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[dik]

Be careful how you paint it... I did a parkade about 40 years back with different concrete colours and the Architect wanted to paint it for appearance... after painting, it looked worse. Coatings are generally referred to as 'stains'.

We've used Nawkaw in Canada with excellent results; you might check to see if they have someone local. If not, there are likely many 'concrete staining' outfits that can do the work.

Dik

 

[sswan60]

> Looks fine to me. It is a concrete wall, not a piece of art. The colour will somewhat even out over time, but if you want it all the same colour, plan to paint it.

I agree, looks good to me. I'd also worry about the integrity of any skim coating fix you may try. Might end up with more problems down the line.

 

[concreteworld]

Please see the latest pictures of wall surface casted showing variable concrete shades. What I suspect is below,

a. Variable rates of formwork oil application
b. Formwork surface
c. Difference in flyash color
d. Variable batch to batch w/c

Kindly share your views.

Thanks in advance

 

[concreteworld]

.....second photo

 

[TehMightyEngineer]

Some visible pour lines, could be better but it happens. Looks like the pours mixed well but obviously can't fully tell from the picture.

The discoloration definitely looks again like differential drying against the forms, no change from the previous thoughts we had.

However, what caused the broken surface finish in those photos? Did they pull the forms too early and peel chunks out of the concrete? That's definitely not something I would allow to go unrepaired.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[epoxybot]

The photo back in May looks 200% better than the more recent, sadly. Is the contractor using Form Release or Form Oil? You might want to check their rate of application. Too much form oil will stain concrete at the surface and too much Form Release can cause water discoloration. Form Oil is cheap and dirty and some form release agents are very sensitive to coverage rates. They should not be using form oil for architectural concrete.

 

[TehMightyEngineer]

epoxybot has good advice on the form oil. We use it on our steel forms and occasionally we have had issues with discoloration. For large pieces we mist it on and then go over it by hand with a rag to get a uniform, thin coating. For smaller pieces or where appearance is critical we use a form release agent, more expensive but definitely has better results. For the release agent we use getting uniform coverage rates aren't nearly as critical as with oil in avoiding discoloration.

a. Variable rates of formwork oil application
b. Formwork surface
c. Difference in flyash color
d. Variable batch to batch w/c

Regarding the above;

(a) seems most likely to me
(b) also likely but I'd rule out oil first
(c) & (d) seems less likely to be the main cause but that one section with the pour lines definitely had an issue with the mix or had too much time between pours.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[concreteworld]

Good day!

Attached is another photo where one piece is totally light and uniform color than the rest of the wall !! The supplied concrete is same and all records are verified okay for w/c and flyash. Still this variation in shade is not understood. ANy ideas to share please

 

[concreteworld]

The wall surface is compared between two contractors A & B. The above issue of variations in color of concrete walls is with contractor A, whereas contractor B (different package in the same project) has been able to produce uniform colour/shade of the walls (please see attached pic). I could found out the below differences as of now with A & B,

Contractor A : a) Uses 5 different subcontractors for formwork & concreting operations b)formworks are not properly oiled by release agent. c) Flyash has slight color variations

Contractor B : a) Uses only one subcontractor for formwork & concreting operations b) workmanship is better in all forms c) Flyash has uniform color

 

[epoxybot]

concreteworld - I notice that the upper wall section & lower wall section in post 30 Dec 17 09:10 are two entirely different colors. Rather than variation "within" one production run. This looks like two separate pours at different times. This suggests that the cement used to make the concrete is coming from different geographic sources of manufacture or the mfg process of the cement has changed. Here in the US, concrete on the West Coast has a different hue of gray than in the Mid-west or on the East Coast. The concrete produced in Ohio has a warmer ruddier gray than the colder pinkish gray produced in California. Mostly the geologic sources where the raw materials are mined & used to produce the cement define, the overall color of ordinary portland cement.

Post 30 Dec 17 07:43 seem to indicate in rather remarkable contrast that something in the composition or preparation of the form work is responsible for the color variation. Literally, one looks like it has been prepared with form oil, maybe even diesel fuel & the other with a chemical release agent. If not that then you have to look at the form face, itself. Aluminum, steel, plywood (species)?
Depending on ground water elevations, I'd be more concerned about the two cold-pour lines and the steady delivery of concrete during production & having enough vibrators to join the lifts, if there is a delay. That is really some haphazard placement.

 

[concreteworld]

epoxybot - They are the same color. The upper part is under shade so looking slightly dark color. The point is look at the lower part of the wall which is very uniform in color. There are no variations in concrete color as seen in post 26 Dec 17 12:01.

How can we get uniform color of concrete wall? My corrective actions would be,
1. Clean adhering dust coating or laitance from the formwork
2. Uniform application of suitable form release oil
3. proper placement and good vibration

 

[concreteworld]

To add to above corrective actions;

4. Use of one coloured flyash in all pours

 

[epoxybot]

concreteworld - Correct me if I am wrong but in the photo from your post 26 Dec 17 12:01 - it looks as though there was eventually a lot of bleed water that flowed from the bottom of the forms. Could the contractor have done something as crazy as add water to concrete already in the form in an effort to mitigate a cold joint..., because concrete was delayed? Or did the bottom of the form start leaking after bleed water had risen to the top of the wall and then washed down the inside face of the form redistributing fines and mottling the surface. There are areas in the photo were the aggregate is showing, I had thought this was from mechanical abrading (chipping hammer) but if it is not and the concrete isn't popcorned but merely absent of cement cover, then bleed water exiting the bottom of the form is contributing to the lack of a uniform color by redistributing fines.

It is important to follow the Form Release manufacturers instruction. It has been a while since I have read up on them, so don't take my words as gold but some form release agents react chemically when the concrete is placed and too heavy an application is not good for the finish, other release agents, if I recall correctly rely on a weak hydrophobic action but again an uneven application can result in trapped water.

Are the forms faced with ordinary mdf plywood and if so, how old is it? Could it be swelling from water uptake & relaxing/distorting? Plywood that is past it good usage would also tend to make an even application of form release difficult to gauge since the open grain of the sheet will soak up the release agent and the denser grained areas will hold it at the surface.

 

[concreteworld]

Epoxybot - No, there was no addition of water in the forms or bleed water escaped. The slump was little higher around 240mm but the mix was cohesive with no segregation or bleeding. The area shown with exposed aggregate is due to chipping done for some repair and that is not so important. The problem is one contractor is able to give a great surface and another cannot.
Please see latest photo (1) that shows dark and light shades. Why is this happening? any clues please...

1. too much release agent
2. too less release agent
3. no release agent
4. mix gradation issue
5. high slumps or variable w/c
6. different form face
7. vibrator near to surface/ over vibration / under vibration
8. flyash
9. reaction of form release with concrete

what???