Wetting of Construction Joints 9

[Kramer]

I have designed a wet well for a pump station (water only) with 18" thick walls on a 24" thick base slab. I have shown a 3 1/2" wide by 1 1/2" deep keyway in the top of the base slab at the center of the wall with a bentonite water stop in the keyway. Both ACI 318 and 350, Sections 6.4.2 call for construction joints to be wetted prior to the pour and all standing water removed. The walls are over 20'-0" tall. I am concerned that if the top of the base slab is wetted, there is not a practical way for the water to be removed from the keyway. I am also concerned the water may activate the swelling properties of the waterstop prematurely. Any thoughts on this?

 

[JAE]

Don't use a keyway. Simply specify a 1/4" roughened amplitude surface on the joint and use the waterstop over it.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[SlideRuleEra]

...keyway in the top of the base slab at the center of the wall with a bentonite water stop in the keyway.

Agree with JAE, don't use a keyway.
But even if a keyway is used, bentonite waterstop should not be in the keyway:

Concern about getting benonite waterstop wet prematurely is valid:

See: Tech data for this brand of bentonite waterstop.

I don't recall guidance on where to place waterstop in water containing structures with thick walls and two rebar mats (with high water table). We have used two waterstops, both just inside the two rebar mats.

http://www.SlideRuleEra.net

 

[Kramer]

Thanks to both of you for your comments. I agree that if I use a keyway I should move the waterstop up out of the keyway. I would need to check the required clearances though. Better yet to just omit the keyway and roughen to 1/4" as suggested. What is your experience regarding the ACI requirement for wetting? Do wall to footing construction joints typically get wetted in the field?

 

[oldestguy]

If you do not want a bond between the slab and the walls, yes wet it. However if you want a bond between the slab and the walls, create a roughened and BONE DRY surface to form a bond, with immediately before pouring (if possibe) place a paste of Portland cement and water. Sometimes that's better than the strength of new concrete.

 

[BridgeSmith]

If there is concern about a bentonite waterstop, why not use a rubber one instead. Where a waterstop is required on our bridge structures, we've always only specified a neoprene waterstop.

We also specify an epoxy resin bonding compound at all cold joints with normal concrete, but not for silica fume modified concrete.

 

[Kramer]

Oldestguy, ACI 318 and ACI 350, Sections 6.4.2 call for the concrete at Construction joints to be wetted immediately before concrete is placed. I wonder how often this actually happens.

 

[Agent666]

The reason its 'wetted' is so the water from the mix is not sucked into the previously poured surface, resulting in poor concrete quality at the interface. I'd disagree with oldestguys approach of having the concrete bone dry, but I'd agree with wet = water all over the place when you pour is a bad thing which will equally result in a weak zone at the interface due to the additional water mixing in at the interface.

Wetted simply means the concrete should be saturated, but there is no standing water on the surface, the technical term for what is required is called Surface saturated dry.

Try doing a sika grout repair or grouting under a baseplate for example without achieving the surface saturated dry condition and see how long it sticks before coming off.

I wonder how often this actually happens.

less than we like to admit I bet. The assumption is the contractor knows he needs to do it, but does he really understand, I doubt it.

 

[Kramer]

Thank you all for your comments. You have helped a lot! It's nice to have a forum like this.

 

[oldestguy]

I see that Agent666 has not cored and tested a joint done as I describe. Period.

 

[Agent666]

Hi oldestguy, no I haven't, I'm simply discribing what is viewed as the best practice guidance in every construction code I'm personally familiar with (UK, AU, NZ, US), and the reason for the requirement. Happy to review any evidence to the contrary though, does your local code not require any wetting of the concrete? Do they give a basis for taking an alternate approach to wetting the previously poured concrete at construction joints?

 

[BridgeSmith]

OG and Agent666, it seems you guys are proposing alternate, but not antagonistic, methods of preparing the surface. I imagine slathering on cement paste with an adequate amount of water before pouring, has a similar effect to getting the surface hydrated, but without free water, prior to pouring. I suspect both methods will work adequately to provide a good bond; it's just a matter of which method can be accomplished effectively and conveniently in a given situation.

For our bridge concrete repairs and construction joints with standard concrete, our construction specs require the the contractor to "wet [the joint surface] immediately before placing concrete", and apply an epoxy resin bonding compound. When using silica fume modified concrete (SFMC) for bridge deck repairs, we specify to "keep the surface wet for at least two hours before placement", blow out any standing water, and apply a slurry of the SFMC to the surfaces. Our successful practice would indicate perhaps the combination of both approaches (if you're willing to pay for it) is the best way to go.

 

[SlideRuleEra]

The method oldestguy suggested is addressed in Portland Cement Association (PCA) "Design and Control of Concrete Mixtures", Fifteenth Edition (2011), Pages 281 and 282:

Don't take my word for this, buy the current (Sixteenth Edition) from PCA,
or Google on "Design and Control of Concrete Mixtures"... amazing what several of the .pdf links contain.

http://www.SlideRuleEra.net

 

[BridgeSmith]

Thanks, SRE. According to the PCA, the surface can be "dry or damp" as long as there is "no free water", and slurry can be "scrubbed" in to the surface (which sounds like a lot of work) or a bonding agent may be used.

Given the labor-intensive nature of applying the slurry to the surfaces of our deck repairs and overlays, I doubt that ever happens in practice. The high-slump consistency of the SFMC, probably makes it unnecessary anyway, though. I'm pretty sure they always make sure the concrete is damp, but without any free water, before they pour.

 

[Agent666]

Thanks all, for any method there seems to be some caveats on 'getting it right' that the contractors methodology needs to address!

Addressing the original query, given the lack of access at the base of a vertical wall pour I'd imagine getting the paste scrubbed in would be a bit of a non starter, as would using an epoxy tie coat as these also need to be applied (usually) immediately prior to placing the concrete (I know from experience Sikadur 32 has a very short application window and most contractors hate using it). When I have seen it used there is access to the joint once the formwork is up, or the formwork is hurriedly erected after the application of the tie coat. If something goes wrong that takes extra time you need to follow the manufacturers instructions for reapplication.

Having the keyway the other way up might address at least some of the issues with water collecting there if you need the keyway/shear key for design reasons, obviously a bit more work in the first pour to form the upstand, but it could be an option. For watertight structures (mainly tanked building work I've been involved in) I'd usually see a small kicker/upstand nib the thickness of the wall poured as part of the slab pour, this gives you something to fix/butt up the formwork for the wall against and keeps the waterstops out of any water that naturally pools on the slab. This also aids in addressing the natural weakness from a watertightness perspective of the construction joint being right at the floor level. Usually this type of detail lets the water drain out via the natural gaps in the formwork (though use foam closure strips between the kicker and the formwork to prevent grout loss, water used to wash out and wet the concrete will drain through them).

 

[hokie66]

Dry or damp, I doubt it makes much difference. The part I cringe about is the suggestion of an epoxy bonding agent, which for the reasons Agent666 described, can instead be an effective debonder.

 

[BridgeSmith]

Yeah hokie66, we're pretty wordy with our directives for the use of the epoxy bonding compound. The gist of it is, if it gels before they're ready to pour the concrete, they have to sandblast it off and try again. We also don't rely on it for shear strength, more to provide a watertight seal than anything else. Anything that requires substantial shear capacity gets a keyway or at least an intentionally roughened surface (1/4" minimum relief).

 

[hokie66]

Wordy doesn't get results. I hope you are there watching it. Instead of specifying those bonding agents, I prohibit them.

 

[BridgeSmith]

We have field engineers and inspectors who are supposed to be watching it all carefully. As I said, we don't rely on the strength of the bond at the joints where we use the bonding compound, anyway. There's always reinforcing crossing the joint, and where shear capacity is required we make them form in or cut in a keyway.

 

[hokie66]

I hesitate to change subjects, so you don't rely on the concept of "shear friction"?