I want to design some concrete mixes with different fly ash/binder ratios (can be as high as 80%) but the same strength level. The water/binder ratios (together with superplasticizer dosages) have to be adjusted in different mixes to achieve comparable 28-day compressive strength.
To have a fair comparison of the material cost of different concrete mixes, should I fix the paste/aggregate volume ratio or the total binder content (in kg/m3)? If the paste/aggregate volume ratio is fixed, the limitation of the max/min cementitious content may not be fulfilled; if the total binder content is fixed, some mixes may not have enough paste to fill the gaps among aggregates.
I prefer to fix the paste/aggregate volume ratio, but I am not sure whether there is any other consideration.
Hi Jing,
In cement, flyash can be replaced upto 35%. So upto 35%, there will be no change as long you use good quality of cement. But flyash cement ratio should not be more than 0.35. As you can see EN 197 code CEM IIB type cement has 35 % flyash. Volume of Fine and Coarse aggregate will not be changed. You can compare cost with that condition.
So for that you have to trial a mix with limited slump (50mm) and use admixture because using flyash that much will reduce consistency. Then that crushing strength will be approximate designed strength.
You can design such concrete, use the concept of the "k" factor (specified in the standard for concrete design, such a possibility is provided for in the Polish and Eropean standard).
Fly ash is a cheap substitute for cement binder that can be used up to 33% of cement mass. Different proportions of fly ash to cement can be used. In this way I will obtain several variants of recipes. The amount of fly ash is calculated according to the concept of the "k" factor.
In European standards we use the concept of the "k" factor (it is a factor depending on the type of additive, binder, fly ash, blast furnace slag, silica dust).
In my country, in Poland, we often design concretes to use waste binders to reduce the use of cement (zeolite, metakaolinite, micrometakaolinite, calcareous and silica fly ash, fluidized ash, blast furnace slag, silica dust, natural pozzolans, resins, asphalt).
You should be familiar with the European standard for concrete technology: "EN 206 - Requirements, properties, production and compliance".
The concept of the "k" factor is to compare reference concrete with concrete in which a certain amount of cement is replaced with an additive.
The most important criterion is the durability of concrete or a surrogate criterion for concrete strength.
The concept of the "k" factor makes it possible to include type II additions by:
1). Replacement of water / cement ratio
water / (cement + k × additive),
2). Fulfillment of the condition: (cement + k × additive) ≥ minimum cement content required due to the exposure class.
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