Wastewater Basin Mat Slopes 1

[profplan]

I am designing a 28" min. thick concrete mat foundation approx. 200'x250' (L-shaped in plan) for a 25' deep oxidation ditch. Typical basins for each zone are 40' x 40'

Mud valves/drains are provided to drain each basin and the slab slopes 1/8" or about 6"+/- from perimeter high points to interior valves. Typ. top reinforcing is # 10 @ 12" each way and 2" min. cover. The design satisfies ACI 350-06. My questions related to constructability and crack control are:

1. Should top reinforcing bars slope or remain flat.
2. Is it recommended to bend top bars to match change in slopes and maintain 2" cover for crack control.
3. Is concrete poured to match the slopes or poured flat with secondary pour preferred for the slopes. (6" approx. elevation difference)
4. If a secondary pour is used to match the slope, what crack control reinforcing is recommended and the minimum thickness at feathered ends.
5. Should the drain low points be reversed and placed on the perimeter of the slab to make bar placement and concrete placement to the slopes easier.

Many of these are somewhat subjective but your insight based on experience will be valuable.

 

[JedClampett]

Answers:
1) Slope
2) Bend the bars
3) Either way will work. Usually grout is used for leveling or sloping. For a 28 inch think mat, I would slope the mat.
4) No reinforcing is required in the grout. Sometimes we place a surface layer of #5's @ 12" each way for very think grout pours. You don't care if it cracks a little.
5) If I understand you, no. You want the thickest concrete adjacent to the walls. Slope away from them.

 

[profplan]

JedClampett,

Thanks. I thought the bending of bars at slope changes would be difficult to coordinate for the contractor but it seems it is not a concern. Do you typically show all the slopes and high and low point elevations on the plans or just show the low and high points, and the 1/8" slope requirement.

 

[JedClampett]

I learned a long time ago that the more you worry about ease of construction, the less it's appreciated. The reinforcing detailers are almost always very good and can do that kind of adjustment in their sleep.
I wouldn't show more information than necessary. Show high points, low points and direction of slopes. If drainage is very important, make the contractor test the basin and fill in any "bird baths".

 

[profplan]

Thanks JC, including your perspective on construction.

Another issue regarding large wastewater basins is the placement of enamel interior linings. I don't have experience with this, but I do have a reliable and tested specification.

In your experience, for oxidation ditches, anoxic/oxic basins, sludge holding tanks, would the lining be applied full height of concrete walls and including the entire mat? There is some question as to whether it should only extend from the top down to a few feet below the service fluid level line, or, further to the base of the walls, but not the mat/floor.

Is this purely a question of performance or maintenance cost/benefit probably best left to the owner or is there a standard or norm usually provided for most of these types of facilities?

 

[JedClampett]

The first question is, why are you coating the concrete at all? If the volume over raw sewage is enclosed or poorly ventilated, a good epoxy mortar lining, like Sauereisen, is appropriate above the water line. As the wastewater treatment progresses, the less chance there will be hydrogen sulfide released and the necessity of coating is reduced. For open oxidation ditches, coating is not required. For sludge holding tanks, it depends on the sludge treatment type.
Sewage is not normally corrosive to concrete. The stinky gas released, however, can chemically evolve into sulfuric acid whcih damages concrete.
While I think coatings have their place in wastewater treatment plants, there is a bigger chance that the coating will give you a problem before the concrete is attacked.

 

[profplan]

I read through portions of the specifications for coatings. I think the answer to your question is that the prime consultant and owner have traditionally used coatings on these facilities. Interestingly, the owner wants to explore the use of concrete admixtures instead of coatings.

These are all open basin/tanks. Thanks.

 

[JedClampett]

That's fine, but if they really wanted to protect the concrete they could give me the money they're going to spend on coatings, admixtures and magic beans and I would do a concrete dance around the basins.