Concrete nib to existing slab - concrete or grout? 1

[NathanNZ]

I am designing a new concrete nib that will sit on an existing suspended slab or beams, 150mm high, 90mm wide. The nib is a retrofit for waterproofing and durability of the non load bearing exterior walls to sit on. There is roughly 600m of nibs to be constructed throughout the 3 storey building. The structural design of it is straight forward with threaded or reinforced bars being epoxy anchored into the existing slab and tied to a straight bar that runs along the nib for shrinkage and crack control. Sometimes the nibs will stop and start between RC columns, sometimes they will be a continuous length up to 20-30m. The nib will be coated in an epoxy membrane to prevent moisture ingress.

The architect has nominated a grout, however they are saying if a concrete could be used there would be a huge saving.

My question is could this be constructed from concrete that is a fine aggregate, flowable concrete mix - say 30-40MPa, or should we be using a low shrinkage high strength grout?

The issues I see with concrete are shrinkage and adhesion to the existing structure, but i'm not sure if it will cause any problems as it is reinforced and has a membrane over it. Scabbling of the existing slab wont be done as the loads are very small.

 

[STrctPono]

Well the "Nib" terminology is a new one for me.

I would definitely ditch the idea of the non-shrink grout. Grout is great and has its uses but I would say this is not one of them. Too much cement, too fine of aggregate, no coarse aggregate, low durability, prebagged, expensive.

My choice would be to spec a 30 Mpa mix and require a shrinkage reducing admixture to be added to the mix. We prescribe a specific type and dosage but I know that is not common. Limit the water/cement ratio AND specify a MAX cement content. Don't let them add too much cement. However, at this thickness, they will probably need to go with a smaller aggregate (maybe something like 3/8" aggregate). They will inherently need more cement with the smaller aggregate and this will thus increase your shrinkage.

Place control joints in the nib every 5 meters or so. I would utilize a 1/4" thick masonite board at about 1/2" deep with chamfer strips either side built onto the inside of the forms. They can then score the top while the concrete is still plastic at the control joint locations.

You mentioned adding some horizontal reinforcing. Good idea. I would add a #3 or #4 bar top and bottom and make sure that it is non-ferrous. GFRP would be my choice. You can either stop it at the control joint locations or run it continuously through.

Have them roughen the surface of the exist. slab before placing the nib and make sure that it is saturated surface dry. Since you won't be able to really properly waterproof the construction joint between the nib and footing, I would utilize a hot-dip galvanized anchor rod for your drill and epoxy application.

The concrete supplier can add a superplasticizer to the mix for workability/flowability if the masons would like. In my office, us Engineers never control the slump and leave this up to the Contractor/Ready Mix supplier.

What is the red thing you sketched on the drawing? Flashing? Waterproofing?

 

[NathanNZ]

STrctPono, thanks for your response. The 'nib' terminology must be a New Zealand term, very commonly used here . What would you call it?

Couple of queries:
[ul]
[li]Why would you specify a non ferrous longitudinal bar? I was going to specify a #4 (12mm diameter) standard reinforcing bar . The cover is minimal but because it is on the interior side it is okay (it is drawn on the wrong side on my drawing).[/li]
[li]The red line is the epoxy membrane that will be applied over the 'nib' for waterproofing. This will prevent moisture and water ingress so the use of galvanized anchors aren't necessarily required.[/li]
[li]What does saturated surface dry mean? Saturate surface to clean, then dry? Perhaps a primer could be laid down?[/li]
[/ul]

I do like the idea of specifying a low shrinkage concrete, cheers.

 

[Tomfh]

We generally do these with normal concrete, with a longitudinal bar central, tied to L bars which go into the slab.

New concrete won’t generally stick very well to the existing concrete. If you really want new concrete to stick like shit to existing concrete use an epoxy primer. Megapoxy or the Parchem version or something like that.

The nib/hob doesn’t need to be 100% waterproof though does it? Isn’t there a waterproofing skin doing that job?

 

[r13]

Will masonry block work?

 

[Ron]

Your nib is our curb. Common for shower enclosures and other waterproofing confinement. I agree with StrctPono's recommendations.

@retired13....not a good application for masonry.

 

[r13]

Ron,

Why? Because it is exterior application, or other reasons? I don't think grout is a good idea, and while I think concrete is good but needs to pay attention to surface preparation, usually I prefer to roughen the existing concrete surface, but for 600 m, the cost seems high.

 

[STrctPono]

Why would you specify a non ferrous longitudinal bar? I was going to specify a #4 (12mm diameter) standard reinforcing bar . The cover is minimal but because it is on the interior side it is okay (it is drawn on the wrong side on my drawing).
The red line is the epoxy membrane that will be applied over the 'nib' for waterproofing. This will prevent moisture and water ingress so the use of galvanized anchors aren't necessarily required.
What does saturated surface dry mean? Saturate surface to clean, then dry? Perhaps a primer could be laid down?

Well your curb is rather narrow. Even with it being on the interior side, if you are using 1/2" dia. anchor bolts and you place the longitudinal bar on the interior side of the nib, you are looking at 1" (25mm) of cover. Does your code allow that kind of cover even for interior? Also, if you use steel reinforcing, the iron workers will have to break the bars at each control joint location. If you use FRP reinforcing you can run the bar through all the CJ's and you solve your minimal clearance issue.

I still would use hot-dip galvanized hardware. It's pennies more expensive for a lot longer service life. Aren't you guys in a pretty corrosive environment in NZ?

Saturated surface dry (SSD) is nothing more than ensuring the concrete has been wetted sufficiently before application of the new concrete. And I'm not talking about hitting it with the hose from the water truck one time. Ideally, they should spray it multiple times over the course of an hour or so until the concrete doesn't soak up the water anymore. The surface should be visibly dry with no standing water. This is probably the biggest issue I have on site with Contractors. It is almost never done yet is very important to ensure good bond of new concrete to existing concrete. Tomfh mentioned using an epoxy bonding agent. I am not too familiar with Megapoxy but I typically use products made by Sika, Mapei, or Euclid. All those epoxy bonding agents still require that the concrete be SSD. It adds one more step to the process but if done properly will get you tremendous bond.

Do nothing - Poor Bond
Roughen and SSD - Good Bond
Roughen, SSD, and Scrubcoat a Polymer - Great Bond
Roughen, SSD, and Epoxy Modified Bonding Agent - Excellent Bond

 

[jayrod12]

I tend to avoid bonding agents. Do they work? Sure, if installed exactly as per the manufacturer's written specifications. If they aren't, you may as well have put a piece of poly between the two pours because it prevents bond.

roughened and SSD are all you need. Potentially a paste of cement and water can be used and has a higher success rate with less installation requirements. But generally speaking is not necessary. I can't imagine the required shear stress is so high that you need a monolithic level bond between the two pours.

 

[JAE]

We've had similar experience with bonding agents that jayrod12 mentions. A cement slurry seems to work better. Depends a LOT on how careful the contractor is in removing dust and loose debris and preparing the surface (SSD per STrctPono).

 

[Tomfh]

I am not too familiar with Megapoxy but I typically use products made by Sika, Mapei, or Euclid. All those epoxy bonding agents still require that the concrete be SSD.

I thought epoxy primers can go on dry concrete?, as it’s two part epoxy resin, not a cementitious system.

 

[STrctPono]

I thought epoxy primers can go on dry concrete?, as it’s two part epoxy resin, not a cementitious system.

I can't speak for all epoxy primers, but the ones I use are all based on the same premise. Three component, water based epoxy, with cementitious material added. See snipet from Euclid's Duralprep AC data sheet. It's the same for Sika and Mapei's products. They are good products and if applied properly get bond values of >2500 psi.

As jayrod and JAE have said, it is very easy to incorrectly apply the bonding agent. They have multiple mixing components and typically have short open window times so yes, it can be a bond breaker if the Contractor doesn't know what they are doing. I don't specify them unless I need them and/or trust that the Contractor knows what they are doing.

For your application, roughen and SSD should be sufficient.

 

[Tomfh]

Strctpono,

I’m thinking of epoxy primers like this:

https://www.parchem.com.au/public/pdfs/data-sheets/Nitobond-EP-TDS.pdf

These are two part epoxy resins similar to those used to build boats and planes. You mix it, paint it on, wait a half hour or so in til it goes tacky, then apply your new concrete/mortar. It isn’t using cementitious adhesion, it’s epoxy glue connection between the two sides. It says either dry or SSD is ok.

I really like these. I’ve used them at my house to fix my pool. There’s no chance of concrete repairs popping off with this stuff.

I’m not as familiar with the three part ones.

 

[STrctPono]

Tomfh,

This does look like a fine product. I only have 2 concerns... They don't give any of the testing standards for the reported results on their data sheet and this product seems to have a very short open window time. Seems as if they call it a "max overlay time" and it is only 90 minutes. The products I like to use are typically 16 to 24 hours.