huangyhg

what it is the reason for fly ash replacement limits?
looking at table 4.2.2.9 of aci 301 (or table 4.2.3 of aci 318), there are limits for fly ash, slag, silica fume, etc, for concrete subject to deicing chemicals.
does this mean for concrete not subject to deicing (i.e. most indoor concrete), there are no replacment limits for fly ash, etc.? and where can i read about the reason for the limits and why?
thanks guys!

there are various reasons, although i may not be up on all of them. two that i can think of are strength and color. color may be important in some architectural projects.

i would look at the aci references related to that section of the commentary. for aci 318-05, look at items 4.4 - 4.9.
there are limits on how much fly ash can be used in concrete for two principal reasons:
1. fly ash needs cement to react with it to form the desired chemical compounds. replace too much cement with fly ash and concrete properties degrade. with the proper ratio, the concrete is "better" than with cement alone.
2. concrete gets it's strength from the large aggregate. within reason, the more large aggregate the stronger the concrete. however a full range of aggregate sizes is needed to fill the spaces between the large aggregate. fly ash is both microscopic and spherical. these two properties are an ideal addition to the concrete mix. however, use too much, and the mix now has too much small aggregate.
here is one of the best documents that i have seen concerning the advantages of fly ash in concrete:
in the oct. 2007 issue of concrete international (an aci publication), a point of view article by bryce simons, material testing engineer, new mexico dot discusses some of these issues. his viewpoint is of highway construction, usually extreme conditions.
mr. simons advocates the mix be designed for the desired properties, without fly ash, and then fly ash is used as a mineral additive. his argument (which i am in general agreement with) is sufficient heat of hydration, setting time, early strength development and such issues must be obtained in many instances. replacing cement with fly ash may produce an inadequate concrete.
mr. simons does use a fair bit of fly ash in most mixes. he states the nmdot stipulates that a minimum of 20% be used, with no restrictions on the maximum amount!!!!
the nmdot concrete mixes reportedly use the w/c to proportion the mixtures for the desired early properties and then use w/cm to develop the longer-term properties.
this seems logical to me.
slideruleera,
thanks for the tech data...very informative
our specs had a 50% replacement limit. we were seeing a lot of 7 day breaks which appeared low, but would generally come up to strength at 28 days. i lowered the replacement limit to 35% to try to eliminate the 7 day problem and it seems to be working. if anyone has a similar experience i would like to hear about it.
7-d strengths are not uncommonly low - because the fly ash does not hydrate at the same rate as the cement so you would expect it to be lower than for opc. the 28 day strengths may likely be a bit lower too than you would expect also do to the fly ash reacting over the longer term. i do not believe you need to reduce the fly ash only to meet a 7-d "estimate" especially since the 7-d strength is not the acceptance criterion.

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