huangyhg

the use of fly ash in concrete for leeds certified building
we are currently managing the design build of a leeds certified office building. the building is aprox. 30000sf and its foundation is comprised of piles, pilecaps, and grade beams, aswell as a 13' high foundation wall below grade in the lab portion of the building.
1) what are the structural and placement issues involved in incresing the amount of fly ash in the concrete mix.
2) what are the costs involved including unforseen hidden costs.
3) what overall inpact will incresing the fly ash content of the concrete have on the leeds point system. (aprox. 1200 cu.yds of concrete. aprox 4 million dollar building)
thanks
tourbillon
1)
quote:
what are the structural and placement issues involved in incresing the amount of fly ash in the concrete mix.
increased set times, delayed strength gain, and i have heard finishing difficulty. avoid applications with de-icing salt exposure. make sure it is adequately cured. some differences depend on if you are choosing to use class c or class f ash.
2)
quote:
what are the costs involved including unforseen hidden costs.
some fly ash may cost more than portland cement if it is not locally available and has to be shipped in. also any additives that are required in the concrete to compensate for the delays mentioned above or shoring in place for longer times. it depends on how much replacement you are going for and the amount of strength needed and when. there may be other expenses
3)
quote:
what overall inpact will incresing the fly ash content of the concrete have on the leeds point system. (aprox. 1200 cu.yds of concrete. aprox 4 million dollar building).
it depends on the replacement quantity and the cost of the cementitious materials versus the cost of all the materials on the building. i am dealing with the same situation you are in right now.
the aia masterspec has a reference to 40% minimum substitution fly ash for portland cement. that's pretty steep.
i've never permitted more than 30% fly ash substitution for portland.
i am not fond of that "40% minimum" default. i think some people may be going with that (engineers and contractors) without any extra thought. the leed reference manual uses 20 to 25% in their example calculations. obviously, the more ash you go with the more it helps the project toward the leed 4.1 or 4.2 credit as a whole. but the recycled content can possibly be made up in other areas besides the cement mix, such as in the recycled steel content in the reinforcing and in the frame steel.
using high volumes of ash is alright (even >40%) if it is done correctly and the application fits. for instance, it may require a longer curing time to reach the intended design strength and durability needed.
this is a very interesting article on the use of flyash.
lsmfse, the link did not work. was that article in the october 2007 issue of concrete international?
ismfe,
yes concur with the you re article in concrete international. class f fly ash in particular, have not observed reduction in set times or rate of strength gain, or workability which can be handled a number of ways. however we have substituted 70% blast furnace slag and there you do get a substaintial decrease in set time and decreased rate of strength gain, that of course is what your looking for though ie reduced heat of hydration when you use this substitution. if aci memeber try 'control of temperatures in mass concrete, concrete international/jan 2002'. wouldn't do it in a high rise type structure unless we seriously re-evalauated the shoring re-shoring aspects of the project re blast furnace slag. another useful source of info along this line is 'control of cracking, state of the art, transportation research circular no: e-c107, oct. 2006, transportation research board of the national academies'.
tourbillion,
might be easier to add a bicycle rack at the front of the building re leeds, suggest this to the architect.
yes, you can overcome delayed strength gain by messing with the mix or specifying longer times to design strength required. here are some random excerpts from a presentation i gave on hvfac (>50% fly ash) using class f ash.
-fly ash increases workability when compared with conventional concrete with the same water content. however, hvfac normally incorporates a very low water to cementitious material ratio to achieve comparable early strengths as conventional portland cement mixtures. therefore, the use of superplasticizers is common.
(jiang and v.m. malhotra, 2000)
-hvfac often requires higher doses of air entraining admixtures due to adsorption of the aea by carbon in the fly ash.
(malhotra, 1994)
-the low cement content of hvfac and the slow reacting property of fly ash increases setting times.
-an additional 1 to 2 hours to final set has been shown for hvfac. special measures may be required when using hvfac in cold weather to avoid significant strength retardation.
(ramachandran, 1996)
-the creep strains of hvfac can be higher or lower than conventional concrete depending on the age of the concrete when loaded.
-the quality of fly ash can also influence the strength gain and therefore the creep strains.(bilodeau and malhotra, 2000)
-hvfac requires lower w/cm ratios to obtain comparable early age compressive strengths as conventional concrete.
-adequate curing of hvfac is critical to strength development. a minimum of 7 days of moist curing of hvfac required for optimum strength and durability (for continued pozzolanic reactions).
-the early compressive strength is a function of the coarseness of the fly ash used and the amount of cement replaced with fly ash.
(bilodeau and malhotra, 2000)
(chindaprasirt, 2004)
-higher values of cement replacement with fly ash will require lower water contents to achieve the same compressive strength.
-the long term compressive strength of hvfac normally exceeds that of conventional concrete. longer term (56 day) compressive strength requirements are often specified. the ratios of the flexural and tensile strength to the compressive strength are comparable to conventional concrete.
(langley, carette and malhotra)
-de-icing salt scaling has been shown to be a problem for
hvfac in the lab. hvfac is not recommended for applications where there will exposure to de-icing salts. (bilodeau and malhotra 2000)
don't know much about flyash, but if you wanted to use it in the %'s suggested in a precast plant, it really screws up the one day accelerated curing cycle.
seems kind of funny to me that we have people running around, trying to get as much flyash into the mix as possible to get more points for leed, which increases the demand for such a product, which is supposed to be a waste byproduct.
so if more and more buildings started requiring max flyash to meet leed stds, we need to produce more flyash to keep up with the demand??
i thought about too, but they only give you 1/2 value towards the leed credit for pre-consumer items like ash. i think they are trying to encourage post-consumer (ex. recycled steel) which counts 100% towards the recycled content value.

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