huangyhg

fly ash replacing cement
is it proper to use class "f" fly ash to replace cement content? i thought that only class "c" has cementious properties, but i keep seeing concrete mix designs using class "f" as part of the w/c ratio. is this a mistake made by the concrete supplier?
if i recall correctly, class "c" is produced from burning hard coal; class "f" is produced from burning soft coal. it is my understanding that the carbon content in the flyash affects the air entrained. class "c" produces consistent results with air entrainment. class "f" does not.
class "f" can be used but only if the supplier has done enough testing and trial batches to know what he has to do to properly control the air entrainment.
just today, we had some guys in our office promoting fly ash subgrade stabilization and they explained the differance between class "c" and class "f". class "c" is self-activating which means that it has cementitous properties on its own without the help of plain ol portland cement. class "f" on the otherhand is not self-activating, which means that it needs to be combined with a portland cement to have cementitous properties.
as far as your question goes, we usually allow the contractor to choose either "c" or "f" for structural concrete. i think the only situations that would require exclusively class "c" would be where no portland cement is used such as for controlled density fill or subgrade stabilization.
class f has been widely used in southern ca. astm c 618 recognizes three classes, namely: n, f and c.
i've seen the water-cementitious ratio expressed 2 ways:
1. w/c where w = water lbs and c = cementitious weight lbs (cement, fly ash etc)
or w = water gals and c = cementitious weight in equivalent sacks of cement, and
2. w/[c + p + s + sf] where w = water lbs, c = cement lbs, p = pozzalan (fly ash) lbs, s = slag lbs, and sf = silica fume lbs
the fly ash must meet astm c 618 and the amount to be used in computing the w/c ratio will depend on several factors such as durability provisions in aci 318 sec 1904 and jurisdictional requirements.
there are some jurisdictions that require an evaluation report and allow no more than 15% to 20% cement replacement by fly ash for computing the w/c ratio even though aci 318 permits up to 25% replacement for durability. so for instance if cement= 450 lbs, fly ash = 150 lb and the cement replacemt allowed is 15%, the fly ash used in computing w/c is [450 x 15]/85 about 79 lb.
fly ash even in small amounts is sometimes utilized in the concrete mixture for pumpability.
getting confused again with ubc and aci 318. aci 318 section 1904 should read aci 318 chapter 4. the ubc equivalent is section 1904

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