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I am looking at a wastewater clarifier. The concrete slab structure is about 35m in diameter, 400mm deep slab and a slope of 1 in 6.

It is an inverted cone shape and I am looking for ideas as to how best to pour it?

Has anyone done one of these before and how did you do It




I poured a bunch of concrete caps on circular cofferdams - these sloped to drain - and a bunch of tank foundations. At the cofferdams we worked from atop the bridge and placed the concrete with buckets; starting at the low end of the perimeter and working our way up and in. In your case pumping would be the way to go; work low to high.


Thank-you for your reply and your ideas, Bridgebuster

The pour would probably be with a concrete pump and start from the base.

I am attaching a photo of a similar structure but the one I am doing is much larger and steeper: 1 in 6


The problems I can see though are:

1. The slab is 400mm of re-bar so it will have to be pokered and have adequate workability. This brings the problem
of concrete flowing towards the base after it is placed pokered and tamped to levels and falls.

2. Where should I place my screed rails to maintain, as well as possible, the curved formation?. If I break it up into separate pours I will have to include water-bar at each joint. This is a possibility but I do not know if the client will accept it.The radial bays would give a "hexagonal" outline to the inverted cone; as they would have to be tamped level between the screed rails. I would pour alternate bays one day; infills the next etc. I could possibly split it into two from top to bottom; a minimum of twenty radial bays at the top and six in the bottom circle.(See the very very rough sketch). however i would prefer not to go down this route if possible.


I am putting a method statement together for submission. I have plenty time on my side. but I would appreciate any thoughts anyone would have.

Please advise

Cheers peakpilgrim


peakpilgrim - Large clarifiers, identical to wastewater clarifiers, are used as part of the flue gas desulfurization (FGD) system at certain coal fired electric generating stations. I agree with bridgebuster, concrete is placed from low to high. For a qualified Contractor, accurately placing properly vibrated (pokered) concrete on a 6:1 slope is not difficult. Concrete slump and the concrete placement rate have to be controlled. The Engineer needs to stay out of this "means & methods" aspect of the project. The Contractor has to have the authority to quickly alter his in-progress field operations depending of variables such as weather, concrete delivery, and manpower.

Concerning your paragraph #2... I believe you need to review how a clarifier works. There are radial rakes just above the bottom that rotate slowly to gather sludge and direct it to the center (underflow). The concrete bottom has to be constructed to fairly tight tolerances (in the radial direction in particular), or the equipment won't work. I suggest you forget about detailing 20+ construction joints and approximations of the required slope.

As a former Owner's Representative, I would reject that (Engineer's) concept based on its needless complexity alone.

As a former Bridge / Heavy Construction Contractor, the bid I would submit (to perform work to that detail) would be so high that that the Owner would reject the bid based on excessive price. Concrete volume of 385 cubic meters (35 meters, diameter, 0.4 meters thick) is not enough for a Contractor to economically justify very many concrete placements.

I suggest your drawing / specification show the slab, including appropriate construction tolerances, without any construction joints. A prominent note states that a Bidder should summit proposed construction joints, if any, with the bid. This submittal will be evaluated by the Engineer during the bid review process.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2


Thank-you SlideRuleEra

Your comments regarding the construction tolerances due to the radial rakes are very informative and extremely relevant.
Your comments regarding the number of bays are also accurate; certainly not a preferred solution; as I stated in my previous post.
As you say I will have to reconsider this all again.

I have not done one of these before and I have had problems with concrete flowing on slopes previously; though the tolerances were not so critical ( ie: storm-water spillways).

I cannot guarantee the quality of the work-force and I want to break this down to as simple and effective an operation as possible so that it will be done right. So, unfortunately, I cannot stay out of the "means and methods"

Fortunately I have plenty time to put a plan together

I would prefer to pour the whole slab together so I am back to the questions:

1. How do I control the pour (ie: screed rails)to maintain accuracy?
2. What slump do I use?

Any further suggestions would be appreciated.

Please advise



peakpilgrim - I see, you do have a challenging project. With those restrictions you certainly don't want to commit to placing too much concrete at one time. Consider making concrete placements as minor sectors of a circle:

In principle, screeding would be performed radially (Line "AO", sweeping on the minor arc to Line "BO"). See if this is practical for you project. There may be complications because of underflow collection details and piping at the center (Point "O"), but you should be able to work around that.

Depending on the workforce, four or more sectors may be best. The sectors do not all have to be the same size. Perhaps begin with a small sector to see how things go, then adjust the size as experience dictates.

Concrete slump will depend on the mix design. Of course you will need a fairly "stiff" mix. Because of the limitations you have, it may be worth making some test placements on a simulated 6:1 slope - both to select a suitable slump and to train the workforce. A "rule-of-thumb" that my father taught me is that structural concrete should be able to flow down the discharge chute of a concrete truck by gravity alone. Any mix with lower slump than that will cause more problems than it will solve.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2


Thank-you again SlideRulEra,

I was starting to think along similar lines myself. using very accurately established circular screed rails; possible using curved steel; taking time to set them up properly and screeding radially. 385m3 is a lot to pour in one day and achieve an adequate finish. There could be pressure on the work-force to rush through the finishing; resulting in a sub-standard product.

The "rule of thumb" that you mention, regarding concrete, is also very true and accurate.

My next problem: if I am going to pump the concrete it would have to be in excess of 100mm slump which would cause flow
problems. I will have to find a solution to that; possibly 75% of the concrete placed by pump and 25% by crane and skip; a stiffer mix, just flowing down the chute for the finish.

Any further ideas would be appreciated.

Kind regards,



peakpilgrim- I would not be comfortable using two mix designs in the same placement for several reasons:

All of the concrete is on a 6:1 slope - slump and placement rate of all concrete have to be controlled, not just some of it.
One mix may require finishing at a different time than the other mix.
Even if blending the two mixes is ok, under typical jobsite conditions having the right mix at the right place at the right time (logistics) will be complex and subject to error.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2


Changing mixes and placement methods is not a good choice since different placement/handling methods may need different amount of water and there is really no good control which load of concrete goes to which place in real life.

A "nit-picking" personal experience is to try to avoid starting too soon since the aggregate storage at a plant may be drier or wetter in the morning than later in the day depending on the weather and storage capacity of the plant/plants.

Granted, 384 cubic meters could be easily batched and delivered by a good supplier, but the placement methods and daily weather does have an effect. As a student and summer bridge inspector, I saw the effects on something as simple a slope protection between an abutment and the initial pier for 3 identical bridges a 1/2 mile apart on the same day created problems. The supplier had the concrete coming from the same plant, but even for a large concrete producer but there is a limit to the controlled aggregate storage (inside and outside) and batching systems if the needs are more critical for slow pour.


Engineer and international traveler interested in construction techniques, problems and proper design.


peakpilgrim - A 4" slump is pumpable. NYSDOT uses a range of 3 to 5 inches for pump mix. You can use a water reducer to increase the slump. Pumping can create slump loss. You could specify that the slump is to be measured at the pump discharge, which allows you to use a higher slump coming out of the truck. However, you would have to do a trial run to get it right.

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