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Wetting of Construction Joints
7

Wetting of Construction Joints

Wetting of Construction Joints

(OP)
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?

RE: Wetting of Construction Joints

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

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RE: Wetting of Construction Joints

Quote (Kramer)

...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.

www.SlideRuleEra.net idea

RE: Wetting of Construction Joints

(OP)
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?

RE: Wetting of Construction Joints

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.

RE: Wetting of Construction Joints

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.

RE: Wetting of Construction Joints

(OP)
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.

RE: Wetting of Construction Joints

2
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.

Quote (Kramer)

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.

RE: Wetting of Construction Joints

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

RE: Wetting of Construction Joints

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

RE: Wetting of Construction Joints

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?

RE: Wetting of Construction Joints

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.

RE: Wetting of Construction Joints

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.

www.SlideRuleEra.net idea

RE: Wetting of Construction Joints

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.

RE: Wetting of Construction Joints

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).



RE: Wetting of Construction Joints

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.

RE: Wetting of Construction Joints

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).

RE: Wetting of Construction Joints

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

RE: Wetting of Construction Joints

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.

RE: Wetting of Construction Joints

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

RE: Wetting of Construction Joints

"...so you don't rely on the concept of "shear friction"?"

We don't rely on the shear capacity of concrete at joints with epoxy bonding compound. If we require substantial shear capacity at a joint, we provide either a keyed joint or a rough construction joint with reinforcing crossing the joint that is developed on both sides. For a shear-resisting construction joint, we check the shear friction capacity per AASHTO. We would not use the bonding compound for such a joint - it would be wetted as I described earlier.

RE: Wetting of Construction Joints

Like reinforcement and jointing of slabs on ground, there are lots of ideas about how to make joints in walls. I doubt there will ever be a definitive answer, or at least one that everyone can agree on.

RE: Wetting of Construction Joints

5
I have followed OG and his repeated quest that 'bone-dry substrate is best' for some time. As a guy who get lots of cement paste under my fingernails, along with direct hands-on experience with pre-bagged cementitious concrete and field-batched concrete, alike (mainly for concrete repairs and deck overlays), and somewhat uniquely, with bond testing of said products and procedures, my n=1 experience is: substrate to be ALWAYS SSD.

ACI's Concrete International magazine in 2018 had a recent article on this and related factors:

Effect of Common Surface Pretreatments on Shear Strength of Bonded Concrete Overlays by Andrew S. Pultorak and Frederick R. Rutz Article available here: ==> Link

They looked at various substrate pretreatments:



With some informative results, that I highlighted:




OG's position is (was?) a common one, based upon research at the time by Felt from 1956:

Quote (CI)

Felt investigated substrate moisture content and the use of bonding agents. He concluded that a “dry” substrate was preferable to a “damp” one and that a cement or cement-sand slurry bonding agent produced a superior bond.

However, the Bureau of Reclamation research disputed Felt's findings:

Quote (CI)

However, some studies contradict Felt’s 1956 conclusions regarding moisture content of the substrate. Wall and Shrive found that some moisture in the substrate was beneficial to bond strength. In a report for the Bureau of Reclamation, Bissonnette et al. found that optimum moisture content in the
substrate surface lies somewhere between 55 and 90% of saturation, though they state that “…fundamental issues remain unsolved with regard to moisture conditioning of the concrete substrate…”.

Anyway, the article is a good read. Form you own opinion. I shall continue to do what I have been doing for decades - substrate is SSD.

And finally, as a contractor I hate epoxy-based bonding agents, so therefore as an engineer, I similarly hate them! Office-based engineers need to listen to their experienced field contractors when it comes to bonding agents, and don't always listen to the manufacturer's rep on the 'magic' they sell.



RE: Wetting of Construction Joints

"And finally, as a contractor I hate epoxy-based bonding agents, so therefore as an engineer, I similarly hate them!"

As one of those "office engineers" (I have some field experience, just not much with concrete pours), I have to ask, why do you hate them? Is it because they are difficult to use or because they don't perform well?

RE: Wetting of Construction Joints

No argument from OG. Now that's the way to prove or disprove your arguments. Thanks. We need more of these types of statements backing up opinions. I'm adding my star.

RE: Wetting of Construction Joints

Using an epoxy bonding agent inside of form work isn't something you want to do unless a dual function of the bonding agent is desired, e.g., as a waterstop. Greater requirements for reinforcement in seismic zones, post Loma Preita, have pretty much rendered this application defunct anyway.

Back in the day, we would spray the epoxy from the end of a 6 ft. spray bar. There are not many contractors that have the equipment or capability to do so. It is definitely a specialty contractor job.

Epoxies are still great for bonding a topping slab, subject to a lot of traffic or slab-to-slab repairs. You just have to pick the right formula. It should have an potlife for a gallon mass of at least an hour and an open-time (time to place concrete) of 2 to 4 hours. Epoxy formulas based on Polysulfides, Amidoamines or Amidoamine/Aliphatic Amine blends work best. Something like MasterEmaco ADH 326 (formerly: Concresive Liquid LPL) or Chemco Systems Liquid-LWL, is what to look for.

RE: Wetting of Construction Joints

Quote (HotRod10)

As one of those "office engineers" (I have some field experience, just not much with concrete pours), I have to ask, why do you hate them? Is it because they are difficult to use or because they don't perform well?

Sorry, HotRod10, upon re-reading my reference to 'office-based engineers' may have come across as derogatory - if it did, I apologize.

To answer your question, it is as hokie66 repeatedly states in these type of discussions over the years - in way-too-many occasions, they become de-bonders.

On concrete placement day (somewhat regardless of how small or large the placement is) the most experienced concrete contractor has a very dynamic scope of work to undertake - manpower, equipment, materials delivery, weather, etc. etc., so add-in an epoxy bonding agent and it is another item that needs to be accommodated flawlessly. Based upon the post-installed field bond testing I have done, those projects that used epoxy-based bonding agents have a higher proportion of failure at the bond-line on the NEW 'wet' concrete side, indicative of epoxy set issues.


Quote (OG)

No argument from OG. Now that's the way to prove or disprove your arguments. Thanks. We need more of these types of statements backing up opinions. I'm adding my star.
Thank you.


Quote (epoxybot)

Epoxies are still great for bonding a topping slab, subject to a lot of traffic or slab-to-slab repairs. You just have to pick the right formula. It should have an potlife for a gallon mass of at least an hour and an open-time (time to place concrete) of 2 to 4 hours. Epoxy formulas based on Polysulfides, Amidoamines or Amidoamine/Aliphatic Amine blends work best. Something like MasterEmaco ADH 326 (formerly: Concresive Liquid LPL) or Chemco Systems Liquid-LWL, is what to look for.

Absolutely, and all the manufacturer names you have dropped are great products - especially and notably, IMO, Chemco (small team, great resins and technology).

Unfortunately, those tasked with writing specifications on such uses are often engineers/specifiers who do not have direct/relevant experience with such products for the given application and the relevant field variables.


RE: Wetting of Construction Joints

Thanks for the explanation, Ingenuity. I definitely see the point you guys are making. Hopefully, our field guys are holding the contractors to what we specify in our notes so that we don't have those issues. Given the nature and location of the work where we employ the epoxy bonding material, I'm not confident we'd do any better attempting to reach an SSD condition at the interface, and it might be more difficult to identify whether the condition actually exists.

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