Pouring cement grout instead concrete in containtment slab

[patelam]

I have a 2ft thick concrete containment slab for a lube oil tank. The slab has layer of rebars at top and bottom surface. The slab was exposed to the fire. The concrete from slab will be tested to determine if the concrete is deteriorated or not. My guess is that the portion of the concrete is deteriorated, and the damaged concrete will
have to be replaced.

The operation team is talking about using a cement or epoxy grout instead of concrete to replace the damaged portion of concrete slab. We don't know how much thickness of concrete will be replaced. But for worst case scenario, assume replacing full thickness (2ft) of concrete. The new rebars in the grout will be embedded into adjacent existing concrete.

Can you please tell me what are the drawbacks of using cement/epoxy grout instead of concrete? The degree of shrinkage/expansions under temperature may cause a crack at junction. Any other drawback you see?

Any other better suggestions as an alternative to using concrete?

 

[JAE]

Why would you not just patch or replace with concrete? What's the issue with that?
Not sure I understand why you are looking at alternative materials.

 

[AskTooMuch]

I think he's debating to use "regular" concrete as opposed to "grout" like five star structural concrete or equivalent. I normally use the later for concrete repair unless the volume to repair is huge.
Either way, you should properly prepare the surface by roughening it and making it saturated surface dry (SSD) before pouring new concrete or grout.

 

[Tomfh]

It all depends on the extent and thickness.

For patching we typically specify bagged concrete repair mortar.

For large volumes regular concrete makes sense.

Even regular sand and cement can work in some circumstances.

The bond to the existing concrete it important. For important structures a liquid epoxy primer works best.

 

[patelam]

Sorry for not giving the complete information. The intent to replace concrete with alternate material is to avoid long wait setting time of concrete.

 

[TLHS]

What's the loading situation here? Is it basically just pure bearing through the concrete into the ground or is there any sort of significant expected bending from the various load effects? How long was it in place beforehand? How big is the tank and how high is the center of gravity? Does this bear directly on grade?

At full thickness replacement you'd definitely want to be pouring concrete. You need the aggregate at that thickness. If you're just chipping surface concrete and replacing the top couple of inches, a repair grout of some sort might make sense, but you would want to look at durability considerations.

If you're just worries about repair time, you can get a high strength concrete or any of the high early strength mixes that are available now. You'll have to think about the extra heat and make sure your curing is well thought through, but you can hit target strength really quickly these days if you really really need to. The mixes aren't free, but they're not insanely expensive either.

If this is on grade, the strength you need for operational load is potentially minimal and maybe regular concrete is fine if you have a couple of days and look at it carefully.

There are lots of concrete repair specifications out there. Take a look at them. I don't have an example in mind right this second, but when I've had to look at this before there are a lot of full on specifications and design guides for concrete repair by various industry groups.

 

[AskTooMuch]

If you want less curing time, then choose high strength concrete. Call the concrete batch plant where you usually buy concrete for their recommended concrete mix for faster curing time.

If you choose "grout" like five star, etc., then call the manufacturer or read their product specs which one has faster curing.

You sound like a mechanical/maintenance engineer doing civil/structural stuff.

 

[LittleWheels]

Most epoxy repair materials and most high early strength cementitious repair materials are quite exothermic while curing and thick sections (e.g. 2' thick) will be excessively cooked from their own heat production.

Look at removing fire-damaged concrete (fairly easy to determine) and probably to behind the outer layer of reinforcement. Reinstate with a repair mortar, either epoxy or cementitious. Read the product data sheets pretty thoroughly before choosing a material as limited working times can be impractical for large repair areas. Maximum and minimum thickness limitations need to be considered. The reduced thickness (compared to complete replacement) should mitigate most issues with excessive heat while curing but that should be confirmed with the material manufacturer.

Check that the reinforcement isn't fire-affected. Unlikely but if so, that would significantly change the repair requirements.

Differential stiffness/ strength between repair material and substrate can be a problem but usually is not. You are considering this issue already.

 

[patelam]

@TLHS - The tanks sits on this slab (no pedestals for support), exerting bearing load on it. I don't expect any bending or other load than bearing.

@AskTooMuch - The operation team wants to use epoxy grout instead of concrete for repair. I am trying to find out the reasons to justify that the epoxy grout isnt a good option to do such concrete repair work.

And thank you all for the response.

 

[AskTooMuch]

Patelam, I'm assuming this is an expense project. This will be unnecessarily more expensive than needed. Epoxy grout are normally used for vibrating equipment. Unless they meant epoxy the rebar into existing concrete.

Call five star or sika, etc.. and ask what product they recommend for your situation.

If you need to put new rebar and epoxy that rebar into existing concrete, tell operations to give this to an engineering company instead as you are not a structural engineer.

 

[AskTooMuch]

Rereading your last response I have a feeling you and operations may not be understanding the difference between epoxy grout and cementitious grout. Are you sure operarations said or meant epoxy grout?

 

[TLHS]

Epoxy grout is not a cheaper or faster solution than a properly specified concrete would be. The working time on epoxy grout is very aggressive, and you have to coordinate the placement and mixing pretty heavily. You also aren't going to be able to work your surface finish the same way you can with concrete. You're going to end up with a more expensive, harder to install repair with a worse outcome.

Epoxy grout is worth the trouble for the specific areas that it is exceptionally good at. Without those benefits, it's just trouble for no reason.

edit: and if you're talking cementitious grout, your thicknesses are just too much for a grout without large aggregate, if you're really talking about a full depth pour. Once you add aggregate, it's basically just concrete.

 

[patelam]

Thanks @AskTooMuch, I will check with Five Star or Sika if they can recommend any product. I think I will be able to manage with the embedment length of rebar epoxied into the concrete.

Thanks @TLHS. I haven't used epoxy grout before so wasn't sure if than can be an alternative or not. But you have clarified that now. Thank you very much for that.