Type of Cement in WW Treatment Plants 4

[JAE]

What would be appropriate for the concrete used in a small town wastewater treatment plant for the tanks set in the ground. The town is in a region that gets winters to -10 deg. F and snow and summers to 100 deg. F (i.e. Midwest US). This is a very small town.

Studying ACI 350 I get the following:

Types I thru V cements can be used.
Question: Is type II or V preferable?

Use largest practical aggregate.

Use air entrainment.

Use fly ash.

 

[JAE]

Are the above items appropriate for a WW tank? Anyone have an opinion? Any help would be appreciated.

 

[Ginger]

Hi JAE

I am not familiar with the US concrete grades but I can set down what we in the UK would do and perhaps this may compare well with one of your concrete specifications.

In the UK, for water/sewage treatment works we use a grade of concrete called C35A. This has a characteristic compressive strength of 35N/mm2. Its cementitious content is limited generally to a maximum of 400kg/m3 (but is generally specified at 325 kg/m3) with maximum 30% pfa or 70% ggbs as a cement replacement. The water/cement ratio is limited to 0.45 and the aggregate is usually selected to give a low coeff of thermal expansion.

Aggregate size is usually 20mm but reduced to 10mm if the concrete is to be pumped. DO NOT use air entrained concrete as this will create a water path to your rebar, will take an age to cure in the shutter, and the vibration required to compact the concrete in the shutter will destroy any air matrix you have created with the entraining agent.

To cater for the large thermal range I suggest you have movement joints and reinforce with lots of small diameter bars at close centres.

Hope this helps.

Best regards Andy Machon

 

[JAE]

Thanks, Andy:

Are you sure about the air entraiment? We looked at ACI 350R which is "Enviromental Engineering Structures" and they state that it improves the workability and contributes to a dense watertight structure, and should be used in all concrete for watertight, chemical resistant structures.

I understand the fact that with 4.9m high walls 250 mm thick, you could vibrate out a lot of the air. Just wondering.

 

[Ron]

JAE...use Type II cement as it is readily available and provides adequate sulphate resistance for WW treatment plants.

Your other parameters, air entrainment and fly ash are fine, just be careful adding fly ash to Type II cement as Type II has a slightly slower strength gain anyway, and the fly ash will exacerbate that. Ultimate strength will likely be fine, you'll just get a scare early on at 7 or 28 days.

In the weather conditions you noted, air entrainment would be indicated. Further, it will increase the workability and water intrusion resistance of the concrete in general.

As Andy indicated, it's important to protect the rebar. For marine structures and aggressive environments such as WW treatment plants, my premise is add a little more cover and make the concrete as good and as dense as you can. In my opinion, air entrainment does not compromise this. In the specifications, be tight on the control of the concrete. If pumped, have them use a large pump and stay with a large aggregate (#57 preferably). Required vibratory consolidation, but no over-vibrating. Require steel forms if you can. These typically give a denser surface than wood forms.

Ron