Batch plant broke down. . . 1

[rholder98]

. . .40 yards into a 200+ yard pour for a section of slab in a carousel aeration basin. Now a large section of the slab has about half the thickness in place, with most of the top mat of steel exposed, and some of the bottom mat exposed. They at least had the quick-thinking to 'spread out' what they had already placed up to floor level.

Contractor wants to clean out the loose stuff, chip out under the top bars, and continue when the plant is back up. Basically it leaves a horizontal construction joint in the slab for this section. In theory there is no leakage.

Slab thickness is 10", with #5 at 12" each way top and bottom. "Good slab" would be probably about 6" thick on top of this.

I'm leaning toward making him rip it out, but want others' opinions. Thank you in advance.

 

[msquared48]

What's the soil like there - Porous or silts? - looks like you are in a low spot - seasonal high groundwater level?



Mike McCann
McCann Engineering

 

[rholder98]

It's very good soil, mostly caliche at this depth. Groundwater is not an issue.

 

[cvg]

how will you assure a good bond at the cold joint? and if you don't how will the two layer floor slab react to the full load? Will it have sufficient strength? Will he need to add additional steel and make the floor thicker to accomodate the cold joint? Have the contractor provide answers to these two questions (calculations, sealed by structural) and then evaluate his proposal. Charge him for your additional analysis as well.

 

[rholder98]

I think bond would be okay, because the surface is so rough. I could require a bonding agent, as well. The thickened slabs under the walls will act in flexure, but the slab itself won't see much.

What really concerns me most is: a)the roughness that gives me the bond could cause cracking in the surface when the new concrete shrinks (then again, the slab is restrained anyway--would this be any worse?), and b)I don't know if what is there was properly consolidated before they quit, and I know it's not being cured.

 

[LittleWheels]

Cracking due to extra restraint is likely to be frequent but hairline. Autogenous healing may take that up with any luck.

 

[shin25]

"b)I don't know if what is there was properly consolidated before they quit, and I know it's not being cured."

If this part was not really cured then, its structural valus is now in question.

Before you proceed in any direction, evaluate the condition of the already poured slab in depth.

 

[MarcbSE]

If there is sufficient freeboard at the top of the walls, could you consider the concrete in place as a mud mat and place the full 10" thickness above what is in place?

This would be easier and cheaper than having to rip out what is in place.

 

[Qshake]

I agree with MarcbSE and would otherwise note that in the absence of that type of workable solution would not have reservations about taking it out.

I don't think the effort in removing it is that great given the many tools and equipment that contractors have at their disposal to demolish.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[MarcbSE]

I would also have no hesistation in removing the existing concrete if the walls are not tall enough to raise the floor elevation.

 

[rholder98]

Thanks to all for the responses. The bottom line is this tank has to be in use for 50+ years, and has to be watertight. I don't like being the unreasonable engineer, and I always try to work with contractors to resolve issues like these, but this is a big one. What he wants to do might work, but it might not. The might not is too much for me. He's going to have to remove it.

 

[Zambo]

I'm a contractor and would of course always like to find a solution which wouldn't involve complete demolision. In this case I just cannot see an alternative. In my opinion the contractor did exactly the wrong think by spreading the concrete out and creating a horizontal cold joint. He should have created a vertical joint when the concrete supply stopped and then thought of a way to guarantee to you that with some effort a good joint could be formed.

Now he has no option, he has to remove the concrete carefully so that the rebar can be reused.

My suggestion is just call him into your office, tell him and then refuse to discuss any further.

By the way is this event covered by insurance?

 

[BigH]

I sort of like the idea of putting the "design" slab above what he has put down now and call the existing first pour a blinding mat. If the structure can be raised the 300mm or so, this seems to me, as it does to others above, as a suitable method - a lot less costly to the contractor and a lot less time lost to the owner/client.