Fly Ash in Architecturally exposed concrete

[bhiggins]

Hey eng-tippers!

I have a client who was asking about the use of fly ash in his concrete floor slab. This house is very high end with exposed concrete floors so proper appearance is crucial. I have some old standard notes that state "Fly ash shall not be used in architecturally exposed concrete." I did some research to clarify this statement, but I could not find any definitive reasons not to use fly ash in the mix.

It seems to me that the use of fly ash produces beneficial properties across the board with very few drawbacks. According to literature and testing data I pulled from the internet, the use of fly ash may create a narrower "finishing window" for the concrete. Fly ash may create a "stickier" mix which may cause difficulty in finishing. I've also read that a stickier mix may increase the probability of the concrete blistering as it cures. It appears that all of these issues can be easily dealt with. I couldn't come up with any other reasons not to use fly ash in the mix.

Is there any sort of consensus about using fly ash where a high quality finish is desired?

 

[sandman21]

Fly ash even when single sourced has a tendency to have less uniform coloring, typically not desired for exposed conditions.

 

[oldestguy]

I'd flag this post and tell the operators to delete it. After that I'd re-post under the "Concrete Engineering discussion" room to avoid double posting.

 

[hokie66]

The attached may be of assistance, although I don't think it says much about fly ash.

The key to achieving the result desired is to achieve it first in test panels. If you don't do test panels, be prepared for disappointment.

http://www.ccaa.com.au/imis_prod/documents/Library Documents/CCAA Datasheets/DS2010Honed.pdf

 

[epoxybot]

To add to hokie66 reference material, here is a presentation by Tim Cost. P.E. for Holcem. Link Page 52 gives information on fly ash discoloration. Based on hokie66's document and the Holcim presentation, it is probably best to have settled on both a ready-mix supplier & the place and finish contractor AND that they both express confidence in delivering their end of the process.

 

[Ron]

Why add fly ash when the aesthetic consideration is higher than the paltry benefits you'll receive from the fly ash? I would pay more attention to aggregate size/type, water-cement ratio, slump, placement techniques, finishing and curing. If those are proper, your client will be happy 'cause you'll have good quality concrete. If the area is large and you'll need more than one truckload of concrete, pay attention to the consistency between the loads, particularly the water! That will cause variations in coloring as well.

Wet cure the concrete...don't use spray on curing compounds....those can cause significant coloration issues.

 

[dik]

Fly ash has other purposes... best not to use it... and Hokie's comment about test panels is good, but, you should make 2... I nearly always do. It's difficult to hold up something to a single test panel... with 2 you have a range. Good habit to get into.

Dik

 

[dik]

Should have added, as Ron mentions, if you have a large surface, find a good place to make a spot where differences in concrete mix can be non-obtrusive. It's really difficult to have two concrete mixes 'cure' to look the same.

Dik

 

[NorthCivil]

I always defer questions like this to the concrete supplier. They know better than I do, and going commenting on something I don't know is always a sure way to look like a fool

 

[dik]

NorthCivil:

Good suppliers generally do, but, not always.

Dik

 

[bhiggins]

I appreciate all of your comments, this is super helpful and the client is happy!