WWTP - Concrete Structures with Joints 3

[bpstruct]

We have been working on WWTP's (basically a bunch of giant concrete tanks) over the past couple of years, and I am beginning to question how I have detailed some of the concrete joints. I have been requiring PVC waterstops (chemical resistant) on the OUTSIDES of the reinforcing steel (water side and soil side when below grade). My thinking was to prevent contact between the reinforcing steel and chemicals/wastewater. However, these are the only kinds of projects that I have ever placed waterstops like this in details. I normally center the waterstop in the cross section.

Reinforcing is continuous across the joints, and I need it structurally. I don't want the reinforcing to corrode. I have thought about requiring the contractor to coat rebar with coal tar epoxy for some distance above (and/or below) the joint and just centering the single waterstop. I've looked at the Sika website, and it shows the waterstops centered in all of the graphics. But I can't find anything in writing dictating where they are required/allowed. Maybe I'm just missing it.

Any help would be appreciated!

 

[JedClampett]

We either use 12 inch minimum slabs with two layers of bars and the waterstop centered between them or a thinner slab locally thickened to 12 inches and additional bars at the joint to get two layers.

 

[David RP]

The waterstop does not need to be installed at the center of the joint. It is perfectly acceptable to install the waterstop off-center. What you want to bear in mind, is the waterstop should have one half waterstop width in concrete cover. Example: A typical 6-inch waterstop would require 3 inches of concrete cover on all sides of the waterstop. A 9" waterstop would require 4.5 inches.

 

[JAE]

And hydrophilic type waterstops need to be kept a certain distance away from the concrete face as their expansion with exposure to water can blow out the face of concrete along the joint.

 

[ATSE]

bp - good queestions.
I always place 6" PVC waterstops at the centerline, between two curtains of reinforcing. For municipal water and wastewater containment, and raw water conveyance (California practice). Exclude for the moment water with chlorine / chlorides.

Three related questions:

a. I would like to know how much the 6" PVC waterstop (3" above the joint) placed in a narrow wall (say, 12" thick) reduces the out-of-plane shear capacity at the base of the wall. There is a 3" shadow that the waterstop casts across the joint interface (in terms of shear and shear friction evaluation). I have been comfortably numb and not penalized the joint.

b. Are any studies / reports available to compare corrosion of black reinforcing steel bars in a wall with and without waterstops for water and wastewater containment?

c. Does anyone ever coat the bars crossing construction joints in water containing structures? I thinking of joints that are in tension or flexure. For joints with no tension, I expect the maximum crack width is small for typical 0.5% rebar.

 

[JedClampett]

a. I would like to know how much the 6" PVC waterstop (3" above the joint) placed in a narrow wall (say, 12" thick) reduces the out-of-plane shear capacity at the base of the wall. There is a 3" shadow that the waterstop casts across the joint interface (in terms of shear and shear friction evaluation). I have been comfortably numb and not penalized the joint. Technically, you're not supposed to place joints at places of high stress, but sometimes it's unavoidable. But to answer your question, the reduction is commonly ignored

b. Are any studies / reports available to compare corrosion of black reinforcing steel bars in a wall with and without waterstops for water and wastewater containment? Never seen any.

c. Does anyone ever coat the bars crossing construction joints in water containing structures? I thinking of joints that are in tension or flexure. For joints with no tension, I expect the maximum crack width is small for typical 0.5% rebar. No, I've see a lot of corrosion in structures, but it's no more pronounced at joints than away from them.

 

[SlideRuleEra]

I was taught that rebar corrosion in a joint with a waterstop is not a problem for the following reason:
There is no water flowing through the joint. Oxygen in the stagnant water in the joint, needed for corrosion, is quickly exhausted and with no flow, no more oxygen and no more corrosion.

There is a rough analogy from another time-tested field, buried, polyethylene encased ductile iron pipe:

http://www.SlideRuleEra.net

 

[JedClampett]

Ideally, joints should be spaced at about 25 ft. For a 50 ft. square tank, I'd place a joint 12'-6" from each corner. Eight pours, eight joints. And I'd make them wait 7 days between adjacent pours.

 

[STrctPono]

There is no water flowing through the joint. Oxygen in the stagnant water in the joint, needed for corrosion, is quickly exhausted and with no flow, no more oxygen and no more corrosion.

Don't wastewater treatment plants inherently operate off a system that relies on continuously replenished high concentrations of dissolved oxygen in the water in order to facilitate the aerobic activity needed for treatment?

Either way, on the water side, the waterstop is really your suspenders as your belt system is the coating/lining on the inside of the tank. On the soil side, you still have the presence of sulfate ions, oxygen, and water.

I believe that the OP correctly placed the waterstops on the outside of the rebar so long as the clear cover was sufficiently thick and distance between the waterstop and rebar was large enough to allow consolidation of the concrete mix. I too place waterstops on the outside of the rebar, not at the centerline.

 

[r13]

In the hydro and waste water treatment projects I'd dealt with, we usually place the water stop in the center, or equally spaced between outer and inner reinforcement, when 2 water stops were used. The minimum concrete cover was 4", no coating on the bars. However, the reinforcing were protected through cathodic protection using grounding grids at regular intervals.

Since only Jed answered my question on vertical construction joint, I guess the 25' spacing is the common practice for water retention structures, just curios to know if reference is available to back up this practice. Thanks ahead.

 

[ATSE]

STrctPono: Waterstops are not primarily for corrosion resistance. This is a secondary effect that is usually discounted.
Waterstops placed near the concrete surface is not standard practice in the US (at least Western US), and is susceptible to spalling.

If 2" to 3" thick (high pH) concrete cover will not sufficiently protect your black steel reinforcing bars from corrosion in your environment, you need to do something different than what ACI 350 assumes. Most likely this is a high performance coating at the surface, or different reinforcing steel (epoxy coated or galv or GFRP). Not near-surface waterstops.