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Fly Ash In Structural Concrete

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lma2h

lma2h

Structural
Sep 13, 2005
8
My question is whether anyone can give me some history on the use of fly ash in structural concrete. As an office standard, we do not allow the use of fly ash according to our specifications. I have never really been given a satisfactory answer as to why that is the case. I was wondering if anyone else does not allow fly ash in your mix designs and why. If you do allow it, maybe you could tell me why you do so as well. Thanks for any information.
 
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I think flyash is used mostly in highway paving, if not exclusively for that purpose.

-Mike
 
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In our environs, Portland cement is often blended with flyash for economy. There's nothing wrong with using it and you might consider revising your specs... just have to watch the proportions. It tends to retard strength a bit, but is capable of achieving very high strength concrete.

Fol
Dik
 
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I worked for an electric utility that promoted (and sold) fly ash for use in concrete. To "practice what we preached", we have been using it in all concrete for power plant construction since the 1970's.

Concrete with fly ash sets and gains strength more slowly than the same mix without fly ash. It will reach the same final strength, however - just takes longer. In hot weather slowing down hydration is an advantage, not as much heat generated. In colder weather, fly ash is a disadvantage, it takes what seems to be "forever" for the concrete to cure.

Fly ash is included in Type IP Cement.

[idea]
 
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As SRE and others have pointed out, fly ash has been used in portland cement concrete for many years. There are several other advantages to using fly ash as well as those mentioned. In particular, it has the potential to reduce alkali-silica (ASR) and alkali-carbonate (ACR) reactions because it reduces the total alkali available for such reactions when substituted for a portion of the portland cement.

Codes recognize this substitution and consider fly ash to be part of the cementitious material in the mix as it does hydrate and impart strength to the concrete, albeit slower in gain as stated. When you read or hear the term water-cement ratio, it is really water-cementitious material ratio which might include fly ash, ground blast furnace slag cement, or other materials having a pozzolanic effect.

With appropriate consideration and fitting to the application, there is no reason fly ash shouldn't be allowed in certain concretes.
 
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Fly ash (or PFA=pulverized fly ash, as its known here) gets extensively used in South Africa. Over and above the ASR & ACR benefits, experiments have found that it leads to denser concrete. Except for slower strength gain (which with our local PFA is marginal) there's no disadvantage. I'd recommend that you investigate the use of it.

Alten

 
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Fly ash is nearly always used for "non-architectural"concrete during the warmer construction months in my area. The use of fly ash in the colder months is optional because of reasons stated above.
There is a color difference between regular concrete and fly ash concrete. This is the reason stated by architects for not using fly ash concrete for exposed concrete. Apparently the archtiects really like gray. Slag is also starting to be introduced for regular use in my area. It has not caught on as of yet.
 
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I had the opportunity to design flow able fill which has fly ash in it. I think you should consider American Coal Ash Association publications as your reference. You can call there engineers. I found them very cooperative.
 
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We allow fly ash in our structural concrete but limit its content to a maximum of 20% of the cementitious material.
 
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What I've noticed in years past was that engineers and ready-mix suppliers in large metropolitan areas seemed pretty comfortable using fly ash mixes. And in small towns, they shied away from it- perhaps just lack of experience with it at smaller companies. This was generally foundation applications.
 
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I have noticed that the fly ash appears to retard curing time. that causes the contractor to wait a few extra days before removing forms for columns and elevated framing. anyone else have that experience? otherwise, no problems with using fly ash of 20% or less of total cementitious material.
 
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archeng59...yes, that's characteristic of fly ash addition. You can compensate with admixtures or different cement type (type III) if necessary.
 
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In my area it is relatively common to see fly ash in the mix, and we had been allowing it in our specs. However, on several jobs there was a big problem getting the propper air content. We spoke with a cement manufacturer, and they said that variability in the fly ash is the probable cause. If I remember right, it was variablity with the carbon content in the fly ash.

The cement mfr said that they do not see the same problem with ground granulated blast furnace slag. Since they get the slag from a blast furnace there is less variablity because of the tighter controls on what goes into a blast furnace versus burning of coal.

For these reasons, our preference is to allow slag in the mix.


 
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bjb...when using slag, you might want to request strength gain curves from the ready mix supplier. Strength gain is retarded with GBFS cement as well.
 
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If we're using slag in something structural we alert the contractor to the fact that strength gains will be delayed, which will affect when forms can be stripped.
 
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Fly ash in concrete has many advantages.

We do not allow Type F since this has created problems with air content. We allow only Type C.

We use it as a 1:1 substitute for cement up to 20%. As Ron said you need to measure "water to cement + flyash" ratio since fly ash is considered cementatious material.

Retardation of set time for flatwork is a problem.
 
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I didn't know that the air content problem was only associated with Type F. Several of the ready mix suppliers in my area have told me they prefer using slag over fly ash.

I agree with the beneficial aspects of fly ash. Slag also has many similar benefits.
 
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There has been a tremendous amount of work in Canada on the use of fly ash in Concrete - I remember using it back in 1979 - understand that it was used as far back as 1929 on the Hoover Dam. Malhotra of CANMET is a name that comes to my mind - you might wish to do a search on google for him and or CANMET. Fly Ash Concrete is also called Green Concrete - a PC term for Environmentalists I presume. CANMET actually pushes for High Volume Fly Ash - upping the percentage of Fly ash used in a mix than is currently (something like 30%) used.
 
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