Epoxy grout poured too low beneath skid

[tab101]

Hi all, after some advice here. We have an issue whereby our grouting sub-contractor has poured epoxy grout too low in some sections below a large skid (approx. 15m * 10m, 100tonne). The top of grout is currently between about 10mm and 40mm below underside of skid (it has been poured correctly under skid beams on the outside, but they didn’t notice the issue for the skid beams hidden away towards the middle). The skid has already been fully connected up to surrounding pipework etc and is held down by about 50 cast in hold down bolts.

The epoxy grout supplier has had an expert out who has said that as the top surface of the epoxy grout is extremely smooth and more than 24 hours has passed (so no chemical bond will develop), any grout poured on top will simply act like as a slip plane. Owner/design engineer has therefore instructed us to remove all the epoxy grout and redo the work (they are particularly cautious as due to the large vibrations caused by the equipment on the skid). This will require the skid to be fully disconnected and lifted not to mention the removal of the high strength epoxy grout without damaging anything else (the underlying concrete, the operational plant <10m away, etc). Obviously a significant amount of time and money involved in that rectification work

First question is – even if we do have to lift the skid up, surely simply the scabbling the epoxy grout and pouring on top will develop as much strength as the interface between the concrete and the epoxy grout or the epoxy grout and the skid? Removing it completely doesn’t seem necessary.

Second question is – would there be some way to achieve a satisfactory result without having to remove the skid? For example, some way to roughen the top of the existing epoxy sufficiently, whilst in place under the skid beams (note that the gap width is between 10mm and 40mm).

Any advice would be much appreciated.

 

[racookpe1978]

If the bolts are going to prevent shear movement, and serve as a vertical clamping force, then a suitable thin enough epoxy seems like it would not be harmed if the (now too smooth) epoxy_1 layer to epoxy_2 layer joint were sub-strength.

 

[Ron]

Go back to the epoxy manufacturer/supplier. Suggest the scabbling/roughening the surface for bond. As a further shear interface, small pins can be installed in the existing epoxy to mitigate a slippage plane, then encapsulted in the 2nd layer of epoxy.

Multiple layers of epoxy should not be an issue as long as interlayer bond is obtained and the manufacturer agrees. If you can't get the manufacturer to agree, then remove and replace.

 

[thaidavid40]

How is it that this grout can bond to the concrete, but not to itself? This seems counter-intuitive. And I agree with @racookpe1978, in that if you have a sufficient number of anchor bolts, you should be okay. Wouldn't the grout's sole purpose be simply to transmit bearing pressure from the underside of the skid plates to the top of the concrete? I fail to comprehend the problem with simply pouring (or injecting) a second "fill-in" layer of grout. Of course, this is conditioned upon the second grout layer being thick enough to meet the manufacturer's minimum layer-thickness limitations.
Dave

Thaidavid

 

[tab101]

We did ask about the bolts taking the shear and the epoxy being for bearing. I don't remember exactly what the design/owner engineers said, but essentially the epoxy needs to take the shear (the bolts are canister bolts -

http://rlrowan.com/images/pdfs/products/rowan adjustable canister anchor bolt.pdf

- so they are not really held in a set position laterally, they are free to move somewhat).

I will see if I can propose scabbling the existing grout and using pins, but this still raises a couple of questions:

1. Is it possible to scabble the existing grout whilst the skid is in position? Is there any chemical that will be able to roughen the surface sufficiently without damaging the skid steel which is only 10mm above in some spots? If not can this be done mechanically given the size of the gap width? Any ideas at all?
2. What types of pins do you think would be necessary and how would I go about determining or justifying the number of pins and their spacings?

Thanks

 

[Ron]

I would blast the surface with dry ice to roughen and leave no residue for cleanup. 10mm is tight, but doable.

 

[dhengr]

Tab101 :
You said the grout gap depth was 10 - 40mm in height. You didn’t say how wide the skid beam bottom flanges were, in terms of height and width for clearance for roughening. But it would seem to me that you should be able to get some sort of tool or flexible grinding disk in a 10mm gap to do some roughening of the first layer of epoxy grout. Just provide some means of protecting the A.B’s. from being nicked, stay an inch or so away from them. You also didn’t say what the various bearing pad vert. or lateral loads were. But, I would assume that the grout primarily transmits compression to the concrete, and the A.B’s. transmit any tension and hold down. How does the grout transmit much shear, even if done right in the first place, not very well in bearing at the canister A.B’s. or through bond or friction at the beam bot. flg.? What are the lateral loads which need to be transmitted, and at what locations on the skid? Why not provide some auxiliary shear blocks at a number of strategic locations, which bump up against the outside/edge of the bot. flgs. of some of the beams, and are anchored to the conc. for the shear loads. Then just fill the grout pockets for good compressive bearing and be done with it.

You have to prepare a plan of attack and explanations/answers for the questions which you can anticipate the epoxy manufacture or the owner’s engineer will bring up. Don’t say you don’t remember what they said, it’s your job to remember those discussions, and be reasonable able to refute their objections. Or, you will be doing it over.

 

[tab101]

Just as a bit of background, I work for the construction contractor, which means I don't have access to any detailed design calculations eg loads. In terms of what was said at the meeting etc, I'm not even the contractor engineer looking after/responsible for this scope. I decided to come along to the meeting at the last minute purely out of interest, and it would be the responsible engineer's job to take minutes (they didn't). At the meeting, the designer/owner engineers have pretty much said they want us to lift the skid and remove the grout. Everyone from my company (construction contractor) has just accepted this, but I feel that we only received this direction because the designer/owner engineers are a bit lazy and this is an easy direction for them to given. Hence I am looking into other options.

Bottom skid beams are up to 400mm wide, but there's also a non-structural steel plate spanning between beams in some sections, which further limits access here to 1.6m width of grout (edge of beam to edge of beam) - ie the furthest that grout is from any opening is 800mm. I imagine this would limit use of tools such as grinding disk.

Regarding use of shear blocks, might be a good idea in theory for structural purposes but I think this would be a hard sell, because all of a sudden you have these concrete blocks around the skid that don't appear anywhere else - you'd have to find suitable locations (ie away from pedestrians, pipe, etc) and then whose going to do the engineering calculations? Designer/owner won't want to do it, but we don't have any of the available info to do these calcs.

They don't want to change the design, so I just need to find the most cost-effective way to achieve the design intent. I'll call the designer tomorrow to get more clarity on how the shear is transmitted etc.

 

[racookpe1978]

We are reading this with interest - let us know what was brought up.