Pouring new concrete slabs in brownfields 2

[MatBrener]

I am an agricultural engineer working on a development in a brownfield site in Philadelphia. The project is the construction of a 10,000 square foot hydroponics facility that will include several 100'x 20' low-walled reservoirs. The site is over 75 years old, the original slab is still present and in terrible shape. Is there a manner in which new concrete slabs can be formed on-site without removing the old concrete slabs. It is vital that the reservoirs not crack.

 

[phuduhudu]

It will depend not so much on the quality of the existing slab as on the quality of the subgrade. I would suggest drilling some holes through the slab and doing some tests on the subgrade of the slab to a depth of about 450mm. Then if the subgrade turns out to be to the sort of spec you would want if you were building new reservoirs on it go ahead and cast the reservoirs right onto the old slab. It will give you a nice surface for chairing your reinforcement on and working on generally. As it is you will have a fairly stiff structure because the side walls of your reservoirs will be 100'long beams. The problem might be if one end of the reservoir has slightly more compressible soil than the other end. Your reservoir would have to crack to relieve that kind of stress. The important thing then for your soil investigation is uniformity over a large area. Small pockets of poor soil will easily be bridged by the beams. Carl Bauer

http://www.bauerconsultbotswana.com

 

[dik]

What is a brownfield site?

 

[MatBrener]

Carl-

Thank you for that information. I hadn't considered the strength the side beams will be adding to the structure, but it makes great sense.

There are areas on the old slab where settlememt has taken place, where chunks are missing and where pieces of old foundation walls have been left in place. I am assuming that we will be able to grade the area with fill to keep the thickness of the new slab from variating too greatly over its area. Do you think this is important, and do you think it will work (again the goal is to keep the slab from cracking due to inequalities in thickness).

dik-

a brownfield is an urban parcel that was the former site of some industrial facility. Normally these parcels (to be dubbed brownfields) have undergone some demolition, and an environmental assessment that has characterized the property as a major environmental risk. Often they are cleaned by the USEPA or state agency to some degree of safety but are still difficult properties to redevelop - often because they are located in blighted neighborhoods. Philadelphia, like a lot of large cities has many acres, often hundreds of acres, of these properties available for development. Ironically, agriculture may be one of the best uses of these lots.

 

[dik]

Is it possible to 'proof roll' the area to determine any 'soft spots', add compacted engineered fill and construct on top using a mat type foundation? The existing slab areas could help bridge soft spots, athough they may cause problems by creating 'hard' areas.

Can you advise the client that due to the uncertainty of the founding material that differential movement may occur; this is unlikely to be catastrophic and that if movement occurs then it will have to be addressed at that time. Can you detail a level of ductility into the structure that will accommodate slight movement (using liners, etc.?)

 

[phuduhudu]

Pieces of old foundation wall could be more troublesome because these will give you very localised hard spots. The idea of putting a layer of fill over the whole lot sounds sensible if it is so variable. Perhaps it would be an idea to break down the old foundation walls to say 400mm below underside of reservoir level and fill to give you some cushion above these.

The reservoir will either have to be designed as waterproof concrete (limiting crack widths) or have liners. As dik suggested the liner option might be the one to go for since you just can't be sure of the subgrade. This would then allow some cracking. Carl Bauer

http://www.bauerconsultbotswana.com

 

[MatBrener]

I am inclined to use a waterproof concrete design rather than liners for a number of reasons that have to do with the detailed design of the hydroponics system itself. I have been looking into a line of waterproofing materials produced by Xypex. The literature claims self-repair of hair line fractures. They also sell a product for repair of larger cracks. I am hoping to be able to design the foundation for the reservoirs in such a way that we will have a fairly good certainty that cracks that do develop will not prove to be a major problem over time. Do you have a suggestion for a waterproofing product (admix)?

When you suggest a mat type foundation - are you suggesting that the reinforcement be greater than for an ordinary slab. I had been expecting that we should have to do this. Is there a guide I can follow for increasing the reinforcement. As you know, there won't be much weight to bear (12" of water)but if increased reinforcement will help our chances in avoiding multiple cracks I would like to investigate it. Is it also advisable to strike contraction joints (these will not interfere with the function of the reservoir).

We are about to begin construction of a prototype to test the design. It will only be one-quarter of the size of the design model but hopefully we will learn a lot about the capabilities of the waterproofing, and our ability to adequately prepare the old surface to support the new slabs.

 

[dik]

I've used Xypex before... works well... been around for decades... Added reinforcement won't hurt, but it's better to put the effort into preparation of the base.

The 12" of water does not seem to be a very high load and should be able to be supported by almost any reasonable base material.

Will take a gander to see what waterproofing admixtures can be used... even for Olympic sized pools, I've just used regular concrete...