fly ash replacing cement
fly ash replacing cement
(OP)
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?






RE: fly ash replacing cement
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.
RE: fly ash replacing cement
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.
RE: fly ash replacing cement
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.
RE: fly ash replacing cement