Fly Ash in Theoretical Concrete Mix Design 3

[SirAl]

I generally apply the ACI volumetric mix design approach when calculating theoretical porportions. Fly ash, being a pozzolan, is commonly substituted for a portion of Portland cement in the mix. Accepting that the fly ash composition meets all ACI/ASTM/CSA criteria, can the fly ash be substituted at a 1:1 ratio with Portland cement (up to a limit of course) or should I be using a reduced equivalency ratio of say 1.0 lb fly ash : 0.75 lb Portland cement.

Thank you.

 

[JedClampett]

We usually use a equivalency of 1.2 lb. fly ash per every lb. of cement replaced.
I believe it's conservative, but fly ash is pretty cheap.

 

[dicksewerrat]

and unless you are not worried about strengths never go over 15% replacement. Remeber also that Fly ash will not give you anywhere as much heat as the portland. If you are working in a cold climate (as in Minnesota or Canada in the winter) this may be a deciding factor.

 

[stanier]

You need to consider tha make of fly ash. I am not a civil engineer but have pumped many tonnes of fly ash as dense pastes. The changes in rheological aprameters that led to a chemcial analysis of the fly ash. The constituents are really unreliable. They vary with the coal that is burnt , firing temperature of the boilers, ash content of the feed, type of boiler.

The only sure way of working up a concrete value is do some testing and establish a set of book ends of ash variability.

Sharing knowledge is a way to immortality

 

[rmccabe]

If the fly ash is reactive, you will gain long-term strength and durability by replacing cement 1:1 at 25% or more. I have seen replacements up to 66% that were very successful. To gauge the reactivity of FA, test it against the best and most consistent out there: Micron3 from Boral. (One mine, one plant, 3 micron avg. particle size.)Type C more reactive than Type F, but main thing is the amount of amorphous silica. The supplier should be able to provide this information. If available, compare pozzolanic index.

FA does lower heat of hydration--making a more durable concrete--but it also reduces water demand, increases workability and provides sacrificial silica for ASR.

Richard

 

[BigH]

I read a newsletter from PCI the other day - from about 2 years ago - don't have it in front of me so this is from memory (so treat it as such): They used 318kg/m3 of Type 1 Portland cement, 41kg/m3 of fly ash, 20kg/m3 or so of granulated slag etc and got 41MPa out of it. If you want I can look up for you and give more details.

 

[BigH]

I found the newsletter and want to confirm the correct mix proportions alluded to in my previous thread. Nice to have a few mix designs to compare to. See PCA's Concrete Technology Today (CT013 - Nov 2001, Vol 22 #3).

Portland Cement Type 1 311kg/m3 (525 lb/yd3)
Fly Ash, Class F: 31kg/m3 (53 lb/yd3)
silica flume (dry): 16kg/m3 (27)
Ground granulated blast furnace slag: 47kg/m3 (79)
fine agg 676 (1140)
coarse agg 1068 (1800)
water 150.8 (254)
water Reducer Type A: 1.6litre/m3 (41 oz/yd3)
HRWR Type F: 2.1 litre/m3 (55oz/yd3)
AE - enough to get 7% air

Peformance - 28d min = 41.4MPa (6000psi)
28d max = 65.5MPa (9500 psi)

For a double deck road (Wacker Drive in Chicago)

 

[SirAl]

Thank you, all, for your valuable input. Flyash is certainly an enhansement to PCC for most applications. Thanks BigH for digging out the mix design example. I will archive the design for future reference. Blast slag is not a common product in my neck of the woods. What benefits would it provide to the mix?

 

[chicopee]

Isn't the composition of cinder block fly ash?

 

[BigH]

I've used slag cement in Hamilton Ontario - a steel town. Basically, it can be used in areas with mild sulphate problems - also chloride. M.S. Shetty has a coupld of pages in his book that are quite informative on slag cement - if you want, send me co-ordinates and I will forward to you. bohica@indiatimes.com

 

[BryanStein]

Type C fly ash is reactive and most common on in western us. It is more reactive than type F because type C has more free calcium ---- more alkalinity. Calcium hydroxide (lime) is a by product of cement hydration. Fly ash in the presence of free lime and moisture and alkalinity will hydrate.

Type F fly ash is low in free calcium (alkalinity) and must depend solely on the cement hydration to hydrate.

Type C fly ash is often replaced 1:1 or 1.1:1 with cement by weight. Up to 25% replacement is allowed.
Type F fly ash is often replaced 1.2:1 with cement by weight. Up to 25% replacement is allowed.

specific gravity of cement is 3.15, of fly ash is about 2.2 to 2.5. Keep this in mind when proportioning mixes.

In the last 5 years concrete with up to 60 and 70% fly ash has been successfully used.