huangyhg

fly ash in structural concrete
my question is whether anyone can give me some history on the use of fly ash in structural concrete. as an office standard, we do not allow the use of fly ash according to our specifications. i have never really been given a satisfactory answer as to why that is the case. i was wondering if anyone else does not allow fly ash in your mix designs and why. if you do allow it, maybe you could tell me why you do so as well. thanks for any information.
i think flyash is used mostly in highway paving, if not exclusively for that purpose.
-mike
it is being used to achieve leed points in buildings. we have specified a percentage for slab on grade to help get the certification before.
-horace
in our environs, portland cement is often blended with flyash for economy. there's nothing wrong with using it and you might consider revising your specs... just have to watch the proportions. it tends to retard strength a bit, but is capable of achieving very high strength concrete.
fol
dik
i worked for an electric utility that promoted (and sold) fly ash for use in concrete. to "practice what we preached", we have been using it in all concrete for power plant construction since the 1970's.
concrete with fly ash sets and gains strength more slowly than the same mix without fly ash. it will reach the same final strength, however - just takes longer. in hot weather slowing down hydration is an advantage, not as much heat generated. in colder weather, fly ash is a disadvantage, it takes what seems to be "forever" for the concrete to cure.
fly ash is included in type ip cement.
as sre and others have pointed out, fly ash has been used in portland cement concrete for many years. there are several other advantages to using fly ash as well as those mentioned. in particular, it has the potential to reduce alkali-silica (asr) and alkali-carbonate (acr) reactions because it reduces the total alkali available for such reactions when substituted for a portion of the portland cement.
codes recognize this substitution and consider fly ash to be part of the cementitious material in the mix as it does hydrate and impart strength to the concrete, albeit slower in gain as stated. when you read or hear the term water-cement ratio, it is really water-cementitious material ratio which might include fly ash, ground blast furnace slag cement, or other materials having a pozzolanic effect.
with appropriate consideration and fitting to the application, there is no reason fly ash shouldn't be allowed in certain concretes.
fly ash (or pfa=pulverized fly ash, as its known here) gets extensively used in south africa. over and above the asr & acr benefits, experiments have found that it leads to denser concrete. except for slower strength gain (which with our local pfa is marginal) there's no disadvantage. i'd recommend that you investigate the use of it.
alten

pfa is common in australian concrete, for the reasons noted above.
fly ash is nearly always used for "non-architectural"concrete during the warmer construction months in my area. the use of fly ash in the colder months is optional because of reasons stated above.
there is a color difference between regular concrete and fly ash concrete. this is the reason stated by architects for not using fly ash concrete for exposed concrete. apparently the archtiects really like gray. slag is also starting to be introduced for regular use in my area. it has not caught on as of yet.
i had the opportunity to design flow able fill which has fly ash in it. i think you should consider american coal ash association publications as your reference. you can call there engineers. i found them very cooperative.

__________________
借用达朗贝尔的名言：前进吧，你会得到信心!
[url="http://www.dimcax.com"]几何尺寸与公差标准[/url]