Green Cutting of Blinding Concrete / Mud Mat 1

[BigH]

G'day all,

Have a question - mainly due to what my counterparts are pushing . . .

Contractor has placed a blinding concrete for a foundation (some call it a mud mat, some lean concrete). Basically, it is a thin slab, as you all know, to provide a nice working platform for the placing of reinforcement and to prevent disturbance of the approved foundation due to precipitation and construction activity.

For construction joints to provide for a monolithic structure, green cutting operations would be carried out. For the blinding concrete - do you think it necessary to green cut? I am of the opinion, along with a very wised and experienced engineer of 60 years, do not think so. I've never seen green cutting of blinding concrete. Just seeking what you all's position would be.

p.s., if you have documentation to back up your opinion (either way) that would be useful. The counterparts with whom I am working have a "to the letter" mentality.

 

[Ron]

BigH.....No need to green cut the mud mat. It is a construction aid, not a true necessity (except in some cases). Will see if I have any documentation.

Depending on reinforcing of main slab, could lead to reflective cracking.

 

[hokie66]

I don't have a reason to sawcut a blinding layer, have never seen it done, never heard of it done. So I would not do it.

 

[phamENG]

No need. Your lean concrete is going to be incredibly weak - especially relative to the building slab. The only "restraint" it could provide would be through friction. So you can start there - look at the shrinkage strains in the building slab and determine the restraining force that would be required to prevent movement. Is it more than the friction between the two slabs? If yes, then point proven. If somehow it isn't, look at what kind of force would do to a couple inches of unreinforced, lean concrete. It'll destroy it. If anything in this scenario is restraining anything else, the building slab is going to restrain the mud slab.

 

[STrctPono]

I suppose that the advocates for sawcutting the lean concrete would say that reflective cracking could occur in the structural slab above. I don't buy it nor do I think that it is necessary.

 

[darthsoilsguy2]

No... but i am sitting here imagining what this nonsense would look like. since we're being conservative by the books... on that end of the rules of thumb you make a 1/3 slab thickness cut spaced 24x the slab thickness. so lets say we have a 1" thick mud mat. you're making 3/8" cuts at 2 ft on center... Checkers anyone?

 

[BigH]

Maybe my terms are different than others - but green cutting is not saw cutting. Green cutting is washing off the laitance and producing a surface where the coarse aggregate is exposed and the "paste" has been removed for a few millimetres.

 

[hokie66]

Now that I have refreshed my memory, I think BigH is correct in the terminology. And I apologize for interpreting "green cutting" as saw cutting, whether that was the OP's intention or not.

But green cutting of a mud mat would be even less logical than saw cutting.

 

[dik]

I still wouldn't cut it... in the unlikely event the cracks telegraph through... who's going to see them? If a SOG, then there should be aggregate between the two.

Dik

 

[r13]

Why try to boundbond the mud slab to the SOG if that's the intention of "green cutting"? I don't see the benefit of boundingbonding lower quality material to the higher one. You already have backup document - the design calculation. Did it include the mud mat for strength, or any purpose other than serving as a working surface?

[ADD] Green cutting is required if needs to transfer shear force, or increase friction, between new and existing concrete layers.
[ADD] BTW, I've never heard of "green cutting", I thought it is something relate to vegetation, turns out just like power/jet blasting, isn't it?

 

[Ron]

I did the same as hokie66. Still no reason to do it. No need to bond mud slab to slab.

 

[BigH]

I agree with you all - the mud made just replaces a bit of soil - we do not expect the mud slab to be monolithically part of the structure. Thanks for all your comments. Be glad you don't deal with rigid education.

 

[darthsoilsguy2]

I definitely thought your use of the words 'green cutting' must be a way others talk about sawcutting while the concrete was still green...
now my thoughts are:
1-is there a term for the cognitive bias where you come in thinking something is a bad idea so you don't consider the possibility that it could be a completely different bad idea?
2-i wonder if any of the inspection jobs i've worked on in the last 20 years had this technique called out on the plans... if so... [insert twiddle thumbs emoji]

 

[STrctPono]

Thanks for the clarification. I too thought "Green Cutting" = "Sawcutting Slab when Concrete was Still Green".

The only reason I would think you would need bond from the SOG to the mudslab would be if there were specific friction values you needed to maintain for sliding or something. Perhaps this would be required under a retaining wall footing, but a large SOG mat with likely downturned edges.... I would not think that exposing the aggregate would be necessary.

 

[BigH]

The Company for which I worked some 40 years always, in their hydro projects added a clause in the concrete specifications as given below - or something quite similar. And, no, we wouldn't do the mud slab/blinding concrete (for the Brits).