Concrete Mix design 2

[rowingengineer]

I have a few questions so please bear with me if you have the time. I am not expert in mix design, normally just specifying standard mix’s ie N40 or N32. am before you think i am over stepping the mark, I will be getting My mix design reviewed and tested by the concrete supplier however I need to have something that is +/- 20% for costing purposes.

I have two buildings to design, one is a tank for the storage of wastewater and the other is a heating airbox.

The waste water has a PH of 5.5, So I am thinking that i will need some Silica fume and will use siliceous Agg.
My Mix design is S40, 330kg/m3 of cement, 50kg/m3 of silica fume and w/c of 0.4. 20mm agg
Does this sound ok, and what type of aggregate would be siliceous?

For the heating air box, I have a building that will be heated to 95deg then cooled rather quickly in 12hr cycles.
I would like to densify the concrete thus am thinking of 30kg/m3 silica fume, 420kg/m3 of cement w/c 0.3. I am also considing using some fly ash as replacement for cement. 20mm agg.
Soes this sounds ok? Does fly ass help and what replacement would you use, say 350kg/m3 cement with 70kg/m3 flyash?

Any thoughts wuld be appricated, as the closer i get now the better the costing and testing.


When in doubt, just take the next small step.

 

[concretemasonry]

rowing -

Where are you located? The cost figures will be greatly influenced by what you specify. Silica fume could be very costly with minor benefits for your criteria and costly to use depending on the batching facilities. For economy, the local aggregates will be the most economical because of the high cost of transporting aggregates. If you are considering a ready-mix or central mix plant the cost of delivery of all concrete is all about the same irregardless of the properties, but testing and sampling will raise the cost in the real world.

Why the desire to densify the concrete just because of the short term temperature cycles? The mass of concrete is so great that a 12 hour cycle from 95F to whatever you think it might be due to the "thermal inertia" or mass of the concrete and the surrounding materials. If it is totally isolated, it could be a factor for good or bad.

Any fly ash will increase the rate of curing and in some areas, it can increase the cost of controlled concrete, especially since it can be a variable uncontrolled waste product in some areas.

Dick

 

[rowingengineer]

Dick,
thankyou for you reply,
I am located in Australia, Brisbane.
I was under the impression that silica would increase the resistance of the concrete to degradation by the low PH level, is this not correct.

My apologies for using metric units in my post, I should have used US. The concrete will be subject to 200F cycles, The concrete will be heated for 12 hrs to 200 deg, then the place will be open to the outside air to cool for about 1hr, I am concerned about temp shock as this temperature could be as low as 45F.

I was hoping to increase the density of the concrete thus that temperature shock would be less of a problem, I would have thought differently cooling problems would exist. I was hoping the denser concrete would cool slower, hence less thermal shock problems.


When in doubt, just take the next small step.

 

[LittleWheels]

In Queensland, silica fume is rarely used. Flyash replacement of cement is standard practice (decent quality too).

Stick with SI units, one of these days the USA will catch up with the rest of the world ;-)

 

[patswfc]

Rowing

Would denser concrete not cool faster?

 

[concretemasonry]

Dense concrete has more mass, therefore more thermal inertia, so the internal temperature would change slower. This is in spite of the slightly higher heat transmission rate.

Your conditions are really not that severe and the minor increase in density you are trying to achieve would not be a factor. The thickness and mass of the concrete is also important.

Depending on the conditions, the moisture change could be a big factor.

 

[rowingengineer]

Dick,
Good point in regards to the moisture change, due to the heating process there will be effectively zero moisture on one side of the walls and 60-80% on the other. I will have to do some more research.

By the sounds of things, I may be over reacting to the heat and PH level, and I should just includ the temperature affects in my normal structural analysis and not get to concerned about the concrete mix’s.

Should I start to get concerned when the PH gets below say 4, is this the time to start looking at concrete mix’s? Or is it just a cover problem, as per the codes?

Dave,
Thanks for the heads up, I will look at using fly ash in future.

Pat,
Please refer to dicks post, but I also think denser concrete will cool more evenly as well.

Thankyou everyone for the great replies, have a great day.


When in doubt, just take the next small step.

 

[LittleWheels]

Concrete has a limited service life in acidic water. How limited depends on a lot of things including placement quality. Poor placement will kill concrete durability faster than any other factor.

I guess you've already had a look at the information available from the various Australian cement and concrete associations? Useful overviews include

http://filer.case.edu/slr21/Bridge/acid.htm

 

[rowingengineer]

Thanks Dave, your document had the information I was looking for, basically it is PH of 5 when things start to get out of hand. Good to know.

and yes i had taken a look at most of the cement and concrete associations, but they don’t really refer to PH mostly.


When in doubt, just take the next small step.